Photoperiod and temperature responses in early-maturing, near-isogenic soybean lines

While photoperiod responses have been studied in soybean [Glycine mar (L.) Merr.] isolines, identification of temperature and photothermal responses a re lacking in early-maturing soybean. This study was conducted to quantify photoperiod and temperature responses of early-maturing soybean. Six 'Haros oy' isolines with different combinations of alleles at the E1, E3, E4, and E7 loci were grown in growth cabinets with 10-, 12-, 14-, 16-, and 20-h pho toperiods and with either 18 or 28 degreesC constant temperature. Under the most inductive conditions (10 and 12 h, 28 degreesC, all isolines flowered in about 26 d. Under the least inductive conditions (20 h, 28 degreesC, th ere was a 50 d difference in flowering time between the early- and late-flo wering isolines. Interestingly, the late-flowering isolines flowered earlie r under cool than under warm temperatures. A mathematical model was develop ed to quantify the effects of temperature and photoperiod on days to first flower. This model related the rate of phenological development from planti ng to flowering to temperature, photoperiod and the interaction between tem perature and photoperiod. The equation was integrated analytically, resulti ng in an inverse time (1/time) equation, or numerically resulting in the de velopment of a Growing Photothermal Day (G(PTD)) similar to a heat unit. Th e model had a base temperature (5.8 degreesC below which the rate of phenol ogical development was zero, a critical or base photoperiod (13.5 h) below which photoperiod had no effect, and two genetic coefficients, one of which varied with isoline. The isoline photoperiod sensitivity coefficient was l inearly related to the number of dominant (late flowering, photoperiod sens itive) alleles. The model fit the observed data well (R-2 = 0.96).