Effects of phosphate and pH stress on the growth and function of apple roots

Summary Effects of phosphate and pH stress on the growth and uptake functions of apple roots were studied over a period of fourteen days using split-root (2-way) seedlings in solution culture. The level of P fed to either or both halves of the root system was varied and a demineralized water control was also included. pH treatments consisted of using acidic nutrient solutions (pH 3 to 4) or nutrient solutions adjusted to pH 5.0 before use.Solution pH proved of paramount importance for the expression of P deficiency effects on root growth and water uptake. Where initial solution pH was favourable for root growth (pH 5), P deficiency stimulated root growth and water uptake per seedling even if the stress was localized. On the other hand, acidic solutions and the water control inhibited root growth and water uptake compared with +P controls. Where solution pH was favourable, P stress also led to an increase in the mean length per root versus the +P control suggesting that the plant adapted to stress by developing an exploratory type of root.Water use per seedling was predominantly a function of root size rather than leaf area since the treatments influenced root size to a much greater extent than leaf area. Uptake was positively related to root size in that adjusted solutions gave a higher water use than nonadjusted solutions. However, efficiency of water use per unit weight of root was consistently higher in the nonadjusted solutions and this appeared to be due to the presence of a larger number of root tips per unit weight of root in such solutions compared with root systems in pH adjusted solutions.Uptake of P per half root was higher from pH adjusted than from nonadjusted solutions and was also increased by increasing the P concentration. Further, for any one treatment P uptake per half root increased throughout the experiment indicating that uptake was influenced by root growth. However, in contrast to water uptake, uptake of P per unit weight or per unit surface area of root was not changed by pH adjustment nor was this parameter of uptake concentration dependent. That is, the above-mentioned pH and concentration effects on P uptake were mediated through effects on root growth.Comparing localized versus uniform placement of P, uptake of P was significantly higher from the uniform application. However, uptake from localized placement at pH 5 was markedly higher than uptake under pH stress and therefore if the pH of the medium remains favourable for root growth then the lower value for localized placement could probably be compensated for by further increasing the concentration of P applied.