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Leaf Gas Exchange in Water-Stressed Common Bean and Tepary Bean

Station de Recherches, Agriculture Canada, 2560 boul. Hochelaga, Sainte-Foy, PQ, Canada, GIV 213
Dep. of Horticultural Sci., Univ. of Minnesota, St. Paul, Minnesota 55108

^* Corresponding author.

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 Pn 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 (C₁) led to higher water-use efficiency in P. acutifolius than in P. vulgaris. Analysis of the initial slope of P_n vs. C₁, curves indicated that P. acutifolius bad higher carboxylation efficiency (CE) at low C₁. 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 C₁ 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.

Received for publication July 1, 1991.