Cell growth in expanding primary leaves ofPhaseolus

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Abstract
Plants were grown at 25 and 20° C in 6, 12, and 18 h daylengths. Final area of the primary leaf pair ranged from 105 to 209 cm2, and for a given temperature was greatest in the 12 h and least in 6 h daylength. Cell numbers per leaf were similar for all treatments. In the 6 h daylength leaves were thinner, contained less chlorophyll and ethanol-insoluble dry matter, and had considerably smaller cells than leaves on plants in the longer daylengths; final levels of protein and cell-wall material per cell were also low, although levels of nucleic acid per cell were as high as, or higher than, those for leaves in 12 and 18 h days. It is concluded that the low levels of protein and cell-wall material are associated with a low level of photosynthesis, and that the small area of these leaves is a result of the reduction in cell size. In the 12- and 18-h daylenghts, protein and cell wall per cell increased linearly with time, and when expansion of the lamina was completed, values for these parameters were found to be similar. Cell size, as measured by fresh weight, was also similar at this stage, although small differences in lamina thickness were found. Thus the smaller area for leaves in 18-h days was not due to a reduction in mean cell size, although differences in epidermal cell dimensions must be involved. From consideration of simple models it is concluded that increase in cell wall material during lamina expansion is associated with increase in wall area, but that the continued formation of wall material after lamina expansion has ceased is accounted for by deposition on already existing walls. This continued increase in wall material occurs at a time when protein and nucleic acid levels per cell are declining.
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