STABILIZATION OF APOPROTEINS OF LIGHT-HARVESTING CHLOROPHYLL-A/B PROTEIN COMPLEX BY FEEDING 5-AMINOLEVULINIC ACID UNDER INTERMITTENT ILLUMINATION

Effect of 5-aminolevulinic acid (ALA) on the accumulation of apoproteins of light-harvesting Chl a/b-protein complexes of photosystem II (LHCII) in cucumber (Cucumis sativus L. cv. Aonagajibai) cotyledons was studied. Etiolated cucumber cotyledons were placed on a filter paper containing buffer with or without ALA (control). They were preilluminated, then incubated for 4 h in the dark, and exposed to 4 cycles of intermittent light. ALA treatment increased Chl a and Chl b 2.4- and 3.3-fold over the control, respectively. LHCII apoproteins were also increased by this treatment. When cotyledons were returned to darkness after intermittent illumination, almost all Chl b disappeared in controls during 6 h of the dark incubation, while about 70 % of Chl b remained in ALA-fed cotyledons. Pulse-chase experiments showed that increased supply of Chl by ALA feeding did not affect the incorporation of newly synthesized LHCII apoproteins into the membrane fraction, but inhibited their breakdown. These results suggest that increased supply of Chl stimulated the accumulation of LHCII by stabilizing its apoproteins, and that the stoichiometric relationship between Chl and LHCII apoproteins was maintained by decomposing excess Chl and apoproteins during early stage of greening.