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

Abstract

ABSTRACTHeterogeneous distribution of PSI and PSII in thick grana in shade chloroplasts is believed 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 (PSI) in the leaf discs of spinach, cucumber, and shade tolerantAlocasia 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). WhenFIand FIIparameters between State 1 and State 2, induced by weak PSI and PSII light, were compared, PSII maximum fluorescence (FIIm) was greater andFIαwas smaller in State 1 by up to 30 %. The spillover ratio,FIβ/FIm, ranging from 0.25 to 0.4, was greater in State 1. In a barleyb-less mutant, bothFIImandFIαwere insensitive to PSI or PSII light, andFIβ/FImwas 0.33.A. odoraleaf discs that were frozen after NPQ induction showed lowerFIβandFIβ/FImthan the dark controls, whileFIαdid not change. In contrast to the dogma, the spillover ratios were not lower in low light grown leaves. Although extensive grana in low light grown plants would hint that PSII and PSI are too separated for spillover, in chloroplasts in high and low light grownA. odoraleaves, the ratio of non-appressed thylakoid membranes/ total thylakoid membranes was little affected by growth light and more than 40%.