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Photosynthetic Rate and Diffusion Conductance as a Function of Age in Leaves of Bean Plants¹

Bush beans (Phaseolus vulgaris L.) were grown and tested under constant environmental conditions. Simultaneous measurements of CO₂ exchange rates and H₂O diffusion conductances were made on individual leaves, throughout all stages of leaf and plant development. Both exchange rate and leaf conductance (reciprocal of resistance) declined with leaf age. In both cases the data for all leaves fell on a single straight line, when plotted against time expressed as a percentage of the time from maturity to death. The decline in conductance was not sufficient to explain the decline in the exchange rate. There was a proportionate decline in the internal photosynthetic capacities of the leaves. Since the CO₂ concentration in the substomatal cavity was found to remain almost constant, and the external conditions (light, temperature, water status) were kept constant, the decline in conductance was either due to a real decrease in the abilities of the stomates to stay open or a compensating adjustment of stomates by a CO₂ feedback process.

Key Words: CO₂ exchange • Leaf stomata mechanism

² Former graduate research assistant, Dep. of Biology (now assistant professor, Natural Science Division, Pepperdine Univ., Malibu, CA 90265); and associate professor, Dep. of Soil and Crop Sciences, Texas Agric. Exp. Sin., Texas A&M Univ., College Station, TX 77843.

Received for publication March 28, 1977.

This article has been cited by other articles:

				E. ZEIGER and A. SCHWARTZ Longevity of Guard Cell Chloroplasts in Falling Leaves: Implication for Stomatal Function and Cellular Aging Science, November 12, 1982; 218(4573): 680 - 682. [Abstract] [PDF]