Nitrogen storage and mobilisation for spring growth in Ligustrum cultivated in container

In soilless cultures, fertiliser leaching is particularly large in spring, when fertilisation is applied during rainy periods. Therefore, a better knowledge of the N uptake periods and of the amount of N mobilised from the storage organs could improve fertilisation management during spring growth. Nitrogen allocation was studied for two consecutive years in Ligustrum ovalifolium L. These shrubs were cultivated in containers, fertilised (with or without 15N) in spring and autumn of the first year, and received either fertilisation or only tap water on the subsequent spring growth. Nitrogen assimilated in spring was preferentially allocated to the leaves whereas a large part of N assimilated in autumn was directly stored in the perennial organs, mainly in roots. During the subsequent spring period, N was mobilised from all perennial organs, with a proximity gradient, to sustain bud break and shoot growth. Nitrogen assimilated in autumn was preferentially used to sustain shoot growth, with a rate of 68% of its winter N content against 55% for N assimilated the previous spring. When plants were subjected to spring fertilisation, growth was initially sustained by N mobilisation before N assimilation resumption, but N mobilisation was 15% lower than in unfertilised plants. Overall results are discussed in terms of the improvement of the N-fertilisation efficiency relative to spring growth of ornamental shrubs cultivated in container.