Photoperiod and Temperature Responses in Early‐Maturing, Near‐Isogenic Soybean Lines

While photoperiod responses have been studied in soybean [Glycine max (L.) Merr.] isolines, identification of temperature and photo–thermal responses are lacking in early‐maturing soybean. This study was conducted to quantify photoperiod and temperature responses of early‐maturing soybean. Six ‘Harosoy’ 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 photoperiods and with either 18 or 28°C constant temperature. Under the most inductive conditions (10 and 12 h, 28°C), all isolines flowered in about 26 d. Under the least inductive conditions (20 h, 28°C), there was a 50 d difference in flowering time between the early‐ and late‐flowering isolines. Interestingly, the late‐flowering isolines flowered earlier under cool than under warm temperatures. A mathematical model was developed to quantify the effects of temperature and photoperiod on days to first flower. This model related the rate of phenological development from planting to flowering to temperature, photoperiod and the interaction between temperature and photoperiod. The equation was integrated analytically, resulting in an inverse time (1/time) equation, or numerically resulting in the development of a Growing Photothermal Day (GPTD) similar to a heat unit. The model had a base temperature (5.8°C) below which the rate of phenological 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 linearly related to the number of dominant (late flowering, photoperiod sensitive) alleles. The model fit the observed data well (R2 = 0.96).