Differential photosynthetic performance and photoprotection mechanisms of three Mediterranean evergreen oaks under severe drought stress

Incorporating photosynthetic acclimation improves stomatal optimisation models

Stomatal opening in plant leaves is regulated through a balance of carbon and water exchange under different environmental conditions. Accurate estimation of stomatal regulation is crucial for understanding how plants respond to changing environmental conditions, particularly under climate change. A new generation of optimality-based modelling schemes determines instantaneous stomatal responses from a balance of trade-offs between carbon gains and hydraulic costs, but most such schemes do not account for biochemical acclimation in response to drought. Here, we compare the performance of six instantaneous stomatal optimisation models with and without accounting for photosynthetic acclimation. Using experimental data from 37 plant species, we found that accounting for photosynthetic acclimation improves the prediction of carbon assimilation in a majority of the tested models. Photosynthetic acclimation contributed significantly to the reduction of photosynthesis under drought conditions in all tested models. Drought effects on photosynthesis could not accurately be explained by the hydraulic impairment functions embedded in the stomatal models alone, indicating that photosynthetic acclimation must be considered to improve estimates of carbon assimilation during drought.

Mesophyll conductance limits photosynthesis in fluctuating light under combined drought and heat stresses

Drought and heat stresses usually occur concomitantly in nature, with increasing frequency and intensity of both stresses expected due to climate change. The synergistic agricultural impacts of these compound climate extremes are much greater than those of the individual stresses. However, the mechanisms by which drought and heat stresses separately and concomitantly affect dynamic photosynthesis have not been thoroughly assessed. To elucidate this, we used tomato (Solanum lycopersicum) seedlings to measure dynamic photosynthesis under individual and compound stresses of drought and heat. Individual drought and heat stresses limited dynamic photosynthesis at the stages of diffusional conductance to CO2 and biochemistry, respectively. However, the primary limiting factor for photosynthesis shifted to mesophyll conductance under the compound stresses. Compared with the control, photosynthetic carbon gain in fluctuating light decreased by 38%, 73%, and 114% under the individual drought, heat, and compound stresses, respectively. Therefore, compound stresses caused a greater reduction in photosynthetic carbon gain in fluctuating light conditions than individual stress. These findings highlight the importance of mitigating the effects of compound climate extremes on crop productivity by targeting mesophyll conductance and improving dynamic photosynthesis.

Proteomic and Metabolomic Analysis of the Quercus ilex-Phytophthora cinnamomi Pathosystem Reveals a Population-Specific Response, Independent of Co-Occurrence of Drought

Holm oak (Quercus ilex) is considered to be one of the major structural elements of Mediterranean forests and the agrosilvopastoral Spanish "dehesa", making it an outstanding example of ecological and socioeconomic sustainability in forest ecosystems. The exotic Phytophthora cinnamomi is one of the most aggressive pathogens of woody species and, together with drought, is considered to be one of the main drivers of holm oak decline. The effect of and response to P. cinnamomi inoculation were studied in the offspring of mother trees from two Andalusian populations, Cordoba and Huelva. At the two locations, acorns collected from both symptomatic (damaged) and asymptomatic (apparently healthy) trees were sampled. Damage symptoms, mortality, and chlorophyll fluorescence were evaluated in seedlings inoculated under humid and drought conditions. The effect and response depended on the population and were more apparent in Huelva than in Cordoba. An integrated proteomic and metabolomic analysis revealed the involvement of different metabolic pathways in response to the pathogen in both populations, including amino acid metabolism pathways in Huelva, and terpenoid and flavonoid biosynthesis in Cordoba. However, no differential response was observed between seedlings inoculated under humid and drought conditions. A protective mechanism of the photosynthetic apparatus was activated in response to defective photosynthetic activity in inoculated plants, which seemed to be more efficient in the Cordoba population. In addition, enzymes and metabolites of the phenylpropanoid and flavonoid biosynthesis pathways may have conferred higher resistance in the Cordoba population. Some enzymes are proposed as markers of resilience, among which glyoxalase I, glutathione reductase, thioredoxin reductase, and cinnamyl alcohol dehydrogenase are candidates.

Unveiling resilience mechanisms of Quercus ilex seedlings to severe water stress: Changes in non-structural carbohydrates, xylem hydraulic functionality and wood anatomy

Over the last few decades, extensive dieback and mortality episodes of Quercus ilex L. have been documented after severe drought events in many Mediterranean forests. However, the underlying physiological, anatomical, and biochemical mechanisms remain poorly understood. We investigated the physiological and biochemical processes linked to embolism formation and non-structural carbohydrates (NSCs) dynamics in Q. ilex seedlings exposed to severe water stress and rewatering. Measurements of leaf gas exchange, water relations, non-structural carbohydrates, drought-related gene expression, and anatomical changes in wood parenchyma were assessed. Under water stress, the midday stem water potential dropped below - 4.5 MPa corresponding to a ~ 50 % loss of hydraulic conductivity. A 70 % reduction in stomatal conductance led to a strong depletion of wood NSCs. Starch consumption, resulting from the upregulation of the β-amylase gene BAM3, together with the downregulation of glucose (GPT1) and sucrose (SUC27) transport genes, suggests glucose utilization to sustain cellular metabolism in the wood parenchyma. After rewatering, the presence of residual xylem embolism led to an incomplete recovery of leaf gas exchanges. However, the partial restoration of photosynthesis allowed the accumulation of new starch reserves in the wood parenchyma and the production of new narrower vessels. In addition, changes in the cell wall composition of the wood parenchyma fibers were observed. Our findings indicate that thirty days of rewatering were sufficient to restore the NSCs reserves and growth rates of Q. ilex seedlings and that the carryover effects of water stress were primarily caused by hydraulic dysfunction.

Reversible down-regulation of photosystems I and II leads to fast photosynthesis recovery after long-term drought in Jatropha curcas

Jatropha curcas is a drought-tolerant plant that maintains its photosynthetic pigments under prolonged drought, and quickly regains its photosynthetic capacity when water is available. It has been reported that drought stress leads to increased thermal dissipation in PSII, but that of PSI has been barely investigated, perhaps due to technical limitations in measuring the PSI absolute quantum yield. In this study, we combined biochemical analysis and spectroscopic measurements using an integrating sphere, and verified that the quantum yields of both photosystems are temporarily down-regulated under drought. We found that the decrease in the quantum yield of PSII was accompanied by a decrease in the core complexes of PSII while light-harvesting complexes are maintained under drought. In addition, in drought-treated plants, we observed a decrease in the absolute quantum yield of PSI as compared with the well-watered control, while the amount of PSI did not change, indicating that non-photochemical quenching occurs in PSI. The down-regulation of both photosystems was quickly lifted in a few days upon re-watering. Our results indicate, that in J. curcas under drought, the down-regulation of both PSII and PSI quantum yield protects the photosynthetic machinery from uncontrolled photodamage.

