Physiological and biochemical responses to severe drought stress of nine Eucalyptus globulus clones: a multivariate approach

Effects of Drought Stress and Rehydration on Physiological and Biochemical Properties of Four Oak Species in China

Quercus fabri Hance, Quercus serrata Thunb, Quercus acutissima Carruth, and Quercus variabilis BL are four Chinese oak species commonly used for forestation. To ensure the survival of seedlings, we first need to understand the differences in drought resistance of the four oak species at the seedling stage, and comprehensively evaluate their drought resistance capabilities. The four oak seedlings were divided into drought-rewatering treatment group and well watered samples (control group). For the seedlings of the drought-rewatering treatment group, drought stress lasting 31 days was used, and then re-watering for 5 days. The water parameters, osmotic solutes content, antioxidant enzyme activity and photosynthesis parameters of the seedlings in the two groups were measured every 5 days. Compared with the control group, the relative water content, water potential, net photosynthetic rate, transpiration rate, and stomatal conductance levels of the four oaks all showed a downward trend under continuous drought stress, and showed an upward trend after rehydration. The soluble protein, soluble sugar, proline, peroxidase, superoxide dismutase and catalase content of the four oaks increased first and then decreased under drought stress, and then increased after rehydration. The content of glycine betaine and malondialdehyde continued to increase, and gradually decreased after rehydration. The weight of each index was calculated by principal component analysis, and then the comprehensive evaluation of each index was carried out through the membership function method. The drought resistance levels of the four oak species were as follows: Q. serrata > Q. fabri > Q. variabilis > Q. acutissima.

Changes in Watering Frequency Stimulate Differentiated Adaptive Responses among Seedlings of Different Beech Populations

Simple Summary

Future precipitation changes are expected to affect plant populations’ adaptive responses. In southern Europe, annual precipitation is expected to decline and become unpredictable with occasional extreme rainfall events. Although there are many studies investigating water deficit effects in beech populations, they mainly refer to water withholding and rewatering or limited watering for prolonged periods. There is a lack of information considering the effect of simulated changes in monthly precipitation distribution on plants. In our study, we aimed to elucidate whether simulated distribution differences in monthly precipitation, expected to prevail in 2050, affects the response of various adaptive traits in beech seedlings originating from sites with contrasting climatic conditions. We found significant population differences according to watering interactions in most of the stem anatomical traits, but only for leaf circularity regarding the morphological traits. Our results indicate that beech populations in the southernmost region of their European distribution may demonstrate high variability in adaptive responses towards climate change conditions.

Abstract

Seasonality, rather than annual precipitation levels, is expected to affect the adaptive responses of plant populations under future climate change. To estimate adaptive traits’ variation, we conducted a common garden experiment with two beech populations from contrasting climatic origins (Evros with longer drought intervals during summer and higher precipitation seasonality, and Drama representing a more temperate ecosystem). We simulated two different watering treatments (frequent vs. non-frequent) on beech seedlings, according to predicted monthly precipitation levels expected to prevail in 2050 by the CSIRO MK3.6 SRESA1B model, considering as reference area a natural beech stand in Mt. Rodopi, Greece. A series of morphological and stem anatomical traits were measured. Seedling survival was greater for the Evros population compared to that of Drama under non-frequent watering, while no difference in survival was detected under frequent watering. Leaf morphological traits were not generally affected by watering frequency except for leaf circularity, which was found to be lower under non-frequent watering for both populations. Stomata density in leaves was found to be higher in the Evros population and lower in the Drama population under non-frequent watering than frequent. Stem anatomical traits were higher under non-frequent watering for Evros but lower for the Drama population. Multivariate analyses clearly discriminated populations under non-frequent rather than frequent watering, indicating genetic adaptation to the population’s environment of origin.

Photosynthesis-Related Responses of Colombian Elite Hevea brasiliensis Genotypes under Different Environmental Variations: Implications for New Germplasm Selection in the Amazon

The objective of this study was to evaluate photosynthetic performance based on gas exchange traits, chlorophyll a fluorescence, and leaf water potential (Ψ_L) in nine Hevea brasiliensis genotypes from the ECC-1 (Élite Caquetá Colombia) selection and the cultivar IAN 873 (control) in response to different climatic (semi-humid warm and humid warm climates), seasonal (dry and rainy periods), and hourly (3:00 to 18:00) variations that can generate stress in the early growth stage (two-year-old plants) in two large-scale clonal trials in the Colombian Amazon. The photosynthetic performance in 60% of the Colombian genotypes was slightly affected under the conditions with less water availability (dry period, semi-humid warm site, and between 9:00 and 15:00 h), as compared with IAN 873, whose affectation was moderate in terms of photosynthesis rates, but its water conservation strategy was strongly affected. The ECC 90, ECC 83, and ECC 73 genotypes had the best photosynthetic performance under conditions of greater water limitation, and ECC 35, and ECC 64 had a higher water status based on the leaf water potential, with intermediate photosynthetic performance. This germplasm has a high potential for selection in rubber tree breeding programs in future scenarios of climate change in the Colombian Amazon.

