Aridity drives clinal patterns in leaf traits and responsiveness to precipitation in a broadly distributed Australian tree species

Aridity shapes species distributions and plant growth and function worldwide. Yet, plant traits often show complex relationships with aridity, challenging our understanding of aridity as a driver of evolutionary adaptation. We grew nine genotypes of Eucalyptus camaldulensis subsp. camaldulensis sourced from an aridity gradient together in the field for ~650 days under low and high precipitation treatments. Eucalyptus camaldulesis is considered a phreatophyte (deep-rooted species that utilizes groundwater), so we hypothesized that genotypes from more arid environments would show lower aboveground productivity, higher leaf gas-exchange rates, and greater tolerance/avoidance of dry surface soils (indicated by lower responsiveness) than genotypes from less arid environments. Aridity predicted genotype responses to precipitation, with more arid genotypes showing lower responsiveness to reduced precipitation and dry surface conditions than less arid genotypes. Under low precipitation, genotype net photosynthesis and stomatal conductance increased with home-climate aridity. Across treatments, genotype intrinsic water-use efficiency and osmotic potential declined with increasing aridity while photosynthetic capacity (Rubisco carboxylation and RuBP regeneration) increased with aridity. The observed clinal patterns indicate that E. camaldulensis genotypes from extremely arid environments possess a unique strategy defined by lower responsiveness to dry surface soils, low water-use efficiency, and high photosynthetic capacity. This strategy could be underpinned by deep rooting and could be adaptive under arid conditions where heat avoidance is critical and water demand is high.

The interplay between irrigation and fruiting on branch growth and mortality, gas exchange and water relations of coffee trees

The overall coordination between gas exchanges and plant hydraulics may be affected by soil water availability and source-to-sink relationships. Here we evaluated how branch growth and mortality, leaf gas exchange and metabolism are affected in coffee (Coffea arabica L.) trees by drought and fruiting. Field-grown plants were irrigated or not, and maintained with full or no fruit load. Under mild water deficit, irrigation per se did not significantly impact growth but markedly reduced branch mortality in fruiting trees, despite similar leaf assimilate pools and water status. Fruiting increased net photosynthetic rate in parallel with an enhanced stomatal conductance, particularly in irrigated plants. Mesophyll conductance and maximum RuBisCO carboxylation rate remained unchanged across treatments. The increased stomatal conductance in fruiting trees over nonfruiting ones was unrelated to internal CO2 concentration, foliar abscisic acid (ABA) levels or differential ABA sensitivity. However, stomatal conductance was associated with higher stomatal density, lower stomatal sensitivity to vapor pressure deficit, and higher leaf hydraulic conductance and capacitance. Increased leaf transpiration rate in fruiting trees was supported by coordinated alterations in plant hydraulics, which explained the maintenance of plant water status. Finally, by preventing branch mortality, irrigation can mitigate biennial production fluctuations and improve the sustainability of coffee plantations.

Melatonin Enhances Drought Tolerance by Regulating Leaf Stomatal Behavior, Carbon and Nitrogen Metabolism, and Related Gene Expression in Maize Plants

It is commonly known that exogenously applied melatonin can alleviate the impact of drought stress, but the mechanism used by melatonin to regulate stomatal behavior and carbon (C) and nitrogen (N) metabolism to increase drought resistance remains elusive. Herein, our aim was to investigate the influence of exogenous melatonin on the regulation of C and N metabolism in maize plants under water deficit. In this study, we analyzed stomatal behavior, the key components of C and N metabolism, and the gene expression and activity of enzymes involved in the C and N metabolism in maize plants. The results showed that the application of melatonin (100 μM) significantly increased maize growth and sustained the opening of stomata, and secondarily increased the photosynthetic capacity in maize. Under drought stress, foliar application of melatonin induced the gene transcription and activities of sucrose phosphate synthetase, ADP-glucose pyrophosphorylase, phosphoenolpyruvate carboxylase, and citrate synthase, resulting in the enhancement of sucrose and starch synthesis and the tricarboxylic acid (TCA) cycle. This enhancement in sugar biosynthesis and the TCA cycle might lead to stronger N assimilation. As anticipated, NO₃ ^- reduction and NH₄ ⁺ assimilation were also strengthened after melatonin treatment under drought stress. An increase was observed in some key enzymatic activities and transcription involved in nitrogen metabolism, such as that of nitrate reductase, nitrite reductase, glutamate synthase, and glutamine synthetase, in melatonin-treated, drought-stressed maize. Moreover, melatonin attenuated the drought-induced damage by reducing protein degradation and increasing the level of proline. Conclusively, our results indicate that exogenous melatonin enhances drought tolerance in maize via promoting stomatal opening and regulating C and N metabolism and related gene expression.

