| Abstract: | The fixation, utilization, and translocation of carbon and thenet import and export of phosphorus by three leaves of Cucumissativus over the course of their lives were measured in a controlledenvironment. The rate of photosynthesis of a leaf followed a regular dailypattern, rising to a maximum during the first 2 hrs. of thelight period and subsequently falling. Dark respiration wasusually highest at the beginning of the dark period and fellthroughout it. The daily rate of photosynthesis per unit areaof a leaf fell during its later life partly as a result of shadingby upper leaves and also because of an independent age factor.The rate of dark respiration per unit area was high in veryyoung leaves but fell rapidly with age. The amount of phosphorus in each leaf reached a maximum beforethe leaf had reached its maximum dry weight. There was thensubstantial net loss of phosphorus from the leaf. The changing function of each leaf as a sink or source of carbohydrateand mineral nutrients was determined. Four stages were recognized:(1) early development from inception until some time after unfolding,when the leaf was dependent upon imported carbohydate; (2) aperiod of rapid expansion, associated with a high rate of uptakeor mineral nutrients, during which translocation of assimilatedcarbon from the leaf was most rapid; (3) a time of decliningrates of growth, photosynthesis and export of carbon, associatedwith substantial loss of phosphorus; (4) finally, a short sensescentphase with net loss of CO2, terminating in the death of theleaf. |
