Osmotic adjustment and hormonal regulation of stomatal responses to vapour pressure deficit in sunflower

Dynamic variation of the stomatal pore in response to changes in leaf–air vapour pressure difference (VPD) constitutes a critical regulation of daytime gas exchange. The stomatal response to VPD has been associated with both foliage abscisic acid (ABA) and leaf water potential (Ψ l); however, causation remains a matter of debate. Here, we seek to separate hydraulic and hormonal control of stomatal aperture by manipulating the osmotic potential of sunflower leaves. In addition, we test whether stomatal responses to VPD in an ABA-deficient mutant (w-1) of sunflower are similar to the wild type. Stomatal apertures during VPD transitions were closely linked with foliage ABA levels in sunflower plants with contrasting osmotic potentials. In addition, we observed that the inability to synthesize ABA at high VPD in w-1 plants was associated with no dynamic or steady-state stomatal response to VPD. These results for sunflower are consistent with a hormonal, ABA-mediated stomatal responses to VPD rather than a hydraulic-driven stomatal response to VPD.

Leaf anatomical adaptations have central roles in photosynthetic acclimation to humidity

Examination of long-term photosynthetic acclimation of two tomato cultivars (Jinpeng and Zhongza) to leaf-to-air vapour pressure difference reveals that independent changes in epidermal and mesophyll cell size maintain photosynthetic homeostasis in Jinpeng.

El effecto del microambiente en la conductividad estomática de Buddleia cordata H. B. K., en la Reserva del Pedregal de San Ángel

Precipitation seasonality and substratum in Pedregal de San Angel can play a key role on plant water status. Therefore, stomatal conductance (g), water potential (Ψ h), photosynthetically photon flux density (Q), air temperature (Ta) and leaf-air vapour pressure difference (VPD), were measured on leaves of Buddleia cordata H.B.K., because its perennial character and its dominance in the Pedregal. These measurements were carried out during the wet and dry seasons in the Pedregal which is a plant community developing in a lava substratum. The highest values of stomatal conductance were registered in the wet season (330 mmol m-2 s-1), however during the dry season, stomatal conductance was reduced by 54%. Upper limit enveloping curve technique and multiple regression analysis were performed to the data in order to study how g responded to the microenvironment and (Ψ h). Results clearly showed that there was a significant effect of Q, Ta and VPD on g and stomata were more sensitive on humid season than during dry season. (Ψ h), effect was not significant during the humid season, nor the dry season, but annually. Results from these kind of analysis may be very useful during the formulation of mathematics models to simulate or predict stomatal conductance.

Flux Enhancement in Membrane Distillation Using Nanofiber Membranes

Membrane distillation (MD) is an emerging separation technology, whose largest application potential lies in the desalination of highly concentrated solutions, which are out of the scope of reverse osmosis. Despite many attractive features, this technology is still awaiting large industrial application. The main reason is the lack of commercially available membranes with fluxes comparable to reverse osmosis. MD is a thermal separation process driven by a partial vapour pressure difference. Flux, distillate purity, and thermal efficiency are always in conflict, all three being strictly connected with pore size, membrane hydrophobicity, and thickness. The world has not seen the ideal membrane yet, but nanofibers may offer a solution to these contradictory requirements. Membranes of electrospun PVDF were tested under various conditions on a direct contact (DCMD) unit, in order to determine the optimum conditions for maximum flux. In addition, their performance was compared to commonly available PTFE, PE, and PES membranes. It was confirmed that thinner membranes have higher fluxes and a lower distillate purity and also higher energy losses via conduction across the membrane. As both mass and heat transfer are connected, it is best to develop new membranes with a target application in mind, for the specific membrane module and operational conditions.