Rationale: Photosynthesis of Vascular Plants in Dim Light

Light dominates the earth's climate and ecosystems via photosynthesis, and fine changes of that might cause extensive material and energy alternation. Dim light (typically less than 5 μmol photons m^-2 s^-1) occurs widely in terrestrial ecosystems, while the frequency, duration, and extent of that are increasing because of climate change and urbanization. Dim light is important for the microorganism in the photosynthetic process, but omitted or unconsidered in the vascular plant, because the photosynthesis in the high-light adapted vascular leaves was almost impossible. In this review, we propose limitations of photosynthesis in vascular plant leaves, then elucidate the possibility and evidence of photosynthesis in terms of energy demand, stomatal opening, photosynthetic induction, and photosynthesis-related physiological processes in dim light. This article highlights the potential and noteworthy influence of dim light on photosynthesis in vascular plant leaves, and the research gap of dim light in model application and carbon accounting.

Effect of Light/Dark Cycle on Photosynthetic Pathway Switching and CO2 Absorption in Two Dendrobium Species

Many Dendrobium species are both ornamental and medicinal plants in China. Several wild species have been exploited to near extinction, and facility cultivation has become an important way to meet the great market demand. Most Dendrobium species have evolved into crassulacean acid metabolism (CAM) pathways in adapting to harsh epiphytic environment, leading to low daily net CO₂ absorption. Photosynthetic pathways of many facultative CAM plants are regulated by various environmental factors. Light/dark cycle plays an important role in regulating the photosynthetic pathway of several CAM species. The aims of this study were to investigate whether the photosynthetic pathway of Dendrobium species could be regulated between C3 and CAM by changing light/dark cycles and the daily net CO₂ absorption could be enhanced by shortening light/dark cycle. In this study, net CO₂ exchange rates of D. officinale and D. primulinum were monitored continuously during two different light/dark cycles conversion compared to Kalanchoe daigremontiana as an obligate CAM plant. The net CO₂ exchange pattern and stomatal behavior of D. officinale and D. primulinum were switched from CAM to C3-like by changing the light/dark cycle from 12/12 h to 4/4 h. However, this switching was not completely reversible. Compared to the original 12/12 h light/dark cycle, the dark, light, and daily net CO₂ exchange amount of D. officinale were significantly increased after the light/dark cycle was changed from 4/4 h to 12/12 h, but those in D. primulinum was opposite and those in K. daigremontiana was not affected. Daily net CO₂ exchange amount of D. officinale increased by 47% after the light/dark cycle was changed from 12/12 h to 4/4 h, due to the sharp increase of light net CO₂ exchange amount. However, the large decrease of dark net CO₂ exchange amount could not be offset by increased light net CO₂ exchange amount, leading to reduced daily net CO₂ exchange amount of D. primulinum. In conclusion, the 4/4 h light/dark cycle can induce the photosynthetic pathway of D. officinale and D. primulinum to C3-like, and improve the daily CO₂ absorption of D. officinale.

Do Aphids Alter Leaf Surface Temperature Patterns During Early Infestation?

Arthropods at the surface of plants live in particular microclimatic conditions that can differ from atmospheric conditions. The temperature of plant leaves can deviate from air temperature, and leaf temperature influences the eco-physiology of small insects. The activity of insects feeding on leaf tissues, may, however, induce changes in leaf surface temperatures, but this effect was only rarely demonstrated. Using thermography analysis of leaf surfaces under controlled environmental conditions, we quantified the impact of presence of apple green aphids on the temperature distribution of apple leaves during early infestation. Aphids induced a slight change in leaf surface temperature patterns after only three days of infestation, mostly due to the effect of aphids on the maximal temperature that can be found at the leaf surface. Aphids may induce stomatal closure, leading to a lower transpiration rate. This effect was local since aphids modified the configuration of the temperature distribution over leaf surfaces. Aphids were positioned at temperatures near the maximal leaf surface temperatures, thus potentially experiencing the thermal changes. The feedback effect of feeding activity by insects on their host plant can be important and should be quantified to better predict the response of phytophagous insects to environmental changes.

