STOMATAL RESPONSE TO HUMIDITY IN COMMON BEAN (PHASEOLUS-VULGARIS) - IMPLICATIONS FOR MAXIMUM TRANSPIRATION RATE, WATER-USE EFFICIENCY AND PRODUCTIVITY

Twelve common bean (Phaseolus vulgaris L.) cultivars were grown under greenhouse conditions to study the response of net photosynthesis (A) and transpiration (E) to variation in the leaf-to-air humidity gradien t (nu). Large differences were observed between cultivars in maximum r ates of A and E. The variation in A correlated with both leaf nitrogen content and specific leaf area. Thin leaves had higher nitrogen conte nts per unit dry weight, but thick leaves had higher nitrogen content per unit surface area. Photosynthetic nitrogen-use efficiency did not correlate with nitrogen content on either a mass or a surface area bas is. Very little variation was found between cultivars in the sensitivi ty of total leaf conductance (g) to increasing nu, when sensitivity wa s defined as the slope of ln(g) versus nu (partial derivative ln(g)/pa rtial derivative nu). No significant correlation existed between parti al derivative ln(g)/partial derivative nu and variation in maximum con ductance values. Much steeper slopes (greater sensitivity) were found in the response of stomatal conductance alone (g(s)) to the leaf-leaf surface humidity gradient (nu(s)). The sensitivity of stomatal respons e correlated positively with variation in maximum conductance among cu ltivars, and, since stomatal conductance was in series with a fixed bo undary layer conductance, this positive correlation made possible the uniform sensitivity of g((total)) with respect to nu((leaf-air)) despi te the wide variation in g(max). All cultivars reached their maximum E at very similar v values, and all showed a relatively constant E over a wide range of high nu. The implications of this relative homeostasi s in E are discussed in the context of possible hydraulic limitations on E. Considerable recent interest has focussed on the use of carbon i sotope discrimination (Delta) as a useful screening character in crop breeding programmes, and Delta has been found to correlate positively with yield in P. vulgaris. We found that Delta, measured on bulk leaf tissue, positively correlated with both A(max) and g(max) between bean cultivars, but did not correlate with instantaneous measures of inter cellular CO2 (c(i)) when nu was held constant across all measurements. This apparent discrepancy may be due, at least in part, to variation in leaf temperatures among cultivars under normal growing conditions. Leaf-energy-budget simulations indicated that the observed range of ma ximum leaf conductance at low nu would generate up to 3.0 degrees C va riation in leaf temperatures under field conditions of low to moderate windspeed. Given the strong stomatal response to nu, this variation i n leaf temperature could cause variation in carbon isotope discriminat ion, which reflects long-term c(i) values. Such a mechanism of produci ng variation in c(i) would not be apparent in c(i) measurements taken under cuvette conditions where leaf temperature was held constant.