Field Evaluation of a Water Relations Model for Soybean. I. Validity of Some Basic Assumptions1

Abstract

AbstractBasic assumptions made in a model of stomatal feedback control, transpiration, and carbon dioxide exchange rate (CER) were tested on soybean (Glycine max L. Merr cv. Williams) grown in the field on an Arredondo fine sand soil. Intensive measurements were made of diurnal fluctuations in leaf water potential, stomatal resistance, transpiration rate (TR), and canopy CER during a drying cycle corresponding to the mid‐to‐late pod‐filling period of the crop. A threshold leaf water potential initiating stomatal closure at −1.6 MPa was measured in the field and verified the relationship assumed in the model. Linear relationships between transpiration rate and both radiation flux and vapor pressure deficit were found to exist only in well‐watered plants having low stomatal resistances (1 to 3 sec/cm). Following the time when stomatal closure was initiated, the dependence of transpiration rate on climatic variables quickly dissipated. Transpiration rate was independent of stomatal resistance for fully open stomata, quickly decreased following stomata closure and remained essentially constant at high stomatal resistances. This overall behavior was satisfactorily described by a modified Penman equation which includes a stomatal resistance term. A curvilinear relationship between CER and photosynthetically active radiation flux (PAR) existed as long as stomata were fully open. Following stomata closure, CER was independent of PAR above 500 µ/m2‐sec (full sunlight = 2,100 µE/m2‐sec). The dependence of CER on stomatal resistance was similar to that of transpiration rate but with greater sensitivity.