Vapour Pressure Deficit (VPD) Drives the Balance of Hydraulic-Related Anatomical Traits in Lettuce Leaves

Abstract

The coordination of leaf hydraulic-related traits with leaf size is influenced by environmental conditions and especially by VPD. Water and gas flows are guided by leaf anatomical and physiological traits, whose plasticity is crucial for plants to face environmental changes. Only a few studies have analysed how variations in VPD levels influence stomatal and vein development and their correlation with leaf size, reporting contrasting results. Thus, we applied microscopy techniques to evaluate the effect of low and high VPDs on the development of stomata and veins, also analysing leaf functional traits. We hypothesized that leaves under high VPD with a modified balance between veins and stomata face higher transpiration. We also explored the variability of stomata and vein density across the leaf lamina. From the results, it was evident that under both VPDs, plants maintained a coordinated development of stomata and veins, with a higher density at low VPD. Moreover, more stomata but fewer veins developed in the parts of the lettuce head exposed to light, suggesting that their differentiation during leaf expansion is strictly dependent on the microclimatic conditions. Knowing the plasticity of hydraulic-related morpho-functional traits and its intra-leaf variability is timely for their impact on water and gas fluxes, thus helping to evaluate the impact of environmental-driven anatomical variations on productivity of natural ecosystems and crops, in a climate change scenario.

Keywords: VPD; leaf anatomical traits; leaf hydraulic conductance; stomatal density; vein density.

Figure 1

(a) Stomatal density per leaf size (SD × LS) and (b) vein density per leaf size (VLA × LS) of green and red Salanova lettuces grown under low and high VPD. Mean values ± standard errors are shown. Different letters correspond to statistically significant differences according to Tukey test (p < 0.05).

Figure 2

(a) Vein density per leaf size (VLA × LS) and 1√leaf size, (b) stomatal density per leaf size (SD × LS) and 1/leaf size and (c) total epidermal cell number per leaf size (ED × LS) and leaf size of grey (circles) and red (triangles) Salanova lettuces grown under low and high VPDs. Mean values ± standard errors are shown. Broken line represents the proportional relationships.

Figure 3

Vein density per leaf size (VLA × LS) and stomatal density per leaf size (SD × LS) of grey (circles) and red (triangles) lettuces grown under low and high VPDs. Regression lines and R2 values are also shown.

Figure 4

(a–c) Vein and stomatal density relationships of grey (circles) and red (triangles) lettuce grown under low VPD in the bottom, LVb (a); middle, LVm (b); and apex, LVa (c) part of the leaf. (d–f) Vein and stomatal density relationships of grey (circles) and red (triangles) lettuce grown under high VPD in the bottom, HVb (d); middle, HVm (e); and apex, HVa (f) part of the leaf. Regression lines and R² values are also shown.

Figure 5

Schematic representation of leaf visual division into three portions: bottom (e,h), middle (d,g) and apex (c,f), from a representative plant grown under (a) high and (b) low VPD. Representative micrographs of leaf veins for both conditions are shown. Scale bars: 500 µm.