Post-flowering supply of P, but not K, is required for maximum canola seed yields

The effect of limited P or K supply during and after flowering on canola (Brassica napus L.) seed yields is not known. To determine the growth stage at which canola had accumulated sufficient P or K for maximum yields, we conducted two sand culture experiments in which external P (Experiment 1) or K (Experiment 2) supply was removed at four growth stages (GS 4.7; GS 4.9–5.5; GS 6.2 and maturity). Two levels of P/K supply (adequate or high) were included in each experiment. Plants had accumulated enough K by early flowering (GS 4.7) for maximum seed yields at both adequate and high K supply. Under high P supply, canola plants had accumulated sufficient P by early flowering (GS 4.7) for maximum yields. Under adequate P supply, yield loss occurred when P was removed at early flowering (GS 4.7), predominantly due to a combination of fewer siliques and fewer seeds per silique on branches. Yield loss when P supply was removed at late flowering (GS 4.9–5.5) was due to fewer seeds per silique on branches. Early P removal caused more P remobilization from vegetative organs to the seed. When K was supplied until later growth stages, K concentration in the silique walls increased under both adequate and high K supply. Stem K concentrations also increased under high supply. The results suggest that when P supply is near-limiting, canola requires a continued supply of P post-flowering for maximum seed yields.