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Problems with Porometry: Measuring Net Photosynthesis by Leaf Chamber Techniques

U.S. Water Conserv. Lab., 4331 E. Broadway, Phoenix, AZ 85040

B. J. Choudhury

Hydrological Sci. Branch, Natl. Aeronautics and Space Administration (NASA), Goddard Space Flight Ctr., Greenbelt, MD 20771

^* Corresponding author.

Prior experiments with cotton (Gossypium hirsutum L.) and water hyacinth [Eichhornia crassipes (Mart.) Solms] demonstrated that porometer chamber conditions may significantly perturb the measurement of leaf stomatal conductance. Thus, this study was conducted to determine if net photosynthesis rate measurements of water hyacinth and cotton leaves were similarly perturbed by leaf chamber conditions and, if so, to devise a method to correct for the instrument-induced error. Several net photosynthesis rate and concurrent canopy environmental data sets for these two plant species were analyzed at Phoenix, AZ within the context of the non-water-stressed baseline paradigm, which relates foliage-to-air temperature difference to air vapor pressure deficit. It was found that leaf chamber measurements of net photosynthesis rate were subject to the same type of measurement error as that associated with porometer measurements of leaf stomatal conductance. For plants transpiring at potential rates, photosynthesis rates can be corrected by using ancillary data obtained by the leaf chamber system. However, the proper adjustment of net photosynthesis rates of water-stressed plants measured with leaf chambers required free-air foliage and air temperatures, as well as the free-air humidity. It was demonstrated that concurrent infrared radiation thermometry, as well as simultaneous measurements of air wet- and dry-bulb temperatures, are required to correctly evaluate, by leaf chamber techniques, the net photosynthesis rates of plants that are experiencing any degree of stomatal closure due to some aspect of their normal environment.

Received for publication February 29, 1988.