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Articles

Mesophyll Resistance and Carboxylase Activity

A Comparison under Water Stress Conditions

^a Department of Vegetable Crops, Cornell University, Ithaca, New York 14850

The response of several leaf gas exchange parameters were monitored with decreasing leaf water potential in Phaseolus vulgaris L. leaflets. These included photosynthesis, transpiration, CO₂ compensation point, ribulose 1,5-diphosphate carboxylase activity, boundary layer plus stomatal, and mesophyll resistance to diffusion of CO₂. Mesophyll resistance was calculated under two assumptions: (a) the CO₂ concentration at the chloroplast was zero, and (b) it was equal to the CO₂ compensation point.

Contrary to some reports on bean, the estimates of mesophyll resistance (both models) increased with decreasing leaf water potential. Concurrently, the CO₂ 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.

² Present address: Department of Agronomy and Plant Genetics, University of Minnesota, St. Paul, Minn. 55108.

³ Present address: International Institute for Tropical Agriculture, Ibadan, Nigeria.

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