Root pressurization affects growth-induced water potentials and growth in dehydrated maize leaves

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Abstract
Profiles of water potential (Ψw) were measured from the soil to the tips of growing leaves of maize (Zea mays L.) when pressure (P) was applied to the soil/root system. At moderately low soil Ψw, leaf elongation was somewhat inhibited, large tensions existed in the xylem, and Ψw were slightly lower in the elongating leaf tissues than in the xylem, i.e. a growth‐induced Ψw was present but small. With P, the tension was relieved, enlarging the difference in Ψw between the xylem and the elongating tissues, i.e. enlarging the growth‐induced Ψw, which is critical for growth. Guttation occurred, confirming the high Ψw of the xylem, and the mature leaf tissue rehydrated. Water uptake increased and met the requirements of transpiration. Leaf elongation recovered to control rates. Under more severe conditions at lower soil Ψw, P induced only a brief elongation and the growth‐induced Ψw responded only slightly. Guttation did not occur, water flow did not meet the requirements of transpiration, and the mature leaf tissues did not rehydrate. A rewatering experiment indicated that a low conductance existed in the severely dehydrated soil, which limited water delivery to the root and shoot. Therefore, the initial growth inhibition appeared to be hydraulic because the enlargement of the growth‐induced Ψw by P together with rehydration of the mature leaf tissue were essential for growth recovery. In more severe conditions, P was ineffective because the soil could not supply water at the required rate, and metabolic factors began to contribute to the inhibition.

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