TOWARDS THE RELIABLE PREDICTION OF TIME TO FLOWERING IN 6 ANNUAL CROPS .2. SOYBEAN (GLYCINE-MAX)

Eleven genotypes of soyabean (Glycine max) of tropical, sub-tropical a nd temperate origin and one accession of G. soja were grown in six loc ations in Australia during 1986-88, and at one location in Australia a nd two in Taiwan during 1989-91. Dates of sowing were varied within an d among locations so as to expose plants to as many as 32 environments of widely different diurnal temperature and daylength. Times from sow ing to flowering (f) were recorded, from which rates of progress towar ds flowering (1/f) were calculated. These derived data were then relat ed to mean pre-flowering values of temperature (TBAR) and photoperiod (P) using a three-plane linear model developed from controlled environ ment data. Among genotypes, mean values off varied between 24-49 d and between 134-291 d in the most- and least-inductive environments, resp ectively. These differences were associated with variations in P from about 11 to 16 h d-1, in daily mean maximum temperatures from about 17 -degrees to 36-degrees-C, in daily mean minimum temperatures from abou t 5-degrees to 25-degrees-C, and in TBAR from about 11-degrees to 30-d egrees-C, that is, a very wide range of photothermal regimes. The rela tions of 1/f to TBAR and P can be described in photoperiod-insensitive genotypes by a thermal plane defined by two constants, a and b, and a dditionally by a photothermal plane defined by three constants, a', b' and c', in the more numerous photoperiod-sensitive genotypes. If phot operiod-sensitive genotypes are grown in sufficiently long days then a third photoperiod and temperature-insensitive plane is exposed, defin ed by a constant, d'; this plane indicates the maximum delay in flower ing 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, P(c), above which increase in daylength delays flowering, and the ceiling photoperiod, P(ce), above which there is no further delay. The values of the six constants for any genotype can b e estimated from observations of fin several natural environments. Com parisons between years in Australia and between Australia and Taiwan s how that these genotypic constants can predict 1/f, and so the time ta ken to flower, given data on latitude, sowing date and daily values of maximum and minimum air temperatures. This model is more accurate tha n an alternative logistic model; we also believe that all six constant s in the three-plane rate model described here have biological meaning . They indicate separate genetic control of flowering responses to P a nd TBAR and could form a rational basis for the genetic characterizati on and analysis of these responses in the soyabean germplasm.