Accurate photosynthetic parameter estimation at low stomatal conductance: effects of cuticular conductance and instrumental noise.

Accurate estimation of photosynthetic parameters is essential for understanding plant physiological limitations and responses to environmental factors from the leaf to the global scale. Gas exchange is a useful tool to measure responses of net CO2 assimilation (A) to internal CO2 concentration (Ci), a necessary step in estimating photosynthetic parameters including the maximum rate of carboxylation (Vcmax) and the electron transport rate (Jmax). However, species and environmental conditions of low stomatal conductance (gsw) reduce the signal-to-noise ratio of gas exchange, challenging estimations of Ci. Previous works showed that not considering cuticular conductance to water (gcw) can lead to significant errors in estimating Ci, because it has a different effect on total conductance to CO2 (gtc) than does gsw. Here we present a systematic assessment of the need for incorporating gcw into Ci estimates. In this study we modeled the effect of gcw and of instrumental noise and quantified these effects on photosynthetic parameters in the cases of four species with varying gsw and gcw, measured using steady-state and constant ramping techniques, like the rapid A/Ci response method. We show that not accounting for gcw quantitatively influences Ci and the resulting Vcmax and Jmax, particularly when gcw exceeds 7% of the total conductance to water. The influence of gcw was not limited to low gsw species, highlighting the importance of species-specific knowledge before assessing A/Ci curves. Furthermore, at low gsw instrumental noise can affect Ci estimation, but the effect of instrumental noise can be minimized using constant-ramping rather than steady-state techniques. By incorporating these considerations, more precise measurements and interpretations of photosynthetic parameters can be obtained in a broader range of species and environmental conditions.

Hydraulic plasticity and water use regulation act to maintain the hydraulic safety margins of Mediterranean trees in rainfall exclusion experiments.

Hydraulic failure due to xylem embolism has been identified as one of the main mechanisms involved in drought-induced forest decline. Trees vulnerability to hydraulic failure depends on their hydraulic safety margin (HSM). While it has been shown that HSM globally converges between tree species and biomes, there is still limited knowledge regarding how HSM can adjust locally to varying drought conditions within species. In this study, we relied on three long-term partial rainfall exclusion experiments to investigate the plasticity of hydraulic traits and HSM for three Mediterranean tree species (Quercus ilex L., Quercus pubescens Willd., and Pinus halepensis Mill.). For all species, a homeostasis of HSM in response to rainfall reduction was found, achieved through different mechanisms. For Q. ilex, the convergence in HSM is attributed to the adjustment of both the turgor loss point (Ψtlp) and the water potential at which 50% of xylem conductivity is lost due to embolism (P50). In contrast, the maintenance of HSM for P. halepensis and Q. pubescens is related to its isohydric behavior for the first and leaf area adjustment for the latter. It remains to be seen whether this HSM homeostasis can be generalized and if it will be sufficient to withstand extreme droughts expected in the Mediterranean region.

Cuticular conductance of adaxial and abaxial leaf surfaces and its relation to minimum leaf surface conductance.

Cuticular conductance to water (gcw ) is difficult to quantify for stomatous surfaces due to the complexity of separating cuticular and stomatal transpiration, and additional complications arise for determining adaxial and abaxial gcw . This has led to the neglect of gcw as a separate parameter in most common gas exchange measurements. Here, we describe a simple technique to simultaneously estimate adaxial and abaxial values of gcw , tested in two amphistomatous plant species. What we term the 'Red-Light method' is used to estimate gcw from gas exchange measurements and a known CO2 concentration inside the leaf during photosynthetic induction under red light. We provide an easy-to-use web application to assist with the calculation of gcw . While adaxial and abaxial gcw varies significantly between leaves of the same species we found that the ratio of adaxial/abaxial gcw (γn ) is stable within a plant species. This has implications for use of generic values of gcw when analysing gas exchange data. The Red-Light method can be used to estimate total cuticular conductance (gcw-T ) accurately with the most common setup of gas exchange instruments, i.e. a chamber mixing the adaxial and abaxial gases, allowing for a wide application of this technique.

On the minimum leaf conductance: its role in models of plant water use, and ecological and environmental controls.