Towards a unified theory of plant photosynthesis and hydraulics

The global carbon and water cycles are governed by the coupling of CO₂ and water vapour exchanges through the leaves of terrestrial plants, controlled by plant adaptations to balance carbon gains and hydraulic risks. We introduce a trait-based optimality theory that unifies the treatment of stomatal responses and biochemical acclimation of plants to environments changing on multiple timescales. Tested with experimental data from 18 species, our model successfully predicts the simultaneous decline in carbon assimilation rate, stomatal conductance and photosynthetic capacity during progressive soil drought. It also correctly predicts the dependencies of gas exchange on atmospheric vapour pressure deficit, temperature and CO₂. Model predictions are also consistent with widely observed empirical patterns, such as the distribution of hydraulic strategies. Our unified theory opens new avenues for reliably modelling the interactive effects of drying soil and rising atmospheric CO₂ on global photosynthesis and transpiration.

Management of Phytophthora cinnamomi Using Fungicides and Host Plant Defense Inducers Under Drought Conditions: A Case Study of Flowering Dogwood

Phytophthora cinnamomi is considered one of the most destructive pathogens of ornamental crops. Different fungicides and host plant defense inducers were tested for their efficacy in managing Phytophthora root rot in drought conditions. In this study, the drought conditions were maintained by evaluating the moisture-holding capacity of the pine bark in a 10.2-cm nursery container. Four controls and nine different treatments were used in two trials for this greenhouse study. All treatments were drench-applied as a preventative or curative treatment. Seedlings were artificially inoculated with P. cinnamomi. Regular irrigation was carried out using overhead irrigation for 1 month after inoculation. Irrigation was regulated by drip irrigation after the first month. A moisture level of 15% to 18% of total moisture-holding capacity was maintained using the gravimetric method throughout the drought period. Physiological parameters of the seedlings were recorded a week after seedlings were drought stressed. In both trials of preventative and curative treatments, all treatments were able to suppress the disease significantly except Orkestra Intrinsic. Orkestra Intrinsic had a disease severity statistically similar to the inoculated and stressed control in trial 1 of the curative treatment. Net photosynthesis, stomatal conductance, and leaf moisture potential were significantly greater in seedlings treated with Subdue MAXX, Signature Xtra, and Empress Intrinsic in both trials of preventative and curative treatments. Effective quantum yield of Photosystem II was significantly lower in the inoculated stressed control in both trials of preventative and curative treatments. Net chlorophyll content through the SPAD meter showed higher values for Subdue MAXX treated seedlings compared with the noninoculated nonstressed controls in trial 1 as both a curative and preventative application. In trial 2, Subdue MAXX and Signature Xtra were the best curative treatments, whereas Empress Intrinsic, Interface, and Subdue MAXX were the best preventative treatments for higher chlorophyll content. This case study will help growers perform successful management of Phytophthora root rot in woody ornamental crops during drought or water deficit conditions.

Short-term warming does not affect intrinsic thermotolerance but induces strong sustaining photoprotection in tropical evergreen citrus genotypes

Consequences of warming and postwarming events on photosynthetic thermotolerance (P_T ) and photoprotective responses in tropical evergreen species remain elusive. We chose Citrus to answer some of the emerging questions related to tropical evergreen species' P_T behaviour including (i) how wide is the genotypic variation in P_T ? (ii) how does P_T respond to short-term warming and (iii) how do photosynthesis and photoprotective functions respond over short-term warming and postwarming events? A study on 21 genotypes revealed significant genotypic differences in P_T , though these were not large. We selected five genotypes with divergent P_T and simulated warming events: T_max 26/20°C (day-time highest maximum/night-time lowest maximum) (Week 1) < T_max 33/30°C (Week 2) < T_max 36/32°C (Week 3) followed by T_max 26/16°C (Week 4, recovery). The P_T of all genotypes remained unaltered despite strong leaf megathermy (leaf temperature > air temperature) during warming events. Though moderate warming showed genotype-specific stimulation in photosynthesis, higher warming unequivocally led to severe loss in net photosynthesis and induced higher nonphotochemical quenching. Even after a week of postwarming, photoprotective mechanisms strongly persisted. Our study points towards a conservative P_T in evergreen citrus genotypes and their need for sustaining higher photoprotection during warming as well as postwarming recovery conditions.

Effect and Response of Quercus ilex subsp. ballota [Desf.] Samp. Seedlings From Three Contrasting Andalusian Populations to Individual and Combined Phytophthora cinnamomi and Drought Stresses

Quercus ilex L. is the dominant species in the Mediterranean forest and agrosilvopastoral ecosystem "dehesa." Currently, this forest species is threatened by natural and anthropogenic agents, especially by the decline syndrome, which is caused by Phytophthora cinnamomi and drought periods. Although the morphological and physiological responses of Q. ilex to combined stress (P. cinnamomi and drought) have been examined already, little is known at the molecular level. In this study, we studied the effect and response of 8-month seedlings from three contrasting Andalusian populations (Seville [Se], Granada [Gr], and Almeria [Al]) to the individual and combined stresses of P. cinnamomi and drought from morphological, physiological, biochemical, and proteomics data. Whereas, seedling damage (leaf chlorosis and necrosis) and mortality were greater under the combined stresses in the three populations, the effect of each individual stress was population-dependent. Resilient individuals were found in all the populations at different percentages. The decrease in leaf chlorophyll fluorescence, photosynthetic activity, and stomatal conductance observed in undamaged seedlings was greater in the presence of both stresses, the three populations responding similarly to drought and P. cinnamomi. Biochemical and proteomic analyses of undamaged seedlings from the two most markedly contrasting populations (Se and Al) revealed the absence of significant differences in the contents in photosynthetic pigments, amino acids, and phenolics among treatments. The Se and Al populations exhibited changes in protein profile in response to the different treatments, with 83 variable proteins in the former population and 223 in the latter. Variable proteins belonged to 16 different functional groups, the best represented among which were protein folding, sorting and degradation, carbohydrate, amino acid, and secondary metabolism, photosynthesis, and ROS scavenging. While photosynthetic proteins were mainly downaccumulated, those of stress-responsive were upaccumulated. Although no treatment-specific response was observed in any functional group, differences in abundance were especially marked under the combined stresses. The following variable proteins are proposed as putative markers for resilience in Q. ilex, namely, aldehyde dehydrogenase, glucose-6-phosphate isomerase, 50S ribosomal protein L5, and α-1,4-glucan-protein synthase [UDP-forming].

Photoprotective Role of Photosynthetic and Non-Photosynthetic Pigments in Phillyrea latifolia: Is Their "Antioxidant" Function Prominent in Leaves Exposed to Severe Summer Drought?

Carotenoids and phenylpropanoids play a dual role of limiting and countering photooxidative stress. We hypothesize that their “antioxidant” function is prominent in plants exposed to summer drought, when climatic conditions exacerbate the light stress. To test this, we conducted a field study on Phillyrea latifolia, a Mediterranean evergreen shrub, carrying out daily physiological and biochemical analyses in spring and summer. We also investigated the functional role of the major phenylpropanoids in different leaf tissues. Summer leaves underwent the most severe drought stress concomitantly with a reduction in radiation use efficiency upon being exposed to intense photooxidative stress, particularly during the central hours of the day. In parallel, a significant daily variation in both carotenoids and phenylpropanoids was observed. Our data suggest that the morning-to-midday increase in zeaxanthin derived from the hydroxylation of ß-carotene to sustain non-photochemical quenching and limit lipid peroxidation in thylakoid membranes. We observed substantial spring-to-summer and morning-to-midday increases in quercetin and luteolin derivatives, mostly in the leaf mesophyll. These findings highlight their importance as antioxidants, countering the drought-induced photooxidative stress. We concluded that seasonal and daily changes in photosynthetic and non-photosynthetic pigments may allow P. latifolia leaves to avoid irreversible photodamage and to cope successfully with the Mediterranean harsh climate.

