Comparison of Photosynthetic Activities of Spinach Chloroplasts with Those of Corn Mesophyll and Corn Bundle Sheath Tissue

Abstract

Bundle sheath and mesophyll chloroplasts from Zea mays showed comparable rates of O2 evolution, which amounted to about half of the rate observed in spinach (Spinacia oleracea) chloroplasts. Ratios of 4.5, 4.6 and 6.2 Mn2+ atoms/400 chlorophylls were observed in mesophyll, bundle sheath and spinach chloroplasts, respectively. These ratios roughly correspond to the observed O2 evolution rates. Rates of electron transport from water to methylviologen (photosystem I and II) in both types of corn chloroplasts were about 1/3 that in spinach. Compared to spinach, transport rates from reduced diaminodurene to methylviologen (photosystem I) were about 1/3 and greater than 1/2 in mesophyll and bundle sheath material, respectively. In both types of corn chloroplasts, electron flow from photosystem II to P700 was abnormal. This observation, together with the low rates of all activities, suggests that damage occurred during isolation. Such damage may limit the quantitative significance of observations made with these materials (including the following data). Measurements of flash yields of O2 evolution or O2 uptake showed that the size of the photosynthetic unit was the same in photosystems I and II and in all 3 types of chloroplasts (about 400 chlorophylls per equivalent). Similarity of of the photochemical cross-section of the 2 photosystems in the 3 preparations was also found in optical experiments: that is the half-times of the fluorescence rise in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) (photosystem II) and of the photooxidation of P7000 (photosystem I). The ratio of P700 to chlorophyll appeared to be about 2-fold higher in bundle sheath chloroplasts than in the other materials (1/200 vs. 1/400).