Seeds of doubt: re-assessing the impact of grain P concentrations on seedling vigor

Interest in developing crop varieties with low grain phosphorus (P) in order to minimize the removal of P from fields in harvested grain has been limited due to the view that a low-P grain trait may impair subsequent seedling vigor. This perception is based on relatively few studies, which typically investigated seedling growth on infertile soils, and used seed that may have differed in attributes other than P concentration. To investigate whether these anomalies cast sufficient doubt to warrant renewed research in this field, we compared the growth of rice seedlings from seed low in P obtained from P-starved plants (P-starved seed) vs. high-P seeds (obtained from P-fertilized plants) in P-deficient and P-replete soils. While plants from high-P seed were superior in the P-deficient soil, plants grown from P-starved seed overcame an initial lull in early vigor to obtain similar biomass at maturity as plants grown from high-P seed. Subsequent experiments were undertaken using high-P seed vs. seed low in P from a range of rice genotypes that was not obtained from P-stressed plants (low-P seed): There was no reduction in seedling vigor or biomass and grain yields at maturity of plants from low-P seeds in low-P soil compared to plants from high-P seed, though responses were genotype-specific. The results suggest that multiple factors can confound the results of seed P × seedling vigor studies, and that a renewed research effort to define the minimum P levels in seeds required for adequate seedling growth across a range of environments is warranted.