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Functional and molecular changes in the photosynthetic apparatus during senescence of flag leaves from field‐grown barley plants

Changes in the function and composition of the photosynthetic apparatus were analysed during maturation and senescence of barley (Hordeum vulgare L.) flag leaves under field conditions. At about 8 d post-anthesis, photosynthetic capacity, measured as oxygen evolution under light-saturating conditions, and the D 1 protein level started to decrease. At about 14 d post-anthesis, the pigment content, photosystem II efficiency (F v /F m ) and levels of cytochrome f and the large subunit of ribulose-1,5-bis-phosphate carboxylase started to decline. The levels of cytochrome b559 and the small subunit of ribulose-1,5-bis-phosphate carboxylase decreased later, at about 22 d post-anthesis. High levels of the light-harvesting complex of photosystem II were still detectable at an even later stage of senescence, at 26 d post-anthesis. There were also differences in the kinetics of the declines in the levels of transcripts coding for components of the photosynthetic apparatus. rbcS mRNA abundance had already decreased when flag leaves reached their final lengths, at about 4 d post-anthesis, whereas levels of psbA and petC transcripts did not decrease until 16 d post-anthesis. In contrast, a senescence-related transcript, pHvS40 (Becker & Apel 1993), accumulated in the flag leaves 14 d post-anthesis. Several senescence-related processes which differ in their kinetics are described. The interrelationships among these processes and their regulation are discussed.