Light Response of CO₂ Assimilation, Dissipation of Excess Excitation Energy, and Zeaxanthin Content of Sun and Shade Leaves

Abstract

Intact attached sun leaves of Hellanthus annuus and shade leaves of Monstera deliciosa and Hedera helix were used to obtain light response curves of CO2 uptake, the content of the carotenoid zeaxanthin (formed by violaxanthin de-epoxidation), as well as nonphotochemical quenching (qNp), and the rate constant of radiationless energy dissipation (kD). The latter two parameters were calculated from the decrease of chlorophyll a fluorescence at closed photosystem 11 traps in saturating pulses in the light. Among the three species, the light-saturated capacity of CO2 uptake differed widely and light saturation of CO2 uptake occurred at very different photon flux densities. Fluorescence quenching and zeaxanthin content exhibited features which were common to all three species: below light-saturation of CO2 uptake nonphotochemical quenching occurred in the absence of zeaxanthin and was not accompanied by a decrease in the yield of instantaneous fluorescence. Nonphotochemical quenching, qw., increased up to values which ranged between 0.35 and 0.5 when based on a control value of the yield of variable fluorescence determined after 12 hours of darkness. As light saturation of CO2 uptake was approached, qNp showed a secondary increase and the zeaxanthin content of the leaves began to rise. This was also the point from which the yield of instantaneous fluorescence began to decrease. The increase in zeaxanthin was paralleled by an increase in the rate constant for radiationless energy dissipaffon kD, which opens the possibility that zeaxanthin is related to the rapidly relaxing "high-energy-state quenching" in leaves.When leaves are exposed to light levels in excess of those which can be used to drive photosynthetic electron transport, excitation energy may accumulate and damage the photochemical system. Protection from such damage is thought to be afforded by thermal dissipation of excess excitation energy within the photochemical system (3,13,16,23), which results in nonphotochemical fluorescence quenching. This dissipative quenching must be distinguished from other components of nonphotochemical quenching which may be related, for example, to changes in energy distribution between PSII and PSI (10) In these studies (3, 6, 7), fluorescence measurements were performed after leaves had been exposed to 5 min of darkness to remove the rapidly responding component of qNp.Nevertheless, there are some intriguing similarities between the rapidly relaxing high-energy-state quenching in leaves and the more slowly relaxing component which showed the correlation with the zeaxanthin content (6, 7). The rapidly relaxing and the slowly relaxing components ofnonphotochemical fluorescence quenching in leaves were found to have the same fluorescence characteristics, with respect to a quenching of Fo, and, in addition, the proportion of the rapidly relaxing component decreased at the expense of the more slowly relaxing component during prolonged exposures (5). Furthermore, the induction of the rapidly relaxing high-energy-state que...

Adaptation of the Photosynthetic Apparatus in Maize Leaves as a Result of Nitrogen Limitation

“…Nitrogen deficiency resulted in a large increase in the zeaxanthin content of the leaves. Since the formation of zeaxanthin via the xanthophyll cycle has been consistently shown to correlate with the appearance of the nonphotochemical quenching process, qI (4,5) and to influence the conditions necessary for qE formation (21), we are drawn to the conclusion that this change in the xanthophyll composition of the antenna makes a major contribution to the enhanced qNP observed in nitrogen-deficient maize leaves. More importantly, the increased capacity for excitation energy dissipation in PSII displayed by these shade-grown and nitrogen-regulated plants affords protection after transfer to potentially photoinhibitory conditions.…”

“…It has been clearly demonstrated that shade plants grown with low nitrogen are more susceptible to photoinhibition than those that are nitrogen replete (8,12). However, it is important to note that it is difficult to distinguish between light-induced damage to the PSII reaction center and the regulated decreases in PSII efficiency via the processes that facilitate thermal dissipation ofexcitation energy (4,5,21), using measurements of Fv/Fm and quantum yield alone. Recent advances in our understanding of the mechanisms by which the utilisation of excitation energy is controlled suggest that dissipation is a major cause of decrease in PSII quantum efficiency in vivo (9,21 of PSII photochemistry in the reactions centers, e.g.…”

Analysis of Changes in Minimal and Maximal Fluorescence Yields with Irradiance and O2 Level in Tobacco Leaf Tissue

“…Although oversimplifying the mechanism of energy utilization and heterogeneous nature of PSII, the bipartite kinetic model of Butler and Kitajima (5) has provided a useful conceptual basis for interpretation of changes in fluorescence quenching. Regardless of the postulated locus of quenching (antennae complex [9][10][11] versus reaction center [29,30]), the model predicts that when traps are open photochemistry competes effectively with fluorescence and thermal deactivation for excitation. Furthermore, the relative impact ofnonradiative deactivation on fluorescence yield is substantially increased when traps are closed (5,11,16,23).…”

“…In the light the redox states of these carriers will depend on the quantum efficiences of PSII and PSI together with the magnitude of the rate-limiting resistance to electron transfer from plastoquinol to Cyt b/f (27). As irradiance increases QA and plastoquinone become more reduced (8,11,20,25,29,30) while P700 becomes progressively more oxidized (12,14,29). Upon darkening, electrons "trapped" in the intersystem chain will distribute according to the relative stoichiometries and redox potentials of the respective carriers.…”

“…"nonphotochemical" quenching coefficient, qN, increases from 0 to 1) (4,15,25). This is due mainly to increases in nonradiative (thermal) deactivation of absorbed photosynthetically active excitation in the chl antennae complex (9)(10)(11) and/or at the reaction center (29,30). An increase in the transfer of energy from PSII to nonfluorescent PSI at the level of the light harvesting Chl complexes can also result in a lowering of maximal fluorescence yield (1,15).…”