Correlations between Carbon Isotope Discrimination and Leaf Conductance to Water Vapor in Common Beans

Carbon isotope discrimination (Delta) was measured in the field on 10 cultivars of common bean (Phaseolus vulgaris L.). There was substantial variation (more than 2 per thousand) in leaf Delta values and these differences were maintained between vegetative and reproductive developmental stages. These bean lines also exhibited substantial differences in leaf conductance to water vapor, and again these differences were maintained across developmental stages. The differences in leaf conductance were positively correlated with Delta values, whether conductance was measured as total leaf conductance or as the individual conductances of either upper or lower leaf surfaces. The observed differences in leaf conductance were not associated with differences in stomatal density. There were small differences among bean lines in their leaf Kjeldahl nitrogen contents, which is interpreted as indicating that photosynthetic capacity among bean lines was similar. Thus, because Delta values and leaf conductance were positively correlated, these data suggested that there may have been differences among bean lines in the extent to which stomata limited photosynthetic gas exchange rates.