Mesophyll Resistance and Carboxylase Activity: A Comparison under Water Stress Conditions.

The response of several leaf gas exchange parameters were monitored with decreasing leaf water potential in Phaseolus vulgaris L. leaflets. These included photosynthesis, transpiration, CO(2) compensation point, ribulose 1,5-diphosphate carboxylase activity, boundary layer plus stomatal, and mesophyll resistance to diffusion of CO(2). Mesophyll resistance was calculated under two assumptions: (a) the CO(2) concentration at the chloroplast was zero, and (b) it was equal to the CO(2) compensation point.Contrary to some reports on bean, the estimates of mesophyll resistance (both models) increased with decreasing leaf water potential. Concurrently, the CO(2) compensation point increased and ribulose 1,5-diphosphate carboxylase activity decreased as internal water stress increased.It is suggested that the mesophyll resistance term has been too liberally interpreted in the past and that future use be verified by an indicator of biochemical response to environmental stress such as the assay for ribulose 1,5-diphosphate carboxylase activity.