Proteomic analyses unraveling water stress response in two Eucalyptus species originating from contrasting environments for aridity

Eucalyptus are widely cultivated in several regions of the world due to their adaptability to different climatic conditions and amenable to tree breeding programs. With changes in environmental conditions pointing to an increase in aridity in many areas of the globe, the demand for genetic materials that adapt to this situation is required. Therefore, the aim of this work was to identify contrasting differences between two Eucalyptus species under water stress through the identification of differentially abundant proteins. For this, total protein extraction was proceeded from leaves of both species maintained at 40 and 80% of field capacity (FC). The 80% FC water regime was considered as the control and the 40% FC, severe water stress. The proteins were separated by 2-DE with subsequent identification of those differentially abundant by liquid nanocromatography coupled to high resolution MS (Q-Exactive). Comparative proteomics allowed to identify four proteins (ATP synthase gamma and alpha, glutamine synthetase and a vacuolar protein) that were more abundant in drought-tolerant species and simultaneously less abundant or unchanged in the drought- sensitive species, an uncharacterized protein found exclusively in plants under drought stress and also 10 proteins (plastid-lipid, ruBisCO activase, ruBisCO, protease ClpA, transketolase, isoflavone reductase, ferredoxin-NADP reductase, malate dehydrogenase, aminobutyrate transaminase and sedoheptulose-1-bisphosphatase) induced exclusively in the drought-tolerant species in response to water stress. These results suggest that such proteins may play a crucial role as potential markers of water stress tolerance through the identification of species-specific proteins, and future targets for genetic engineering.

Rising [CO2] effect on leaf drought-induced metabolome in Pinus pinaster Aiton: Ontogenetic- and genotypic-specific response exhibit different metabolic strategies

Rising atmospheric CO₂ concentrations ([CO₂]) together with water deficit can influence ecological interactions of trees through an array of chemically driven changes in plant leaves. In four drought stressed Pinus pinaster genotypes, grown under two levels of atmospheric [CO₂] (ambient (aCO₂) and enriched (eCO₂)) the metabolome of adult and juvenile needles was analyzed to know if the metabolic responses to this environmental situation could be genotype-dependent and vary according to the stage of needle ontogeny. Drought had the highest incidence, followed by needle ontogeny, being lower the eCO₂ effect. The eCO₂ reduced, eliminated or countered the 50 (adult needles) - 44% (juvenile) of the drought-induced changes, suggesting that CO₂-enriched plants could perceived less oxidative stress under drought, and proving that together, these two abiotic factors triggered a metabolic response different from that under single factors. Genotype drought tolerance and ontogenetic stage determined the level of metabolite accumulation and the plasticity to eCO₂ under drought, which was mainly reflected in antioxidant levels and tree chemical defense. At re-watering, previously water stressed plants showed both, reduced C and N metabolism, and a "drought memory effect", favoring antioxidants and osmolyte storage. This effect showed variations regarding genotype drought-tolerance, needle ontogeny and [CO₂], with remarkable contribution of terpenoids. Chemical defense and drought tolerance were somehow linked, increasing chemical defense during recovery in the most drought-sensitive individuals. The better adaptation of trees to drought under eCO₂, as well as their ability to recover better from water stress, are essential for the survival of forest trees.

Dynamics of photosynthetic responses in 10 rubber tree (Hevea brasiliensis) clones in Colombian Amazon: Implications for breeding strategies

The rubber tree [Hevea brasiliensis (Willd. Ex Adr. de Juss.) Muell.-Arg] is the main source of natural rubber in the world. However, in the Amazon region, its production is reduced by biotic and abiotic limitations, which have prompted breeding programs in order to identify desirable agronomic and physiological indicators. The objective of this study was to analyze the temporal dynamics of photosynthetic responses based on the parameters of leaf gas exchange and chlorophyll a fluorescence in 10 rubber tree clones during the immature phase (pre-tapping) in three large-scale clone trials, during daily cycles and under two climatic periods (dry and rainy) in the Caquetá region (Colombian Amazon). The variables A, LT, Φ_PSII, ETR and qP were significantly higher in the dry period, where the highest values of PAR, AT and VPD were seen. In San Vicente del Caguán and Florencia, the highest averages were estimated for A, E and g_s, as compared with Belén de los Andaquíes. In Florencia, the highest fluorescence parameters of chlorophyll a were recorded. At 9:00 h and 12:00 h, the highest means of A, E, Φ_PSII and ETR were observed. The majority of the clones displayed the highest F_v/F_m mean (0.82–0.84) in the dry period. The clones FX 4098, FDR 4575, MDF 180, GU198 and FDR 5788 represent genotypes with the best photosynthetic performance (greater photosynthetic rates and better ability of the photosynthetic apparatus to capture, use and dissipate light energy). These desirable genotypes constitute a promising gene pool for expanding the genetic resource of rubber trees in the Colombian Amazon.

