Excitation spillover from PSII to PSI measured in leaves at 77 K.

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 77 K 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; thereby, the spillover ratio, FIβ/(FIα + FIβ), was greater in State 1. When nonphotochemical quenching (NPQ) was induced, the spillover ratio decreased. Since analyses of Fv/Fmspectra tentatively suggested that ∼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 separate for spillover, the ratios of nonappressed thylakoid membranes/total thylakoid membranes in A. odora chloroplasts were little affected by growth light and >40%. Spillover would occur efficiently in abundant nonappressed thylakoids and in the margins of appressed thylakoids.