Evaluation of Morpho-Physiological Traits Adjustment of Prosopis tamarugo Under Long-Term Groundwater Depletion in the Hyper-Arid Atacama Desert

Water extraction from the underground aquifers of the Pampa del Tamarugal (Atacama Desert, Chile) reduced the growing area of Prosopis tamarugo, a strict phreatic species endemic to northern Chile. The objective of this work was to evaluate the effect of various architectural and morpho-physiological traits adjustment of P. tamarugo subjected to three groundwater depletion intervals (GWDr): <1 m (control), 1-4 m and 6-9 m. The traits were evaluated at three levels, plant [height, trunk cross-section area, leaf fraction (fGCC), and crown size], organ [length of internodes, leaf mass per unit area (LMA), leaflet mass and area], and tissue level [wood density (WD), leaf ¹³C, ¹⁸O isotope composition (δ), and intrinsic water use efficiency (iWUE)]. In addition, soil water content (VWC) to 1.3 m soil depth, pre-dawn and midday water potential difference (ΔΨ), and stomatal conductance (g_s) were evaluated. At the deeper GWDr, P. tamarugo experienced significant growth restriction and reduced fGCC, the remaining canopy had a significantly higher LMA associated with smaller leaflets. No differences in internode length and WD were observed. Values for δ¹³C and δ¹⁸O indicated that as GWDr increased, iWUE increased as a result of partial stomata closure with no significant effect on net assimilation over time. The morpho-physiological changes experienced by P. tamarugo allowed it to acclimate and survive in a condition of groundwater depletion, keeping a functional but diminished canopy. These adjustments allowed maintenance of a relatively high g_s; ΔΨ was not different among GWDrs despite smaller VWC at greater GWDr. Although current conservation initiatives of this species are promising, forest deterioration is expected continue as groundwater depth increases.

Isohydric species are not necessarily more carbon limited than anisohydric species during drought

Isohydry (i.e., strong regulation of leaf water potential, Ψl) is commonly associated with strict stomatal regulation of transpiration under drought, which in turn is believed to minimize hydraulic risk at the expense of reduced carbon assimilation. Hence, the iso/anisohydric classification has been widely used to assess drought resistance and mortality mechanisms across species, with isohydric species being hypothetically more prone to carbon starvation and anisohydric species more vulnerable to hydraulic failure. These hypotheses and their underlying assumptions, however, have rarely been tested under controlled, experimental conditions. Our objective is to assess the physiological mechanisms underlying drought resistance differences between two co-occurring Mediterranean forest species with contrasting drought responses: Phillyrea latifolia L. (anisohydric and more resistant to drought) and Quercus ilex L. (isohydric and less drought resistant). A total of 100 large saplings (50 per species) were subjected to repeated drought treatments for a period of 3 years, after which Q. ilex showed 18% mortality whereas no mortality was detected in P. latifolia. Relatively isohydric behavior was confirmed for Q. ilex, but higher vulnerability to cavitation in this species implied that estimated embolism levels were similar across species (12-52% in Q. ilex vs ~30% in P. latifolia). We also found similar seasonal patterns of stomatal conductance and assimilation between species. If anything, the anisohydric P. latifolia tended to show lower assimilation rates than Q. ilex under extreme drought. Similar growth rates and carbon reserves dynamics in both species also suggests that P. latifolia was as carbon-constrained as Q. ilex. Increasing carbon reserves under extreme drought stress in both species, concurrent with Q. ilex mortality, suggests that mortality in our study was not triggered by carbon starvation. Our results warn against making direct connections between Ψl regulation, stomatal behavior and the mechanisms of drought-induced mortality in plants.

