Biophysical Characterization of Mesophyll and Bundle Sheath Chloroplasts Isolated From the Leaves of Eleusine coracana, an Aspartate-Type C₄ Plant. II. Photosynthetic Electron Transfer and Energy Conservation Reactions

Abstract

Mesophyll and bundle sheath chloroplasts isolated from the leaves of E. coracana, an aspartate-type C₄ plant, have been utilized to study various photochemical activities in an effort to understand their high rates of photosysnthetic CO₂ fixation. Bundle sheath chloroplasts were relatively more active in the overall process of photosynthetic electron transfer from water to NADP. Though photosystem II (PS II) and the associated non-cyclic photophosphorylation activities were of the same magnitude in both types of chloroplast, PS I and the associated cyclic photophosphorylation activities were three to four times more active in bundle sheath chloroplasts than in mesophyll chloroplasts. The influence of different illuminance on the electron and energy transfer reactions has been studied. At all the light levels tested, cyclic photophosphorylation activity was higher than non-cyclic in bundle sheath chloroplasts, while sigmoid kinetics were observed with mesophyll chloroplasts. Acid-base phosphorylation activities of the two types of chloroplast were similar. Heat treatment of the chloroplasts resulted in a selective inhibition of PS II in both types. Total leaf chlorophyll was approximately equally destributed between mesophyll and bundle sheath chloroplasts, which were found to have chlorophyll a-chlorophyll b ratios of 4.50 and 3.50 respectively. A working hypothesis for the energy requirements in the mesophyll and bundle sheath cells for the maintenance of various biochemical processes is proposed. It is also concluded that a greater part of the total leaf photochemical potential is associated with bundle sheath chloroplasts of E. coracana.