Anatomical and ultrastructural damage to citrus leaves from phosphite spray depends on phosphorus supply to roots

Background and aims Although phosphite (PO 3 , Phi) cannot replace phosphate (PO 4 , Pi) as a nutritional source of phosphorus (P) for plants, decisions about using foliar Phi application in citrus groves scarcely take into account P availability and plant nutritional status. Accordingly, we studied the interactive effects of P supply in the rooting medium and foliar sprays of Pi or Phi on citrus tree performance and leaf anatomy and ultrastructure. Methods Young sweet orange trees were grown hydroponically using P-deficient (PD) or P-sufficient (PS) nutrient solutions for 15 weeks in combination with 3 foliar spray treatments: 0.16 M P in the form of Pi (KH 2 PO 4 ) or Phi (KH 2 PO 3 ) and a control spray of 0.16 M potassium chloride (KCl). Six foliar sprays were applied to the drip point at 15-day intervals during the experimental period. Before the trees were harvested to estimate growth, mature leaves were sampled to study the anatomical and ultrastructural organization and to evaluate P concentration. Results Under PD, the Pi spray stimulated tree growth compared with the control, whereas Phi sprays impaired tree performance, suggesting toxic effects. Both Pi and Phi applications increased the leaf P concentration relative to that of the control. Leaves of PD trees exposed to Phi exhibited pronounced damage to the epidermis, and stomata showed deformed ostioles and surface flaking. Moreover, phloem and xylem vessels were disorganized, cell wall presented sinuosity, cell membranes were plasmolyzed, and chloroplast thylakoids were disrupted, with accumulation of starch and plastoglobuli. However, no similar changes were observed either in PD trees under Pi spray or in PS trees independent of foliar treatment. PD trees receiving KCl exhibited intermediate responses between those of Pi and Phi applications. Conclusions This was the first study to demonstrate that Phi spray disrupts the anatomical and ultrastructural organization of citrus leaves under P deficiency. Our results provide insights into the mechanisms explaining the poor nutritional value of Phi and support information to guide the use of Phi-based products.