Resource Allocation in Relation to Leaf Retention Time of the Wintergreen Rhododendron Lapponicum

Abstract

Extended leaf longevity has been explained both as an adaptation to conserve nutrients and to increase seasonal or life—time carbon gain. The relative importance of these possible adaptations was investigated in the wintergreen dwarf shrub Rhododendron lapponicum by analyses of responses of growth and nutrient allocation to early and late spring defoliation and to natural reduction of leaf biomass following flowering. Removal of old leaves at the time of budbreak had no effect on biomass accumulation and nutrient pools of the new shoots 1 mo later, although the defoliation prevented the transport of resorbed nutrients from old leaves to the new shoots. This transport accounted for by far the largest part of the increase in shoot N and P in underfoliated controls. Extra nutrients were not transported from stems and roots to compensate for the lost import from old leaves. Hence, defoliated plants compensated for the losses solely by increasing nutrient uptake. In contrast, defoliation 3 wk earlier caused a 29% reduction of annual shoot growth and a proportional decline of nutrient pools in the shoots, while the nutrient concentrations, reflecting the shoot nutrient status, did not decline. Almost identical responses were observed in unmanipulated plants the year after heavy flowering. Reproductive branches do not form leaves, resulting in a decreasing photosynthetic capacity and a reduction of resource storage and allocation to next year's leaf cohort. Although early and late spring defoliation caused equal losses of mobile nutrients, only the early defoliation decreased growth, suggesting that it was the loss of carbon rather than nutrients that affected growth. This support earlier observations that the extended retention time of leaves in wintergreen and evergreen species plays a greater role in improving the carbon balance than in supplying the new leaves with nutrients.