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The fresh produce industry is constantly growing, due to increasing consumer demand. The shelf-life of some fruit, however, is relatively short, limited by microbial contamination or visual, textural and nutritional quality loss. Thus, techniques for reducing undesired microbial contamination, spoilage and decay, as well as maintaining product’s visual, textural and nutritional quality are in high demand at all steps within the supply chain. The postharvest use of signalling molecules, i.e. jasmonates and salicylates, seems to have unexplored potential. The focus of this review is on the effects of treatment with jasmonates and salicylates on the fresh produce quality, defined by decay incidence and severity, chilling injury, maintenance of texture, visual quality, taste and aroma, and nutritional content. Postharvest treatments with jasmonates and salicylates have the ability to reduce decay by increasing fruit resistance to diseases and reducing chilling injury in numerous products. These treatments also possess the ability to improve other quality characteristics, i.e. appearance, texture maintenance and nutritional content. Furthermore, they can easily be combined with other treatments, e.g. heat treatment and ultrasound treatment. A good understanding of all the benefits and limitations related to the postharvest use of jasmonates and salicylates is needed, and relevant information has been reviewed in this paper.

A comparison has been made between the changes in absorption spectra and chlorophyll fluorescence emission occurring upon the induction of non-photochemical dissipation of excitation energy (qE) in isolated thylakoids and those accompanying the aggregation of detergent-solubilised spinach light-harvesting complex (LHCII). In support of a recent hypothesis for the mechanism of qE (Horton et al. (1991) FEBS Lett. 292, 1–4), it was found that absorbence changes at 530 nm were associated with qE and LHCII aggregation. Antimycin A inhibited these changes and prevented LHCII aggregation, as indicated by the electrophoretic mobility of the complex and its low-temperature fluorescence spectrum. An antimycin-insensitive partial aggregation of LHCII was associated with an absorbance change at 505 nm. Low concentration of detergent caused disaggregation of LHCII and the reversal of qE. These data are discussed in terms of the relationship between structural change in LHCII and the mechanism of non-photochemical quenching of chlorophyll fluorescence in thylakoids.

The pH dependence of maximum chlorophyll fluorescence yield (Fm) was examined in spinach thylakoids in the presence of nigericin to dissipate the transthylakoid pH gradient. 3-(3',4'-dichlorophenyl)-1,1-dimethylurea (DCMU) was present to eliminate photochemical quenching. Thylakoids were prepared from dark adapted leaves ('dark' thylakoids) or preilluminated leaves ('light' thylakoids). In the latter there had been approximately 50% conversion of the xanthophyll violaxanthin to zeaxanthin, while no conversion had occurred in the former. In the presence of a reductant such as ascorbate, antimycin A sensitive quenching was observed (half maximal quenching at 5 μM), whose pH dependence differed between the two types of thylakoid. Preillumination of leaves resulted in more quenching at pH values where very little quenching was observed in 'dark' thylakoids (pH 5-7.6). This was similar to activation of high-energy-state quenching (qE) observed previously (Rees D, Young A, Noctor G, Britton G and Horton P (1989) FEBS Lett 256: 85-90). Thylakoids isolated from preilluminated DTT treated leaves, that contained no zeaxanthin, behaved like dark thylakoids. A second form of quenching was observed in the presence of ferricyanide, that could be reversed by the addition of ascorbate. This was not antimycin A sensitive and showed the same pH dependence in both types of thylakoid. The former type of quenching, but not the latter, showed similar low temperature fluorescence emission spectra to qE, and was considered to occur by the same mechanism.

A new hypothesis is presented to explain the major molecular process that regulates the efficiency of light harvesting by chloroplast membranes. It is proposed that in excess light the decrease in the thylakoid lumen pH causes an increase in aggregation of the light harvesting complexes of photosystem II resulting in formation of an efficient pathway for non-radiative dissipation of excitation energy. The aggregation is potentiated by the conversion of violaxanthin to zeaxanthin. This hypothesis is based upon (i) similarity between the spectroscopic changes associated with energy dissipation and those observed upon aggregation of isolated light harvesting complex; and (ii) the link between changes in light scattering and increased energy dissipation.

