Excitation Spillover from PSII to PSI 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 (mostly 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 (FIα) and fluorescence caused by excitations spilt over from PSII (FIβ). 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 FIβ were greater, and FIα was smaller in State 1 and thereby the spillover ratio, FIβ/(FIα+FIβ), was greater in State 1. When non-photochemical quenching (NPQ) was induced, the spillover ratio decreased. Since analyses of Fv/Fm spectra tentatively suggested that about 15% of Fm at 760 nm was from PSII, all data were corrected accordingly. Even after the correction, the spillover ratio in FIm in State 1 ranged from 16 to 28%. The spillover ratios did not greatly differ between the species or growth light levels. Although extensive grana in low light grown plants would suggest that PSII and PSI are too separated for spillover, the ratios of non-appressed thylakoid membranes/total thylakoid membranes in A. odora chloroplasts were little affected by growth light and more than 40%. Abundant non-appressed thylakoids and margins of appressed-thylakoids would enable efficient spillover. (250 words).