PLANT GROWTH AND THE AERIAL ENVIRONMENT

Abstract

SummaryThe analytical methods developed in the earlier papers of this series are applied to the effects of daylength on the pattern of growth under controlled environmental conditions, with particular reference to ontogenetic drifts. Longer days are found to decrease the leaf weight ratio (8–16 hours by 9%, 16–24 hours by 8%), confirming suggestions from the field observations. Unit leaf rate is nearly proportional to daylength, and if allowance be made for respiration, at a fixed light intensity the daily addition of dry weight per unit leaf area is closely proportional to daylength, in spite of changes in plant form and leaf structure. At a fixed total dry weight of plants grown under fixed conditions of temperature and spectral composition, the specific leaf area is found to be much more closely associated with unit leaf rate than with light intensity or total daily light. Apart from ontogenetic changes, the relationship appears to be influenced by daylength, temperature and the spectral composition of the light. The possibilities of a direct causal relationship are discussed, and the consequences of some anomalous observations are considered. Changing daylength produces changes in unit leaf rate, specific leaf area and leaf weight ratio which have compensatory effects on relative growth rate: at a fixed light intensity an increase of the daylength from 8 to 16 to 24 hours produced increases in relative growth rate of about 20 % and 4 % only. The ontogenetic drifts of dry‐matter content of leaf, stem and root follow their characteristic pattern, higher values being associated with higher unit leaf rates.It is concluded that the growth pattern of the plant is very closely bound up with the rate of accumulation of new material per unit leaf area, over a considerable range of leaf structure. It is noteworthy that Impatiens parviflora was found to make normal growth in constant temperature and humidity conditions with continuous light and flowered normally in 8, 16 or 24 hour days.