Water transport to older leaves in maize ( Zea mays L.) was significantly impaired compared with that to younger leaves under conditions of soil moisture stress

In crop plants subjected to soil moisture stress, older leaves wilt more severely than younger leaves on the same stem. This study investigated the differential responses of older and younger leaves to water stress, as well as the underlying mechanisms, using maize grown in pots. As soil moisture decreased, turgor pressure (TP) declined more markedly in older leaves than in younger leaves, whereas osmotic potential showed almost the same decrease across all leaves. Thus, the pronounced reduction in TP in the older leaves was attributed to a substantial decrease in water potential (Ψleaf). Stomatal conductance also decreased more strongly in older leave, supporting the conclusion that the greater decrease in Ψleaf resulted from reduced water transport to these leaves. Using the rehydration kinetics method, we estimated hydraulic conductance from root to leaf (Kroot-leaf), from stem to leaf (Kstem-leaf) and within the leaf (Kleaf) in plants under water stress, and calculated hydraulic conductance in the root (Kroot) and stem (Kstem). The values of Kstem and Kleaf for older leaves decreased to approximately 50% of those for younger leaves, whereas the decrease in Kroot for the older leaves was only 15%. Consequently, the larger reduction of Ψleaf in older leaves under soil moisture stress was primarily attributed to decreases in Kstem and Kleaf. Additionally, xylem vulnerability to cavitation in the stem and leaf under water stress likely explains the observed difference in dehydration between younger and older leaves.