Phosphite Is Not a Good Source of Phosphorus Nutrition for Citrus Rootstock Seedlings

Phosphite (PO3; Phi), a reduced form of P, can prevent plant infection by Phytophthora spp. and thereby reduce crop loss in many agricultural commodities including citrus. Although some commercial formulations of Phi have a nutritional label as a source of P, Phi can have negative effects on plant growth. We tested the nutritional value of Phi in Carrizo citrange (CC) and Smooth Flat Seville (SFS) citrus rootstock seedlings growing in either aerated hydroponic nutrient solution or in fertigated native Candler sand soil in a green house.There were four P treatments: No P; 0.5 mmol.L-1 PO4 (Pi); 0.25 mmol.L-1 PO4 + 0.25 mmol.L-1 PO3 (Ppi+Phi); or 0.5 mmol.L-1 PO3 (Phi). Leaf net gas exchange was measured near the end of the experiment and after 83 days, plants were harvested, separated into leaves, stems, and roots and growth parameters were evaluated. Overall, hydroponically-grown roots were longer and thinner than sand-grown roots so hydroponically-grown plants had a lower root/shoot dry weight ratio than sand-grown plants. Growth of the two rootstocks responded in a similar way to P treatments but leaf N and chlorophyll were lower in SFS than in CC leaves. The Pi seedlings had the highest leaf P, net assimilation of CO2 (A(CO2)) and total growth while the No P seedlings had the highest root/shoot dry weight ratio. Growth responses, leaf N, and A(CO2), of the Ppi+Phi treatment were intermediate betvveen the Pi and Phi treatments. Total plant growth was positively related to leaf P in Pi plants but growth was negatively related to leaf P in Phi plants. Phi reduced root and shoot growth even though seedlings had higher leaf P than the No P seedlings. Phi plants had comparable leaf chlorophyll and A(CO2) to No P plants but leaf N was reduced by Phi. Thus, Phi was not a source of nutritionally available P for seedlings of citrus rootstocks.