| 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 leastin 6 h daylength. Cell numbers per leaf were similar for alltreatments. In the 6 h daylength leaves were thinner, containedless chlorophyll and ethanol-insoluble dry matter, and had considerablysmaller cells than leaves on plants in the longer daylengths;final levels of protein and cell-wall material per cell werealso low, although levels of nucleic acid per cell were as highas, or higher than, those for leaves in 12 and 18 h days. Itis concluded that the low levels of protein and cell-wall materialare associated with a low level of photosynthesis, and thatthe small area of these leaves is a result of the reductionin cell size. In the 12- and 18-h daylenghts, protein and cell wall per cellincreased linearly with time, and when expansion of the laminawas completed, values for these parameters were found to besimilar. Cell size, as measured by fresh weight, was also similarat this stage, although small differences in lamina thicknesswere found. Thus the smaller area for leaves in 18-h days wasnot due to a reduction in mean cell size, although differencesin epidermal cell dimensions must be involved. From consideration of simple models it is concluded that increasein cell wall material during lamina expansion is associatedwith increase in wall area, but that the continued formationof wall material after lamina expansion has ceased is accountedfor by deposition on already existing walls. This continuedincrease in wall material occurs at a time when protein andnucleic acid levels per cell are declining. |
