Effect of phosphite-phosphate interaction on growth and quality of hydroponic lettuce (Lactuca sativa)

Although the fungicidal properties of phosphite have been recognized, its potential as a fertilizer is still being debated. The information on how phosphite affects the growth and quality of plants in relation to phosphate (Pi) also remains unknown. This study was conducted to investigate the effect of phosphite in relation to Pi on growth and quality parameters of lettuce (Lactuca sativa L.). The results showed that addition of phosphite to the nutrient solution at different rates ranging from 0.05 to 2 mM significantly increased total P, water-extractable Pi, and phosphite in both shoots and roots, but did not improve plant growth under various Pi supplies (0.05, 0.1, 0.15, and 0.3 mM as Pi levels for approximately 50%, 80%, 90%, and 100% of the maximum plant growth, respectively), indicating that phosphite was well absorbed by roots and mobile inside the plants, but did not provide any P nutrition. Also, no stimulating effect of any Pi-phosphite combination was observed. The effect of phosphite on plant growth was strongly dependent on the level of Pi supply. In general, application of phosphite up to 2 mM did not influence the growth of Pi-sufficient plants. However, plants fertilized with Pi for about 90% of maximum growth were still vulnerable to phosphite at 2 mM. The negative effect of phosphite was found even at concentrations as low as 0.2 mM, when plants were supplied with Pi adequate for about 80% of maximum growth or less. At 0.05 mM, phosphite had marginal effects on plant growth under all the Pi levels. Although phosphite itself had little influence on the ascorbate and mineral concentrations of lettuce, its application to Pi-deficient plants may decrease the mineral concentrations of plants brought about by the inhibitory effect of phosphite on root growth and hence nutrient uptake. Since phosphite is an effective fungicide for lettuce, care should be taken on Pi supplies prior to application of phosphite products to minimize the harmful effects.