The effect of irradiance level on cocoa (Theobroma cacao L.): II. Gas exchange and chlorophyll fluorescence

Abstract

The effects of full sunlight and 50% sunlight on pigment content, gas exchange, and chlorophyll fluorescence were compared, with the aim of identifying physiological characteristics which can enhance tolerance to high irradiance in cocoa. Chlorophyll and carotenoid levels were reduced in full sunlight, but the mean chlorophyll-carotenoid ratio was smaller in plants grown in full sunlight compared with those grown in 50% sunlight. Chlorophyll fluorescence measurements indicated a reduction in the photochemical efficiency of photosystem II (PSII) and the rate of electron transport under full sunlight. Net photosynthesis (Pn) was increased in 100% sunlight even though the half time for fluorescence rise and the variable: maximum fluorescence ratio were reduced. Stomatal conductance and transpiration rate also were greater in plants grown in full sunlight. There was variability in Pn between cultivars, and the results suggest that differences in leaf morphology may influence gas-exchange rates under high irradiance.