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Light Acclimation During and After Leaf Expansion in Soybean ¹

Randall S. Alberte^b,3

^a Department of Botany, Duke University, Durham, North Carolina 27706, Department of Biology, University of California, Los Angeles, California 90024

Soybean plants (Glycine max var. Ransom) were grown at light intensities of 850 and 250 µeinsteins m^–2 sec^–1 of photosynthetically active radiation. A group of plants was shifted from each environment into the other environment 24 hours before the beginning of the experiment. Net photosynthetic rates and stomatal conductances were measured at 2,000 and 100 µeinsteins m^–2 sec^–1 photosynthetically active radiation on the 1st, 2nd, and 5th days of the experiment to determine the time course of photosynthetic light adaptation. The following factors were also measured: dark respiration, leaf water potential, leaf thickness, internal surface area per external surface area, chlorophyll content, photosynthetic unit size and number, specific leaf weight, and activities of malate dehydrogenase, and glycolate oxidase. Comparisons were made with plants maintained in either 850 or 250 µeinsteins m^–2 sec^–1 environments. Changes in photosynthesis, stomatal conductance, leaf anatomy, leaf water potential, photosynthetic unit size, and glycolate oxidase activity occurred upon altering the light environment, and were complete within 1 day, whereas chlorophyll content, numbers of photosynthetic units, specific leaf weight, and malate dehydrogenase activity showed slower changes. Differences in photosynthetic rates at high light were largely accounted for by internal surface area differences with low environmental light associated with low internal area and low photosynthetic rate. An exception to this was the fact that plants grown at 250 µeinsteins m^–2 sec^–1 then switched to 850 µeinsteins m^–2 sec^–1 showed lower photosynthesis at high light than any other treatment. This was associated with higher glycolate oxidase and malate dehydrogenase activity. Photosynthesis at low light was higher in plants kept at or switched to the lower light environment. This increased rate was associated with larger photosynthetic unit size, and lower dark respiration and malate dehydrogenase activity. Both anatomical and physiological changes with environmental light occurred even after leaf expansion was complete and both were important in determining photosynthetic response to light.

³ Present address: Department of Biology, Barnes Laboratory, The University of Chicago, Chicago, Illinois 60637.

¹ This research was supported by National Science Foundation Grants GI-39229 and DEB 76-04150. R. S. A. was supported by a National Science Foundation Energy Related Postdoctoral Fellowship.

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