Early Holocene greening of the Sahara requires Mediterranean winter rainfall

The greening of the Sahara, associated with the African Humid Period (AHP) between ca. 14,500 and 5,000 y ago, is arguably the largest climate-induced environmental change in the Holocene; it is usually explained by the strengthening and northward expansion of the African monsoon in response to orbital forcing. However, the strengthened monsoon in Early to Middle Holocene climate model simulations cannot sustain vegetation in the Sahara or account for the increased humidity in the Mediterranean region. Here, we present an 18,500-y pollen and leaf-wax δD record from Lake Tislit (32° N) in Morocco, which provides quantitative reconstruction of winter and summer precipitation in northern Africa. The record from Lake Tislit shows that the northern Sahara and the Mediterranean region were wetter in the AHP because of increased winter precipitation and were not influenced by the monsoon. The increased seasonal contrast of insolation led to an intensification and southward shift of the Mediterranean winter precipitation system in addition to the intensified summer monsoon. Therefore, a winter rainfall zone must have met and possibly overlapped the monsoonal zone in the Sahara. Using a mechanistic vegetation model in Early Holocene conditions, we show that this seasonal distribution of rainfall is more efficient than the increased monsoon alone in generating a green Sahara vegetation cover, in agreement with observed vegetation. This conceptual framework should be taken into consideration in Earth system paleoclimate simulations used to explore the mechanisms of African climatic and environmental sensitivity.

Specific leaf metabolic changes that underlie adjustment of osmotic potential in response to drought by four Quercus species

Osmotic adjustment is almost ubiquitous as a mechanism of response to drought in many forest species. Recognized as an important mechanism of increasing turgor under water stress, the metabolic basis for osmotic adjustment has been described in only a few species. We set an experiment with four species of the genus Quercus ranked according to drought tolerance and leaf habit from evergreen to broad-leaved deciduous. A cycle of watering deprivation was imposed on seedlings, resulting in well-watered (WW) and water-stressed (WS) treatments, and their water relations were assessed from pressure-volume curves. Leaf predawn water potential (Ψpd) significantly decreased in WS seedlings, which was followed by a drop in leaf osmotic potential at full turgor (Ψπ100). The lowest values of Ψπ100 followed the ranking of decreasing drought tolerance: Quercus ilex L. < Quercus faginea Lam. < Quercus pyrenaica Willd. < Quercus petraea Matt. Liebl. The leaf osmotic potential at the turgor loss point (ΨTLP) followed the same pattern as Ψπ100 across species and treatments. The pool of carbohydrates, some organic acids and cyclitols were the main osmolytes explaining osmotic potential across species, likewise to the osmotic adjustment assessed from the decrease in leaf Ψπ100 between WW and WS seedlings. Amino acids were very responsive to WS, particularly γ-aminobutyric acid in Q. pyrenaica, but made a relatively minor contribution to osmotic potential compared with other groups of compounds. In contrast, the cyclitol proto-quercitol made a prominent contribution to the changes in osmotic potential regardless of watering treatment or species. However, different metabolites, such as quinic acid, played a more important role in osmotic adjustment in Q. ilex, distinguishing it from the other species studied. In conclusion, while osmotic adjustment was present in all four Quercus species, the molecular processes underpinning this response differed according to their phylogenetic history and specific ecology.

Molecular Research on Stress Responses in Quercus spp.: From Classical Biochemistry to Systems Biology through Omics Analysis

The genus Quercus (oak), family Fagaceae, comprises around 500 species, being one of the most important and dominant woody angiosperms in the Northern Hemisphere. Nowadays, it is threatened by environmental cues, which are either of biotic or abiotic origin. This causes tree decline, dieback, and deforestation, which can worsen in a climate change scenario. In the 21st century, biotechnology should take a pivotal role in facing this problem and proposing sustainable management and conservation strategies for forests. As a non-domesticated, long-lived species, the only plausible approach for tree breeding is exploiting the natural diversity present in this species and the selection of elite, more resilient genotypes, based on molecular markers. In this direction, it is important to investigate the molecular mechanisms of the tolerance or resistance to stresses, and the identification of genes, gene products, and metabolites related to this phenotype. This research is being performed by using classical biochemistry or the most recent omics (genomics, epigenomics, transcriptomics, proteomics, and metabolomics) approaches, which should be integrated with other physiological and morphological techniques in the Systems Biology direction. This review is focused on the current state-of-the-art of such approaches for describing and integrating the latest knowledge on biotic and abiotic stress responses in Quercus spp., with special reference to Quercus ilex, the system on which the authors have been working for the last 15 years. While biotic stress factors mainly include fungi and insects such as Phytophthora cinnamomi, Cerambyx welensii, and Operophtera brumata, abiotic stress factors include salinity, drought, waterlogging, soil pollutants, cold, heat, carbon dioxide, ozone, and ultraviolet radiation. The review is structured following the Central Dogma of Molecular Biology and the omic cascade, from DNA (genomics, epigenomics, and DNA-based markers) to metabolites (metabolomics), through mRNA (transcriptomics) and proteins (proteomics). An integrated view of the different approaches, challenges, and future directions is critically discussed.

Contrasting functional strategies following severe drought in two Mediterranean oaks with different leaf habit: Quercus faginea and Quercus ilex subsp. rotundifolia

Nowadays, evergreen sclerophyllous and winter-deciduous malacophyllous oaks with different paleogeographical origins coexist under Mediterranean-type climates, such as the mixed forests of the evergreen Quercus ilex subsp. rotundifolia Lam. and the winter-deciduous Quercus faginea Lam. Both Mediterranean oaks constitute two examples of contrasting leaf habit, so it could be expected that they would have different functional strategies to cope with summer drought. In this study, we analysed photosynthetic, photochemical and hydraulic traits of different organs for Q. faginea and Q. ilex subsp. rotundifolia under well-watered conditions and subjected to very severe drought. The coordinated response between photosynthetic and hydraulic traits explained the higher photosynthetic capacity of Q. faginea under well-watered conditions, which compensated its shorter leaf life span at the expense of higher water consumption. The progressive imposition of water stress evidenced that both types of Mediterranean oaks displayed different functional strategies to cope with water limitations. Specifically, the decrease in mesophyll conductance associated with edaphic drought seems to be the main factor explaining the differences found in the dynamics of net CO2 assimilation throughout the drought period. The sharp decline in photosynthetic traits of Q. faginea was coupled with a strong decrease in shoot hydraulic conductance in response to drought. This fact probably avoided extensive xylem embolism in the stems (i.e., 'vulnerability segmentation'), which enabled new leaf development after drought period in Q. faginea. By contrast, leaves of Q. ilex subsp. rotundifolia showed effective photoprotective mechanisms and high resistance to drought-induced cavitation, which would be related with the longer leaf life span of the evergreen Mediterranean oaks. The co-occurrence of both types of Mediterranean oaks could be related to edaphic conditions that ensure the maintenance of soil water potential above critical values for Q. faginea, which can be severely affected by soil degradation and climate change.

