The phosphorus nutrition of the apple tree. I. Influence of rate of application of superphosphate on the performance of young trees

Abstract

A pot experiment was established to determine the influence of rate of superphosphate (applied at planting only) on the growth and fruiting responses of young apple trees (Jonathan/3428) in four successive growing seasons. Increasing the rate of application of superphosphate stimulated tree growth, fruit set and yield over much of the range tested. A critical leaf phosphorus value of 0.32 % in mid-shoot leaves in midsummer is suggested for newly planted apple trees. Phosphate-deficient trees showed delayed bud burst and a retarded rate of development of a reduced number of vegetative and floral meristems. The pronounced effect of phosphorus level on percentage fruit set was also at least partially attributable to its effect on the rate of flower cluster emergence since delayed emergence reduced the opportunity for cross-pollination and fruit set. Fruit from phosphorus-deficient trees contained fewer but larger cells than fruit from non-deficient trees. These results are consistent with the hypothesis that phosphorus level is an important regulator of the meristematic activity of apple tree tissue. Leaves and fruit on phosphorus-deficient trees showed comparatively early senescence. It is suggested that retarded bud development on deficient trees in spring and early senescence of deficient tissues in autumn could result from reduced cytokinin production by deficient trees. Leaf analysis was superior to fruit analysis as an index of the phosphorus status of apple trees. Fruit phosphorus values tended to be constant with time for a given treatment, but leaf phosphorus values declined with time and progressively more of the trees in the low phosphorus treatments developed visual symptoms of phosphorus deficiency. Analysis of soil each year proved of little value as a predictor of the phosphorus status of the trees, since deficiency symptoms were seen on trees grown in soil containing over 1000 ppm phosphorus. Magnesium deficiency symptoms developed on trees in the high phosphorus treatments, and the possible reason for this is briefly discussed.