Contents Summary 693 I. Introduction 693 II. Comparison of various definitions and measurement techniques of minimum conductance 694 III. Cuticular conductance 695 IV. Contribution of stomata 696 V. Environmental and ecological variation in minimum conductance 696 VI. Use of minimum conductance in models 698 VII. Conclusions 703 Acknowledgements 703 References 703 SUMMARY: When the rate of photosynthesis is greatly diminished, such as during severe drought, extreme temperature or low light, it seems advantageous for plants to close stomata and completely halt water loss. However, water loss continues through the cuticle and incompletely closed stomata, together constituting the leaf minimum conductance (gmin ). In this review, we critically evaluate the sources of variation in gmin , quantitatively compare various methods for its estimation, and illustrate the role of gmin in models of leaf gas exchange. A literature compilation of gmin as measured by the weight loss of detached leaves is presented, which shows much variation in this trait, which is not clearly related to species groups, climate of origin or leaf type. Much evidence points to the idea that gmin is highly responsive to the growing conditions of the plant, including soil water availability, temperature and air humidity - as we further demonstrate with two case studies. We pay special attention to the role of the minimum conductance in the Ball-Berry model of stomatal conductance, and caution against the usual regression-based method for its estimation. The synthesis presented here provides guidelines for the use of gmin in ecosystem models, and points to clear research gaps for this drought tolerance trait.

Hydraulic conductance, resistance, and resilience: how leaves of a tropical epiphyte respond to drought.

PREMISE: Because of its broad range in the neotropical rainforest and within tree canopies, the tank bromeliad Guzmania monostachia was investigated as a model of how varying leaf hydraulic conductance (Kleaf ) could help plants resist and recover from episodic drought. The two pathways of Kleaf , inside and outside the xylem, were also examined to determine the sites and causes of major hydraulic resistances within the leaf. METHODS: We measured leaf hydraulic conductance for plants in the field and laboratory under wet, dry, and rewetted conditions and applied physiological, anatomical, and gene expression analysis with modeling to investigate changes in Kleaf . RESULTS: After 7 d with no rain in the field or 14 days with no water in the glasshouse, Kleaf decreased by 50% yet increased to hydrated values within 4 d of tank refilling. Staining to detect embolism combined with modeling indicated that changes outside the xylem were of greater importance to Kleaf than were changes inside the xylem and were associated with changes in intercellular air spaces (aerenchyma), aquaporin expression and inhibition, and cuticular conductance. CONCLUSIONS: Low values for all conductances during drying, particularly in pathways outside the xylem, lead to hydraulic resilience for this species and may also contribute to its broad environmental tolerances.

Stem photosynthesis and hydraulics are coordinated in desert plant species.

Coordination between stem photosynthesis and hydraulics in green-stemmed desert plants is important for understanding the physiology of stem photosynthesis and possible drought responses. Plants with photosynthetic stems have extra carbon gain that can help cope with the detrimental effects of drought. We studied photosynthetic, hydraulic and functional traits of 11 plant species with photosynthetic stems from three California desert locations. We compared relationships among traits between wet and dry seasons to test the effect of seasonality on these relationships. Finally, we compared stem trait relationships with analogous relationships in the leaf economics spectrum. We found that photosynthetic and hydraulic traits are coordinated in photosynthetic stems. The slope or intercept of all trait relationships was mediated by seasonality. The relationship between mass-based stem photosynthetic CO2 assimilation rate (Amass ) and specific stem area (SSA; stem surface area to dry mass ratio) was statistically indistinguishable from the leaf economics spectrum. Our results indicate that photosynthetic stems behave like leaves in the coordination of multiple traits related to carbon gain, water movement and water loss. Because of the similarity of the stem Amass -SSA relationship to the leaf Amass -specific leaf area relationship, we suggest the existence of a photosynthetic stem economic spectrum.

Coordinated hydraulic traits influence the two phases of time to hydraulic failure in five temperate tree species differing in stomatal stringency.

Worldwide, forests are increasingly exposed to extreme droughts causing tree mortality. Because of the complex nature of the mechanisms involved, various traits have been linked to tree drought responses with contrasting results. This may be due to species-specific strategies in regulating water potential, a process that unfolds in two distinct phases: a first phase until stomatal closure, and a second phase until reaching lethal xylem hydraulic thresholds. We conducted dry-down experiments with five broadleaved temperate tree species differing in their degree of isohydry to estimate the time to stomatal closure (tsc) and subsequent time to critical hydraulic failure (tcrit). We measured various traits linked to tree drought responses, such as the water potentials at turgor loss point (Ptlp), stomatal closure (Pgs90), and 12%, 50% and 88% loss of xylem hydraulic conductance (P12, P50, P88), hydraulic capacitance (C), minimum leaf conductance (gmin), hydroscape area (HSA) and hydraulic safety margins (HSM). We found that Pgs90 followed previously recorded patterns of isohydry and was associated with HSA. Species ranked from more to less isohydric in the sequence Acer pseudoplatanus < Betula pendula < Tilia cordata < Sorbus aucuparia < Fagus sylvatica. Their degree of isohydry was associated with leaf safety (Ptlp and gmin), drought avoidance (C) and tsc, but decoupled from xylem safety (HSM and P88) and tcrit. Regardless of their stomatal stringency, species with wider HSM and lower P88 reached critical hydraulic failure later. We conclude that the duration of the first phase is determined by stomatal regulation, while the duration of the second phase is associated with xylem safety. Isohydry is thus linked to water use rather than to drought survival strategies, confirming the proposed use of HSA as a complement to HSM for describing plant drought responses before and after stomatal closure.