Southeastern Rear Edge Populations of Quercus suber L. Showed Two Alternative Strategies to Cope with Water Stress

Climate change models predict an increase in aridity, especially in the regions under Mediterranean-type climates such as the Mediterranean Basin. However, there is a lack of ecophysiological studies supporting the selection of the more drought-adapted ecotypes for reforestation programs. In this study, we analyzed the anatomical and functional adaptations of 18-month-old seedlings to drought on 16 Quercus suber L. populations from the southeastern rear edge of the species distribution in northern Tunisia growing in a common garden, in order to identify the most appropriate material to use in reforestations. The results evidenced that populations from more xeric habitats displayed the highest leaf dry mass per area (LMA) and lowest leaf area (LA) values, together with the largest increase in the bulk modulus of elasticity (Δε) in response to drought (i.e., elastic adjustment). On the other hand, some populations with intermediate values of aridity, LMA and LA displayed the sharpest increase in proline concentration (ΔPro), with a concomitant increase in osmotic potential at full turgor (Δπo) (i.e., osmotic adjustment). Therefore, two different strategies seem to drive the within-species variation of the studied Q. suber populations in response to water scarcity: (i) a water saver strategy for improving water stress tolerance through the maximization of the elastic adjustment; and (ii) a water spender strategy for maintaining water absorption and photosynthetic activity under moderate water stress through the maximization of the osmotic adjustment. We concluded that the higher elastic adjustment, together with reduced LA and increased LMA, implied a better performance under drought stress in the populations of Q. suber from more xeric habitats, which can be considered the most drought-adapted ecotypes and, consequently, the most appropriate for reforestation programs under an eventual increase in aridity.

The Light Dependence of Mesophyll Conductance and Relative Limitations on Photosynthesis in Evergreen Sclerophyllous Rhododendron Species

Mesophyll conductance (g_m) limits CO₂ diffusion from sub-stomatal internal cavities to the sites of RuBP carboxylation. However, the response of g_m to light intensity remains controversial. Furthermore, little is known about the light response of relative mesophyll conductance limitation (l_m) and its effect on photosynthesis. In this study, we measured chlorophyll fluorescence and gas exchange in nine evergreen sclerophyllous Rhododendron species. g_m was maintained stable across light intensities from 300 to 1500 μmol photons m^-2 s^-1 in all these species, indicating that g_m did not respond to the change in illumination in them. With an increase in light intensity, l_m gradually increased, making g_m the major limiting factor for area-based photosynthesis (A_N) under saturating light. A strong negative relationship between l_m and A_N was found at 300 μmol photons m^-2 s^-1 but disappeared at 1500 μmol photons m^-2 s^-1, suggesting an important role for l_m in determining A_N at sub-saturating light. Furthermore, the light-dependent increase in l_m led to a decrease in chloroplast CO₂ concentration (C_c), inducing the gradual increase of photorespiration. A higher l_m under saturating light made A_N more limited by RuBP carboxylation. These results indicate that the light response of l_m plays significant roles in determining C_c, photorespiration, and the rate-limiting step of A_N.

Living in Drylands: Functional Adaptations of Trees and Shrubs to Cope with High Temperatures and Water Scarcity

Plant functioning and survival in drylands are affected by the combination of high solar radiation, high temperatures, low relative humidity, and the scarcity of available water. Many ecophysiological studies have dealt with the adaptation of plants to cope with these stresses in hot deserts, which are the territories that have better evoked the idea of a dryland. Nevertheless, drylands can also be found in some other areas of the Earth that are under the Mediterranean-type climates, which imposes a strong aridity during summer. In this review, plant species from hot deserts and Mediterranean-type climates serve as examples for describing and analyzing the different responses of trees and shrubs to aridity in drylands, with special emphasis on the structural and functional adaptations of plants to avoid the negative effects of high temperatures under drought conditions. First, we analyze the adaptations of plants to reduce the input of energy by diminishing the absorbed solar radiation through (i) modifications of leaf angle and (ii) changes in leaf optical properties. Afterwards, we analyze several strategies that enhance the ability for heat dissipation through (i) leaf size reduction and changes in leaf shape (e.g., through lobed leaves), and (ii) increased transpiration rates (i.e., water-spender strategy), with negative consequences in terms of photosynthetic capacity and water consumption, respectively. Finally, we also discuss the alternative strategy showed by water-saver plants, a common drought resistance strategy in hot and dry environments that reduces water consumption at the expense of diminishing the ability for leaf cooling. In conclusion, trees and shrubs living in drylands have developed effective functional adaptations to cope with the combination of high temperature and water scarcity, all of them with clear benefits for plant functioning and survival, but also with different costs concerning water use, carbon gain, and/or leaf cooling.

Correlated evolution of morphology, gas exchange, growth rates and hydraulics as a response to precipitation and temperature regimes in oaks (Quercus)

It is hypothesised that tree distributions in Europe are largely limited by their ability to cope with the summer drought imposed by the Mediterranean climate in the southern areas and by their competitive potential in central regions with more mesic conditions. We investigated the extent to which leaf and plant morphology, gas exchange, leaf and stem hydraulics and growth rates have evolved in a coordinated way in oaks (Quercus) as a result of adaptation to contrasting environmental conditions in this region. We implemented an experiment in which seedlings of 12 European/North African oaks were grown under two watering treatments, a well-watered treatment and a drought treatment in which plants were subjected to three cycles of drought. Consistent with our hypothesis, species from drier summers had traits conferring more tolerance to drought such as small sclerophyllous leaves and lower percent loss of hydraulic conductivity. However, these species did not have lower growth rates as expected by a trade-off with drought tolerance. Overall, our results revealed that climate is an important driver of functional strategies in oaks and that traits have evolved along two coordinated functional axes to adapt to different precipitation and temperature regimes.

Responses and Differences in Tolerance to Water Shortage under Climatic Dryness Conditions in Seedlings from Quercus spp. and Andalusian Q. ilex Populations

Analyzing differences in tolerance to drought in Quercus spp., and the characterization of these responses at the species and individual population level, are imperative for the selection of resilient elite genotypes in reforestation programs. The main objective of this work was to evaluate differences in the response and tolerance to water shortage under in five Quercus spp. and five Andalusian Q. ilex populations at the inter- and intraspecies level. Six-month-old seedlings grown in perlite were subjected to drought treatments by withholding water for 28 days under mean 37 °C temperature, 28 W m−2 solar irradiance, and 41% humidity. The use of perlite as the substrate enabled the establishment of severe drought stress with reduction in water availability from 73% (field capacity) to 28% (dryness), corresponding to matric potentials of 0 and −30 kPa. Damage symptoms, mortality rate, leaf water content, photosynthetic, and biochemical parameters (amino acids, sugars, phenolics, and pigments) were determined. At the phenotypic level, based on damage symptoms and mortality, Q. ilex behaved as the most drought tolerant species. Drought caused a significant decrease in leaf fluorescence, photosynthesis rate, and stomatal conductance in all Quercus spp. analyzed, being less pronounced in Q. ilex. There were not differences between irrigated and non-irrigated Q. ilex seedlings in the content of sugar and photosynthetic pigments, while the total amino acid and phenolic content significantly increased under drought conditions. As a response to drought, living Q. ilex seedlings adjust stomata opening and gas exchange, and keep hydrated, photosynthetically active, and metabolically competent. At the population level, based on damage symptoms, mortality, and physiological parameters, the eastern Andalusian populations were more tolerant than the western ones. These observations inform the basis for the selection of resilient genotypes to be used in breeding and reforestation programs.

