LOG-LOGISTIC MODEL ANALYSIS OF OPTIMAL AND SUPRA-OPTIMAL TEMPERATURE EFFECT ON PHOTOSYSTEM II USING CHLOROPHYLL A FLUORESCENCE IN CHRYSANTHEMUM (DENDRANTHEMA GRANDIFLORA)

Air temperature with modern greenhouse climate control often allows more freedom for dynamic behaviour than standard rigid greenhouse climate regimes. These types of climate control strategies potentially result in severe microclimate conditions such as high crop temperature, which affects photosynthesis and the major sensitive sites in the photosynthetic apparatus of photosystem II (PSII). In two independent experiments i) excised chrysanthemum leaves were treated with optimal and supra-optimal temperatures from 24 to 45°C and ii) intact plants were subjected to high temperature of 32/28, 38/32 and 40/36°C day/night, respectively for ten successive days. The initial fluorescence kinetics (OJIP curve) was used to characterize high temperature effect on PSII on four selected parameters. The log-logistic model of the dose response curve was used to model the maximum quantum efficiency of PSII (F_v/F_m), density of active PSII reaction centers per chlorophyll (RC/ABS), the conformation term for primary photochemistry (F_v/F_o) and performance index (PI), as temperature was the dose function. The model estimated the upper and lower limit and the temperature dose causing 50% reduction in F_v/F_m and F_v/F_o was 41 and 39°C, respectively. The critical temperature limit of thermo-tolerance of PSII in chrysanthemum leaves was estimated to be 38°C. This study suggests that physiological information combined with modelled response curves originating from chlorophyll a fluorescence can be used as early detection of high temperature stress in chrysanthemum, and probably in other ornamental plants in greenhouse production.