Excitation Spillover from PSII to PSI Measured in Leaves at 77K

Heterogeneous distribution of PSI and PSII in thick grana in shade chloroplasts is argued to hinder spillover of chlorophyll excitations from PSII to PSI. To examine this dogma, we measured fluorescence induction at 77K at 690 nm (PSII) and 760 nm (mainly PSI) in the leaf discs of Spinacia oleracea, Cucumis sativus and shade tolerant Alocasia odora, grown at high and low light, and quantified their spillover capacities. PSI fluorescence (FI) consists of the intrinsic PSI fluorescence (FI) and fluorescence caused by excitations spilt over from PSII (FI{beta}). When FI and FII parameters between State 1 and State 2, induced by weak far-red and blue light, were compared, PSII maximum fluorescence (FIIm) and FI{beta} were greater, and FI was smaller in State 1 and thereby the spillover ratio, FI{beta}/(FI +FI{beta}) or FI{beta}/FIm, was greater in State 1. Since the leftover FIIm was found to be about 10% of total Fm at 760 nm, all the data were corrected. Even after the correction, the spillover ratio in FIm in State 1 ranged from 21 to 32%, and the spillover ratios were comparable irrespective of growth light conditions. Although extensive grana in low light grown plants would suggest that PSII and PSI are too separated for spillover, in A. odora chloroplasts, the ratio of non-appressed thylakoid membranes/total thylakoid membranes was little affected by growth light and more than 40%. Abundant non-appressed thylakoids would contribute to efficient spillover.