Phenotypic plasticity in soybean (Glycine max (Merrill)) genotypes with contrasting growth characteristics subjected to planting density stress at different developmental stages

A study was conducted to determine phenotype plasticity responses in soybean genotypes subjected to plant density stress at different phenological stages. Plants planted at supra optimal population density (500,000 plants per hectare) were thinned to 400,000 population density at 4 developmental stages: V1 (Full expansion of first trifoliate leaves), V3 (Four nodes with 3 unfolded trifoliates), V6 (Seven nodes, with 6 unfolded-unifoliate) and R2 (Full bloom with an open flower at the top). The study was conducted in Chilanga, Zambia. Three varieties: Dina, an indeterminate variety and Magoye and Sc Safari determinate varieties were used. A randomized complete block design arranged as a factorial design with two treatments: varieties and thinning stage and four replications were used. Variety had significant effects on days to flowering, biomass, branching, leaf chlorophyll content, plant height, seed weight, and total grain yield. Thinning time influenced number of branches, plant height, above ground biomass and yield. Plant height increased between 32 and 39% for the thinned treatments. Early thinning increased biomass; thinning at the V8 stage increased biomass by 39% compared to the unthinned treatments. Magoye at 2.42 tons/ha had a higher yield, compared to Dina (2.37 tons/ha) and Sc Safari (1.76 tons/ha). Early thinning (T1) reduced yield by 28%. Soybean varieties used in this study exhibited significant vegetative plasticity. Reducing plant density especially in early vegetative phases reduced plant height, increased branching and biomass allocation to vegetative plant parts. Key words: Biomass, determinate, indeterminate, phenology, vegetative plasticity, yield, yield components.