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 degrees C/250 mu mol photons m(-2) s(-1) exhibited a relative reduction state of PS II comparable to that of rye grown at 20 degrees C but high light (800 mu mol photons m(-2) s(-1)) (1-qp = 0.32) whereas winter rye grown at 20 degrees C/250 mu mol photons m(-2) s(-1) exhibited values of 1-qp (approximate to 0.15) comparable to plants grown at 5 degrees C but low light (50 mu mol 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 Delta A(820) 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, Cytf, PC, PsaA/PsaB heterodimer and the B-subunit of ATPase complex exhibited minimal changes on a Chi 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 degrees 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 degrees C/250 mu mol photons m(-2) s(-1) or 20 degrees C/800 mu mol 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.