Volume 106, number 1 FEBS LETTERS October 1979 zyxwvuts THE ~FLUENCE OF PROTON-INDUCED GRANA FO~.A~ON ON PARTIAL ELECTRON- TRANSPORT REACTIONS IN CHLOROPLASTS R. C. JENNINGS, P. D. GEROLA, G. FORTI and F. M. GARLASCHI Centro di Studio de1 CNR per la Biologia Cellulare e Moiecolare delle Piante, Via Giuseppe Colombo 60,20133 Milano, Italy Received 9 Juty 1979 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONML 1. Introduction The bulk of the photosynthetic membranes of higher plants forms closely appressed stacks called grana. Despite the considerable effort expended in recent years to find the function of this characteristic structure, no general consensus of opinion has emerged. Based on studies with ‘sun’ and ‘shade’ plants in which large differences in grana formation were observed, it was suggested [l] that grana formation ‘may be a means of achieving a high density of light harvesting assemblies.’ The same idea was subsequently put for- ward [2] based on experiments with AtripZex plants grown under different light intensity regimes in which different levels of grana fo~ation were observed. However, these workers were unable to demonstrate any difference in the quantum efficiency of dichloro- phenolindophenol reduction of chloroplasts with high or low levels of grana formation. It was also suggested [3,4] that membrane stacking should lead to a higher photoche~c~ efficiency due to a higher light- harvesting pigment density, though no new evidence was presented to support this view. Agranal maize bundle-sheath chloroplasts were reported [5] to have a lower quantum efficiency of NADP reduction with water as the reductant than the granal mesophyll c~oroplasts from the same plant. An agranal mutant of Chlamydomonas was shown [63 to reduce benzo- quinone with a quantum efficiency similar to that of cells containing granal chloroplasts. All the above- mentioned experiments suffer from the defect that Abbreviations: DCMU, 3-(3’,4’-dichlorophenyl)-l,ld~e~yl- urea; MES, 2-(N-morphoIino)ethane sulphonic acid; PS I, photosystem I; PS II, photosystem II EisevierjNorth-Holland Biomedical Press the degree of grana formation is not the only experi- mental variable. Almost certainly, and in some cases this was demonstrated, substantial differences in mem- brane composition occur along with different degrees of grana formation. More recently, a different experimental approach has been used, based on the original observation [7], that reversible grana formation can be induced in isolated chlorop~sts by adding cations to the incuba- tion medium. The importance of the lint-ha~est~g chlorophyll a/b protein complex in cation-induced grana formation has been demonstrated [8--l 11.They have shown a close correlation between membrane stacking and the regulation of energy distribution between the two photo~stems, leading to an increase in the photoche~c~ efficiency of PS II and a decrease of PS I. This idea is in accord with an earlier suggestion [ 121 where, on the basis of freeze fracture studies, that grana formation was proposed to permit a greater degree of interaction between the two photosystems. The major ex~~ent~ problem with work based on the in vitro induction of membrane stacking with cations is that the cations themselves seem to exert a number of effects at the level of the photosystems, which may not be related to membrane stacking [13-l 61, In an attempt to circumvent this problem we have utilized the recent ob~rvation [ 171 that lower- ing the suspension medium to pH 5.4 leads to the formation of grana which are indistinguishable morphologically from those induced by cations at higher pH values and the formation of which seems to involve the same membrane site(s). We show that low pH-educed grana formation does not influence the photochemical efficiency of PS II, though PS I operates at a decreased efficiency. 247