Studies on the Expansion of the Leaf Surface

The fixation, utilization, and translocation of carbon and the net import and export of phosphorus by three leaves of Cucumis sativus over the course of their lives were measured in a controlled environment. The rate of photosynthesis of a leaf followed a regular daily pattern, rising to a maximum during the first 2 hrs. of the light period and subsequently falling. Dark respiration was usually highest at the beginning of the dark period and fell throughout it. The daily rate of photosynthesis per unit area of a leaf fell during its later life partly as a result of shading by upper leaves and also because of an independent age factor. The rate of dark respiration per unit area was high in very young leaves but fell rapidly with age. The amount of phosphorus in each leaf reached a maximum before the leaf had reached its maximum dry weight. There was then substantial net loss of phosphorus from the leaf. The changing function of each leaf as a sink or source of carbohydrate and 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) a period of rapid expansion, associated with a high rate of uptake or mineral nutrients, during which translocation of assimilated carbon from the leaf was most rapid; (3) a time of declining rates of growth, photosynthesis and export of carbon, associated with substantial loss of phosphorus; (4) finally, a short sensescent phase with net loss of CO2, terminating in the death of the leaf.