A Dominant Negative OsKAT2 Mutant Delays Light-Induced Stomatal Opening and Improves Drought Tolerance without Yield Penalty in Rice

Stomata are the main gateways for water and air transport between leaves and the environment. Inward-rectifying potassium channels regulate photo-induced stomatal opening. Rice contains three inward rectifying shaker-like potassium channel proteins, OsKAT1, OsKAT2, and OsKAT3. Among these, only OsKAT2 is specifically expressed in guard cells. Here, we investigated the functions of OsKAT2 in stomatal regulation using three dominant negative mutant proteins, OsKAT2(T235R), OsKAT2(T285A) and OsKAT2(T285D), which are altered in amino acids in the channel pore and at a phosphorylation site. Yeast complementation and patch clamp assays showed that all three mutant proteins lost channel activity. However, among plants overexpressing these mutant proteins, only plants overexpressing OsKAT2(T235R) showed significantly less water loss than the control. Moreover, overexpression of this mutant protein led to delayed photo-induced stomatal opening and increased drought tolerance. Our results indicate that OsKAT2 is an inward- rectifying shaker-like potassium channel that mainly functions in stomatal opening. Interestingly, overexpression of OsKAT2(T235R) did not cause serious defects in growth or yield in rice, suggesting that OsKAT2 is a potential target for engineering plants with improved drought tolerance without yield penalty.

Ectopic expression of a tobacco vacuolar invertase inhibitor in guard cells confers drought tolerance in Arabidopsis

There are several hypotheses that explain stomatal behavior. These include the concept of osmoregulation mediated by potassium and its counterions malate and chlorine and the more recent starch-sugar hypothesis. We have previously reported that the activity of the sucrose cleavage enzyme, vacuolar invertase (VIN), is significantly higher in guard cells than in other leaf epidermal cells and its activity is correlated with stomatal aperture. Here, we examined whether VIN indeed controls stomatal movement under normal and drought conditions by transforming Arabidopsis with a tobacco vacuolar invertase inhibitor homolog (Nt-inhh) under the control of an abscisic acid-sensitive and guard cell-specific promoter (AtRab18). The data obtained showed that guard cells of transgenic Arabidopsis plants had lower VIN activity, stomatal aperture and conductance than that of wild-type plants. Moreover, the transgenic plants also displayed higher drought tolerance than wild-type plants. The data indicate that VIN is a promising target for manipulating stomatal function to increase drought tolerance.

Impaired Stomatal Control Is Associated with Reduced Photosynthetic Physiology in Crop Species Grown at Elevated [CO2]

Physiological control of stomatal conductance (Gs) permits plants to balance CO2-uptake for photosynthesis (PN) against water-loss, so optimizing water use efficiency (WUE). An increase in the atmospheric concentration of carbon dioxide ([CO2]) will result in a stimulation of PN and reduction of Gs in many plants, enhancing carbon gain while reducing water-loss. It has also been hypothesized that the increase in WUE associated with lower Gs at elevated [CO2] would reduce the negative impacts of drought on many crops. Despite the large number of CO2-enrichment studies to date, there is relatively little information regarding the effect of elevated [CO2] on stomatal control. Five crop species with active physiological stomatal behavior were grown at ambient (400 ppm) and elevated (2000 ppm) [CO2]. We investigated the relationship between stomatal function, stomatal size, and photosynthetic capacity in the five species, and then assessed the mechanistic effect of elevated [CO2] on photosynthetic physiology, stomatal sensitivity to [CO2] and the effectiveness of stomatal closure to darkness. We observed positive relationships between the speed of stomatal response and the maximum rates of PN and Gs sustained by the plants; indicative of close co-ordination of stomatal behavior and PN. In contrast to previous studies we did not observe a negative relationship between speed of stomatal response and stomatal size. The sensitivity of stomata to [CO2] declined with the ribulose-1,5-bisphosphate limited rate of PN at elevated [CO2]. The effectiveness of stomatal closure was also impaired at high [CO2]. Growth at elevated [CO2] did not affect the performance of photosystem II indicating that high [CO2] had not induced damage to the photosynthetic physiology, and suggesting that photosynthetic control of Gs is either directly impaired at high [CO2], sensing/signaling of environmental change is disrupted or elevated [CO2] causes some physical effect that constrains stomatal opening/closing. This study indicates that while elevated [CO2] may improve the WUE of crops under normal growth conditions, impaired stomatal control may increase the vulnerability of plants to water deficit and high temperatures.

