Does morphology or the size of the internal nitrogen store determine how Vaccinium spp. respond to spring nitrogen supply?

1. Vaccinium myrtillus (deciduous) and V. vitis-idaea (evergreen) are ericaceous shrubs adapted to low-nitrogen environments. Their comparative responsiveness to N supply was determined in relation to both N storage and developmental constraints.

2. Plants were grown with high or low N in sand culture to condition their N storage, and their growth measured during the first flush of a second year when plants from each treatment were again supplied either high or low N. N-15-labelling was used to quantify remobilization of N taken up in the first year for growth in the second.

3. In both species, the growth response to external N availability was mediated through a change in the number of buds, initially present, which produced shoots, with no alteration of the number of leaves per shoot; but the magnitude of the response was smaller in the evergreen species. The second flush of growth took place more rapidly in V. myrtillus than in V. vitis-idaea, and depended on both external and internal N supply, in terms of the number of shoots produced and the number of leaves per shoot.

4. The amount of labelled N remobilized by both species depended on the level of N reserves, and on the number of initial buds which produced shoots. In V. myrtillus, the total amount of N remobilized to new growth was significantly affected by external N supply. Since the total amount of N remobilized is independent of external N availability during spring growth, this result implies that the age of the N remobilized depended on the number of buds that produced shoots. We explain this result by the perennial nature of N storage in V. myrtillus and the age range of the pool of buds coming into growth each year on an individual plant.

5. Nitrogen remobilization and the growth response to N supply were closely linked with the pattern of bud activation. Species-specific growth responses to N supply in spring were better explained by developmental constraints on growth than by the ability to store and re-use N.