Thylakoids isolated from light-treated spinach leaves (‘light’ thylakoids) which contain significant zeaxanthin levels show amplification of high-energy-state chlorophyll fluorescence quenching (qE) with respect to the transthylakoid pH gradient, when compared to thylakoids from dark-adapted leaves (‘dark’ thylakoids) which contain no zeaxanthin. In 77 K fluorescence emission spectra q E in ‘dark’ thylakoids shows maximal quenching with respect to fluorescence yield near 680 nm, whereas in ‘light’ thylakoids additional quenching occurs near 700 nm, which is associated with the appearance of a chlorophyll species absorbing at long wavelengths. It is suggested that this long-wavelength species is associated with an aggregated state of LHC II.

The quenching of variable fluorescence yield (qN) and the quenching of dark level fluorescence yield (q0) directly atributable to high-energy-state fluorescence quenching (qE) was studied to distinguish between energy dissipation in the antenna and light harvesting complexes (antenna quenching) and energy dissipation at the reaction centres (reaction centre quenching). A consistent relationship was obtained between qN and q0 in barley leaves, the green alga Dunaliella C9AA and in pea thylakoids with 2,3,5,6-tetramethyl-p-phenylene diamine (DAD) as mediator of cyclic electron flow around PS 1. This correlated well with the relationship obtained using m-dinitrobenzene (DNB), a chemical model for antenna quenching, to quench fluorescence in Dunaliella C9AA or pea thylakoids. The results also correlated reasonably well with theoretical predictions by the Butler model for antenna quenching, but did not correlate with the predictions for reaction centre quenching. It is postulated that qE quenching therefore occures in the antenna and light harvesting complexes, and that the small deviation from the Butler prediction is due to PS 2 heterogeneity.

The parameter фp (= rate of photosynthetic O2evolution(incident light × qQ) reflects the efficiency of O2 evolution by PS II reaction centres with respect to incident light, independent of the redox state of the primary electron acceptor QA (Weis, E. and Berry, J.A. (1987) Biochim. Biophys. Acta 894, 198–208). During whole chain electron transfer to methyl viologen in spinach thylakoids decreases in фp of more than 50% were observed in the presence of a transthylakoid pH gradient. Evidence was obtained that there are two components of фp decrease; one is associated with non-photochemical dissipation of chlorophyll excitation, observed as nonphotochemical fluorescence quenching (qNP), while a second component is not associated with fluorescence changes. The evidence for the latter was obtained using thylakoids that were suspended in low magnesium, or in the presence of antimycin A, so that they would develop only low levels of qNP. It is proposed that this component is due to electron cycling around the PS II reaction centre, and that the cycle is closely related to the transthylakoid pH gradient. The cycle can be observed during electron flow through PS II only, to o-phenylene diamine, and therefore does not appear to be controlled by the redox state of components beyond PS II in the electron transfer chain. Reversible фp decreases observed in uncoupled thylakoids at low pH suggest that the cycle may be controlled by the pH of the intrathylakoid space, and not the pH gradient per se. Measurements made during the approach to steady state, in which the development qNP is slower that the formation of the pH gradient, are consistent with the suggestion that the contribution to фp changes by qNP can be determined by the ratio of variable to maximum fluorescence yield (Genty, B., Briantais, J.-M. and Baker, N.R. (1989) Biochim. Biophys. Acta 990, 87–92).

The yield of photosynthetic O2 evolution was measured in cultures of Dunaliella C9AA over a range of light intensities, and a range of low temperatures at constant light intensity. Changes in the rate of charge separation at Photosystem I (PS I) and Photosystem II (PS II) were estimated by the parameters ΦPS I and ΦPS II . ΦPS I is calculated on the basis of the proportion of centres in the correct redox state for charge separation to occur, as measured spectrophotometrically. ΦPS II is calculated using chlorophyll fluorescence to estimate the proportion of centres in the correct redox state, and also to estimate limitations in excitation delivery to reaction centres. With both increasing light intensity and decreasing temperature it was found that O2 evolution decreased more than predicted by either ΦPS I or ΦPS II. The results are interpreted as evidence of non-assimilatory electron flow; either linear whole chain, or cyclic around each photosystem.

