DETERMINATION OF THE RELATIONSHIP BETWEEN NUTRIENT-UPTAKE RATE AND SOLUTION CONCENTRATION AT THE ROOT SURFACE UNDER FIELD CONDITIONS - P-32-LABELED ORTHO-PHOSPHATE UPTAKE BY APPLE ROOTS

The relationships between the rates of uptake of 32P-labeled Pi per unit length, surface area and volume of root and the concentration of 32P-Pi in solution at the root surface were determined in a solution depletion experiment conducted in a root laboratory, using a part of the current season''s roots of a 3.5-yr-old composite apple tree growing under field conditions. The results are compared with those of a previous experiment on young M.9 [Malting 9] rootstocks grown in a controlled environment. The rate of P uptake per unit root of the field-grown tree increased .apprx. linearly with P concentration in the external solution over the range 0.8-10.0 mmol m-3, confirming the results of the growth cabinet experiment. At any given external concentration, uptake rate per unit root of the field-grown tree was lower than that observed in the growth cabinet experiment. Possible reasons for this difference are discussed.