A light-dependent reversible grana stacking-unstacking process, paralleled by a reorganization of thylakoid components, has been noticed in greening etiolated bean (Phaswols vulgaris, var It is proposed that this process does not reflect the turnover of the LHCP component per se, but a regulatory process operating during development, by which the ratio of light-harvesting to PSII reaction center components, determined by the environmental conditions, controls the photosynthetic rate.The assembly of functional and structural components in the thylakoid during chloroplast development follows a step wise process (2,20). For full growth and assembly of the membrane, exposure of the etiolated tissue in the light for a certain period of time is necessary, after which the thylakoid acquires all the characteristics and properties of the fully mature photosynthetic membrane.Under conditions where the thylakoid is still in the process of development, its components seem to undergo a process of reorganization. Such a reorganization has been noticed earlier under certain experimental conditions: (a) in young etiolated leaves exposed to intermittent light for long periods of time after their transfer to CL2 in the presence of the protein synthesis inhibitor chloramphenicol (5); (b) in etiolated leaves exposed to periodic light or to 2 h CL and transferred to darkness for some hours (1).In the first case (5), it was found that even though net ChM synthesis can no longer occur, the Chl a/Chl b ratio drops, new LHCP are detected in the thylakoid, the PSII unit size increases, and the 'Supported partly by NATO PSII activity per mg Chl decreases. These results were proposed to reflect a reorganization of the PSII unit components: some units being destroyed (PSII activity per mg Chl decreased) and new ones organized (increased LHCP incorporation increasing the PSII unit size) (5). In the second case (1), it was found that upon transfer of 2 h CL leaves or intermittent light leaves to darkness the FmaI/Fo as well as the DPC-DCIP activity per mg Chl increased in darkness by about 30%o to 70%o. This was again explained as reflecting the organization of unorganized Chl in new small sized PSII units during the subsequent dark period (1). The rationale behind this proposal was that in those cases where the thylakoid contains greater amount of PSII units than that of the mature green thylakoid (first case [51) some of the PSII units are destroyed to reach the number found in the green plant; their Chl a thus liberated could be used for the growth of the remaining PSII units. On the other hand, in those cases where the content of PSII units is lower than that in the mature thylakoid (second case [1]), their LHCP is destroyed in darkness so that the liberated Chl a as well as the Chl a still not organized into units, could form new PSII units. This proposal, therefore, predicted that in the cases where the thylakoid contains equal concentration of PSII units as that of the mature thylakoid, no change in the LHCP content of the th...

It was shown earlier that in etiolated bean (Phaseolus rulgaris, var. red kidney) leaves exposed to continuous light for a short time and then transferred to darkness a reorpnization of their photosystem II (PSII) unit components occurs. This reorpnization involves disornization of the light-harvesting complex of PSII (LHC-II), destruction of its chlorophyll b and the 25 kilodalton polypeptide, and reuse of its chlorophyll a for the formation of additional, small in size, PSII units (ArgyroudiAkoyunoglou, Akoyunoglou, Kaloskas, Akoyunoglou 1982 Plant Physiol 70: 1242-1248. The present study further shows that parallel to the PSII unit reorganization a reorganization of the PSI unit components also occurs: upon transfer to darkness the 24, 23, and 21 kilodalton polypeptides, components of the light-harvesting complex of PSI (LHC-I), are decreased, the 69 kilodalton polypeptide, component of the chlorophyll a-rich P700-protein complex (CPI), is increased and new smallsized PSI units are formed. Concomitantly, the cytochromeJlchlorophyll and the cytochrome b/chlorophyll ratios are gradually increased. This suggests that the concentration of the electron transport components is also modulated in darkness to allow for adequate electron flow to occur between the newly synthesized PSII and PSI units.Young etiolated bean leaves exposed to CL2 accumulate Chl a and Chl b without lag-phase (3), and they form complete (large in size) photosynthetic units. The concentration of the PSII and PSI units per chloroplast increases with the time of exposure to CL, until it reaches the concentration of the mature chloroplast.Under certain conditions, but always in cases where the thylakoid is still in the process of development, the thylakoid components seem to undergo a process of reorganization (2, 4). Such a case has been noticed recently in the PSII unit components of etiolated bean leaves exposed to CL for a short time and then transferred to darkness (1 1): after transfer to darkness Chl accumulation stops completely; however, the growth of the leaf continues, and thus the Chl per g fresh weight decreases.

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