Use of simultaneous analysis of gas-exchange and chlorophyll fluorescence quenching for analysing the effects of water stress on photosynthesis in apple leaves

A convenient system for the rapid simultaneous measurement of both chlorophyll fluorescence quenching using a modulated light system, and of CO2, and water vapour exchange by leaves is described. The system was used in a study of the effects of water deficits on the photosynthesis by apple leaves (Malus x domestica Borkh.). Apple leaves were found to have low values of steady-state variable fluorescence, and the existence of significant fluorescence with open traps (Fo) quenching necessitated the measurement and use of a corrected Fo in the calculation of quenching components. Long-term water stress had a marked effect on both gas-exchange and chlorophyll fluorescence quenching. Non-photochemical quenching (qn) in particular was increased in water-stressed leaves, and it was particularly sensitive to incident radiation in such leaves. In contrast, rapid dehydration only affected gas exchange. Relaxation of qn quenching in the dark was slow, taking approximately 10 min for a 50% recovery, in well-watered and in draughted plants, and whether or not the plants had been exposed to high light.