Photorespiration is the major alternative electron sink under high light in alpine evergreen sclerophyllous Rhododendron species

Owing to the high leaf mass per area, alpine evergreen sclerophyllous Rhododendron have low values of mesophyll conductance (g_m). The resulting low chloroplast CO₂ concentration aggravates photorespiration, which requires a higher ATP/NADPH ratio. However, the significance of photorespiration and underlying mechanisms of energy balance in these species are little known. In this study, eight alpine evergreen sclerophyllous Rhododendron species grown in a common garden were tested for their g_m, electron flow to photorespiration, and energy balancing. Under saturating light, g_m was the most limiting factor for net photosynthesis (A_N) in all species, and the species differences in A_N were primarily driven by g_m rather than stomatal conductance. The total electron flow through photosystem II (ETRII) nearly equaled the electron transport required for Rubisco carboxylation and oxygenation. Furthermore, blocking electron flow to photosystem I with appropriate inhibitors showed that electron flow to plastic terminal oxidase was not observed. As a result, these studied species showed little alternative electron flow mediated by water-water cycle or plastic terminal oxidase. By comparison, the ratio of electron transport consumed by photorespiration to ETRII (J_PR/ETRII), ranging from 43%∼55%, was negatively correlated to A_N and g_m. Furthermore, the increased ATP production required by enhanced photorespiration was regulated by cyclic electron flow around photosystem I. These results indicate that photorespiration is the major electron sink for dissipation of excess excitation energy in the alpine evergreen sclerophyllous Rhododendron species. The coordination of g_m, photorespiration and cyclic electron flow is important for sustaining leaf photosynthesis.

Comparative photosynthetic responses of Norway spruce and Scots pine seedlings to prolonged water deficiency

Stressors of different natures, including drought stress, substantially compromise the ability of plants to effectively and safely utilize light energy. We investigated the influence of water stress on the photosynthetic processes in Picea abies and Pinus sylvestris, two species with contrasting drought sensitivities. Spruce and pine seedlings were exposed to polyethylene glycol 6000-induced water deficits of different intensities and durations. The maintenance of photosystem I (PSI) oxidation in spruce required increased photosynthetic control and led to the increased reduction of the plastoquinone pool, which was not the case in pine seedlings. As a result of increased excitation pressure, photosystem II (PSII) inactivation was observed in spruce plants, whereas in pine, the decreased PSII photochemistry was likely due to sustained non-photochemical quenching. Downregulation of PSII photochemistry and maintenance of PSI in an oxidized state were linked with the prevention of oxidative stress, even under severe water deficit. The decreased photosynthetic pigment content and photosynthetic gene expression suggested the coordinated downregulation of photosynthetic apparatus components under water stress to reduce light energy absorption. In summary, the observed adaptative mechanisms of pine and spruce to water stress may be similar to the well-studied adaptative mechanisms to winter stress, which may indicate the universality of protective mechanisms under various stresses in conifers.

Differences in isoprenoid-mediated energy dissipation pathways between coastal and interior Douglas-fir seedlings in response to drought

Plants have evolved energy dissipation pathways to reduce photooxidative damage under drought when photosynthesis is hampered. Non-volatile and volatile isoprenoids are involved in non-photochemical quenching of excess light energy and scavenging of reactive oxygen species. A better understanding of trees' ability to cope with and withstand drought stress will contribute to mitigate the negative effects of prolonged drought periods expected under future climate conditions. Therefore we investigated if Douglas-fir (Pseudotsuga menziesii(Mirb.)) provenances from habitats with contrasting water availability reveal intraspecific variation in isoprenoid-mediated energy dissipation pathways. In a controlled drought experiment with 1-year-old seedlings of an interior and a coastal Douglas-fir provenance, we assessed the photosynthetic capacity, pool sizes of non-volatile isoprenoids associated with the photosynthetic apparatus, as well as pool sizes and emission of volatile isoprenoids. We observed variation in the amount and composition of non-volatile and volatile isoprenoids among provenances, which could be linked to variation in photosynthetic capacity under drought. The coastal provenance exhibited an enhanced biosynthesis and emission of volatile isoprenoids, which is likely sustained by generally higher assimilation rates under drought. In contrast, the interior provenance showed an enhanced photoprotection of the photosynthetic apparatus by generally higher amounts of non-volatile isoprenoids and increased amounts of xanthophyll cycle pigments under drought. Our results demonstrate that there is intraspecific variation in isoprenoid-mediated energy dissipation pathways among Douglas-fir provenances, which may be important traits when selecting provenances suitable to grow under future climate conditions.

Quantification of leaf-scale light energy allocation and photoprotection processes in a Mediterranean pine forest under extensive seasonal drought

Photoprotection strategies in a Pinus halepensis Mill. forest at the dry timberline that shows sustained photosynthetic activity during 6-7 month summer drought were characterized and quantified under field conditions. Measurements of chlorophyll fluorescence, leaf-level gas exchange and pigment concentrations were made in both control and summer-irrigated plots, providing the opportunity to separate the effects of atmospheric from soil water stress on the photoprotection responses. The proportion of light energy incident on the leaf surface ultimately being used for carbon assimilation was 18% under stress-free conditions (irrigated, winter), declining to 4% under maximal stress (control, summer). Allocation of absorbed light energy to photochemistry decreased from 25 to 15% (control) and from 50% to 30% (irrigated) between winter and summer, highlighting the important role of pigment-mediated energy dissipation processes. Photorespiration or other non-assimilatory electron flow accounted for 15-20% and ~10% of incident light energy during periods of high and low carbon fixation, respectively, representing a proportional increase in photochemical energy going to photorespiration in summer but a decrease in the absolute amount of photorespiratory CO2 loss. Resilience of the leaf photochemical apparatus was expressed in the complete recovery of photosystem II (PSII) efficiency (ΦPSII) and relaxation of the xanthophyll de-epoxidation state on the diurnal cycle throughout the year, and no seasonal decrease in pre-dawn maximal PSII efficiency (Fv/Fm). The response of CO2 assimilation and photoprotection strategies to stomatal conductance and leaf water potential appeared independent of whether stress was due to atmospheric or soil water deficits across seasons and treatments. The range of protection characteristics identified provides insights into the relatively high carbon economy under these dry conditions, conditions that are predicted for extended areas in the Mediterranean and other regions due to global climate change.

Cavitation Limits the Recovery of Gas Exchange after Severe Drought Stress in Holm Oak (Quercus ilex L.)

Holm oak (Quercus ilex L.) is a Mediterranean species that can withstand intense summer drought through a high resistance to cavitation far beyond the stomatal closure. Besides stomatal limitations, both mesophyll and biochemical limitations to CO2 uptake could increase in holm oak under drought. However, no studies have addressed how hydraulic and non-hydraulic factors may limit the recovery of photosynthesis when re-watering after inducing 50% loss of hydraulic conductivity. We measured photosynthetic traits, xylem embolism, and abscisic acid (ABA) in holm oak with increasing levels of drought stress and seven days after plant re-watering. Drought stress caused a sharp decrease in net CO2 assimilation (AN), stomatal and mesophyll conductance (gs and gm), and maximum velocity of carboxylation (Vcmax). The stomatal closure could be mediated by the rapid increase found in ABA. The high level of xylem embolism explained the strong down-regulation of gs even after re-watering. Therefore, only a partial recovery of AN was observed, in spite of non-hydraulic factors not limiting the recovery of AN, because i/ABA strongly decreased after re-watering, and ii/gm and Vcmax recovered their original values. Therefore, the hydraulic-stomatal limitation model would be involved in the partial recovery of AN, in order to prevent extensive xylem embolism under subsequent drought events that could compromise holm oak survival.

