Phenotypic plasticity in soybeans (Glycine max (L.) Merrill) associated with plant density and phenological stage at thinning

Abstract

Soybean (Glycine max L.) is reported to demonstrate plasticity responses when exposed to either supra or sub optimal plant density stress. However, it is not known whether determinate and indeterminate varieties respond the same and at what phenological stage a soybean plant is able to exhibit adaptive plasticity or elasticity response after thinning. The relationship between plant density stress and plant performance was studied in soybean varieties. The objective of the study was to determine phenotypic plasticity and its consequence on grain yield associated to plant population density-stress recovery capacity on different soybean varieties. The trials were conducted at Mansa Research Station of the Zambia Agricultural Research Institute (Agro-ecological Region III). A split-split plot design was used replicated four times, with variety occupying the main plot, plant density in sub plot and thinning time in sub-subplot. The Zambian soybean varieties used were two determinate types (Lukanga and SC Semeki) and one indeterminate type (Mwembeshi). Planting density stress was imposed by planting at supra optimal densities (700 K, 600 K, and 500 K plants ha-1, where K represented 1000) and stress was removed by thinning to the recommended density (400 K plants ha-1) at different crop phenological stages (V0, V4, R1 and R8). V0 was time of planting, where seeds were sown at the recommended density level of 400K plants ha-1. V4 was the vegetative stage where the fourth trifoliate leaf had completely unrolled and not touching, R1 was the onset of the reproductive growth phase-beginning bloom, where the plant had developed at least one open flower at any node and R8 was a stage when the plant had reached full maturing with at least 95 % of the pods had attained their full maturity colour. In practice V0 and R8 treatments were maintained with no thinning. The results showed that variety had significant effects on plant height, biomass weight, number of grains per pod, number of pods per plant, grain yield and harvest index (HI). Plant density only affected biomass, grain weight and HI. Thinning time influenced root to shoot ratio, number of grains per pod, grain yield and HI. All the interaction levels exerted significant effects on most of the observed parameters. Grain yield and biomass weight were highly influenced by variety. Lukanga had the highest grain yield (2.43 tons.ha-1), followed by Mwembeshi (1.95 tons.ha-1) the least was recorded in SC Semeki (1.17 tons.ha-1). The significantly high yield observed in Lukanga was associated to the significant number of grains per pod while for Mwembeshi was owing its significant yield value to its high number of pods per plant. There was an observed inverse relationship between biomass weight and plant density stress duration, particularly for SC Semeki.