Sebastià Martorell

Anisohydric behaviour in grapevines results in better performance under moderate water stress and recovery than isohydric behaviour

AimsThree grapevine varieties original from different climates: Grenache, from Mediterranean origin; Syrah, from mesic origin and Chardonnay, from the humid zone of Burgundy (France) were used to study differential physiological responses to water deficit and sub-sequent recovery after re-watering. Moreover, the effect of the environmental growing conditions on water use efficiency (WUE) was also studied.MethodsChanges of the lamina hydraulic conductance (Klamina), transpiration, photosynthetic CO2 assimilation (AN), stomatal conductance (gs), mesophyll conductance to CO2 (gm), chlorophyll fluorescence, and their interactions with other environmental conditions were followed during prolonged water stress and subsequent re-watering in Chardonnay, Grenache and Syrah.ResultsGrenache confirmed its reputation as isohydric and Chardonnay as anisohydric, but Syrah, a variety often considered as anisohydric, showed near-isohydric behaviour. Chardonnay displayed higher hydraulic conductance during both irrigation and water stress and a faster recovery after water stress as compared to the two isohydric-behaving varieties. Chardonnay attained lower decreases in stomatal conductance in response to water stress by delaying its adjustment of the lamina hydraulic conductance (Klamina), which in turn resulted in the maintenance of higher photosynthesis and photosynthetic capacity, favoring faster recovery upon re-watering.The results do not support the common assumption that isohydric behaviour results in a better performance under water stress conditions. Indeed, under moderate water stress, Chardonnay showed some advantages over the two varieties displaying near-isohydric behaviour.ConclusionsIntegrated over a period including water stress imposition, acclimation and recovery Chardonnay displayed higher CO2 assimilation than Grenache and Syrah, which implies a higher yield potential under these conditions.

Most stomatal closure in woody species under moderate drought can be explained by stomatal responses to leaf turgor

Reduced stomatal conductance (gs ) during soil drought in angiosperms may result from effects of leaf turgor on stomata and/or factors that do not directly depend on leaf turgor, including root-derived abscisic acid (ABA) signals. To quantify the roles of leaf turgor-mediated and leaf turgor-independent mechanisms in gs decline during drought, we measured drought responses of gs and water relations in three woody species (almond, grapevine and olive) under a range of conditions designed to generate independent variation in leaf and root turgor, including diurnal variation in evaporative demand and changes in plant hydraulic conductance and leaf osmotic pressure. We then applied these data to a process-based gs model and used a novel method to partition observed declines in gs during drought into contributions from each parameter in the model. Soil drought reduced gs by 63-84% across species, and the model reproduced these changes well (r(2)  = 0.91, P < 0.0001, n = 44) despite having only a single fitted parameter. Our analysis concluded that responses mediated by leaf turgor could explain over 87% of the observed decline in gs across species, adding to a growing body of evidence that challenges the root ABA-centric model of stomatal responses to drought.

Stomatal and mesophyll conductances to CO2 in different plant groups: Underrated factors for predicting leaf photosynthesis responses to climate change?

The climate change conditions predicted for the end of the current century are expected to have an impact on the performance of plants under natural conditions. The variables which are foreseen to have a larger effect are increased CO2 concentration and temperature. Although it is generally considered CO2 assimilation rate could be increased by the increasing levels of CO2, it has been reported in previous studies that acclimation to high CO2 results in reductions of physiological parameters involved in photosynthesis, like the maximum carboxylation rate (Vc,max), stomatal conductance (gs) and mesophyll conductance to CO2 (gm). On the one hand, most of the previous modeling efforts have neglected the potential role played by the acclimation of gm to high CO2 and temperature. On the other hand, the effect of climate change on plant clades other than angiosperms, like ferns, has received little attention, and there are no studies evaluating the potential impact of increasing CO2 and temperature on these species. In this study we predicted responses of several representative species among angiosperms, gymnosperms and ferns to increasing CO2 and temperature. Our results show that species with lower photosynthetic capacity - such as some ferns and gymnosperms - would be proportionally more favored under these foreseen environmental conditions. The main reason for this difference is the lower diffusion limitation imposed by gs and gm in plants having high capacity for photosynthesis among the angiosperms, which reduces the positive effect of increasing CO2. However, this apparent advantage of low-diffusion species would be canceled if the two conductances - gs and gm - acclimate and are down regulated to high CO2, which is basically unknown, especially for gymnosperms and ferns. Hence, for a better understanding of different plant responses to future climate, studies are urged in which the actual photosynthetic response/acclimation to increased CO2 and temperature of ferns, gymnosperms and other under-evaluated plant groups is assessed.

Is stomatal conductance optimized over both time and space in plant crowns? A field test in grapevine (Vitis vinifera)

Crown carbon gain is maximized for a given total water loss if stomatal conductance (gs ) varies such that the marginal carbon product of water (∂A/∂E) remains invariant both over time and among leaves in a plant crown, provided the curvature of assimilation rate (A) versus transpiration rate (E) is negative. We tested this prediction across distinct crown positions in situ for the first time by parameterizing a biophysical model across 14 positions in four grapevine crowns (Vitis vinifera), computing optimal patterns of gs and E over a day and comparing these to the observed patterns. Observed water use was higher than optimal for leaves in the crown interior, but lower than optimal in most other positions. Crown carbon gain was 18% lower under measured gs than under optimal gs . Positive curvature occurred in 39.6% of cases due to low boundary layer conductance (gbw ), and optimal gs was zero in 11% of cases because ∂A/∂E was below the target value at all gs . Some conclusions changed if we assumed infinite gbw , but optimal and measured E still diverged systematically in time and space. We conclude that the theorys spatial dimension and assumption of positive curvature require further experimental testing.