The red:far-red ratio of solar radiation as an environmental signal regulating tiller recruitmnet in the bunchgrass Schizachyrium scoparium

Abstract

Changes in the red:far-red ratio (R:FR) of solar radiation are an important environmental signal influencing dicotyledonous plant development. Recent studies conducted mainly in controlled environments also indicate that depressions in the R:FR relative to values typically measured in daylight reduce tiller recruitment from axillary buds in grasses. However, the mechanism responsible for reduced tiller recruitment and the site of photoperception of the R:FR signal on the grass tiller are not known. In addition, few attempts have been made to experimentally verify the significance of the R:FR in influencing tiller recruitment in grasses grown in the field. The experiments reported in this dissertation were designed to address these three problems utilizing Schizachyrium scoparium, a widely distributed native North American perennial bunchgrass. Tiller recruitment in S. scoparium seedlings grown in a controlled environment was temporarily delayed by end-of-day FR. The mechanism of reduced tiller recruitment appears to be assimilate diversion from buds to support end-of-day FR enhancement of leaf growth on existing tillers. Reduced tiller recruitment is therefore only one component of a major shift in allocation priorities induced by end-of-day FR which increases existing tiller height and leaf development. Emerging, immature leaf blades on the tiller were demonstrated to be the exclusive sites of end-of-day FR photoperception in S. scoparium seedlings. However, end-of-day FR enhanced not only the growth of the leaf which perceived the radiation signal, but also subsequent leaves on the same tiller. This carry-over effect allowed each tiller to respond to end-of-day FR as an integrated unit. In a field experiment, supplemental radiation provided beneath undefoliated and defoliated canopies of individual S. scoparium plants measurably altered the R:FR at plant bases, but failed to influence the magnitude of tiller recruitment as predicted. The results of experiments reported in this dissertation suggest that depressions in the R:FR slightly alter the timing of tiller recruitment and not, as has been previously suggested, its magnitude. Consequently, a depressed R:FR signal is probably of greater ecological significance in modifying the architecture of tillers and grass canopies in canopy shade than in regulating tiller recruitment from buds on existing tillers.