Physiological and Ecological Studies in the Analysis of Plant Environment

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Abstract
Pot experiments were carried out in which sunflowers in the early vegetative phase were first grown for a period under three levels of light (1·0, 0·5, and 0·24 daylight). Subsequently pots from each light group were subdivided into three so that in a second period plants could be subjected to the nine possible combinations of the same three light intensities before and after transference. During the post-transference period of adaptation to either a higher or a lower intensity the net assimilation rate is logarithmically proportional to the light received and there is no residual effect of the initial light treatments. Eight days after transference the leaf-area ratios (total leaf area/total plant weight) at each light level become adjusted to a new equilibrium irrespective of the large initial differences in the ratio induced by the pre-transference intensities. In both periods there is an inverse and logarithmic relationship between the leaf-area ratio and falling light intensity; consequently, the greater the degree of shading in the pre-transference period, the higher are the mean ratios in the second period. Since the relative growth rate is the product of the net assimilation rate and the leaf-area ratio, the variations in the leaf-area ratio in the post-transference period induced by the initial light treatments are reflected in the relative growth rates. Thus plants transferred from a lower to a higher light intensity are leafier and initially grow faster than plants maintained at the higher level in both periods: the converse conditions lead to a reduction in the growth rate. Shading depresses the growth of the roots, but the relative growth rate is dependent on the light intensity in both the pre-transference and post-transference periods. With transference from daylight to 0·24 daylight, the roots during the period of adjustment may lose weight, while the growth rate is maximal when plants are moved from the lowest to the highest intensity. In two out of the three experiments the relative growth rate of the shoot in the post-transference period is of the same order at all light intensities and is largely independent of the light received in the initial period. In terms of leaf weight, decreasing the light intensity decreases the relative growth rate and there is no consistent after-effect of the initial light treatments. The rate of expansion in leaf area tends to be highest at the intermediate level of 0·5 daylight and over all the post-transference intensities the rates are maximal for those plants which received initially full daylight. The ratio of leaf area to leaf weight is inversely and logarithmically proportional to the light level. After transference the slopes of the regressions are independent of the initial light treatments, but the mean ratios are inversely correlated with the initial degree of shading. These adaptive changes to a variation in the light level are discussed with particular reference to the control of growth exerted by growth-regulating substances. It is concluded that on the basis of existing knowledge no adequate interpretation is yet possible.