The Photosynthetic Electron Transfer System in Agranal Chloroplasts

Abstract

Plants possessing the C4-dicarboxylic (HATCH-SLACK) pathway of photosynthesis are characterized by the presence of two types of chloroplasts, which differ in their structure, enzymic content and pathway of photosynthesis (HATCH & SLACK 1970). Differences in chloroplast ultrastructure, especially with respect to grana formation are particularly marked in certain of these plants such as sugar cane, maize and Sorghum. Chloroplasts of the mesophyll cells contain grana, while the mature chloroplasts of the bundle-sheath cells are essentially agranal, possessing at most, a few rudimentary grana (WEIER et al 1967). Mesophyll and bundle-sheath chloroplasts of maize are illustrated in Figure 1. In addition to the differences in their ultrastructure and pathway of C02 fixation, the chloroplasts differ in the composition of their lipids (BISHOP et al 1971) Open image in new window Fig. 1 Ultrastructure of mesophyll and bundle-sheath chloroplasts of maize. A, mesophyll chloroplasts; B, bundle-sheath chloroplasts; g, grana, s, starch granule. Arrows indicate areas in the bundle-sheath chloroplasts which appear to contain small grana. and electron transfer proteins (WOO et al 1970). The occurence of two morphologically distinct types of chloroplast in maize, and the availability of methods for their seperation, allows a comparison of the photochemical activities of granal and agranal chloroplasts isolated from the same plant to be made. The studies reported here revolve around two fundamental questions. Firstly, do the agranal chloroplasts of C4 plants such as maize the capacity to photoreduce NADP from water and thereby produce reducing potential for photosynthesis? Secondly. is grana formation necessary for photosystem II activity? As the question of the realationship between photosystem II activity and grana formation is not confined to chloroplasts of C4 plants, the photochemical activities of some algal chloroplasts which do not show the typical granal structure found in the chloroplasts of higher plants, have also been examined.