A Hydraulic Model Is Compatible with Rapid Changes in Leaf Elongation under Fluctuating Evaporative Demand and Soil Water Status      

Abstract

Plants are constantly facing rapid changes in evaporative demand and soil water content, which affect their water status and growth. In apparent contradiction to a hydraulic hypothesis, leaf elongation rate (LER) declined in the morning and recovered upon soil rehydration considerably quicker than transpiration rate and leaf water potential (typical half-times of 30 min versus 1–2 h). The morning decline of LER began at very low light and transpiration and closely followed the stomatal opening of leaves receiving direct light, which represent a small fraction of leaf area. A simulation model in maize (Zea mays) suggests that these findings are still compatible with a hydraulic hypothesis. The small water flux linked to stomatal aperture would be sufficient to decrease water potentials of the xylem and growing tissues, thereby causing a rapid decline of simulated LER, while the simulated water potential of mature tissues declines more slowly due to a high hydraulic capacitance. The model also captured growth patterns in the evening or upon soil rehydration. Changes in plant hydraulic conductance partly counteracted those of transpiration. Root hydraulic conductivity increased continuously in the morning, consistent with the transcript abundance of Zea maize Plasma Membrane Intrinsic Protein aquaporins. Transgenic lines underproducing abscisic acid, with lower hydraulic conductivity and higher stomatal conductance, had a LER declining more rapidly than wild-type plants. Whole-genome transcriptome and phosphoproteome analyses suggested that the hydraulic processes proposed here might be associated with other rapidly occurring mechanisms. Overall, the mechanisms and model presented here may be an essential component of drought tolerance in naturally fluctuating evaporative demand and soil moisture.

Genetic and Physiological Controls of Growth under Water Deficit

“…The daytime depression of leaf elongation rate is the highest during days with high evaporative demand (high light intensity and vapor pressure deficit; Fig. 1) and increased by mild water deficit (Parent et al, 2010a, Caldeira et al, 2014. However, the negative effect of light intensity on expansive growth only applies when the leaf is mature enough to sustain its own carbon demand through photosynthesis.…”

“…Our view is summarized in Figure 2, in which expansive growth (vegetative and reproductive) and photosynthesis are considered to have largely independent environmental and genetic controls. (Caldeira et al, 2014). D and E, Light intensity and whole-plant photosynthesis in a canopy gas exchange platform (redrawn from Fig.…”

“…Leaf elongation rate of monocotyledons changes in less than 30 min, rapidly stabilizes, and returns to its original value when light is switched on in a growth chamber (Munns et al, 2000) or, in the early morning in natural conditions (Caldeira et al, 2014). This is also the case when evaporative demand increases via a change in air vapor pressure deficit with constant light (Sadok et al, 2007) when salt or manitol is added to a nutrient solution (Munns et al, 2000), when droughted plants are rehydrated (Hsiao et al, 1970), or when plants are subjected to a sudden anoxia (Ehlert et al, 2009).…”

Is change in ovary carbon status a cause or a consequence of maize ovary abortion in water deficit during flowering?

“…1, B and E; Table I). Pots were then watered individually with the necessary volume to maintain soil water potential at the targeted value (for details, see Caldeira et al, 2014). Three levels of soil WD were performed in experiment 2, with soil water potential targeted values of 20.3, 20.5, and 20.6 MPa in WD1, WD2, and WD3 treatments, respectively, while only treatment WD1 was applied in experiment 1 (Table I).…”

Focus on Water

“…Scoffoni et al (2014) explore the feedback and connection between leaf hydraulic conductance, turgor loss, and leaf shrinkage in different species and show that the process plays a role in hydraulic decline during mild dehydration. Caldeira et al (2014) close the circle, reporting on model and experimental analysis of leaf growth with diurnal water flux and transpiration. Their studies are consistent with a significant role in leaf expansion of a source-sink balance between aquaporin expression and hydraulic conductance of the root, on the one hand, and of stomatal transpiration from the leaf, on the other.…”