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:fa r-red ratios (R:FR). Plants were grown in canopy microenvironments designed to lower the R:FR by reflection of horizontally propagated FR from neighbo rs and by direct attenuation of R by filters located above plants. Two spec ific hypotheses were tested: (1) Paspalum dilatatum will express greater sh ade avoidance than Schizachyrium scoparium to low R:FR in both sparse and d ense canopies, and (2) low R:FR will produce greater expressions of shade a voidance 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 avoida nce response between species in sparse canopies. P. dilatatum also showed s ignificant reductions in juvenile ramet initiation, juvenile ramet mass, to tal shoot mass, and shoot:root ratios in sparse canopies, but only juvenile ramet initiation was reduced in dense canopies. The suppression of juvenil e ramet initiation in the dense canopy was at least partially modulated by the vertically propagated R:FR because a similar reduction in PFD and horiz ontally propagated R:FR showed 42% greater juvenile ramet initiation in the respective control. S. scoparium only showed a significant reduction in ra met number plant(-)1 and a significant increase in blade length in sparse c anopies, but no significant responses occurred in dense canopies. Consequen tly, neither hypothesis was rejected. Variable shade avoidance responses be tween species and canopy densities indicate that both interspecific variati on and various proportions of vertically and horizontally propagated low R: FR can influence the expression of shade avoidance responses of perennial g rasses in field settings.