Perennial plants perpetually adapt to environmental changes in complex and yet insufficiently understood manner. We aimed to separate the intra-seasonal temperature effects on structure and function from perennial and annual water stress effects. This study focused on grapevine (Vitis vinifera L. `Cabernet Sauvignon') petioles, which being a continuously produced organ, represent the current status of the plant. Field-grown mature plants subjected to multi-annual irrigation treatments (severe water stress, mild water stress and non-stressed) throughout the growing season were compared with greenhouse-grown plants under three temperature regimes (22, 28 and 34 degrees C). Physiological and functional anatomy parameters were measured. A generalized additive model (GAM) based on meteorological and lysimeter-based field data was applied to determine the relative influence of various meteorological parameters on evapotranspiration (ETc) during the growing season in the field experiment. At the beginning of the growing season, in May, petioles in the severe stress treatment showed a stress-related structure (decreased length, safer hydraulic structure and increased lignification), though having high values of stem water potential (SWP). As the season progressed and temperatures increased, all water availability treatments petioles showed similar changes, and at the end of season, in August, were structurally very similar. Those changes were independent of SWP and were comparable to high temperature-induced changes in the greenhouse. In contrast, stems hydraulic structure was strongly influenced by water availability. Regression analyses indicated a relationship between petioles xylem structure and stomatal conductance (g(s)), whereas g(s) (but not SWP) was temperature-dependent. The GAM showed that ETc was mainly dependent on temperature. Our results indicate a perennial water-stress memory response, influencing the petiole structure at the beginning of the following season. Intra-seasonally, the petiole's structure becomes independent of water status, whereas temperature drives the structural changes. Thus, ongoing climate change might disrupt plant performance by purely temperature-induced effects.

Due to climate change, winter temperatures are predicted to increase worldwide. For thermophilic trees, highly sensitive to low temperatures, an increase in winter temperatures may be beneficial for survival and regeneration. Ziziphus spina-christi is a thermophilic tree that has recently become more abundant and widespread in the eastern Mediterranean, presumably due to a gradual increase in winter temperatures. We aim to define the temperature limitations for seed germination and the growth and survival of young seedlings to broaden our understanding of the future geographical distribution of this species. We studied effects of temperature on germination, growth, and photosynthesis in a controlled environment with four different day/night temperature regimes (34/28 degrees C, 28/22 degrees C, 22/16 degrees C and 16/10 degrees C). Effects of endocarp on germination and seed germination in the field were also studied. Results showed that germination has a lower thermal optimum (34-22 degrees C, 63.5-67.5% germination) than growth and photosynthesis (34-28 degrees C). Moderate cold stress (22/16 degrees C), did not affect germination capacity, but strongly reduced seedling growth (71%) and photosynthetic capacity (44.6%). Under severe cold stress (16/10 degrees C), germination still occurs (22%), but seedlings cannot perform growth and photosynthesis. We conclude that slow seedling growth, not germination, is the main barrier for successful establishment of Z. spina-christi under low temperature. Warmer winters could lead to earlier establishment of seedlings and increase their chance of survival the following summer. This may explain the recent increase in the tree's relative abundance and further highlight the potential spread of this species at higher altitudes and latitudes across the Mediterranean.

Photosynthesis is limited by three main factors: stomatal conductance (gs), mesophyll conductance (gm) and maximum capacity for Rubisco carboxylation (Vcmax). It is unclear how limiting factors vary under stress, particularly during long-term stress acclimation. In this work, we compared for the first time photosynthesis limitation resulting from long-term acclimation to three major abiotic stresses: drought, salinity and temperature. We used saplings of Ziziphus spina-christi, a thermophilic and drought-tolerant tree, which recently became more abundant in the Mediterranean, presumably due to increased winter temperatures. Stress acclimation was investigated by measuring growth, gas exchange, chlorophyll fluorescence and leaf structure. For each stress, photosynthesis-limiting factors were compared. We developed an integrative stress index that allowed us to precisely define stress level, enabling a comparison between stress types. Photosynthesis under all stresses was limited mostly by gs and gm (80–90%); whereas biochemistry (Vcmax) made a minor contribution (10–20%). The relative contribution of gs and gm on photosynthetic limitation was influenced by stress type. During acclimation to drought or salinity, photosynthesis was limited by a decline in gs, while intolerance to low temperatures was driven by decline in gm. In all the stresses, gm decreased only under progressive reduction in leaf physiological functionality and was associated with low turgor under drought, an increase in leaf Na+ under salinity and low leaf hydraulic conductance (Kleaf) at low temperatures. Mesophyll structure (mesophyll surface area exposed to the intercellular air spaces, leaf thickness, % intercellular air spaces) did not explain gm acclimation to stress. Current work gives methodology for stress studies, and defines the main factors underlying the plant response to climate change. The ability to minimize mesophyll-imposed limitations on photosynthesis was found as a strong indicator of progressive stress tolerance. Moreover, the results demonstrate how warming climate benefits the photosynthetic function in thermophilic species, such as Ziziphus spina-christi. © The Author(s) 2018.

