Topology of a maize field: distinguishing the influence of end-of-day far-red light and shade avoidance syndrome on plant height.

Correlations were established between plant height and Cartesian position in a field of diverse maize (Zea mays) germplasm. The influence of the shade avoidance syndrome (SAS), a series of responses to lower photosynthetically active radiation (PAR) and red to far-red light ratio (R:FR) at high planting density, was detected by a steep increase of plant height from the edge to interior rows of the field. In addition, a gradual increase in height was observed across the field from east to west. We attribute this result to a R:FR gradient caused by sunlight laterally penetrating the stand at dusk. Furthermore, we hypothesize that the increased height of west-positioned plants may be analogous to responses induced by end-of-day FR (EOD-FR) treatments used by photobiologists to induce SAS in controlled environments. While preliminary, these results nevertheless suggest that a plant's position in a field will influence the impact of daily fluctuations in PAR and R:FR in modulating plant height and, potentially, other agronomically relevant traits.

Physiological and genetic characterization of end-of-day far-red light response in maize seedlings.

Developmental responses associated with end-of-day far-red light (EOD-FR) signaling were investigated in maize (Zea mays subspecies mays) seedlings. A survey of genetically diverse inbreds of temperate and tropical/semitropical origins, together with teosinte (Zea mays subspecies parviglumis) and a modern hybrid, revealed distinct elongation responses. A mesocotyl elongation response to the EOD-FR treatment was largely absent in the tropical/semitropical lines, but both hybrid and temperate inbred responses were of the same magnitude as in teosinte, suggesting that EOD-FR-mediated mesocotyl responses were not lost during the domestication or breeding process. The genetic architecture underlying seedling responses to EOD-FR was investigated using the intermated B73 x Mo17 mapping population. Among the different quantitative trait loci identified, two were consistently detected for elongation and responsiveness under EOD-FR, but none were associated with known light signaling loci. The central role of phytochromes in mediating EOD-FR responses was shown using a phytochromeB1 phytochromeB2 (phyB1 phyB2) mutant series. Unlike the coleoptile and first leaf sheath, EOD-FR-mediated elongation of the mesocotyl appears predominantly controlled by gibberellin. EOD-FR also reduced abscisic acid (ABA) levels in the mesocotyl for both the wild type and phyB1 phyB2 double mutants, suggesting a FR-mediated but PHYB-independent control of ABA accumulation. EOD-FR elongation responses were attenuated in both the wild type and phyB1 phyB2 double mutants when a chilling stress was applied during the dark period, concomitant with an increase in ABA levels. We present a model for the EOD-FR response that integrates light and hormonal control of seedling elongation.