A Modeling Approach to Quantify the Effects of Stomatal Behavior and Mesophyll Conductance on Leaf Water Use Efficiency

Water use efficiency (WUE) is considered as a determinant of yield under stress and a component of crop drought resistance. Stomatal behavior regulates both transpiration rate and net assimilation and has been suggested to be crucial for improving crop WUE. In this work, a dynamic model was used to examine the impact of dynamic properties of stomata on WUE. The model includes sub-models of stomatal conductance dynamics, solute accumulation in the mesophyll, mesophyll water content, and water flow to the mesophyll. Using the instantaneous value of stomatal conductance, photosynthesis, and transpiration rate were simulated using a biochemical model and Penman-Monteith equation, respectively. The model was parameterized for a cucumber leaf and model outputs were evaluated using climatic data. Our simulations revealed that WUE was higher on a cloudy than a sunny day. Fast stomatal reaction to light decreased WUE during the period of increasing light (e.g., in the morning) by up to 10.2% and increased WUE during the period of decreasing light (afternoon) by up to 6.25%. Sensitivity of daily WUE to stomatal parameters and mesophyll conductance to CO2 was tested for sunny and cloudy days. Increasing mesophyll conductance to CO2 was more likely to increase WUE for all climatic conditions (up to 5.5% on the sunny day) than modifications of stomatal reaction speed to light and maximum stomatal conductance.

Elevated-CO2 Response of Stomata and Its Dependence on Environmental Factors

Stomata control the flow of gases between plants and the atmosphere. This review is centered on stomatal responses to elevated CO2 concentration and considers other key environmental factors and underlying mechanisms at multiple levels. First, an outline of general responses in stomatal conductance under elevated CO2 is presented. Second, stomatal density response, its development, and the trade-off with leaf growth under elevated CO2 conditions are depicted. Third, the molecular mechanism regulating guard cell movement at elevated CO2 is suggested. Finally, the interactive effects of elevated CO2 with other factors critical to stomatal behavior are reviewed. It may be useful to better understand how stomata respond to elevated CO2 levels while considering other key environmental factors and mechanisms, including molecular mechanism, biochemical processes, and ecophysiological regulation. This understanding may provide profound new insights into how plants cope with climate change.

Water Relations and Foliar Isotopic Composition of Prosopis tamarugo Phil., an Endemic Tree of the Atacama Desert Growing at Three Levels of Water Table Depth

Prosopis tamarugo Phil. is a strict phreatophyte tree species endemic to the "Pampa del Tamarugal", Atacama Desert. The extraction of water for various uses has increased the depth of the water table in the Pampa aquifers threatening its conservation. This study aimed to determine the effect of the groundwater table depth on the water relations of P. tamarugo and to present thresholds of groundwater depth (GWD) that can be used in the groundwater management of the P. tamarugo ecosystem. Three levels of GWD, 11.2 ± 0.3 m, 10.3 ± 0.3 m, and 7.1 ± 0.1 m, (the last GWD being our reference) were selected and groups of four individuals per GWD were studied in the months of January and July of the years 2011 through 2014. When the water table depth exceeded 10 m, P. tamarugo had lower pre-dawn and mid-day water potential but no differences were observed in minimum leaf stomatal resistance when compared to the condition of 7.1 m GWD; the leaf tissue increased its δ(13)C and δ(18)O composition. Furthermore, a smaller green canopy fraction of the trees and increased foliage loss in winter with increasing water table depth was observed. The differences observed in the physiological behavior of P. tamarugo trees, attributable to the ground water depth; show that increasing the depth of the water table from 7 to 11 m significantly affects the water status of P. tamarugo. The results indicate that P. tamarugo has an anisohydric stomatal behavior and that given a reduction in water supply it regulates the water demand via foliage loss. The growth and leaf physiological activities are highly sensitive to GWD. The foliage loss appears to prevent the trees from reaching water potentials leading to complete loss of hydraulic functionality by cavitation. The balance achieved between water supply and demand was reflected in the low variation of the water potential and of the variables related to gas exchange over time for a given GWD. This acclimation capacity of P. tamarugo after experiencing increases in GWD has great value for the implementation of conservation strategies. The thresholds presented in this paper should prove useful for conservation purposes of this unique species.

