Declining soil-root hydraulic conductance drives stomatal closure of tomato under drought

<p>The fundamental question as to what triggers stomatal closure during soil drying remains contentious. Thus, we urgently need to improve our understanding of stomatal response to water deficits in soil and atmosphere.<strong> </strong>Here, we investigated the role of soil-plant hydraulic conductance (K<sub>sp</sub>) on transpiration (E) and stomata regulation. We used a root pressure chamber to measure the relation between E, leaf xylem water potential (ψ<sub>leaf-x</sub>) and soil water potential (ψ<sub>soil</sub>) in tomato. Additional measurements of ψ<sub>leaf-x</sub> were performed with unpressurized plants. A soil-plant hydraulic model was used to simulate E(ψ<sub>leaf-x</sub>) for decreasing ψ<sub>soil</sub>. In wet soils, E(ψ<sub>leaf-x</sub>) had a constant slope while in dry soils the slope decreased, with ψ<sub>leaf-x</sub> rapidly and nonlinearly decreasing for moderate increases in E. The ψ<sub>leaf-x</sub> measured in pressurized and unpressurized plants matched well, which indicates that the shoot hydraulic conductance did not decrease during soil drying and that the decrease in K<sub>sp</sub> is caused by a decrease in soil-root conductance. The decrease of E matched well the onset of hydraulic nonlinearity. Our findings demonstrate that stomatal closure prevents the drop in ψ<sub>leaf-x</sub> caused by a decrease in K<sub>sp</sub> and elucidate a strong correlation between stomatal regulation and belowground hydraulic limitation.</p>

Incorporating non-stomatal limitation improves the performance of leaf and canopy models at high vapour pressure deficit

Vapour pressure deficit (D) is projected to increase in the future as temperatures rise. In response to increased D, stomatal conductance (gs) and photosynthesis (A) are reduced, which may result in significant reductions in terrestrial carbon, water, and energy fluxes. It is thus important for gas exchange models to capture the observed responses of gs and A with increasing D. We tested a series of coupled A-gs models against leaf gas exchange measurements from the Cumberland Plain Woodland (Australia), where D regularly exceeds 2 kPa and can reach 8 kPa in summer. Two commonly used A-gs models (Leuning 1995 and Medlyn et al. 2011) were not able to capture the observed decrease in A and gs with increasing D at the leaf scale. To explain this decrease in A and gs, two alternative hypotheses were tested: hydraulic limitation (i.e., plants reduce gs and/or A due to insufficient water supply) and non-stomatal limitation (i.e., downregulation of photosynthetic capacity). We found that the model that incorporated a non-stomatal limitation captured the observations with high fidelity and required the fewest number of parameters. While the model incorporating hydraulic limitation captured the observed A and gs, it did so via a physical mechanism that is incorrect. We then incorporated a non-stomatal limitation into the stand model, MAESPA, to examine its impact on canopy transpiration and gross primary production. Accounting for a non-stomatal limitation reduced the predicted transpiration by ~19%, improving the correspondence with sap flow measurements, and gross primary production by ~14%. Given the projected global increases in D associated with future warming, these findings suggest that models may need to incorporate non-stomatal limitation to accurately simulate A and gs in the future with high D. Further data on non-stomatal limitation at high D should be a priority, in order to determine the generality of our results and develop a widely applicable model.

Potential involvement of root auxins in drought tolerance by modulating nocturnal and daytime water use in wheat

Background and Aims The ability of wheat genotypes to save water by reducing their transpiration rate (TR) at times of the day with high vapour pressure deficit (VPD) has been linked to increasing yields in terminal drought environments. Further, recent evidence shows that reducing nocturnal transpiration (TRN) could amplify water saving. Previous research indicates that such traits involve a root-based hydraulic limitation, but the contribution of hormones, particularly auxin and abscisic acid (ABA), has not been explored to explain the shoot–root link. In this investigation, based on physiological, genetic and molecular evidence gathered on a mapping population, we hypothesized that root auxin accumulation regulates whole-plant water use during both times of the day. Methods Eight double-haploid lines were selected from a mapping population descending from two parents with contrasting water-saving strategies and root hydraulic properties. These spanned the entire range of slopes of TR responses to VPD and TRN encountered in the population. We examined daytime/night-time auxin and ABA contents in the roots and the leaves in relation to hydraulic traits that included whole-plant TR, plant hydraulic conductance (KPlant), slopes of TR responses to VPD and leaf-level anatomical traits. Key Results Root auxin levels were consistently genotype-dependent in this group irrespective of experiments and times of the day. Daytime root auxin concentrations were found to be strongly and negatively correlated with daytime TR, KPlant and the slope of TR response to VPD. Night-time root auxin levels significantly and negatively correlated with TRN. In addition, daytime and night-time leaf auxin and ABA concentrations did not correlate with any of the examined traits. Conclusions The above results indicate that accumulation of auxin in the root system reduces daytime and night-time water use and modulates plant hydraulic properties to enable the expression of water-saving traits that have been associated with enhanced yields under drought.

