Regulatory aspects of chloroplast growth in leaves ofXanthium pensylvanicum and etiolated red kidney bean seedling leaves

Abstract

The average chloroplast size was studied as a function of leaf growth in leaves of cocklebur (Xanthium, pensylvanicum) and the primary leaves of 9-day old seedlings of red kidney bean (Phaseolus vulgaris). Diameters of chloroplasts were measured in crude tissue homogenates with the aid of a fluorescence microscope. Chlorophyll content of the leaves was determined spectrophotometrically in acetone extracts. For cocklebur, data are presented to show the relationship of average chloroplast diameter to morphological age of leaves (Leaf Plastochron Index) and are discussed in relation to the available leaf growth analyses. In bean, the increase in chloroplast diameter in response to illumination of etiolated leaves of various size was studied as a function of the duration of continuous illumination. The size of the etiolated bean leaves was varied experimentally by exposing the seedlings in darkness to low energy red light. Average diameter of the chloroplasts was found to be related to the size of leaf lamina. In both cocklebur and bean, a definite relationship of chloroplast size to leaf area and morphological age was established. The observed patterns of chloroplast size increase are interpreted to be a reflection of the integration of growth at three levels of organization: the leaf, its cells and the chloroplasts.