Photoinhibition and Zeaxanthin Formation in Intact Leaves

Abstract

Comparative studies of chlorophyll a fluorescence, measured with a pulse amplitude modulated fluorometer, and of the pigment composition of leaves, suggest a specific role of zeaxanthin, a carotenoid formed in the xanthophyll cycle, in protecting the photosynthetic apparatus aginst the adverse effects of excessive light. This conclusion is based on the following fmdings: (a) (7), photoinhibitory damage to PSII reaction centers, i.e. a decrease in the rate constant for PSII photochemistry, leads to a rise in initial fluorescence at open PSII traps, F0,3 whereas an increase in the rate constant ofnonradiative energy dissipation leads to a decrease in both initial fluorescence, FO, and maximum fluorescence at closed PSII traps, FM. Such an increase in thermal de-excitation could function as an overflow valve to allow for nondestructive dissipation of excess excitation energy and may therefore be viewed as a potentially protective process rather than as an indication of damage, e.g. to proteins (10) of the thylakoid membrane. Increased nonradiative energy dissipation, as indicated by Chl 77 K fluorescence characteristics, was shown to occur under conditions of excess excitation in low light (in 2% 02 and the absence of C02) with no evidence for simultaneous photoinhibitory damage to the PSII reaction centers, as well as at high light levels where there was evidence for simultaneous damage (4). This paper presents a striking parallel between the occurrence of conditions, at both high and low light levels, under which increased nonradiative energy dissipation is observed, and the accumulation of the carotenoid zeaxanthin in leaves exposed to such conditions. Zeaxanthin is known to be formed by deepoxidation of violaxanthin in the so-called violaxanthin or xanthophyll cycle which is present in thylakoid membranes and which is fine-tuned to respond to the balance of electron transport reactions and CO2 uptake (6, 23). The function of this cycle has been unknown thus far. We propose that the formation of zeaxanthin under photoinhibitory conditions is indicative of a specific function of this particular carotenoid compound in the prevention of photoinhibitory damage.

Economic photoprotection in photosystem II that retains a complete light-harvesting system with slow energy traps

“…As expected, enrichment in zeaxanthin caused in the dark adapted sample a decrease in the F v /F m ratio from 0.83 to B0.7 (ref. 30) as well as a high level of NPQ, which took longer than 5 min to fully relax (Fig. 3a).…”

Nonphotochemical Quenching of Chlorophyll Fluorescence inChlamydomonas reinhardtii

“…These are all associated with photosystem (PS) 1 II and are collectively referred to as nonphotochemical quenching mechanisms (NPQ; 1). However, this term comprises at least three processes: (i) qI, mainly related to photoinhibition, a slowly reversible damage to PSII reaction centers (2,3), although data suggest that a zeaxanthindependent quenching might contribute substantially to this process (4,5), (ii) qT, state transitions, a change in the relative antenna sizes of PSII and PSI, due to the reversible phosphorylation and migration of antenna proteins (LHCII) (6), and (iii) qE, also termed "high-energy state quenching", a form of quenching associated with the development of a low pH in the thylakoid lumen (e.g., ref 7). High-energy state quenching is largely thought to be associated with an increase in thermal dissipation within the light-harvesting apparatus (1,8,9), associated with the generation of a ∆pH (7,10) and with the formation of zeaxanthin via deepoxidation of violaxanthin (11).…”

From ecophysiology to phenomics: some implications of photoprotection and shade–sun acclimation in situ for dynamics of thylakoids in vitro

“…However, ecophysiological research was about to shift this focus to photoprotection involving reversible regulation of light-harvesting efficiency in the antenna and its stabilization by xanthophyll pigment inter-conversions [10]. Interestingly, Jan had also participated in one of the earliest studies of light-dependent changes in the pool of violaxanthin (V) [11].…”