Greening of intermittent-light-grown bean plants in continuous light: thylakoid components in relation to photosynthetic performance and capacity for photoprotection

Abstract

Phaseolus vulgaris (cvv. Windsor longpod and snap bean) plants, etiolated during 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 transfer of the plants to continuous light (100 photons mumol m-2 s-1, 24 degrees C), the quantum yield of oxygen evolution increased two-fold in about 30 hr. The chlorophyll content per unit leaf area or unit fresh weight increased dramatically, but the fresh weight per unit leaf area was relatively 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 in continuous light, by a factor of 3 and 8, respectively. While the chlorophyll b content and the contents of apoproteins of light-harvesting chlorophyll-protein complexes (LHCIIb, CP29, CP26 and CP24) increased markedly, neither the total carotenoid content nor the de-epoxidation state of the xanthophylls [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 well correlated with the increases in PS I and PS II reaction centres, and with chlorophyll b and total carotenoid contents. The increase in the quantum yield of oxygen evolution during greening in continuous light is attributed to a more balanced distribution of excitation energy between the two photosystems, facilitated by the increased number of PS II units, the increased antenna size of each unit and the enhancement of grana formation. The chloroplast in intermittent light was found to contain abundant xanthophyll cycle pigments and the psbS gene product, presumably adequate for photoprotection in continuous light as soon as chlorophyll a/b- protein complexes are synthesized. The results suggest that greening in continuous light is accompanied by adjustments that include enhanced quantum efficiency of photosynthesis and development of a capacity for harmless dissipation of excess excitation energy.