Hydraulic differences between flowers and leaves are driven primarily by pressure-volume traits and water loss.

Flowers are critical for successful reproduction and have been a major axis of diversification among angiosperms. As the frequency and severity of droughts are increasing globally, maintaining water balance of flowers is crucial for food security and other ecosystem services that rely on flowering. Yet remarkably little is known about the hydraulic strategies of flowers. We characterized hydraulic strategies of leaves and flowers of ten species by combining anatomical observations using light and scanning electron microscopy with measurements of hydraulic physiology (minimum diffusive conductance (g min) and pressure-volume (PV) curves parameters). We predicted that flowers would exhibit higher g min and higher hydraulic capacitance than leaves, which would be associated with differences in intervessel pit traits because of their different hydraulic strategies. We found that, compared to leaves, flowers exhibited: 1) higher g min, which was associated with higher hydraulic capacitance (C T); 2) lower variation in intervessel pit traits and differences in pit membrane area and pit aperture shape; and 3) independent coordination between intervessel pit traits and other anatomical and physiological traits; 4) independent evolution of most traits in flowers and leaves, resulting in 5) large differences in the regions of multivariate trait space occupied by flowers and leaves. Furthermore, across organs intervessel pit trait variation was orthogonal to variation in other anatomical and physiological traits, suggesting that pit traits represent an independent axis of variation that have as yet been unquantified in flowers. These results suggest that flowers, employ a drought-avoidant strategy of maintaining high capacitance that compensates for their higher g min to prevent excessive declines in water potentials. This drought-avoidant strategy may have relaxed selection on intervessel pit traits and allowed them to vary independently from other anatomical and physiological traits. Furthermore, the independent evolution of floral and foliar anatomical and physiological traits highlights their modular development despite being borne from the same apical meristem.

Radiation and Drought Impact Residual Leaf Conductance in Two Oak Species With Implications for Water Use Models.

Stomatal closure is one of the earliest responses to water stress but residual water losses may continue through the cuticle and incomplete stomatal closure. Residual conductance (g res ) plays a large role in determining time to mortality but we currently do not understand how do drought and shade interact to alter g res because the underlying drivers are largely unknown. Furthermore, g res may play an important role in models of water use, but the exact form in which g res should be incorporated into modeling schemes is currently being discussed. Here we report the results of a study where two different oak species were experimentally subjected to highly contrasting levels of drought (resulting in 0, 50 and 80% losses of hydraulic conductivity) and radiation (photosynthetic photon flux density at 1,500 µmol m-2 s-1 or 35-45 µmol m-2 s-1). We observed that the effects of radiation and drought were interactive and species-specific and g res correlated positively with concentrations of leaf non-structural carbohydrates and negatively with leaf nitrogen. We observed that different forms of measuring g res , based on either nocturnal conductance under high atmospheric water demand or on the water mass loss of detached leaves, exerted only a small influence on a model of stomatal conductance and also on a coupled leaf gas exchange model. Our results indicate that, while understanding the drivers of g res and the effects of different stressors may be important to better understand mortality, small differences in g res across treatments and measurements exert only a minor impact on stomatal models in two closely related species.

Genome-Wide Association Study for Maize Leaf Cuticular Conductance Identifies Candidate Genes Involved in the Regulation of Cuticle Development.

The cuticle, a hydrophobic layer of cutin and waxes synthesized by plant epidermal cells, is the major barrier to water loss when stomata are closed at night and under water-limited conditions. Elucidating the genetic architecture of natural variation for leaf cuticular conductance (gc) is important for identifying genes relevant to improving crop productivity in drought-prone environments. To this end, we conducted a genome-wide association study of gc of adult leaves in a maize inbred association panel that was evaluated in four environments (Maricopa, AZ, and San Diego, CA, in 2016 and 2017). Five genomic regions significantly associated with gc were resolved to seven plausible candidate genes (ISTL1, two SEC14 homologs, cyclase-associated protein, a CER7 homolog, GDSL lipase, and ß-D-XYLOSIDASE 4). These candidates are potentially involved in cuticle biosynthesis, trafficking and deposition of cuticle lipids, cutin polymerization, and cell wall modification. Laser microdissection RNA sequencing revealed that all these candidate genes, with the exception of the CER7 homolog, were expressed in the zone of the expanding adult maize leaf where cuticle maturation occurs. With direct application to genetic improvement, moderately high average predictive abilities were observed for whole-genome prediction of gc in locations (0.46 and 0.45) and across all environments (0.52). The findings of this study provide novel insights into the genetic control of gc and have the potential to help breeders more effectively develop drought-tolerant maize for target environments.