A hydraulic model is compatible with rapid changes in leaf elongation under fluctuating evaporative demand and soil water status.

Plants are constantly facing rapid changes in evaporative demand and soil water content, which affect their water status and growth. In apparent contradiction with 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, 30 min vs 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 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 ZmPIPs aquaporins. Transgenic lines under-producing ABA, with lower hydraulic conductivity and higher stomatal conductance, had a LER declining more rapidly than that of WT plants. Whole genome transcriptome and phosphoproteome suggested that the hydraulic processes proposed here might be associated with other rapidly occurring mechanisms. Overall, mechanisms and models presented here may be an essential component of drought tolerance in naturally fluctuating evaporative demand and soil moisture.