Soil drying rate and gas exchange of three Phaseolus vulgaris L. varieties

Intermittent drought is the main limiting factor for bean production in Mexico. The root system of three bean varieties of different origin and tolerance to water stress was exposed to different drying soil rates. The synthetic varieties were: Pinto Villa (PV, water stress tolerant) and Bayo Madero (BM, water stress susceptible) both of mesoamerican origin and growth habit type III (climbing habit), and ICA Palmar (water stress tolerant) of Andean origin and growth habit type I (shrub). Treatments were three soil drying rates, which were achieved by stopping irrigation and either maintaining or removing the pot cover made with PVC tubes in a split root system. Gas exchange was studied during 32 h. Slow soil drying rate (with cover) generated a gradual decline in stomatal conductance and CO2 assimilation rate. In BM, the three dehydration levels significantly affected the response of the quantified variables. Despite water was available in the soil, plants remained with stomata closed. In PV and ICA Palmar, the most contrasting effects were achieved with intermediate and fast soil drying rates, even though there were differences in soil water availability; in PV, differences among treatments were not expressed. In ICA Palmar, soil water potential did not express the response in stomatal conductance. Both varieties showed high and low values in stomatal conductance. Leaflet temperature increased during the first two hours of sampling. ICA Palmar was the variety with less change in leaflet temperature, greater control of stomatal conductance, and high CO2 assimilation rate. This was possibly a result of its high presence of trichomes on both sides of the leaflet.