31P nuclear magnetic resonance (NMR) saturation transfer has been used to measure enzymatic flux through the creatine phosphokinase reaction in the direction of ATP synthesis in the human forearm muscle flexor digitorum superficialis. Modification of the ratio method for measurement of spin-lattice relaxation (R. Freeman, H. D. W. Hill, and R. Kaptein, J. Mags. Reson. 7, 82 (1972) was tested and used to appreciably shorten the duration of the measurement. Under conditions of steady state work intracellular pH decreased slightly by 0.06 units and the spin-lattice relaxation time of phosphocreatine in muscle was unchanged, while flux from phosphocreatine to ATP was 64 ± 10% of the resting value. This is contrary to the increase in flux of 155% predicted from previous saturation transfer studies carried out in vitro on rabbit skeletal muscle creatine phosphokinase using metabolite concentrations to mimic those in vivo (E. A. Shoubridge, J. L. Bland, and G. K. Radda, Biochim. Biophys. Acta 805, 72 (1984). This discrepancy could be accounted for by an underestimation of the ADP concentrations to which the enzyme is exposed due to inaccurate assumptions about the total metabolite concentrations, or possibly by compartmentation of creatine phosphokinase and its reactants. © 1989 Academic Press, Inc.

The extent of energy-dependent quenching of chlorophyll fluorescence in broken spinach chloroplasts has been quantitatively related to the size of the thylakoid proton gradient as measured by the quenching of 9-aminoacridine fluorescence by titration at constant irradiance with the uncoupler nigericin or by change in irradiance. It was found that chloroplasts prepared from leaves that had been pre-illuminated with strong light for 30 min showed energy-dependent quenching at a lower proton gradient than chloroplasts prepared from dark-adapted leaves. Measurement of the carotenoid composition of the thylakoids showed that light treatment raised the ratio of zeaxanthin: violaxanthin. The possible dependence of energy-dependent quenching on xanthophyll composition and the physiological implications of this light-activation process to the regulation of photosynthetic electron transport are discussed.

The relaxation of the non-photochemical quenching of chlorophyll fluorescence has been investigated in cells of the green alga Dunaliella following illumination. The relaxation after the addition of DCMU or darkening was strongly biphasic. The uncoupler NH4Cl induced rapid relaxation of both phases, which were therefore both energy-dependent quenching, qE. The proportion of the slow phase of qE increased at increasing light intensity. In the presence of the inhibitors rotenone and antimycin the slow phase of qE was stabilised for in excess of 15 min. NaN3 inhibited the relaxation of almost all the qE. The implications of these results are discussed in terms of the interpretation of the non-photochemical quenching of chlorophyll fluorescence in vivo and the mechanism of qE.

The quantitative relationship between energy-dependent quenching of chlorophyll fluorescence (qE) and trans-thylakoid pH difference (ΔpH, estimated with 9-aminoacridine) was compared in chloroplasts from leaves preilluminated at low oxygen in the absence of CO2 with chloroplasts from leaves darkened under the same conditions. The extent of both phenomena was varied by changes in actinic light intensity. Chloroplasts from preilluminated leaves contained high levels of zeaxanthin (up to 15% of total carotenoid) and were capable of forming qE at lower ΔpH values than chloroplasts from dark-adapted leaves, which lacked zeaxanthin. Infusion of dithiothreitol into leaves prior to preillumination prevented the light-induced formation of zeaxanthin; chloroplasts prepared from these leaves showed the same relationship between qE and ΔpH observed in chloroplasts fom dark-adapted leaves. The rate of appearance and disappearance of the change in relationship between qE and ΔpH upon leaf preillumination and darkening was closely matched by the kinetics of synthesis and degradation of zeaxanthin. The quantitative relationship between qE and quenching of the dark-level of fluorescence (F0) was very similar in chloroplasts from preilluminated or dark-adapted leaves, containing disparate amounts of zeaxanthin. In both sets of chloroplasts, qE was inhibited by antimycin A. These latter observations suggest that the formation of qE involves mechanistically similar features in chloroplasts containing or lacking zeaxanthin. Overall, the data are discussed in terms of action of zeaxanthin as a ‘quenching amplifier’, functioning physiologically to allow qE formation in the absence of high ΔpH potentially prohibitive to high rates of CO2 fixation.