Relationship between efficiency of photosynthetic energy conversion and chlorophyll fluorescence quenching in upland cotton (Gossypium hirsutum L.).

The relationship between components of non-photochemical quenching of chlorophyll fluorescence yield (qNP) and dissipation of excessive excitation energy was determined in cotton leaves using concurrent measurements of fluorescence and gas-exchange at 2% and 20% O2 under a range of photon flux densities and CO2 pressures. A nearly stoichiometric relationship was obtained between dissipation of energy not used in photosynthetic CO2 fixation or photorespiration and qNP provided that a component, probably associated with state transitions, was not included in qNP. Although two distinct components of qNP were resolved on the basis of their relaxation kinetics, both components appear effective in energy dissipation. The photon yield of "open" photosystem-II reaction centers decreased linearly with increases in qNP, indicating that much of the energy dissipation occurs in the pigment bed. However, increases in qNP appear dependent on the redox state of these centers. The results are discussed in relation to current hypotheses of the molecular basis of non-radiative energy dissipation. It is concluded that determinations of qNP can provide a quantitative measure of the dissipation of excessive excitation energy if precautions are taken to ensure that the maximum fluorescence yield is measured under conditions that provide complete closure of the photosystem-II reaction centers. It is also concluded that such dissipation can prevent photoinhibitory damage in cotton leaves even under extreme conditions where as much as 80% of the excitation energy is excessive.