Movement of Nutrients in Radiata Pine Needles in Relation to the Growth of Shoots

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Abstract
The pattern of needle growth and the movement of mineral nutrients (nitrogen, phosphorus, potassium and calcium) in needles of radiata pine (Pinus radiata D. Don) were studied from needle initiation to 2 years of age. During this period, very prominent cyclic patterns of nutrient accumulation, retranslocation and replenishment were observed for nitrogen, phosphorus and potassium, which highlighted the potential role of needles as a nutrient reserve for growth. Significant retranslocation occurred from very young needles about 3 months after initiation. The phases of retranslocation coincided with new flushes of shoot growth, and the growth of new shoots on a branch resulted in nutrient withdrawal from pre-existing needles, regardless of needle age and season. Such withdrawal occurred even in fertilized and irrigated trees on high quality sites and under environmental conditions conducive for nutrient uptake. At all times, except for the short period after initiation when needles were actively growing, the nutrients in the needle were readily available for retranslocation. Contrary to the general view, retranslocation of nutrients was not necessarily related to senescence and ageing of needles. Because new shoots are the primary ‘sinks’ for retranslocated nutrients, an ongoing competition between different parts of a branch for internal nutrients can be envisaged, preference being for the youngest shoot in the hierarchy. The relevance of these results to our understanding of ‘optimum nutrition’ of pine trees is discussed.

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