Abscisic acid (ABA) levels increase significantly in plants under stress conditions and ABA is thought to serve as a key stress-response regulator. However, the direct effect of ABA on photosynthesis and the effect of mesophyll ABA on yield under both well-watered and drought conditions are still the subject of debate. Here, we examined this issue using transgenic Arabidopsis thaliana plants carrying a dominant ABA-signaling inhibitor under the control of a mesophyll-specific promoter (FBPase::abi1-1, abbreviated to fa). Under normal conditions, fa plants displayed slightly higher stomatal conductance and carbon assimilation than wild-type (WT) plants; however, these parameters were comparable following ABA treatment. These observations suggest that ABA does not directly inhibit photosynthesis in the short term. fa plants also exhibited a variety of altered phenotypes under optimal conditions, including more vigorous initial growth, earlier flowering, smaller flowers and delayed chlorophyll degradation. Furthermore, under optimal conditions, fa plant seed production was less than a third of that observed for the WT. However, under drought conditions, WT and fa seed yields were similar due to a significant reduction in WT seed and no reduction in fa seed. These findings suggest that endogenous basal ABA inhibits a stress-escape response under non-stress conditions, allowing plants to accumulate biomass and maximize yield. The lack of a correlation between flowering time and plant biomass combined with delayed chlorophyll degradation suggests that this stress-escape behavior is regulated independently and upstream of other ABA-induced effects such as rapid growth and flowering.

Photosynthesis is limited by three main factors: stomatal conductance (gs), mesophyll conductance (gm) and maximum capacity for Rubisco carboxylation (Vcmax). It is unclear how limiting factors vary under stress, particularly during long-term stress acclimation. In this work, we compared for the first time photosynthesis limitation resulting from long-term acclimation to three major abiotic stresses: drought, salinity and temperature. We used saplings of Ziziphus spina-christi, a thermophilic and drought-tolerant tree, which recently became more abundant in the Mediterranean, presumably due to increased winter temperatures. Stress acclimation was investigated by measuring growth, gas exchange, chlorophyll fluorescence and leaf structure. For each stress, photosynthesis-limiting factors were compared. We developed an integrative stress index that allowed us to precisely define stress level, enabling a comparison between stress types. Photosynthesis under all stresses was limited mostly by gs and gm (80-90%); whereas biochemistry (Vcmax) made a minor contribution (10-20%). The relative contribution of gs and gm on photosynthetic limitation was influenced by stress type. During acclimation to drought or salinity, photosynthesis was limited by a decline in gs, while intolerance to low temperatures was driven by decline in gm. In all the stresses, gm decreased only under progressive reduction in leaf physiological functionality and was associated with low turgor under drought, an increase in leaf Na+ under salinity and low leaf hydraulic conductance (Kleaf) at low temperatures. Mesophyll structure (mesophyll surface area exposed to the intercellular air spaces, leaf thickness, % intercellular air spaces) did not explain gm acclimation to stress. Current work gives methodology for stress studies, and defines the main factors underlying the plant response to climate change. The ability to minimize mesophyll-imposed limitations on photosynthesis was found as a strong indicator of progressive stress tolerance. Moreover, the results demonstrate how warming climate benefits the photosynthetic function in thermophilic species, such as Ziziphus spina-christi.

Stomatal oscillations are cyclic opening and closing of stomata, presumed to initiate from hydraulic mismatch between leaf water supply and transpiration rate. To test this assumption, mismatches between water supply and transpiration were induced using manipulations of vapour pressure deficit (VPD) and light spectrum in banana (Musa acuminata). Simultaneous measurements of gas exchange with changes in leaf turgor pressure were used to describe the hydraulic mismatches. An increase of VPD above a certain threshold caused stomatal oscillations with variable amplitudes. Oscillations in leaf turgor pressure were synchronized with stomatal oscillations and balanced only when transpiration equaled water supply. Surprisingly, changing the light spectrum from red and blue to red alone at constant VPD also induced stomatal oscillations – while the addition of blue (10%) to red light only ended oscillations. Blue light is known to induce stomatal opening and thus should increase the hydraulic mismatch, reduce the VPD threshold for oscillations and increase the oscillation amplitude. Unexpectedly, blue light reduced oscillation amplitude, increased VPD threshold and reduced turgor pressure loss. These results suggest that additionally, to the known effect of blue light on the hydroactive opening response of stomata, it can also effect stomatal movement by increased xylem–epidermis water supply. © 2017 John Wiley & Sons Ltd