Adaptation of soybean (Glycine max (L.) Merrill) to the dry season of the tropics. I. Genotypic and environmental effects on phenology

Abstract

Eight soybean genotpes were sown at weekly intervals in three tropical dry season environments to examine genotypic and environmental effects on phenology. A series of simple linear models was used to test the relationships, for individual genotypes, between rate of development for the vegetative (Rf) and reproductive (Rr) phases and photoperiod and temperature. For each genotype, most (> 85%) of the variation across sites, years and sowing dates in Rf and more than half the variation in Rr could be accounted for by variation in photoperiod and/or temperature. Rf was generally negatively associted with mean maximum temperature and mean photoperiod, and positively with mean minimum temperature. Rr was generally positively associated with either mean, or mean maximum, temperature and negatively with mean photoperiod. It was concluded that variations in photothermal regime across sites, years and sowing dates within the tropical dry season are sufficiently large to induce instability in time to flowering of most present soybean cultivars. Most genotypes are also poorly adapted to the relatively long day-warm temperature conditions experienced by dry season crops during podfilling, presumably because they have been developed as summer crops. Breeding soybeans for the dry season will therefore need to place strong emphasis on photothermal effects during post-flowering as well as pre-flowering development.