Greening of intermittent-light-grown bean plants in continuous light: Thylakoid components in relation to photosynthetic performance and capacity forphotoprotection

Phaseolus vulgaris (cvv. Windsor longpod and snap bean) plants, etiolated d uring germination, were exposed to intermittent light (2 min light every 2 hr) for up to 68 hr and then transferred to continuous white light. On tran sfer of the plants to continuous light (100 photons mu mol m(-2) s(-1), 24 degreesC), the quantum yield of oxygen evolution increased two-fold in abou t 30 hr. The chlorophyll content per unit leaf area or unit fresh weight in creased dramatically, but the fresh weight per unit leaf area was relativel y constant. The changes were expressed on the basis of fresh weight or leaf area. On this basis, the contents of photosystem (PS) I and II increased i n continuous light, by a factor of 3 and 8, respectively. While the chlorop hyll b content and the contents of apoproteins of light-harvesting chloroph yll-protein complexes (LHCIIb, CP29, CP26 and CP24) increased markedly, nei ther the total carotenoid content nor the de-epoxidation state of the xanth ophylls [ratio of zeaxanthin(Z) + antheraxanthin(A) to (Z+ A + violaxanthin ) was about 0.4)] responded significantly on transfer to continuous light. The fast rise of the flash-induced electrochromic signal (Delta A518) was w ell correlated with the increases in PS I and PS II reaction centres,and wi th chlorophyll b and total carotenoid contents. The increase in the quantum yield of oxygen evolution during greening in continuous light is attribute d to a more balanced distribution of excitation energy between the two phot osystems, facilitated by the increased number of PS II units, the increased antenna size of each unit and the enhancement of grana formation. The chlo roplast in intermittent light was found to contain abundant xanthophyll cyc le pigments and the psbS gene product, presumably adequate for photoprotect ion in continuous light as soon as chlorophyll a/b- protein complexes are s ynthesized. The results suggest that greening in continuous light is accomp anied by adjustments that include enhanced quantum efficiency of photosynth esis and development of a capacity for harmless dissipation of excess excit ation energy.