BOTH SPILLOVER AND LIGHT-ABSORPTION CROSS-SECTION CHANGES ARE INVOLVED IN THE REGULATION OF EXCITATION-ENERGY DISTRIBUTION BETWEEN THE 2 PHOTOSYSTEMS DURING STATE TRANSITIONS IN WHEAT LEAF

Weak red light-induced changes in chlorophyll fluorescence parameters and in the distribution of PS I and PS II in thylakoid membranes were measured in wheat leaves to investigate effective ways to alter the ex citation energy distribution between the two photosystems during state transition in vivo. Both the chlorophyll fluorescence parameter Fm/Fo and F685/F735, the ratio of fluorescence yields of the two photosyste ms at low temperature (77 K), decreased when wheat leaves were illumin ated by weak red light of 640 nm, however, Fm/Fo decreased to its mini mum in a shorter time than F685/F735. When Photosystem (PS II) thylako id (BBY) membranes from adequately dark-adapted leaves (control) and f rom red light-illuminated leaves were subjected to SDS-polyacrylamide gel electrophoresis under mildly denaturing conditions, PS I was almos t absent in the control, but was present in the membranes from the lea ves preilluminated with the weak red light. In consonance with this re sult, the content of Cu, measured by means of the energy dispersive X- ray microanalysis (EDX), increased in the central region, but decrease d in the margin of the grana stacks from the leaves preilluminated by the red light as compared with the control. It is therefore suggested that: (i) both spillover and absorption cross-section changes are effe ctive ways to alter the excitation energy distribution between the two photosystems during state transitions in vivo, and the change in spil lover has a quicker response to the unbalanced light absorption of the two photosystems than the change in light absorption cross-section, a nd (ii) the migration of PS I towards the central region of grana stac k during the transition to state 2 leads to the enhancement of excitat ion energy spillover from PS II to PS I.