Physiological and Ecological Studies in the Analysis of Plant Environment

Since relative growth rate is the product of net assimilation rate and leaf-area ratio (leaf area/plant weight), it follows that if the effects of shading on both net assimilation rate and leaf-area ratio can be expressed mathematically, then the relationship between light intensity and relative growth rate can be derived from the product of the two mathematical expressions. For all the ten species investigated in field and pot culture experiments, it has been found that during the early vegetative phase both the changes in leaf-area ratio and net assimilation rate, over the range of 0·1 to full daylight, are linearly related to the logarithm of the light intensity. In consequence, the relationship between relative growth rate and the logarithm of light intensity—being the product of the two linear regressions—is curvilinear. For species of shady habitats (Geum urbanum, Solamun dulcamara) neither the levels of assimilation rate nor the ‘compensation-point’ values are very different from those of the eight species from open situations (e.g. Hordeum vulgare, Pisum sativum, Fagopyrum esculentum). Nevertheless the intensity at which growth rate is maximal varies between species: it is 0⋅5 for G. urbanum, 0⋅7 for H. annuus, full daylight for F. esculentum, while for Trifolium subterraneum the calculated value is 1·8 daylight. Such specific differences can be largely accounted for in terms of the differences in leaf-area ratio at the different light levels. On the basis of this analysis of the light factor, a ‘shade’ plant is best redefined as a species in which a reduction of the light intensity causes a rapid rise in the leaf-area ratio from an initial low value in full daylight: for a ‘sun’ plant the converse definition holds.