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 apparatu s were analysed during maturation and senescence of barley (Hordeum vu lgare L.) flag leaves under field conditions, At about 8 d post-anthes is, 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 effic iency (F-v/F-m) and levels of cytochrome f and the large subunit of ri bulose-1,5-bisphosphate carboxylase started to decline, The levels of cytochrome b559 and the small subunit of ribulose-1,5-bisphosphate car boxylase decreased later, at about 22 d post-anthesis, High levels of the light-harvesting complex of photosystem IT were still detectable a t 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 tra nscripts coding for components of the photosynthetic apparatus, rbcS m RNA abundance had already decreased when flag leaves reached their fin al lengths, at about 4 d post-anthesis, whereas levels of psbA and pet C transcripts did not decrease until 16 d post-anthesis, In contrast, a senescence-related transcript, pHvS40 (Becker & Apel 1993), accumula ted in the flag leaves 14 d post-anthesis. Several senescence-related processes which differ in their kinetics are described, The interrelat ionships among these processes and their regulation are discussed.