USING CHLOROPHYLL FLUORESCENCE TO ASSESS THE FRACTION OF ABSORBED LIGHT ALLOCATED TO THERMAL DISSIPATION OF EXCESS EXCITATION

In the present study we explored the possibility of assessing the allo cation of photons absorbed by photosystem II (PSII) antennae to therma l energy dissipation and photosynthetic electron transport in leaves o f several plant species under field conditions. Changes in chlorophyll fluorescence parameters were determined in situ over the course of an entire day in the field in sun-exposed leaves of two species with dif ferent maximal rates of photosynthesis, Helianthus annuus (sunflower) and Vinca major. Leaves of Vinca minor (periwinkle) growing in a deepl y shaded location were also monitored. We propose using diurnal change s in the efficiency of open PSII centers (F-v'/F-m') in these sun and shade leaves to (a) assess diurnal changes in the allocation of absorb ed light to photochemistry and thermal energy dissipation and, further more, (b) make an estimate of changes in the rate of thermal energy di ssipation, an analogous expression to the rate of photochemistry. The fraction of light absorbed in PSII antennae that is dissipated thermal ly (D) is proposed to be estimated from D = 1-F-v'/F-m', in analogy to the widely used estimation of the fraction of light absorbed in PSII antennae (P) that is utilized in PSII photochemistry from P = F-v'/F-m ' x q(P) (where q(P) is the coefficient for photochemical quenching; G enty, B., Briantais, J.-M. & Baker, N. R. 1989. Biochim. Biophys. Acta 990: 87-92). The rate of thermal dissipation is consequently given by D x PFD (photon flux density), again in analogy to the rate of photoc hemistry P x PFD, both assuming a matching behavior of photosystems I and II. Characterization of energy dissipation from the efficiency of open PSII centers allows an assessment from a single set of measuremen ts at any time of day; this is particularly useful under field conditi ons where the fully relaxed reference values of variable or maximal fl uorescence needed for the computation of nonphotochemical quenching ma y not be available. The usefulness of the assessment described above i s compared with other currently used parameters to quantify nonphotoch emical and photochemical chlorophyll fluorescence quenching.