A molecular approach to drought-induced reduction in leaf CO2 exchange in drought-resistant Quercus ilex

Drought-induced reduction of leaf gas exchange entails a complex regulation of the plant leaf metabolism. We used a combined molecular and physiological approach to understand leaf photosynthetic and respiratory responses of 2-year-old Quercus ilex seedlings to drought. Mild drought stress resulted in glucose accumulation while net photosynthetic CO₂ uptake (P_n ) remained unchanged, suggesting a role of glucose in stress signaling and/or osmoregulation. Simple sugars and sugar alcohols increased throughout moderate-to-very severe drought stress conditions, in parallel to a progressive decline in P_n and the quantum efficiency of photosystem II; by contrast, minor changes occurred in respiration rates until drought stress was very severe. At very severe drought stress, 2-oxoglutarate dehydrogenase complex gene expression significantly decreased, and the abundance of most amino acids dramatically increased, especially that of proline and γ-aminobutyric acid (GABA) suggesting enhanced protection against oxidative damage and a reorganization of the tricarboxylic cycle acid cycle via the GABA shunt. Altogether, our results point to Q. ilex drought tolerance being linked to signaling and osmoregulation by hexoses during early stages of drought stress, and enhanced protection against oxidative damage by polyols and amino acids under severe drought stress.

Cell-level anatomical characteristics explain high mesophyll conductance and photosynthetic capacity in sclerophyllous Mediterranean oaks

Leaf mass per area (LMA) has been suggested to negatively affect the mesophyll conductance to CO₂ (g_m ), which is the most limiting factor for area-based photosynthesis (A_N ) in many Mediterranean sclerophyll species. However, despite their high LMA, these species have similar A_N to plants from other biomes. Variations in other leaf anatomical traits, such as mesophyll and chloroplast surface area exposed to intercellular air space (S_m /S and S_c /S), may offset the restrictions imposed by high LMA in g_m and A_N in these species. Seven sclerophyllous Mediterranean oaks from Europe/North Africa and North America with contrasting LMA were compared in terms of morphological, anatomical and photosynthetic traits. Mediterranean oaks showed specific differences in A_N that go beyond the common morphological leaf traits reported for these species (reduced leaf area and thick leaves). These variations resulted mainly from the differences in g_m , the most limiting factor for carbon assimilation in these species. Species with higher A_N showed increased S_c /S, which implies increased g_m without changes in stomatal conductance. The occurrence of this anatomical adaptation at the cell level allowed evergreen oaks to reach A_N values comparable to congeneric deciduous species despite their higher LMA.

Variation in short-term and long-term responses of photosynthesis and isoprenoid-mediated photoprotection to soil water availability in four Douglas-fir provenances

For long-lived forest tree species, the understanding of intraspecific variation among populations and their response to water availability can reveal their ability to cope with and adapt to climate change. Dissipation of excess excitation energy, mediated by photoprotective isoprenoids, is an important defense mechanism against drought and high light when photosynthesis is hampered. We used 50-year-old Douglas-fir trees of four provenances at two common garden experiments to characterize provenance-specific variation in photosynthesis and photoprotective mechanisms mediated by essential and non-essential isoprenoids in response to soil water availability and solar radiation. All provenances revealed uniform photoprotective responses to high solar radiation, including increased de-epoxidation of photoprotective xanthophyll cycle pigments and enhanced emission of volatile monoterpenes. In contrast, we observed differences between provenances in response to drought, where provenances sustaining higher CO2 assimilation rates also revealed increased water-use efficiency, carotenoid-chlorophyll ratios, pools of xanthophyll cycle pigments, β-carotene and stored monoterpenes. Our results demonstrate that local adaptation to contrasting habitats affected chlorophyll-carotenoid ratios, pool sizes of photoprotective xanthophylls, β-carotene, and stored volatile isoprenoids. We conclude that intraspecific variation in isoprenoid-mediated photoprotective mechanisms contributes to the adaptive potential of Douglas-fir provenances to climate change.

Taxonomic and ecological relevance of the chlorophyll a fluorescence signature of tree species in mixed European forests

The variability of chlorophyll a fluorescence (ChlF) parameters of forest tree species was investigated in 209 stands belonging to six European forests, from Mediterranean to boreal regions. The modifying role of environmental factors, forest structure and tree diversity (species richness and composition) on ChlF signature was analysed. At the European level, conifers showed higher potential performance than broadleaf species. Forests in central Europe performed better than those in Mediterranean and boreal regions. At the site level, homogeneous clusters of tree species were identified by means of a principal component analysis (PCA) of ChlF parameters. The discrimination of the clusters of species was influenced by their taxonomic position and ecological characteristics. The species richness influenced the tree ChlF properties in different ways depending on tree species and site. Tree species and site also affected the relationships between ChlF parameters and other plant functional traits (specific leaf area, leaf nitrogen content, light-saturated photosynthesis, wood density, leaf carbon isotope composition). The assessment of the photosynthetic properties of tree species, by means of ChlF parameters, in relation to their functional traits, is a relevant issue for studies in forest ecology. The connections of data from field surveys with remotely assessed parameters must be carefully explored.

Leaf functional plasticity decreases the water consumption without further consequences for carbon uptake in Quercus coccifera L. under Mediterranean conditions

The accumulation of epicuticular waxes over stomata in Quercus coccifera L. contributes to a severe reduction in maximum stomatal conductance (g s,max) under Mediterranean (MED) conditions. However, this phenomenon was not observed in this species under temperate (TEM) conditions, which could lead to differences in the ability to assimilate CO2 between the sites. We hypothesise that the overall importance of such a reduction in gs,max on photosynthesis is modulated by other factors affecting carbon gain, mainly mesophyll conductance to CO2 (g m), through a plastic response to changes in environmental conditions (i.e., vapour pressure deficit, VPD, and mean daily quantum flux density, Q int). The results reveal that leaves grown at the TEM site did not show an increased ability for net CO2 assimilation (A N), mainly due to an equal gm at both sites. This fact is explained by a trade-off between an increased conductance of the gas phase (g ias) and a reduced conductance of the liquid phase (g liq) at the TEM site compared with the MED site. In spite of the reduction in gs,max at the MED site, transpiration (E) did not diminish during midsummer to the levels of the TEM site due to a higher VPD found at the MED site, yielding a higher water use efficiency (AN/E) at the TEM site. Moreover, photosynthetic nitrogen use efficiency was also higher at the TEM site, indicating these leaves can reach similar values of AN with lower nitrogen investment that those at the MED site. These results suggest that Q. coccifera does not always use the main resources (water and nutrients) at leaf level as efficiently as possible. Moreover, the different patterns of resource use (in particular N), together with the functional plasticity, cannot overcome the morpho-functional constraints that limit photosynthetic activity, even under potentially favourable conditions.

