Towards the reliable prediction of time to flowering in six annual crops. II. soyabean (glycine max)

Abstract

Eleven genotypes of soyabean (Glycine max) of tropical, sub-tropical and temperate origin and one accession of G. soja were grown in six locations in Australia during 1986–88, and at one location in Australia and two in Taiwan during 1989–91. Dates of sowing were varied within and among locations so as to expose plants to as many as 32 environments of widely different diurnal temperature and daylength. Times from sowing to flowering (f) were recorded, from which rates of progress towards flowering (1/f) were calculated. These derived data were then related to mean pre-flowering values of temperature (T¯) and photoperiod (P) using a three-plane linear model developed from controlled environment data. Among genotypes, mean values of f varied between 24–49 d and between 139–291 d in the most- and least-inductive environments, respectively. These differences were associated with variations in P from about 11 to 16 h d-1, in daily mean maximum temperatures from about 17° to 36°C, in daily mean minimum temperatures from about 5° to 25°C, and in T¯ from about 11° to 30°C, that is, a very wide range of photothermal regimes. The relations of 1/f to T¯ and P can be described in photoperiod-insensitive genotypes by a thermal plane defined by two constants, a and b, and additionally by a photothermal plane defined by three constants, a′, b′ and c′, in the more numerous photoperiod-sensitive genotypes. If photoperiod-sensitive genotypes are grown in sufficiently long days then a third photoperiod and temperature-insensitive plane is exposed, defined by a constant, d′; this plane indicates the maximum delay in flowering of which the genotype is capable. The constants a′, b′, c′ and d′ define the delay in flowering caused by photoperiod-sensitivity genes. The two intercepts between the three planes define, respectively, the critical photoperiod, Pc, above which increase in daylength delays flowering, and the ceiling photoperiod, Pcc, above which there is no further delay. The values of the six constants for any genotype can be estimated from observations of fin several natural environments. Comparisons between years in Australia and between Australia and Taiwan show that these genotypic constants can predict 1/f, and so the time taken to flower, given data on latitude, sowing date and daily values of maximum and minimum air temperatures. This model is more accurate than an alternative logistic model; we also believe that all six constants in the three-plane rate model described here have biological meaning. They indicate separate genetic control of flowering responses to P and T¯ and could form a rational basis for the genetic characterization and analysis of these responses in the soyabean germplasm.