Plant growth,leaf photosynthesis,and nutrient‐use efficiency of citrus rootstocks decrease with phosphite supply

Some formulations of phosphite (Phi) have been recommended as a source of P nutrition for several crops including citrus even though there are known negative effects of Phi on plant growth. Changes in plant growth and metabolism after Phi application should be reflected in altered nutrient‐use efficiency and leaf photosynthesis. We carried out a greenhouse study using seedlings of two contrasting citrus (Citrus spp.) rootstocks, Carrizo citrange (CC) and Smooth Flat Seville (SFS), growing in either aerated hydroponic culture or sterilized native sandy soil. Plants were subjected to four P treatments: No P (control, P₀); 0.5 mM P_i (PO₄‐P); 0.25 mM P_i + 0.25 mM Phi (P_i + Phi), or 0.5 mM Phi (Phi). Photosynthetic characteristics, concentrations of total P (P_t) and soluble PO₄‐P or PO₃‐P in leaves and roots, and plant growth were evaluated after 80–83 d P treatments. Overall, the P_i plants had the highest P_t (total P) and total plant dry weight while the P₀ plants had the lowest P_t but highest total root length and root‐to‐shoot ratio. Leaf chlorophyll (SPAD readings) and net assimilation of CO₂ (A_CO2) of the P₀ and Phi plants were similarly lower than those of P_i and P_i + Phi plants. Growth responses of the P_i + Phi treatment were intermediate between the P_i and Phi treatments. Although Phi increased P_t and soluble‐PO₄‐P concentration in leaves and roots above the P₀ treatment, this did not translate into increased plant growth. In fact, the Phi treatment had some phytotoxic symptoms, impaired P‐ and N‐utilization efficiency for biomass production as well as lower nutrient‐use efficiency in the photosynthetic process. Thus, these two rootstocks could not use Phi as a nutritional source of P.