Stomatal and growth responses to hydraulic and chemical changes induced by progressive soil drying

A better understanding of physiological responses of crops to drought stress is important for ensuring sustained crop productivity under climate change. Here, we studied the effect on 15-day-old maize (Zea mays L.) plants of a 6 d non-lethal period of soil drying [soil water potential (SWP) decreased from -0.20 MPa to -0.81 MPa]. Root growth was initially stimulated during drying (when SWP decreased from -0.31 MPa to -0.38 MPa, compared with -0.29 MPa in well-watered pots), followed by inhibition during Days 5-6 (SWP from -0.63 MPa to -0.81 MPa). Abscisic acid (ABA) in the root began to accumulate as the root water potential declined during Days 2-3. Leaf elongation was inhibited from Day 4 (SWP less than -0.51 MPa), just after leaf ABA content began to increase, but coinciding with a decline in leaf water potential. The stomatal conductance was restricted earlier in the younger leaf (fourth) (on Day 3) than in the older leaf (third). The ethylene content of leaves and roots decreased during drying, but after the respective increase in ABA contents. This work identified critical timing of hydraulic and chemical changes at the onset of soil drying, which can be important in initiating early stomatal and growth responses to drought.