Contrasting shade avoidance responses in two perennial grasses: a field investigation in simulated sparse and dense canopies

We designed an experiment with potted plants grown outdoors to investigate the expression of shade avoidance in simulated sparse and dense canopies by two perennial grasses known to express contrasting responses to low red:far-red ratios (R:FR). Plants were grown in canopy microenvironments designed to lower the R:FR by reflection of horizontally propagated FR from neighbors and by direct attenuation of R by filters located above plants. Two specific hypotheses were tested: (1) Paspalum dilatatum will express greater shade avoidance than Schizachyrium scoparium to low R:FR in both sparse and dense canopies, and (2) low R:FR will produce greater expressions of shade avoidance in sparse than in dense canopies in both species. P. dilatatum was more responsive to low R:FR than S. scoparium in both the sparse and dense canopies and lower ramet number plant-1 was the only common shade avoidance response between species in sparse canopies. P. dilatatum also showed significant reductions in juvenile ramet initiation, juvenile ramet mass, total shoot mass, and shoot:root ratios in sparse canopies, but only juvenile ramet initiation was reduced in dense canopies. The suppression of juvenile ramet initiation in the dense canopy was at least partially modulated by the vertically propagated R:FR because a similar reduction in PFD and horizontally propagated R:FR showed 42% greater juvenile ramet initiation in the respective control. S. scoparium only showed a significant reduction in ramet number plant-1 and a significant increase in blade length in sparse canopies, but no significant responses occurred in dense canopies. Consequently, neither hypothesis was rejected. Variable shade avoidance responses between species and canopy densities indicate that both interspecific variation and various proportions of vertically and horizontally propagated low R:FR can influence the expression of shade avoidance responses of perennial grasses in field settings.