The photosynthetic apparatus of plants responds to changing light quantity and quality with coordinated changes in both the light-harvesting antennae of the photosystems and the amounts of electron transport components and ATP synthase. These compositional modulations are accompanied by changes in thylakoid membrane organisation and photosynthetic capacity. It is now clear that there is a dynamic continuum of organisation and function of the photosynthetic apparatus from the appressed granal and non-appressed stroma thylakoids within a chloroplast, to different chloroplasts within a leaf, to leaves within and between species. While it is very unlikely that there is a unique solution to photosynthesis in the sun or shade, substantial changes in composition, and hence thylakoid membrane organisation and function, are elicited as part of sun/shade responses.

(u.s.A. SUMMARYClass II spinach chloroplasts were fragmented by passage through the French pressure cell (French press), and the fragments were separated. by fractional centrifugation. Fragments sedimenting between lOQO X g and 10,000 X g (10K) .have a lower chl a/chl b ratio and lower P-700 content than whole chloroplasts.· Fragments -sedimenting between 4o.·ooo x g and 160,000 x g (160K) have a much higher chl a/chl b ratio (6.0) and amuch higher P-700 content (1 P-700/105 chlorophylls) than whole chloroplasts. The chlorophyll and cytochrome contents of the Frenc::hJ>ress fractions are similar to those found in fractions iso-,.lated after digitonin disruption.The 160K fraction performs photosystem 1 but not photosystem 2 reactions. The lOK fraction contains both photosystems. Electrophoresis of sodium dodecylsulfate solubilized lOK and 160K fractions gives further evidence for this distribution of photosystems.

1. Mesophyll and parenchyma-sheath chloroplasts of maize leaves were separated by density fractionation in non-aqueous media. 2. An investigation of the distribution of photosynthetic enzymes indicated that the mesophyll chloroplasts probably contain the entire leaf complement of pyruvate,P(i) dikinase, NADP-specific malate dehydrogenase, glycerate kinase and nitrite reductase and most of the adenylate kinase and pyrophosphatase. The fractionation pattern of phosphopyruvate carboxylase suggested that this enzyme may be associated with the bounding membrane of mesophyll chloroplasts. 3. Ribulose diphosphate carboxylase, ribose phosphate isomerase, phosphoribulokinase, fructose diphosphate aldolase, alkaline fructose diphosphatase and NADP-specific ;malic' enzyme appear to be wholly localized in the parenchyma-sheath chloroplasts. Phosphoglycerate kinase and NADP-specific glyceraldehyde phosphate dehydrogenase, on the other hand, are distributed approximately equally between the two types of chloroplast. 4. After exposure of illuminated leaves to (14)CO(2) for 25sec., labelled malate, aspartate and 3-phosphoglycerate had similar fractionation patterns, and a large proportion of each was isolated with mesophyll chloroplasts. Labelled fructose phosphates and ribulose phosphates were mainly isolated in fractions containing parenchyma-sheath chloroplasts, and dihydroxyacetone phosphate had a fractionation pattern intermediate between those of C(4) dicarboxylic acids and sugar phosphates. 6. These results indicate that the mesophyll and parenchyma-sheath chloroplasts have a co-operative function in the operation of the C(4)-dicarboxylic acid pathway. Possible routes for the transfer of carbon from C(4) dicarboxylic acids to sugars are discussed.

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