Kinetic of lipoquinone and pigment synthesis in green Hordeum seedlings during an artificial day-night rhythm with a prolonged dark phase.

The effect of a prolonged darkness (48 h) with a following re-illumination on the prenyllipid metabolism of chloroplasts is tested in green Hordeum seedlings. Continuous darkness induces thylakoid and prenyllipid breakdown and changes the remaining lipoquinone and carotenoid metabolism of chloroplasts to that of etioplasts. Re-illumination, in turn, reverses the effects of darkness and regenerates the photosynthetic apparatus within 24 hours by a continuous and spe­ cific synthesis of certain thylakoid prenyllipids. 1. The level of chlorophylls, vitamin Kt and that of the oxidized benzoquinone forms, plastoquinone-9 and a-tocoquinone, decreases continuously in darkness. All these prenyllipids are re­ synthesized upon re-illumination, whereby a faster destruction rate in the dark (a-tocoquinone + plastoquinone-9 > vitamin Kj > chlorophylls) corresponds to a faster re-accumulation rate in the light. 2. The concentration of the reduced benzoquinones plastohvdroquinone-9 and a-tocopherol. which are preferentially deposited in the osmiophilic plastoglobuli, increases until 24 hours of dark­ ness and decreases thereafter either continuously (plastohydroquinone-9) or exhibits another strong increase (a-tocopherol). Re-illumination results only in the accumulation of the oxidized quinone forms (mainly plastoquinone-9 and little a-tocoquinone) ; the level of plastohydroquinone remains, however, almost constant with a concomitant and steady decrease in the concentration of a-toco­ pherol. Darkness changes the main metabolite flow in benzoquinone synthesis via the formation of a-tocopherol (a trimethyl-, phythyl-(l,4-benzoquinone derivative) and light (re-illumination) via the production of plastoquinone-9 (a trimethyl, solanesyl-1.4-benzoquinone). 3. Carotenoids are enriched throughout the dark phase (mainly xanthophylls, little /2-carotene) with a higher accumulation rate than after re-illumination, which yields again a higher portion of /5-carotene. The correlation of prenyl chain synthesis (phytyl chain for chlorophylls and vitamin Kt , solanesyl chain for plastoquinone-9) in the light with a reduced rate of carotenoid formation is discussed with respect to prenyl biosynthesis. 4. It is concluded that breakdown and re-synthesis of thylakoids and their prenyllipds, which are described here in a prolonged dark phase with following re-illumination, also occur during natural day-night growth of plants. The turnover of the thylakoid membranes and their lipids does, however, not get visible, since the decomposition in the night is recompensated by new synthesis during day. From the destruction rate one can calculate in a first approximation the biological half life time (r V2) which gives values in the range of 7 to 2,5 days for all thylakoid prenyllipids.