TIME COURSE OF PHOTOSYNTHESIS IN APPLE LEAVES EXPOSED TO CONTINUOUS ILLUMINATION

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Abstract
Continuous measurements were made of the rate of apparent photosynthesis in fully expanded attached leaves of apple exposed to continuous illumination over periods of 3-18 days. The intensity of the illumination was maintained constant for any one expt. In some expts. the leaves were exposed to the usual atmospheric fluctuations in CO2 and in others the CO2 concn. was constant. Temp. was maintained at 25[degree]C [plus or minus] 1.5[degree]C. When the leaves were exposed to the usual atmospheric fluctuations in CO2 a daily cycle in the rate of apparent photosynthesis was found to occur, which could be correlated with a similar fluctuation in CO2 concn. of the atmosphere. The cyclic variation in rate of apparent photosynthesis did not occur when tanks of compressed air, in which the CO2 concn. was nearly constant, were used. In "sun" leaves of apple the rate of apparent photosynthesis was fairly uniform for at least 18 days under continuous illumination of 3200 f. c. At a light intensity of 5800 f. c. the rate had decreased about 40% by the 14th day. In "shade" leaves of apple exposed to continuous illumination of 3800 f. c. a sharp initial decline in the rate of apparent photosynthesis occurred, reaching a minimum value by the 2d day. This value was maintained until the 10th day. Following the 10th day a further steady falling off in rate set in approaching almost a zero value by the 20th day. Bleaching of the chlorophyll was apparent at all light intensities, but was most pronounced at the high intensities.