Differential degradation of the photosynthetic apparatus during leaf senescence in barley (Hordeum vulgare L.)

Senescence processes were investigated in attached seedling leaves of barley (Hordeum vulgare L. cv. Steffi) grown under standardized conditions in growth chambers. Even a few days after reaching full length, total chlorophyll and protein content of first foliage leaves start to decrease, indicating degradation of photosynthetic units. About five weeks later, the final stage of leaf senescence is reached, with almost no chlorophyll and only about 30% of the protein remaining. In spite of the early decrease in total protein and chlorophyll content, efficiency of remaining photosystem II units stays high for about 3 weeks. Then it rapidly decreases, reaching values close to zero two weeks later. The second leaves show similar changes in photosynthetic parameters as the primary leaves, but the kinetics are delayed by about 1 week. immunological analyses reveal specific changes in the composition of the photosynthetic units during leaf senescence. The level of photosystem I reaction centre protein sharply decreases at 35 d after sowing. Levels of photosystem II reaction centre protein D1 and of cytochrome false decrease at this developmental stage, but not as markedly as the photosystem I reaction centre. Lower levels of these proteins can still be detected in later stages of senescence. A preferential loss of photosystem I reaction centre at this developmental stage could also be shown by spectroscopic measurements of concentrations of reaction centre pigments P700 (photosystem I) and Qa (photosystem II, existing in a 1:1 stochiometry to P680). In contrast to reaction centre proteins and cytochrome f, inner light-harvesting complex LHCl of photosystem I is more stable, still showing high levels even in later stages of senescence. Analyses of transcript levels of the corresponding genes show that changes in protein levels are, in part, due to differential down regulation of gene expression during senescence. Whereas the proportions of psaA/B and petA transcripts start to decrease in primary leaves about 3 weeks after sowing, proportions of psbA and Lhcb4 transcripts stay high. even in late stages of senescence.