Effect of foliar applications of phosphites on growth, nutritional status and defense responses in tomato plants

Abstract

The use of phosphite (Phi)-containing products as foliar fertilizers is a common practice in food production systems, yet their actual contribution as a source of phosphorus (P) for plant nutrition has been object of debate. In this study, we aim to determine, not only the potential impact of foliar Phi applications on tomato plants nutritional status and biomass production, but also their effect on the expression of genes related to plant phosphate (Pi)-starvation and defense responses. The effect of Phi was evaluated on hydroponic tomato plants growing under low (0.1 mM) and optimal (1.0 mM) Pi conditions. Phi-treatments consisted of foliar sprayings of KPhi, CaPhi, AlPhi or H3PO3. Gene expression analysis was carried out 24 h, 48 h and 7 days after the last Phi application, while biomass production and nutrient concentration in the plant tissues were determined seven days after the last Phi application. Regardless the Pi concentration in the hydroponic solution, foliar application of Phi influenced neither net biomass production and partitioning nor photosynthesis-related parameters nor nutrient concentration in plant tissues. Phi applications activated the plant-defense mechanisms but suppressed the Pi-starvation response of tomato plants. Foliar applications of Phi on tomato plants can be used to activate plant-defense responses, but they would not contribute to improve growth and nutritional status of tomato plants, indicating that Phi is not a relevant source of nutrients. Phi-containing products might suppress the response mechanism to Pi-starvation in plants growing under low Pi conditions.