Does the presence of neighbouring weeds alter the expression of adaptive plasticity to subsequent drought stress in soybean?

Phenotypic plasticity in response to stress may limit plastic responses to subsequent stresses. This may result in a maladapted phenotype, thereby limiting further expression of plasticity to subsequent stress. It was hypothesised that exposure of soybean (Glycine max (L.) Merr.) to far-red-enriched (FRE) light reflected from neighbouring weeds would reduce plasticity to subsequent drought stress. Soybean seedlings were grown in a field fertigation system and exposed to a simulated delay in early-season weed control followed by one or two drought stresses. In contrast to control plants, stem elongation induced by FRE light was maintained under drought stress. Stem elongation, however, did not result in a cost to fitness. In addition, no further interactions were found between FRE light and drought. Total biomass plant−1 was reduced by drought stress, which also altered biomass allocation between shoots and roots. Vegetative plasticity in response to drought, however, did not recover total biomass and resulted in a rapid, linear decline in seeds plant−1. Reproductive plasticity in response to drought maintained seed weight despite losses in seeds plant−1. These results demonstrate that the frequency, type and severity of stress influences the ability of soybean to express adaptive vegetative and reproductive plasticity such that delays in early-season weed control may not result in a maladapted phenotype to subsequent drought stress.