USE OF COMBINED FLUORESCENCE AND GAS-EXCHANGE MEASUREMENTS TO ASSESS PROCESSES LIMITING PHOTOSYNTHESIS UNDER STRESS

A 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 was used to study the effects of water deficits on photosynthetic limitations. The system was used in a laboratory study to investigate the effect of water stress on photosynthesis in apple, while a range of complementary gas-exchange and fluorescence techniques were applied in an intensive collaborative field study of the effect of water deficits on the photosynthetic physiology of sorghum growing near Rome in Italy. Equipment that was compared in the field included - for chlorophyll fluorescence: Hansatech modulated fluorescence system, Walz modulated fluorimeter. Plant Stress Meter (Biomonitor); and for gas-exchange: Analytical Development Corporation LCA3, LiCor 6200, and LiCor 1600 porometer. The results of the field study during july 1989 showed that gas-exchange was more sensitive to water stress than were any of the chlorophyll-fluorescence parameters. The most marked effect of water stress was to potentiate stomatal closure in response to a "light shock" caused by briefly shading the leaves. The steady-state reduction in stomatal aperture was less, being of the order of 50% under the most severe water deficits encountered. The main effects of water deficit on chlorophyll fluorescence were to cause significant Fo quenching, and there was an indication that it caused a reduction in the absolute levels of fluorescence. In contrast, FvFm ratios were little affected by drought, though there were large variations in response to diurnal changes in irradiance. © 1990 Taylor and Francis Group, LLC.