Effect of Light Quality (Red:Far-red Ratio) at the Apical Bud of the Main Stolon on Morphogenesis of Trifolium repens L.

Abstract

A controlled environment experiment investigated whether the red:far-red (R:FR) ratio of light at the apical bud of the main stolon could alter plant morphogenesis in clonal cuttings of white clover (Trifolium repens L.) The apical bud included the apical meristem, five to six developing leaf primordia with associated axillary bud primordia and stipules and the first emerged folded leaf until development was greater than 0·3 on the Carlson scale. Three light regimes were imposed on the apical bud by collimating light from R or FR light-emitting diodes so that the R:FR ratio of light incident at the apical bud was set at 0·25, 1·6 or 2·1, without significantly altering photosynthetically active radiation. The effect of these light regimes on white clover seedling growth was also tested.At a low R:FR ratio seedling hypocotyl and cotyledon lengths were significantly longer. However, with the cuttings, the light treatments did not alter node appearance rate or internode length of the main stolon, petiole length, area of leaves or total shoot dry matter. There was one significant photomorphogenetic response in the cuttings, a delay of 0·5 of a phyllochron in the appearance of branches from axillary buds in the low R:FR ratio treatment relative to the other treatments. Where branch appearance was delayed plants had fewer branches. This difference could be ascribed solely to a delay in branch appearance as there were no significant treatment effects on either the initiation of axillary bud primordia within the apical bud, the probability of branching or on the rate of growth of branches after appearance. Because treatment of the apical bud induced only one of the many previously observed responses of white clover to a decrease in the R:FR ratio of light, we conclude that other plant organs must also sense the quality of incident light.