Differential effects of light quality, provided by different grass neighbours, on the growth and morphology of Trifolium repens L. (white clover)

The ability to respond in a specific manner to different light conditions imposed by different species of grass is a major factor contributing to white clover persistence in pastures. Gaps in a pasture provide light with a higher red:far-red ratio (R:FR) and higher photosynthetic photon flux density (PPFD) than the light filtered through neighbours. White clover (Trifolium repens L.) was grown under different light conditions in ways that tried to simulate as closely as possible some of the light conditions experienced in a natural field situation, being partially shaded and receiving light reflected from neighbouring grasses. The objective was to determine specifically if the mere presence of neighbouring grasses could influence the growth and morphology of white clover individuals without physically contacting them, and thereby send a signal of impending competition. In the first experiment, white clover was subjected to shading cast from three different grass species. There were differences in both the quantity and quality of light received under the various grass canopies. The canopies reduced overall growth and branching of clones, while increasing the length of and biomass allocation to petioles. Lolium perenne L. canopy shade had different effects compared to Holcus lanatus L. or Dactylis glomerata L., but between the latter two species, no differences were detected. In the second experiment, light reflected from grass neighbours was provided simultaneously with direct light. There was a strong increase in FR and a resulting decrease in the R:FR due to neighbouring D. glomerata, but few consistent effects on white clover growth and morphology; there was evidence of phototropic movement by the leaves. We show that plants must experience partial shading, and not just reflected light, in order to alter their morphology in response to the presence of different species of grass neighbours.