Responses to Drought in Seedlings of European Larch (Larix decidua Mill.) from Several Carpathian Provenances

European larch (Larix decidua Mill.) has been reported either as more tolerant or as more sensitive to drought than conifers with perennial leaves. Previous studies have revealed that Carpathian populations of European larch display a high genetic variability. A comparative study of the responses of these populations to drought stress at the seedling stage might allow the identification of drought tolerant genotypes and reliable drought stress biomarkers, which could be eventually used for the early detection of drought effects in larch, not only under control greenhouse conditions, but also in their natural stands. Growth responses were analyzed in larch seedlings from six Romanian Carpathian populations, submitted to one month of mild drought stress under controlled conditions. Levels of photosynthetic pigments (chlorophylls a and b, and carotenoids), osmolytes (proline and total soluble sugars), monovalent cations (Na+ and K+), and malondialdehyde (MDA) and non-enzymatic antioxidants (total phenolics and flavonoids) were compared with control treatments and between populations. Growth and the pattern of the biochemical responses were very similar in the six populations. Drought stress lead to stem length decrease in all population, whereas reduction of fresh weight of needles was significant only in one population (BVVC), and reduction of water content of needles in two populations (BVVC and GuHo). The optimal biochemical traits for an early detection of drought symptoms in this species is the increase—in most populations—of total soluble sugars, MDA, and total phenolic compounds, whereas K+ reduction was significant in all populations. Photosynthetic pigments remained unchanged, except for the Anin population where they were reduced under stress. Multivariate principal component and hierarchical clustering analyses confirmed the impact of drought in the growth and physiology of European larch, and revealed that the humidity of the substrate was positively correlated with the growth parameters and the levels of K+ in needles, and negatively correlated with the levels of MDA, total soluble sugars, total phenolic compounds, and flavonoids in needles.

Desiccation and Mortality Dynamics in Seedlings of Different European Beech (Fagus sylvatica L.) Populations under Extreme Drought Conditions

European beech (Fagus sylvatica L., hereafter beech), one of the major native tree species in Europe, is known to be drought sensitive. Thus, the identification of critical thresholds of drought impact intensity and duration are of high interest for assessing the adaptive potential of European beech to climate change in its native range. In a common garden experiment with one-year-old seedlings originating from central and marginal origins in six European countries (Denmark, Germany, France, Romania, Bosnia-Herzegovina, and Spain), we applied extreme drought stress and observed desiccation and mortality processes among the different populations and related them to plant water status (predawn water potential, ΨPD) and soil hydraulic traits. For the lethal drought assessment, we used a critical threshold of soil water availability that is reached when 50% mortality in seedling populations occurs (LD50SWA). We found significant population differences in LD50SWA (10.5-17.8%), and mortality dynamics that suggest a genetic difference in drought resistance between populations. The LD50SWA values correlate significantly with the mean growing season precipitation at population origins, but not with the geographic margins of beech range. Thus, beech range marginality may be more due to climatic conditions than to geographic range. The outcome of this study suggests the genetic variation has a major influence on the varying adaptive potential of the investigated populations.

Genotypic differences in architectural and physiological responses to water restriction in rose bush

The shape and, therefore, the architecture of the plant are dependent on genetic and environmental factors such as water supply. The architecture determines the visual quality, a key criterion underlying the decision to purchase an ornamental potted plant. The aim of this study was to analyze genotypic responses of eight rose bush cultivars to alternation of water restriction and re-watering periods, with soil water potential of -20 and -10 kPa respectively. Responses were evaluated at the architectural level through 3D digitalization using six architectural variables and at the physiological level by measuring stomatal conductance, water content, hormones [abscisic acid (ABA), auxin, cytokinins, jasmonic acid, and salicylic acid (SA)], sugars (sucrose, fructose, and glucose), and proline. Highly significant genotype and watering effects were revealed for all the architectural variables measured, as well as genotype × watering interaction, with three distinct genotypic architectural responses to water restriction - weak, moderate and strong - represented by Hw336, 'Baipome' and 'The Fairy,' respectively. The physiological analysis explained, at least in part, the more moderate architectural response of 'Baipome' compared to 'The Fairy,' but not that of Hw336 which is an interspecific hybrid. Such physiological responses in 'Baipome' could be related to: (i) the maintenance of the stimulation of budbreak and photosynthetic activity during water restriction periods due to a higher concentration in conjugated cytokinins (cCK) and to a lower concentration in SA; (ii) a better resumption of budbreak during the re-watering periods due to a lower concentration in ABA during this period. When associated with the six architectural descriptors, cCK, SA and ABA, which explained the genotypic differences in this study, could be used as selection criteria for breeding programs aimed at improving plant shape and tolerance to water restriction.