Elevated-CO2 Response of Stomata and Its Dependence on Environmental Factors

Stomata control the flow of gases between plants and the atmosphere. This review is centered on stomatal responses to elevated CO2 concentration and considers other key environmental factors and underlying mechanisms at multiple levels. First, an outline of general responses in stomatal conductance under elevated CO2 is presented. Second, stomatal density response, its development, and the trade-off with leaf growth under elevated CO2 conditions are depicted. Third, the molecular mechanism regulating guard cell movement at elevated CO2 is suggested. Finally, the interactive effects of elevated CO2 with other factors critical to stomatal behavior are reviewed. It may be useful to better understand how stomata respond to elevated CO2 levels while considering other key environmental factors and mechanisms, including molecular mechanism, biochemical processes, and ecophysiological regulation. This understanding may provide profound new insights into how plants cope with climate change.

Coordination of stomatal physiological behavior and morphology with carbon dioxide determines stomatal control

PREMISE OF THE STUDY

Stomatal control is determined by the ability to alter stomatal aperture and/or the number of stomata on the surface of new leaves in response to growth conditions. The development of stomatal control mechanisms to the concentration of CO₂within the atmosphere ([CO₂]) is fundamental to our understanding of plant evolutionary history and the prediction of gas exchange responses to future [CO₂].

METHODS

In a controlled environment, fern and angiosperm species were grown in atmospheres of ambient (400 ppm) and elevated (2000 ppm) [CO₂]. Physiological stomatal behavior was compared with the stomatal morphological response to [CO₂].

KEY RESULTS

An increase in [CO₂] or darkness induced physiological stomatal responses ranging from reductions (active) to no change (passive) in stomatal conductance. Those species with passive stomatal behavior exhibited pronounced reductions of stomatal density in new foliage when grown in elevated [CO₂], whereas species with active stomata showed little morphological response to [CO₂]. Analysis of the physiological and morphological stomatal responses of a wider range of species suggests that patterns of stomatal control to [CO₂] do not follow a phylogenetic pattern associated with plant evolution.

CONCLUSIONS

Selective pressures may have driven the development of divergent stomatal control strategies to increased [CO₂]. Those species that are able to actively regulate guard cell turgor are more likely to respond to [CO₂] through a change in stomatal aperture than stomatal number. We propose a model of stomatal control strategies in response to [CO₂] characterized by a trade-off between short-term physiological behavior and longer-term morphological response.

The influence of increasing rhizospheric calcium on the ability of Lupinus luteus L. to control water use efficiency

The stomatal behavior of Lupinus luteus L. was examined to determine the ability of a calcifuge to regulate water loss against a background of different rhizospheric Ca concentrations. A previous study had suggested that a cultivar of this species had limited stomatal sensitivity to ABA, and so the physiological events which enable L. luteus to adjust its water use efficiency (WUE) in response to changes in both atmospheric and soil water availability were explored in a number of ways. L. luteus did not lack the ability to control water loss and there was no evidence to suggest that ABA was unable to regulate stomatal behavior during water deficits, Stomatal closure was shown to occur in a linear manner when different concentrations of ABA were applied. The presence of different concentrations of Ca within xylem sap did not influence the degree to which stomata closed in response to applied ABA. The patterns of stomatal behavior in response to changes in atmospheric humidity, declining soil water potentials and applied ABA, were generally similar to those of other species. The presence of high rhizospheric Ca 8 mol m^-3 did not apparently induce stomatal closure when gas exchange measurements were made at a δW of mmol mol^-1 , when compared with plants grown on 1 mol m^-3 Ca. However, when measurements of gas exchange were made over a range of δW values, the relationship between assimilation and stomatal conductance suggested that the degree of coupling between these two variables was less pronounced with plants grown at 8 mol m^-3 compared to those at 1 mol m^-3 Ca. The effect of this on the regulation of leaf conductance was to perturb WUE. There was little evidence here to support a role for changes in the flux of xylem sap Ca as a means of root-to-shoot signaling.