The Effect of Light Intensity on Stem and Leaf Growth in Broad Bean Seedlings

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Abstract
THE final mean size, cell number, and cell size of the three lower internodes and of a leaflet of the three lower true leaves have been determined from seedlings grown in darkness or in light intensities of 0.1, 1, 10, 100, or 1,000 f.c. The results indicate that the effect of light may be divided into two phases, a sensitive effect on cell multiplication and a less sensitive one on cell enlargement. Growth in 0.1 f.c. results in an increase in leaf area over that attained in darkness mainly due to an increase in the number of cells. Further and more marked increases in area at 100 and 1,000 f.c. are due to an effect on cell size. Low intensities also greatly reduce internode length, again owing to an effect on cell multiplication. Both internode length and the number of cells are gradually further reduced as the intensity is raised to 1,000 f.c. Cell length is reduced in the first (basal) internode by 10 f.c. and above, but in the second is reduced only at 1,000 f.c. In the third internode the cells are longer at all intensities than those in the etiolated seedling. It is suggested that the basic effect of light is modified by differences between the successive leaves and internodes already present in the ungerminated seed, and also by a factor tending to promote growth of the leaves and upper internodes. This factor is taken to be growth substances produced under the action of light, or an increased supply of raw materials as a consequence of the suppression of the lower internodes and the beginning of photosynthesis. Cell multiplication and cell enlargement are possibly controlled through two distinct pigment systems, with peaks of maximum efficiency in the red and blue regions of the spectrum respectively.