The mRNA coding for light-harvesting complex of PSII (LHC-11) apoprotein is present in etiolated bean (Phaseolus vulgaris L.) leaves; its level is low in 5-day-old leaves, increases about 3 to 4 times in 9-to 13-day-old leaves, and decreases thereafter. A red light pulse induces an increase in LHC-11 mRNA level, which is reversed by far red light, in all ages of the etiolated tissue tested. The phytochrome-controlled initial increase of LHC-11 mRNA level is higher in 9-and 13-day-old than in 5-and 17-dayold bean leaves. The amount of LHC-11 mRNA, accumulated in the dark after a red light pulse, oscillates rhythmically with a period of about 24 hours. This rhythm is also observed in continuous white light and in the dark following exposure to continuous white light, and persists for at least 70 hours. A second red light pulse, applied 36 hours after initiation of the rhythm, induces a phaseshift, which is prevented by far red light immediately following the second red light pulse. A persistent, but gradually reduced, far red reversibility of the red light-induced increase in LHC-11 mRNA level is observed. In contrast, far red reversibility of the red light-induced clock setting is only observed when far red follows immediately the red light. It is concluded that (a) the lightinduced LHC-11 mRNA accumulation follows an endogenous, circadian rhythm, for the appearance of which a red light pulse is sufficient, (b) the circadian oscillator is under phytochrome control, and (c) a stable Pfr form, which exists for several hours, is responsible for sustaining LHC-11 gene transcription.

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An alternating light-dark system is described under which etiolated bean (Phaseolus vulgaris) leaves form selectively chlorophyll a.This system consists of cycles of 2 minutes of white light alternating with 98-minute dark periods.Etiolated 80 cm above the plants, were used; one light provided to the plants 120 lux (75-w lamp) and the other, 2000 lux (Phillips Ultrilux, 500 w).The chlorophylls were extracted totally by grinding 1 g of leaves in a mortar with 4 ml of pure acetone, followed by several extractions with 80% acetone to a final volume of about 30 ml. The extract was filtered twice, and the absorbance was measured in a Bausch & Lomb Spectronic 505 spectrophotometer. The chlorophyll concentration was calculated accordingto MacKinney (8). In some cases the pigments were transferred to ether, after which absorbancies were measured. The Chl a, Chl b, and protochlorophyllide concentrations were then calculated according to Koski (7). In some samples the pigments were separated by paper chromatography, according to Chiba and Nogushi (5) In some experiments, the leaves, after exposure to the 120-lux light-dark system, were further exposed to 1500 lux for 30 sec. The pigments were then extracted and their concentrations were measured after transfer to ether. It was thus found that exposure of the leaves to 120 lux for 2 min transformed most of the protochlorophyllide to chlorophyllide; only 3 or 4 ,g of protochlorophyllide were left, of which about 1 jg could be phototransformed to chlorophyllide after further exposure of the leaves to 1500 lux for 30 sec.The absolute amounts of Chl a and Chl b synthesized under the light-dark cycles system are shown in Figure 1. It is evident that the amount of Chl a formed increased with the number of light exposures to which the plants were subjected. The rate of Chl a biosynthesis remains constant during the first 40 to 50 cycles. This is expected, since during the 98-min dark period the amount of protochlorophyllide synthesized by the plant tissue has reached its maximal value (4). Eventually a plateau is reached when the net production of protochlorophyllide ceases (4). Figure 1 also indicates a negligible accumulation of Chi b, especially after the first 40 to 50 light-dark cycles. This accumulation was found to start earlier in older plants.In some samples of 4-day-old leaves exposed to less than 30 low intensity light-dark cycles, Chl b was found, spectrophotometrically, to be present in 3-or 4-ug quantities per g fresh weight. However, it was impossible to separate and trace by chromatography any Chl b from these samples. Figure 2 shows the absorption spectrum of an 80% acetone extract of plants subjected to 31 high intensity light-dark cycles. The absorption peaks of pure Chl a are evident, as well as a peak at 473 nm which is due to carotene. Comparison is also made with the spectrum of chromatographically pure chlorophyll a. The concentration of the latter was adjusted in order to obtain equal, maximal absorbancies for the extract and the solution at th...

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...