Nitrogen‐15 in apple nutrition investigations

Young apple trees growing in pots of soil were treated with N‐15 labelled potassium nitrate or urea at different times of year. Over periods of several months whole trees were harvested and divided into their component tissues for total N and N‐15 analyses; the latter were carried out using an emission spectrometer. Following treatments in mid‐October 1972 and harvest in the following February, about 16% of N‐15 from soil‐applied nitrate and almost 47% of that from leafapplied urea was recovered in the trees. A later harvest in May 1973 showed that by then one‐third to one‐half of this fertiliser N had been translocated to the new leaves. Soil applications of nitrate were also made in March or August 1973 and the maximum recoveries of fertiliser N were about 40% and 60% respectively, much of which was in the leaves. In the fruit from the March‐N trees, seeds and flesh were almost equally labelled but in that from the August‐treated trees the seeds contained no excess N‐15, although there was a considerable concentration in the flesh. Urea foliar sprays were also made in June or August 1973. The distributions of isotope were very similar following these treatments, in that the greater part remained in the leaves. The results are discussed in relation to previously published work on N in apple trees, with particular emphasis on the advantages in using N‐15 to follow unambiguously the movement of N between the tissues of the tree.