Selective Stabilization of the PSI-Associated Electron Transport Network Underlies Cytokinin-Mediated Delay of Leaf Senescence in Barley

Leaf senescence progressively remodels the photosynthetic apparatus, leading to impaired electron transport and declining carbon assimilation. Here, we investigated how dark-induced senescence (DIS) and exogenous 6-benzyladenine (BA) affect photosystem function, cyclic electron flow (CEF), photosynthetic protein remodeling and CO2 assimilation in two barley (Hordeum vulgare L.) cultivars differing in senescence characteristics, Carina (spring) and Lomerit (winter). DIS markedly reduced chlorophyll content, PSI and PSII photochemistry, electron transport and CO2 assimilation in both cultivars, although the underlying mechanisms differed. Carina maintained higher CEF despite stronger PSII inhibition, whereas Lomerit exhibited a greater decline in CEF accompanied by stronger donor- and acceptor-side limitations of PSI. These physiological responses coincided with selective remodeling of proteins forming the PSI-associated electron transport network, including coordinated changes in cytochrome f, PGRL1, NdhS, FNR and photosystem antenna proteins, indicating functional reorganization of photosynthetic electron transport rather than uniform chloroplast protein degradation. BA delayed senescence by preserving chlorophyll, maintaining PSI and PSII activity, sustaining CEF and partially alleviating the decline in CO2 assimilation. The protective effects of BA were more pronounced in Carina and coincided with more effective preservation of proteins associated with PSI-dependent electron transport. Collectively, our findings identify selective stabilization of the PSI-associated electron transport network as a central mechanism underlying cytokinin-mediated delay of leaf senescence in barley and demonstrate that cultivar-dependent regulation of this network determines the effectiveness of cytokinin-mediated protection of photosynthesis.