SATURATED RATES OF PHOTOSYNTHESIS IN WATER-STRESSED LEAVES OF COMMON BEAN AND TEPARY BEAN

The tepary bean, Phaseolus acutifolius A. Gray var. latifolius G. Freeman, is adapted to arid conditions and is a potential source of genes to improve the drought tolerance of common bean, P. vulgaris (L.). The mechanisms of drought tolerance in P. acutifolius are still unclear. Light and CO2 saturated rates of photosynthetic O2 evolution (Pn) measured in a Hansatech leaf-disk chamber and room-temperature chlorophyll fluorescence transients at 740 nm were used to assess the mesophyll tolerance to dehydration. Detached P. acutifolius leaves maintained higher relative water contents (RWC) and water potentials that those of P. vulgaris. Rapidly dehydrated leaves from both species showed identical responses of their saturated rates of photosynthesis to RWC changes. Mesophyll photosynthetic capacity was very tolerant to dehydration even at water deficits known to cause severe reduction of CO2 assimilation. Similar results were obtained with samples taken from plants acclimated to water-deficit stress. Apparent quantum yield of photosynthetic O2 evolution and variable fluorescence suggest that the slow Pn decline with dehydration was not related to an inhibition of photosynthetic electron transport. These results indicate that genotypic variability in drought tolerance between P. vulgaris and P. acutifolius is not related to differences in mesophyll tolerance to dehydration. P. acutifolius adaptation to an arid environment relies more on dehydration postponement than on tolerance.