Nutrient inflows into apple roots. I. 32 P‐orthophosphate uptake from solution by M.9 rootstocks and Worcester Pearmain seedlings

. The rates of uptake of 32 P‐labelled orthophosphate by whole root systems of young apple trees (M.9 rootslocks and Worcester Pearmain seedlings) were measured in solution culture. Using a solution depletion technique, the 32 P‐phosphate uptake rates per unit length, surface area or fresh weight of roots were determined as a function of 32 P‐phosphate concentration in solution at the root surface over the range 0.25–10 mmol m −3 . The effect of P concentration within various plant parts on the relation between uptake rate and external P concentration was studied using plants differing in internal P levels. The apparent minimutn P concentration below which P uptake ceased was of the order of 0.25–0.50 mmol m −3 . Fluxes, inflows and unit absorption rates increased approximately proportionately with solution concentration up to 10mmolm −3 . Except perhaps in the case of the low‐P M.9 plant, there was no evidence of a diminishing returns type of relationship over the range of solution concentrations examined. The threshold P concentration in solution above which uptake rates cease to increase thus appears to be higher for apples than for other species. At any given P concentration, fluxes, inflows and unit absorption rates were higher for M.9 than for Worcester and for low‐P plants than for high‐P plants. The difference between plants of different P status was more marked for M.9 and seems to be more closely related to shoot P levels than to root P.