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Zeaxanthin has been correlated with high-energy non-photochemical fluorescence quenching but whether antheraxanthin, the intermediate in the pathway from violaxanthin to zeaxanthin, also relates to quenching is unknown. The relationships of zeaxanthin, antheraxanthin and ΔpH to fluorescence quenching were examined in chloroplasts ofPisum sativum L. cv. Oregon andLactuca sativa L. cv. Romaine. Data matrices as five levels of violaxanthin de-epoxidation against five levels of light-induced lumen-proton concentrations were obtained for both species. The matrices included high levels of antheraxanthin as well as lumen-proton concentrations induced by subsaturating to saturation light levels. Analyses of the matrices by simple linear and multiple regression showed that quenching is predicted by models where the major independent variable is the product of lumen acidity and de-epoxidized xanthophylls, the latter as the sum of zeaxanthin and antheraxanthin. The interactions of lumen acidity and xanthophyll concentration are shown in three-dimensional plots of the best-fit multiple regression models. Antheraxanthin apparently contributes to quenching as effectively as zeaxanthin and explains quenching previously not accounted for by zeaxanthin. Hence, we propose that all high-energy dependent quenching is xanthophyll dependent. Quenching requires a threshold lumen pH that varies with xanthophyll composition. After the threshold, quenching is linear with lumen acidity or xanthophyll composition.

Artificially mediated linear (methylviologen) and cyclic (phenazine methosulfate) electron transport induced zeaxanthin-dependent and independent (constitutive) nonphotochemical quenching in osmotically shocked chloroplasts of pea (Pisum sativum L. cv Oregon). Nonphotochemical (15), which also appears to depend oii the redox state of a membrane component (20). Recently, qE has been related to "down regulation" of photochemistry at PSII (32).The mechanism for qE is unclear. Exchange of protons for Mg2+ (16), conversion of PSII from fluorescent to nonfluorescent forms (32), and zeaxanthin formation have been implicated (9). Zeaxanthin is formed from violaxanthin (34) by action of violaxanthin deepoxidase whose activity requires an acidified lumen (11). Depending on treatment, zeaxanthin formation results in increased irreversible or reversible qN (8, 9). Irreversible or slowly reversible zeaxanthin-dependent nonphotochemical quenching may be related to photoinhibition (8). Rapidly reversible zeaxanthin-dependent qN is concluded to be qE (9, 10). Zeaxanthin-dependent qN appears to have a photoprotective function (3).Whether qE comprises more than one component is controversial. The results of several laboratories support the view that zeaxanthin-dependent qE adds to an underlying zeaxanthin-independent qN (3, 9, 10). Other studies conclude instead that zeaxanthin sensitizes qE to ApH and that, at saturating ApH, zeaxanthin does not increase total qN (18). Both qN (5) and zeaxanthin-dependent qE (3, 9) have been correlated with Fo quenching (qo) which, according to the Butler-Kitajima model (7), suggests quenching occurs in the pigment bed.2 Abbreviations: qN., coefficient for nonphotochemical quenching; q, coefficient for energy-dependent nonphotochemical quenching: Q^, primary electron acceptor of PSII; MV, methylviologen (1,1'-dimethyl-4,4'-bipyridinium dichloride); DBMIB, dibromothymoquinone (2,5-dibromo-3-methyl-6-isopropyl-p-

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The relationship between the diadinoxanthin cycle and changes in fluorescence yield in the diatom Chaetoceros muelleri Lemm. (clone CH10, Amorient Aquafarm, Inc., Hawaii) was investigated. High‐light‐induced changes in fluorescence yield and xanthophyll de‐epoxidation occurred very rapidly (first order rate constant 1.60 min−1). The observed light‐induced changes in diatoxanthin and diadinoxanthin concentration were consistent with a two‐pool scheme for diadinoxanthin, one of which does not undergo de‐epoxidation. Changes in xanthophyll concentration correlated with changes in in vivo absorbance indicating that diadinoxanthin cycle activity in vivo can be monitored spectrophotometrically. However, changes in cell absorbance were small relative to total optical absorption cross section. Increases in the concentration of diatoxanthin were linearly correlated with increases in the rate constant for thermal de‐excitation in the antenna of photosystem II (PSII). Antenna quenching produced or mediated by diatoxanthin may, thus, protect the PSII reaction center in diatoms. Changes in the maximum fluorescence yield suggested that changes in the reaction center also contributed to nonphotochemical quenching of fluorescence. Thus, reaction center quenching affected the relationship between antenna quenching and changes in photochemical efficiency producing the effect of a decrease in fluorescence yield without a decrease in photochemical efficiency.

The biochemistry of the violaxanthin cycle in relationship to photosynthesis is reviewed. The cycle is a component of the thylakoid and consists of a reaction sequence in which violaxanthin is converted to zeaxanthin (de-epoxidation) and then regenerated (epoxidation) through separate reaction mechanisms. The arrangement of the cycle in the thylakoid is transmembranaus with the de-epoxidation system situated on the loculus side and epoxidation on the outer side of the membrane.