THE SIGNIFICANCE OF INTERNAL NUTRIENT CYCLING IN BRANCHES FOR GROWTH AND REPRODUCTION OF RHODODENDRON-LAPPONICUM

The significance of internal nitrogen and phosphorus cycling for growth and reproduction was evaluated for the evergreen dwarf shrub Rhododendron lapponicum in a subarctic environment. The evaluation was made by analysing the seasonal dynamics of branch nitrogen and phosphorus pools and studying the effects of experimentally depleting the resources available from one-year-old leaves. Resource depletion was induced by shading the old foliage or removing it entirely. Responses were measured as growth and changes in nutrient pools (N and P) in new vegetative and reproductive parts. Non-reproductive branches maintained relatively constant resource levels over the growing season. Before leaf abscission these branches resorbed 78-79% of the leaf maximum N pool and 80-85% of the P pool and 30-36% of the leaf dry mass. Branch internal circulation of N and P provided the new vegetative growth with about 77 and 66%, respectively, of its N and P requirements. The maximum resource pool of reproductive branches was 1.4 (N) and 1.7 (P) times that of non-reproductive branches. Nutrient resorption from senescing leaves on reproductive branches was similar in magnitude to that on non-reproductive branches. For reproductive branches old leaves provided 50% of the N needed and 32% of the required P. Shading non-reproductive branches had a marginal effect on the growth of the current-year shoot whereas the removal of all one-year-old leaves had a marked effect. Seed weight decreased by 53% after defoliation but was not significantly affected by shading. By contrast shading and defoliation had similar effects on reproductive support tissues (i.e., all reproductive parts except seed) reducing their dry weight to 70% of control. Shading and defoliation had about equal effects on the new vegetative growth of reproductive branches. It was concluded that old leaves are an important source of resources for current-year growth and reproduction in R. lapponicum. For non-reproductive branches only the mineral nutrient supplies had a significant influence, whereas for reproductive branches the carbon supplied from old leaves also seemed to be important.