Potential involvement of root auxins in drought tolerance by modulating nocturnal and daytime water use in wheat

The ability of wheat genotypes to save water by reducing their transpiration rate (TR) under times of the day with high vapour pressure deficit (VPD) has been linked to increasing yields in terminal drought environments. Further, recent evidence shows that reducing nocturnal transpiration (TRN) could amplify water-saving. Previous research indicates that such traits involve a root-based hydraulic limitation, but the contribution of hormones, particularly auxin and abscisic acid (ABA) has not been explored to explain the shoot-root link. In this investigation, based on physiological, genetic and molecular evidence gathered on a mapping population, we hypothesized that root auxin accumulation regulates whole-plant water use during both times of the day. Eight double-haploid lines were selected from a mapping population descending from two parents with contrasted water-saving strategies and root hydraulic properties. These spanned the entire range of slopes of TR responses to VPD and TRN encountered in the population. On those lines, we examined daytime/night-time auxin and ABA contents in the roots and the leaves in relation to hydraulic traits that included whole-plant TR, plant hydraulic conductance (KPlant), slopes of TR responses to VPD and leaf-level anatomical traits. Root auxin levels were consistently genotype-dependent in this group irrespective of experiments and times of the day. Daytime root auxin concentrations were found to be strongly and negatively correlated with daytime TR, KPlant and the slope of TR response to VPD. Night-time root auxin levels significantly and negatively correlated with TRN. In addition, daytime and night-time leaf auxin and ABA concentrations did not correlate with any of the examined traits. The above results indicate that accumulation of auxin in the root system reduces daytime and night-time water use and modulates plant hydraulic properties to enable the expression of water-saving traits that have been associated with enhanced yields under drought.

Changes in Allometric Attributes and Biomass of Forests and Woodlands across an Altitudinal and Rainfall Gradient: What Are the Implications of Increasing Seasonality due to Anthropogenic Climate Change?

Canonical correspondence analysis and linear regressions were used to relate height, diameter, and dispersion measurements of 36,380 stems from 197 species recorded in 2,341 plots against both climatic and landscape variables. Above ground biomass increased in wetter and cooler locations that ameliorate the seasonal rainfall deficits. Taller and greater diameter trees with lower wood densities occur at higher altitudes. Differences between locations are based on a change in the composition of species rather than a change in the allometric properties within a species. The results support the hydraulic limitation and species packing hypotheses. These interrelationships may be affected by the interactions of fire frequency and drought which are a common feature of much of the study area. Under current climate change scenarios it is likely that there will be a reduction in above ground biomass, the number of stems per hectare, average height, average diameter, and basal area due to increasing seasonality of rainfall, temperatures, and the intensity and frequency of fires. The largest of trees are likely to be removed early due to their inability to cope with increased drought stress. The results suggest a marked reduction in carbon storage will occur across the study region in eastern Australia.

Do wide crowns in arid woodland trees reflect hydraulic limitation and reduction of self-shading?

In arid regions many tree species develop broad crowns. A number of hypotheses involve trade-offs between growth in height and horizontal spreading, but there is no explanation for the switch from vertical to horizontal growth during development. Using Acacia papyrocarpa Benth as a model, we measured tree height and crown shape across different sites and topographic positions. We also measured δ13C of phyllodes from crown tops and lateral spreading branches. Trees were significantly taller at the base of a hill, where water availability is typically greater, than on the adjacent steep hillslope. In contrast, δ13C from the treetops was not significantly different across this topographic gradient, despite variation in tree height. In addition, δ13C was higher at treetops than in lower, lateral branches. These observations are consistent with hydraulic limitation to tree height. The shape of mature and young crowns in open environments was not symmetrical. At all sites, branches were shortest, but tree crowns tallest, on south-facing (i.e. shadiest) aspect of crowns. This suggests that light limitation may also affect crown development. If upper branches become water-limited and lower branches light-limited, then middle lateral branches become the less-stressed part of the crown and may grow more, producing a broad crown.

Is transpiration efficiency a viable plant trait in breeding for crop improvement?

Increased transpiration efficiency – commonly the ratio of mass accumulation to transpiration – is often suggested as a critical opportunity for genetic improvement for increased crop yields in water-limited environments. However, close inspection of transpiration efficiency (TE) shows that it is a complex term that is explicitly dependent upon both physiological and environmental variables. Physiological variables include leaf photosynthetic capacity, biochemical composition of the plant productions and possible hydraulic limitation on water flow in the plant. Environmental variables include atmospheric CO2 concentration and atmospheric vapour pressure deficit. To complicate the resolution of transpiration efficiency, a weighted integration over the daily cycle and over the dates of interest needs to be resolved. Consequently, it is concluded that transpiration efficiency is not a variable easily resolved for use in many breeding programs. Instead, component traits contributing to TE need to be studied to increase the effective use of available water through the growing season to ultimately maximise growth and yield of the crop.