Low R:FR Light Quality Delays Flowering of E7E7 Soybean Lines

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Abstract
A new locus (E7) controlling flowering and maturity time, and photoperiod response in soybean [Glycine max (L.) Merr.] was identified by means of 20‐h days of low (0.7) red:far‐red (R:FR) light quality. The objective of this study was to characterize light quality and photoperiod responses of E7E7 and e7e7 soybean near‐isogenic lines. E7E7 and e7e7 lines, in two genetic backgrounds, were grown in 20‐h photoperiods with four R:FR light qualities (0.7, 1.2, 2.3, 2.7) and in 12‐h photoperiods with 0.7 and 2.7 R:FR light quality. Under high R:FR, all isolines flowered early and similarly in both 12‐ and 20‐h photoperiods. In 20‐h photoperiods, as R:FR light quality approached that of natural day light (1.2), E7E7 lines flowered 6 to 7 d later than e7e7 lines, while under low R:FR (0.7), E7E7 lines flowered about 15 d later than e7e7 lines. Under low R:FR, all isolines flowered later under 20‐h compared with 12‐h photoperiods. Six ‘Harosoy’ isolines (e7e7 and 5 E7E7 lines) were grown in 1998 and 1999 in the field under natural daylength and daylength extended to 20 h with incandescent lamps (ILD). The e7e7 line matured 6 d earlier than the E7E7 line under natural and ILD photoperiod. An ILD treatment was simulated in growth cabinets with 4+12+4 h of incandescent light and was compared with 12‐, 20‐, and 24‐h photoperiods. All lines flowered early and similarly under 12‐h photoperiod. The 4+12+4‐h treatment delayed flowering less than the 20‐h photoperiod. Flowering was not delayed further when the photoperiod was extended from 20 to 24 h. The E7E7 lines were sensitive to light quality. They did not perceive long days of high R:FR light quality, but under natural daylight and under low R:FR light quality, E7 delayed flowering.