Soybean photoperiod-sensitivity loci respond differentially to light quality

Soybean [Glycine mar (L.) Merr.] genotypes have been identified that show differential sensitivity to light quality. The use of different lamp types emitting light of different quality changed flowering responses to long days. The objective of this study was to investigate the flowering response of three photoperiod-sensitivity loci to long days of various light qualities. 'Harosoy' near-isogenic lines were grown under 20-h-long days of different light qualities, as measured by the bichromatic ratio of red to far-red quanta (R:FR), and under light quality gradients. Generally, the photoperiodic response was greater, i.e., later flowering, with decreased R:FR. The E(1) allele was most sensitive to light quality and required an R:FR approximating that of natural daylight for response to long days. The E(3) allele was the least sensitive, and the E(4) allele showed intermediate sensitivity to light quality. The earliest-flowering near-isogenic line (e(1) e(3) e(4)), previously found to be insensitive to long day length, showed sensitivity to long days of low R:FR light. Long days of high R:FR light were not effective in delaying flowering for some genotypes. Sensing the R:FR ratio is the function of phytochrome in light-grown plants. The three loci studied in this work responded differentially to changes in R:FR, suggesting either a close relationship between soybean E alleles and phytochrome or the possibility that some photoperiod-sensitivity loci are part of the phytochrome family of genes.