Phosphorus and Carbon Budgets of Eucalyptus pilularis Smith Seedlings During Their Cotyledonary Phase of Growth

Abstract

The budgeting of phosphorus and carbon in Eucalyptus pilularis Smith seedlings raised in a phosphorus-deficient sandy podzol was examined for the first 35 days of seedling growth. During this period, the photosynthetic cotyledons were a major source of both phosphorus and carbon. At 7 days from sowing, the cotyledons were gross importers of assimilated phosphorus but switched to gross (day 15) and then net (day 20) export in response to the current rate of phosphorus acquisition and the phosphorus demand by other organs. However, the cotyledons retained the capacity to revert to phosphorus import. This strategy would permit the seedlings to take advantage of any favourable fluctuations that may occur in soil phosphorus supply. Initial cotyledonary phosphorus export was directed basipetally to the roots and soil. From 20 days after sowing the partitioning pattern altered, with exported phosphorus moving solely to the expanding leaves. Carbon exported from the cotyledons initially supplied all sinks although, once the first leaf pair was self-supportive for carbon, the roots and stem received a higher proportion of exported cotyledonary carbon. The efficiency of phosphorus redistribution was considered to minimize the consequences of a rapidly depleted pool of readily available soil phosphorus on seedling growth. The seedlings also possessed a strategy of maximizing the investment of carbohydrate into dry matter by keeping respiratory losses to a minimum. A proposed link between the distribution of the limiting phosphate ion and carbon distribution may be a contributing factor to the success of E. pilularis in the phosphorus-deficient soil.