PHOTOINHIBITION OF STEM ELONGATION BY FULL SOLAR RADIATION

Abstract

Lockhart, James A. (U. Hawaii, Honolulu.) Photoinhibition of stem elongation by full solar radiation. Amer. Jour. Bot. 48(5): 387–392. Illus. 1961.—Stem growth response of ‘Pinto’ bean (Phaseolus vulgaris) to full solar radiation and to various degrees of shading has been studied. Maximum stem elongation occurred at light intensities of approximately 40,000 lux, under the conditions used here. Lower growth rates were found when light intensities were greater or less than this level. When the plants are saturated with gibberellin A3, stem growth is maximum at the highest light intensity, and less at all lower light intensities. Sucrose sprays promoted growth at low light intensities. Apparently, slower growth at low light intensities is due to a deficiency of photosynthetic products, while growth inhibition at high intensities is due to a deficiency of gibberellin. Growth of ‘Alaska’ peas, which are more nearly saturated with endogenous gibberellin, is much less inhibited by high light—or much less promoted by partial shading. This appears to be a general relationship. Dwarf Zea mays (d1), which is very deficient in gibberellin, responds markedly to shading, but the normal segregate (D1) responds little to shading. When the dwarfs are saturated with gibberellin they, too, respond little to shading. Experiments are presented indicating that the high‐intensity light inhibition of stem growth and low‐energy red light inhibition act on the same step in the gibberellin system.