Water Relations of Cotton. II. Continuous Estimates of Plant Water Potential from Stem Diameter Measurements1

Abstract Information about plant water potential is often required at very frequent intervals in studies with small numbers of plants. Two methods for inferring plant water potential continuously and nondestructively are described for cotton ( Gossypium hirsutum L.) plants. Both methods require continuous monitoring of stem diameter and occasional destructive sampling of tissue for determination of water potential by conventional methods. The computed results are compared with experimental measurements of plant water potential. The shrinkage modulus method requires calibration points obtained from conventional water potential measurements in early morning and mid‐afternoon in order to relate a measured diameter change to a corresponding measured water potential difference. The dynamic flux method requires an initial estimate of tissue conductivity (between 0.7 and 1.4 ✕ 10 −6 cm 2 sec −1 bar −1 for cotton) and a half‐time for xylem‐phloem equilibration (30 min for cotton). These parameters are used to calculate the stem xylem water potentials which would have been required to bring about measured changes in stem diameter. Both techniques permit plant water status to be monitored continuously in either field or laboratory experiments with minimum destructive sampling of tissue; once accurate parameter estimates are available, the dynamic flux method requires less frequent field calibration.