Nitrogen levels in xylem sap of apple trees rose following application of fertiliser nitrogen to the soil either in spring or summer but relative proportions of the main nitrogenous compounds in sap did not change. The total nitrogen in leaves also rose soon after nitrogen was applied to the soil. By the end of the season trees fertilised in either spring or summer had attained the same leaf nitrogen level. Application of nitrogen at either time produced higher levels of nitrogen (presumably as mobilisable reserves) in dormant shoots in the following winter.
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Hill‐Cottingham et al. (1965) studied this question.
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