Carbon and Phosphorus Assimilation and Deployment in Eucalyptus pilularis Smith Seedlings With Special Reference to the Role of the Cotyledons

Early growth of glasshouse-raised Eucalyptus pilularis Smith seedlings in a phosphorus-deficient soil was divided into a cotyledonary and a post-cotyledonary phase. The delineation was made on the basis of cotyledon excision experiments which ascertained the stage (between 32 and 39 days from sowing) beyond which the cotyledons no longer made an essential contribution to normal seedling development. The relative growth rate of the E. pilularis seedlings during the cotyledonary phase was at least comparable with that of many fertilized tree species. The expression of this characteristic in such a phosphorus-deficient soil was considered a consequence of the contributions of both carbon and phosphorus from the cotyledons. Since the seed reserves of both elements were low, these organs were established as major sources largely through immediate post-emergence events. The cotyledons rapidly developed photosynthetic capacity and were the dominant sink for assimilated phosphorus during the week following emergence. Phosphorus gains by the seedlings over this period (up to 16 days from sowing) doubled the seed reserve level of phosphorus (7.5 μg). Beyond this time, however, due to the rapid depletion of readily available soil phosphorus, phosphorus uptake abruptly declined. Concurrent with this decline, the cotyledons switched from being net importers to net exporters of phosphorus. Transfers of seedlings between high and zero phosphorus conditions demonstrated that this switch in cotyledonary role was induced when demand for phosphorus by other sinks could not be met by the current rate of phosphorus assimilation.