In the current research, we used chlorophyll fluorescence measurements and photosynthetic pigment content to evaluate onset and rate of the flag leaf senescence every 7 d beginning at the flowering stage (0 d after flowering, DAF) until late senescence stage (35 DAF) on three winter wheat field-grown varieties (Alka, Žitarka, and Olimpija) with the similar maturation time. OJIP curves implied that flag leaves of variety Alka detained the longest photosynthetic efficiency, while the earliest

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... toms of senescence onset were indicated by positive L-band at 7 DAF and K-band at 14 DAF in variety Olimpija. A shift in I-P phase in the wheat flag leaves suggested the reduction of first acceptors in PSI, implying degradation of photosynthetic apparatus. Performance index (PItotal) proved to be the sensitive indicator of senescence; it decreased very early at 7 DAF in the flag leaves of variety Olimpija compared to Alka and Žitarka. Principal component analysis elucidated quantum yield of energy dissipation and dissipated energy flux per reaction centre as the earliest indicators of senescence in wheat flag leaves and thus showed that multivariate approach may be useful in detection of senescence onset. Sustainability indexes (SIs) corroborated the statistical significance results of evaluated parameters implying that SIs can be used as user friendly data analysis for screening purposes. Selection for functional stay-green trait could contribute to increasing crop yields. Abbreviations: ABS/RC -absorption flux (of antenna chlorophylls) per reaction centre; Car -carotenoids; ChlF -chlorophyll a fluorescence; DAF -days after flowering; DI0/RC -dissipated energy flux per reaction centre; ET0/RC -electron transport flux (further than QA − ) per reaction centre; F0 -minimal fluorescence intensity (50 µs); FM -fresh mass; FV -maximal variable fluorescence; OEC -oxygen-evolving complex; PCA -principal component analysis; PIABS -performance index (potential) for energy conservation from exciton to the reduction of intersystem electron acceptors; PItotal -performance index (potential) for energy conservation from exciton to the reduction of PSI end acceptors; RC -reaction centre; RC/CS0 -density of reaction centres per cross section; RE0/RCelectron flux reducing end electron acceptors at the PSI acceptor side per reaction centre; SI -sustainability index; TR0/RC -trapping flux (leading to QA reduction) per reaction centre; WOJ -relative variable fluorescence between O-and J-steps; WOK -relative variable fluorescence between O-and K-steps; WOP -relative variable fluorescence between O-and P-steps; δR0 -probability that an electron from the electron transport chain is transferred to reduce end electron acceptors at the PSI acceptor side; φD0 -quantum yield of energy dissipation at t = 0; φP0 -maximum quantum yield for primary photochemistry at t = 0; φR0 -quantum yield for reduction of end electron acceptors at the PSI acceptor side; φE0 -quantum yield of electron transport at t = 0; ψE0 -probability that a trapped exciton moves an electron into the electron transport chain beyond QA -.