Application of physiological understanding in soybean improvement. II. Broadening phenological adaptation across regions and sowing dates

This paper describes the implementation of a strategy to develop high-yielding soybean cultivars with wider adaptation across latitudes and sowing dates using the ‘long juvenile’ (LJ) trait to ‘convert’ elite temperate cultivars to subtropical and tropical adaptation. In an initial proof-of-concept evaluation, temperate semi-dwarf cultivars from Ohio in the Mid-West of the USA (40°N) were converted into genotypes adapted to the subtropics of southern Queensland (25–28°S), of which cv. Melrose was the first to be released for commercial production. The effect of the LJ trait was to delay flowering of the new genotypes by 10–14 days depending on temperature, while retaining the high yield potential and lodging resistance of the temperate varieties. The temperate cultivars were insensitive to photoperiod in the subtropics, and this attribute was largely retained in cv. Melrose. The LJ trait was also used to convert temperate culinary soybean varieties from eastern Asia to subtropical–tropical adaptation, although susceptibility to disease required the simultaneous introgression of resistance genes from additional sources. Several elite LJ oilseed and culinary varieties with broad adaptation in eastern Australia have since been developed. Like Melrose, these varieties are earlier maturing (110–125 days duration) than traditional, full-season cultivars (120–140 days depending on sowing date), less sensitive to photoperiod, and require higher plant populations than full-season varieties for maximum yield. However, they can be grown over a wider range of latitudes and sowing dates than full-season varieties. Similarly, the LJ trait was used to delay flowering of very early flowering, photoperiod-insensitive soybean varieties used in Asian farming systems, increasing yield potential without changing photoperiod insensitivity. The broadening of varietal adaptation over latitudes and sowing dates has allowed public soybean breeding resources to be rationalised, with one national Australian program replacing four previous, regionally focused programs. The research provides a tangible example of how physiological understanding of genotype × environment interaction contributed to soybean improvement in eastern Australia.