LEAF GAS-EXCHANGE IN WATER-STRESSED COMMON BEAN AND TEPARY BEAN

Tepary bean, Phaseolus acutifolius A. Gray var. latifolius G. Freeman, is adapted to hot arid conditions and might be a valuable source of genes to improve the drought tolerance of P. vulgaris L. We tested the hypothesis that P. acutifolius adaptation to arid environments relies on differences in gas exchange characteristics leading to greater water-use efficiency. Both species were subjected to controlled levels of water stress by enclosing the root systems in semipermeable membranes immersed in nutrient solutions osmotically adjusted with polyethylene glycol. Phaseolus acutifolius had higher net photosynthetic rates (P(n)) than P. vulgaris at high to moderately low water potentials. The P(n) rates of P. acutifolius declined more rapidly with lower water potential than those of P. vulgaris. This response was related to increased stomatal closure with decreased water potential. Higher P(n) rates at any given internal CO2 partial pressure (C(i)) led to higher water-use efficiency in P. acutifolius than in P. vulgaris. Analysis of the initial slope of P(n) vs. C(i) curves indicated that P. acutifolius had higher carboxylation efficiency (CE) at low C(i). In both species, water stress significantly reduced light- and CO2-saturated P(n) rates and only slightly lowered the CE. Similar mesophyll palisade surface area in P. acutifolius and P. vulgaris suggested that differences in P(n) rates at low C(i) might be related to biochemical rather than anatomical leaf characteristics. Higher water-use efficiency at moderate water deficits may contribute to P. acutifolius adaptation to arid environments.