Climatic origins predict variation in photoprotective leaf pigments in response to drought and low temperatures in live oaks (Quercus series Virentes)

Climate is a major selective force in nature. Exploring patterns of inter- and intraspecific genetic variation in functional traits may explain how species have evolved and may continue evolving under future climate change. Photoprotective pigments play an important role in short-term responses to climate stress in plants but knowledge of their long-term role in adaptive processes is lacking. In this study, our goal was to determine how photoprotective mechanisms, morphological traits and their plasticity have evolved in live oaks (Quercus series Virentes) in response to different climatic conditions. For this purpose, seedlings originating from 11 populations from four live oak species (Quercus virginiana, Q. geminata, Q. fusiformis and Q. oleoides) were grown under contrasting common environmental conditions of temperature (tropical vs temperate) and water availability (droughted vs well-watered). Xanthophyll cycle pigments, anthocyanin accumulation, chlorophyll fluorescence parameters and leaf anatomical traits were measured. Seedlings originating from more mesic source populations of Q. oleoides and Q. fusiformis increased the xanthophyll de-epoxidation state under water-limiting conditions and showed higher phenotypic plasticity for this trait, suggesting adaptation to local climate. Likewise, seedlings originating from warmer climates had higher anthocyanin concentration in leaves under cold winter conditions but not higher de-epoxidation state. Overall, our findings suggest that (i) climate has been a key factor in shaping species and population differences in stress tolerance for live oaks, (ii) anthocyanins are used under cold stress in species with limited freezing tolerance and (iii) xanthophyll cycle pigments are used when photoprotection under drought conditions is needed.

Is there a species spectrum within the world-wide leaf economics spectrum? Major variations in leaf functional traits in the Mediterranean sclerophyll Quercus ilex

The leaf economics spectrum is a general concept describing coordinated variation in foliage structural, chemical and physiological traits across resource gradients. Yet, within this concept,the role of within-species variation, including ecotypic and plastic variation components, has been largely neglected. This study hypothesized that there is a within-species economics spectrum within the general spectrum in the evergreen sclerophyll Quercus ilex which dominates low resource ecosystems over an exceptionally wide range. An extensive database of foliage traits covering the full species range was constructed, and improved filtering algorithms were developed. Standardized data filtering was deemed absolutely essential as additional variation sources can result in trait variation of 10–300%,blurring the broad relationships. Strong trait variation, c. two-fold for most traits to up to almost an order of magnitude, was uncovered.Although the Q. ilex spectrum is part of the general spectrum, within-species trait and climatic relationships in this species partly differed from the overall spectrum. Contrary to world-wide trends, Q. ilex does not necessarily have a low nitrogen content per mass and can increase photosynthetic capacity with increasing foliage robustness. This study argues that the within-species economics spectrum needs to be considered in regional- to biome-level analyses.

Morphological and physiological divergences within Quercus ilex support the existence of different ecotypes depending on climatic dryness

BACKGROUND AND AIMS

Several studies show apparently contradictory findings about the functional convergence within the Mediterranean woody flora. In this context, this study evaluates the variability of functional traits within holm oak (Quercus ilex) to elucidate whether provenances corresponding to different morphotypes represent different ecotypes locally adapted to the prevaling stress levels.

METHODS

Several morphological and physiological traits were measured at leaf and shoot levels in 9-year-old seedlings of seven Q. ilex provenances including all recognized morphotypes. Plants were grown in a common garden for 9 years under the same environmental conditions to avoid possible biases due to site-specific characteristics.

KEY RESULTS

Leaf morphometry clearly separates holm oak provenances into 'ilex' (more elongated leaves with low vein density) and 'rotundifolia' (short and rounded leaves with high vein density) morphotypes. Moreover, these morphotypes represent two consistent and very contrasting functional types in response to dry climates, mainly in terms of leaf area, major vein density, leaf specific conductivity, resistance to drought-induced cavitation and turgor loss point.

CONCLUSIONS

The 'ilex' and 'rotundifolia' morphotypes correspond to different ecotypes as inferred from their contrasting functional traits. To the best of our knowledge, this is the first time that the combined use of morphological and physiological traits has provided support for the concept of these two holm oak morphotypes being regarded as two different species.

Physiological and proteomic analyses of drought stress response in Holm oak provenances

Responses to drought stress by water withholding have been studied in 1 year old Holm oak (Quercus ilex subsp. ballota [Desf.] Samp.) seedlings from seven provenances from Andalusia (southern Spain). Several physiological parameters, including predawn xylem water potentials and relative water content in soil, roots, and leaves as well as maximum quantum efficiency and yield of PSII were evaluated for 28 days in both irrigated and nonirrigated seedlings. The leaf proteome map of the two provenances that show the extreme responses (Seville, GSE, is the most susceptible, while Almerı́a, SSA, is the least susceptible) was obtained. Statistically significant variable spots among provenances and treatments were subjected to MALDI-TOF/TOF-MS/MS analysis for protein identification. In response to drought stress, ~12.4% of the reproducible spots varied significantly depending on the treatment and the population. These variable proteins were mainly chloroplastic and belonged to the metabolism and defense/stress functional categories. The 2-DE protein profile of nonirrigated seedlings was similar in both provenances. Physiological and proteomics data were generally in good agreement. The general trend was a decrease in protein abundance upon water withholding in both provenances, mainly in those involved in ATP synthesis and photosynthesis. This decrease, moreover, was most marked in the most susceptible population compared with the less susceptible one.

Physiological and proteomics analyses of Holm oak (Quercus ilex subsp. ballota [Desf.] Samp.) responses to Phytophthora cinnamomi

Phytophthora cinnamomi is one of the agents that trigger the decline syndrome in Quercus spp., this being a serious threat to Mediterranean Holm oak forest sustainability and reforestation programs. Quercus ilex responses to Phytophthora cinnamomi have been studied in one-year olds seedlings from two Andalucía provenances, assessing the physiological water status and photosynthesis-related parameters. Upon inoculation with mycelium a reduction in water content, chlorophyll fluorescence, stomatal conductance and gas exchange was observed along a 90 days post inoculation period in both provenances. The reduction was higher in the most susceptible (SSA) provenance, than in the most tolerant (PCO), being these typical plant responses to drought stress. Leaf protein profiles were analyzed in non-inoculated and inoculated seedlings from the two provenances by using a 2-DE coupled to MS proteomics strategy. Ninety seven proteins changing in abundance in response to the inoculation were successfully identified after MALDI-TOF-TOF analyses. The largest group of variable identified proteins were chloroplasts ones, and they were involved in the photosynthesis, Calvin cycle and carbohydrate metabolism. It was noted that a general tendency was a decrease in the protein abundance as a consequence of the inoculation, being it less accused in the least susceptible, the Northern provenance (PCO), than in the most susceptible, the Southern provenance (SSA). This trend is clearly manifested in photosynthesis, amino acid metabolism and stress/defence proteins. On the contrary, some proteins related to starch biosynthesis, glycolysis and stress related peroxiredoxin showed an increase upon inoculation. These changes in protein abundance were correlated to the estimated physiological parameters and have been frequently observed in plants subjected to drought stress.

