A vertical gradient of the chloroplast abundance among leaves of Chenopodium album

The abundances of chloroplasts in leaves on the main stems of Chenopodium album at different height levels were investigated in relation to the photosynthetic capacity and light environment of the leaves. (1) The number of chloroplasts per mesophyll cell decreased with descending position of leaves, except for young developing leaves at the top of plants that bad smaller chloroplast numbers per cell than matured leaves beneath them. Contents of chlorophyll and ribulose-1,5-bisphosphate carboxylase/oxygenase per leaf area that were highest in the topmost young leaves and decreased with decreasing height level indicate that there is a vertical gradient of chloroplast abundance per leaf area decreasing from the top of the leaf canopy with depth. (2) Light-saturating rate of photosynthetic oxygen evolution per leaf area of matured leaves decreased more steeply with decreasing leaf position than the chloroplast number per cell. Gradients of chlorophyll and the enzyme protein contents were also steeper than that of the chloroplast number. Loss of photosynthesis in lower leaves is, therefore, ascribed partly to loss of whole chloroplasts and partly to reduced photosynthetic capacities of the remaining chloroplasts. (3) The chloroplast number per cell in newly expanded second leaves was comparable to those in leaves that have developed at later stages of the plant growth but decreased gradually during leaf senescence both in the dark and light. The formation of the vertical gradient of chloroplast abundance is, therefore, ascribed to loss of whole chloroplasts during senescence of leaves. (4) Irradiance a leaf receives decreased sharply from the top of the canopy with depth. The physiological or ecophysiological significance of the vertical distribution of chloroplasts among leaves was discussed taking light environments of leaves into consideration.