Simultaneous recording of diurnal changes in leaf turgor pressure and stem water status of bread wheat reveal variation in hydraulic mechanisms in response to drought.

Abstract

Information about water relations within crop canopies is needed to improve our understanding of canopy resource distribution and crop productivity. In this study, we examined the dehydration/rehydration kinetics of different organs of wheat plants using ZIM-probes that continuously monitor water status non-destructively. ZIM-probes were clamped to the flag leaf and penultimate leaf of the same stem to monitor changes in turgor pressure, and a novel stem probe was clamped to the peduncle (just below the spike of the same stem) to monitor changes in stem water status. All organs behaved similarly under well-watered conditions, dehydrating and recovering at the same times of day. When water was withheld, the behaviour diverged, with the leaves showing gradual dehydration and incomplete recovery in leaf turgor pressure during the night, but the stem was affected to a lesser extent. Penultimate leaves were the most severely affected, reaching turgor loss point before the flag leaf. Upon rewatering, turgor pressure recovered but the output patch-pressure of the probes (Pp) oscillated at ~30min periods in all organs of most plants (n=4). Oscillations in Pp were attributed to oscillations in stomatal opening and appear to only occur above a threshold light intensity. The mechanisms identified in this study will be beneficial for crop productivity because the flag leaf is the source of most photoassimilates in developing grains, so the plant's ability to maintain flag leaf hydration at the expense of older leaves should moderate the impact of drought on yield. Stomatal oscillations could increase water use efficiency as the plant attempts to rehydrate after drought.