Adjustment of thylakoid plastoquinone content and Photosystem I electron donor pool size in response to growth temperature and growth irradiance in winter rye (Secale cereale L.)

Winter rye (Secale cereale L. cv Musketeer) grown at 5 °C/250 µmol photons m^–2 s^–1 exhibited a relative reduction state of PS II comparable to that of rye grown at 20 °C but high light (800 µmol photons m^–2 s^–1) (1-q_P = 0.32) whereas winter rye grown at 20 °C/250 µmol photons m^–2 s^–1 exhibited values of 1-q_P ( 0.15) comparable to plants grown at 5 °C but low light (50 µmol photons m^–2 s^–1). The apparent size of the electron donor pool to PS I, estimated either in vivo or in vitro in the presence of methylviologen by A₈₂₀ was positively correlated with the relative reduction state of PS II under the steady-state growth conditions. Immunoblotting of rye thylakoid polypeptides indicated that the relative contents of Lhcb1, Lhcb2, D1, Cyt f, PC, PsaA/PsaB heterodimer and the -subunit of ATPase complex exhibited minimal changes on a Chl basis. In contrast, a 2-fold increase in plastoquinone A content was associated with increasing growth irradiance at growth temperatures of either 5 or 20 °C. We suggest that the increases in the apparent size of the electron donor pool to PS I associated with rye grown at either 5 °C/250 µmol photons m^–2 s^–1or 20 °C/800 µmol photons m^–2 s^–1 may be explained by an increased thylakoid plastoquinone A content, coupled with possible enhanced PS I cyclic electron transport and/or increased capacity for electron donation from the stroma to the intersystem electron transport chain. The results are discussed with respect to photosynthetic adjustment to changes in PS II excitation pressure in winter rye.