Successive degradation of the light-harvesting system of the photosynthetic apparatus during senescence of barley flag leaves

Levels of light-harvesting complexes were investigated during maturation an d senescence of flag leaves derived from field-grown barley plants (Hordeum vulgare L.). In mature flag leaves possessing photosynthetically highly ac tive chloroplasts levels of the major light-harvesting complex LHCII and al so of the minor light-harvesting complexes CP29 of photosystem II and LHCI of photosystem I were high. During growth of barley plants in the field the light conditions drasticall y changed. Variations in photosynthetically active radiation (PAR) in the r ange from 200 to 1500 mu mol*m(-2)*s(-1) did neither affect photosynthetic capacity nor photosystem II efficiency in the mature nag leaves, indicating that the photosynthetic apparatus was sufficiently protected against photo damage. The changes in PAR did also not affect levels of LHCII, LHCI and CP 29. In contrast, levels of another member of the chlorophyll a/b-binding pr otein family, ELIP, drastically fluctuated showing high ELIP levels at days with high PAR. During senescence of flag leaves of field-grown barley plants levels of min or light-harvesting complexes CP29 and LHCI decreased much faster than thos e of the bulk LHCII. The early decline in the minor complexes coincided wit h a decrease in photosystem II efficiency. The decrease in the levels of mi nor complexes LHCI and CP29 was however retarded by a decrease in ambient l ight intensity. The senescence-specific decrease in minor LHCs relative to the bulk LHCII r esults in a decrease in energy transfer to the reaction centers and seems t o represent an additional protection mechanism against photodamage, the ris k of which is increasing during senescence. In contrast, ELIP levels fluctu ated independently from the stage of development, being high at days with h igh PAR and being low at days with low PAR, in mature as well as in senesci ng flag leaves. Northern analyses using a cDNA clone specific for CP29 and another one spec ific for ELIP revealed that both, the senescence-specific decline in the mi nor complex CP29 and the light dependent fluctuations in ELIP level are reg ulated at the transcript level.