Water redistribution determines photosynthetic responses to warming and drying in two polar mosses

Predicting impacts of climate change requires an understanding of the sensitivity of species to temperature, including conflated changes in humidity. Physiological responses to temperature and clump-to-air vapour pressure difference (VPD) were compared in two Antarctic moss species, Ceratodon purpureus (Hedw.) Brid. and Schistidium antarctici (Cardot) L.I. Savicz & Smirnova. Temperatures from 8 to 24°C had no significant effects on photosynthesis or recovery from drying, whereas high VPD accelerated drying. In Schistidium, which lacks internal conduction structures, shoots dried more slowly than the clump, and photosynthesis ceased at high shoot relative water content (RWC), behaviour consistent with a strategy of drought avoidance although desiccation tolerant. In contrast, shoots of Ceratodon have a central vascular core, but dried more rapidly than the clump. These results imply that cavitation of the hydroid strand enables hydraulic isolation of extremities during rapid drying, effectively slowing water loss from the clump. Ceratodon maintained photosynthetic activity during drying to lower shoot RWC than Schistidium, consistent with a strategy of drought tolerance. These ecophysiological characteristics may provide a functional explanation for the differential distribution of Schistidium and Ceratodon along moisture gradients in Antarctica. Thus, predicting responses of non-vascular vegetation to climate change at high latitudes requires greater focus on VPD and hydraulics than temperature.

Hydraulically based stomatal oscillations and stomatal patchiness in Gossypium hirsutum

Slow stomatal oscillations (70–95 min), associated with feedback within the plant hydraulic systems, were studied in cotton (Gossypium hirsutum L.). Oscillations were only evident when the whole plant was exposed to light, and were not influenced by reductions in intercellular CO2 concentrations (Ci) in intact, attached leaves. Oscillations were synchronised among different leaves of the same plant, even when the leaf-to-air vapour pressure difference (VPD) was reduced in a cuvette enclosing one of the leaves. In the trough phase of stomatal oscillations the apparent Ci was higher than expected from the combination of the observed assimilation rate and the A(Ci) relationship measured in the absence of oscillations. Using chlorophyll fluorescence imaging we found evidence of stomatal heterogeneity in this phase. Finally, we found that stomatal oscillations appeared to be correlated with xylem embolism, with more vessels filled with gas at the peak than at the troughs of stomatal oscillations.

Influence of season, drought and xylem ABA on stomatal responses to leaf-to-air vapour pressure difference of trees of the Australian wet-dry tropics

This paper reports the results of two experiments undertaken to investigate the influence of season and soil drying on stomatal responses to leaf-to-air vapour pressure differences. We examined the response of stomatal conductance to increasing leaf-to-air vapour pressure difference, in the wet and dry seasons, of five tropical tree species. We also examined leaves of these species for anatomical differences to determine whether this could explain differences in stomatal sensitivity to leaf-to-air vapour pressure differences. Finally, we conducted a split-root experiment with one of those species to look for interactions between xylem abscisic acid concentration, predawn water potential, leaf area to root mass ratio and stomatal responses to leaf-to-air vapour pressure differences. Stomatal conductance declined linearly with increasing leaf-to-air vapour pressure difference in all species. Leaves that expanded in the ‘dry’ season were more sensitive to leaf-to-air vapour pressure differences than those that had expanded in the ‘wet’ season. The value of leaf-to-air vapour pressure difference where 50% of extrapolated maximum stomatal conductance would occur was 5.5 kPa for wet season but only 3.4 kPa for dry season leaves. In the wet season, transpiration rate increased with increasing leaf-to-air vapour pressure difference in most example species. However, in the dry season, transpiration was constant as leaf-to-air vapour pressure differences increased in most cases. There were significant changes in the proportion of cell wall exposed to air space in leaves, between wet and dry seasons, in three of four species examined. In the split-root experiment, a very mild water stress increased stomatal sensitivity to leaf-to-air vapour pressure differences, and stomatal conductivity declined linearly with decreasing predawn water potential. However, levels of ABA in the xylem did not change, and stomatal sensitivity to exogenous ABA did not change. The ratio of leaf area to root mass declined during water stress and was correlated to changes in stomatal sensitivity to leaf-to-air vapour pressure differences.