Three pools of zeaxanthin in Quercus coccifera leaves during light transitions with different roles in rapidly reversible photoprotective energy dissipation and photoprotection

Under excess light, the efficient PSII light-harvesting antenna is switched into a photoprotected state in which potentially harmful absorbed energy is thermally dissipated. Changes occur rapidly and reversibly, enhanced by de-epoxidation of violaxanthin (V) to zeaxanthin (Z). This process is usually measured as non-photochemical quenching (NPQ) of chlorophyll (Chl) fluorescence. Using instrumentation for instantaneous leaf freezing, NPQ, spectral reflectance, and interconversions within the xanthophyll cycle with time resolution of seconds were recorded from Quercus coccifera leaves during low light (LL) to high light (HL) transitions, followed by relaxation at LL. During the first 30 s of both the LL to HL and HL to LL transitions, no activity of the xanthophyll cycle was detected, whereas 70-75% of the NPQ was formed and relaxed, respectively, by that time, the latter being traits of a rapidly reversible photoprotective energy dissipation. Three different Z pools were identified, which play different roles in energy dissipation and photoprotection. In conclusion, ΔpH was crucial to NPQ formation and relaxation in Q. coccifera during light transitions. Only a minor fraction of Z was associated to quenching, whereas the largest Z pool was not related to thermal dissipation. The latter is proposed to participate in photoprotection acting as antioxidant.

Stomatal encryption by epicuticular waxes as a plastic trait modifying gas exchange in a Mediterranean evergreen species (Quercus coccifera L.)

The adaptive benefit of stomatal crypts remains a matter of controversy. This work studies the effect on gas exchange of cuticular rims that overarch the stomatal pore in the Mediterranean species Quercus coccifera L. growing under Mediterranean (lower relative humidities and high summer temperatures) or oceanic conditions (higher daily relative humidities and mild temperatures). After microscopic assessment of the leaf surfaces and stomatal architecture, the impact of the cuticular 'cup' on gas exchange was evaluated by employing three-dimensional finite element models. Here, we provide evidence for a high plasticity of the Q. coccifera cuticular cup, with much larger vents under oceanic conditions compared to small vents under Mediterranean conditions. This structure adds a substantial fixed resistance thereby strongly decreasing gas exchange under Mediterranean conditions. The cuticular cup, which also increases leaf internal humidity, might buffer the rapid changes in vapour pressure deficit (VPD) often observed under Mediterranean conditions. Since water loss of guard and adjacent epidermal cells regulates stomatal aperture, we suggest that this structure allows an efficient regulation of stomatal conductance and optimum use of resources under high VPD. This study provides evidence that plasticity of stomatal architecture can be an important structural component of hydraulic adaptation to different climate conditions.

Photosynthetic responses of a C(3) and three C(4) species of the genus Panicum (s.l.) with different metabolic subtypes to drought stress

Young plants of Panicum bisulcatum (C(3)), Zuloagaea bulbosa [NADP-malic enzyme (ME)-C(4)], P. miliaceum (NAD-ME-C(4)) and Urochloa maxima [phosphoenolpyruvate carboxykinase (PCK)-C(4)] were subjected to drought stress (DS) in soil for 6 days. The C(3) species showed severe wilting symptoms at higher soil water potential (-1.1 MPa) and relative leaf water content (77 %) than in the case of the C(4) species (-1.5 to -1.7 MPa; 58-64 %). DS decreased photosynthesis, both under atmospheric and under saturating CO(2). Stomatal limitation of net photosynthesis (P(N)) in the C(3), but not in the C(4) species was indicated by P(N)/C(o) curves. Chlorophyll fluorescence of photosystem II, resulting from different cell types in the four species, indicated NADPH accumulation and non-stomatal limitation of photosynthesis in all four species, even under high CO(2). In the NAD-ME-C(4) and the PCK-C(4) species, DS plants showed increased violaxanthin de-epoxidase rates. Biochemical analyses of carboxylating enzymes and in vitro enzyme activities of the C(4) enzymes identified the most likely non-stomatal limiting steps of photosynthesis. In P. bisulcatum, declining RubisCO content and activity would explain the findings. In Z. bulbosa, all photosynthesis enzymes declined significantly; photosynthesis is probably limited by the turnover rate of the PEPC reaction. In P. miliaceum, all enzyme levels remained fairly constant under DS, but photosynthesis can be limited by feedback inhibition of the Calvin cycle, resulting in asp inhibition of PEPC. In U. maxima, declines of in vivo PEPC activity and feedback inhibition of the Calvin cycle are the main candidates for non-stomatal limitation of photosynthesis under DS.

Climate change (elevated CO₂, elevated temperature and moderate drought) triggers the antioxidant enzymes' response of grapevine cv. Tempranillo, avoiding oxidative damage

Photosynthetic carbon fixation (A(N) ) and photosynthetic electron transport rate (ETR) are affected by different environmental stress factors, such as those associated with climate change. Under stress conditions, it can be generated an electron excess that cannot be consumed, which can react with O₂, producing reactive oxygen species. This work was aimed to evaluate the influence of climate change (elevated CO₂, elevated temperature and moderate drought) on the antioxidant status of grapevine (Vitis vinifera) cv. Tempranillo leaves, from veraison to ripeness. The lowest ratios between electrons generated (ETR) and consumed (A(N) + respiration + photorespiration) were observed in plants treated with elevated CO₂ and elevated temperature. In partially irrigated plants under current ambient conditions, electrons not consumed seemed to be diverted to alternative ways. Oxidative damage to chlorophylls and carotenoids was not observed. However, these plants had increases in thiobarbituric acid reacting substances, an indication of lipid peroxidation. These increases matched well with an early rise of H₂O₂ and antioxidant enzyme activities, superoxide dismutase (EC 1.15.1.1), ascorbate peroxidase (EC 1.11.1.11) and catalase (EC 1.11.1.6). Enzymatic activities were maintained high until ripeness. In conclusion, plants grown under current ambient conditions and moderate drought were less efficient to cope with oxidative damage than well-irrigated plants, and more interestingly, plants grown under moderate drought but treated with elevated CO₂ and elevated temperature were not affected by oxidative damage, mainly because of higher rates of electrons consumed in photosynthetic carbon fixation.

Differential regulation of photorespiratory gene expression by moderate and severe salt and drought stress in relation to oxidative stress

The tolerant C3 plant Pancratium maritimum L. was used to investigate the contribution of photorespiration to the oxidative load under salt and drought stress. 7-weeks old plants were salt-stressed by 150 or 300 mM NaCl or drought-stressed by withholding water for 11 or 21 days. The growth, photosynthesis (A) and transpiration rates (E) were reduced by all stress treatments proportionally to the severity of stress. The rate of photorespiration was remarkably higher under moderate stresses than under severe stresses as revealed by large increase in the photorespiratory indicators Gly/Ser ratio and glyoxylate content and as substantiated by higher expression levels of photorespiratory enzymes. Nonetheless, indicators of oxidative stress namely, malondialdehyde (MDA) and hydrogen peroxide (H₂O₂) contents showed greater increase under severe stresses suggesting that the increase in the oxidative load under severe stresses is independent of photorespiration rate. The lower contents of MDA and H₂O₂ under moderate stresses (compared to these under severe stresses) in spite of the strong upregulation of photorespiration indicate efficient detoxification of photorespiration-generated H₂O₂ as shown by efficient upregulation of catalase (CAT) and peroxidase (POD). It is thus concluded that photorespiration may not be major contributor to the oxidative load under salt and drought stress.