Phosphate Deficiency Increases the Rate Constant of Thermal Dissipation of Excitation Energy by Photosystem II in Intact Leaves of Sunflower and Maize.

Sunflower (Helianthus annuus L.) and maize (Zea mays L.) plants were grown in controlled environment chambers either with adequate supply or no external supply of inorganic phosphate. On the third fully-expanded leaves, chlorophyll fluorescence from photosystem II (PSII) was measured using a modulated fluorescence measuring system at various photon flux densities at room temperature. Phosphate deficiency resulted in an increase in the coefficient of non-photochemical quenching and a decrease in the coefficient of photochemical quenching of variable fluorescence. The efficiency of excitation energy capture by open PSII reaction centres and quantum yield of PSII photochemistry were decreased with phosphate deficiency. There was a significant effect of phosphate deficiency on in vivo PSII photochemistry which was independent of changes in thylakoid membrane energisation induced by the actinic light. An increase in the non-photochemical quenching of variable fluorescence with phosphate deficiency was due to an increased rate constant of thermal dissipation of excitation energy by PSII. Analyses of fluorescence signals suggest that phosphate deficiency decreased the rate constant of PSII photochemistry as well as the probability of excitation energy transfer from PSII antenna to PSII reaction centre. These effects were more apparent at low photon flux densities than at high photon flux densities. Regulation of energy transduction in the thylakoid and in vivo PSII activity in response to the physical environment of the plant are important aspects of environmental regulation of photosynthesis.