Soil and hydroponic culture experiments were conducted to investigate the effects of phosphite (Phi) as phosphorus (P) fertilizer via root and foliar applications on the growth and P supply of komatsuna. In both experiments, root P treatments were combinations of Phi and phosphate (Pi) at different Pi:Phi ratios, for a total of high P level (92 mg P pot-1; the soil experiment) or low P level (0.05 mM P; the hydroponic experiment). Foliar P treatments were deionized water (control), a Pi solution and a Phi solution at low concentration of 0.05% P2O5. In both experiments, shoot dry weight of plants significantly decreased as Pi:Phi ratio decreased. In the soil experiment, plants grew abnormally at a Pi:Phi ratio of 25:75 and died when P was applied to soil entirely as Phi form (0:100 treatment). In the hydroponic experiment, no visible damage was found in shoot but root growth was strongly inhibited with severe damage symptoms at low Pi:Phi ratios. Total P concentration in plant decreased significantly with decreasing Pi:Phi ratio, especially in the hydroponic experiment. Foliar application of Phi although greatly increased total P of plants compared to that of Pi in both experiments, it did not improve but further decreased plant growth at low Pi:Phi ratios in the soil experiment and at all Pi:Phi ratios in the hydroponic experiment. The results of this study clearly indicated that Phi could not be used as P fertilizer by komatsuna plants via both application methods and could not substitute P at any rate at either low or high level. No beneficial effect of Phi was detected even when it was applied at low rate or applied in combination with Pi at different ratios. The effects of Phi were strongly dependent on the P nutrition status of plants; and plants that were not sufficiently fertilized with Pi may become vulnerable to Phi even at low levels.
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