Ika. Galyuon et al., THE EFFECT OF IRRADIANCE LEVEL ON COCOA (THEOBROMA-CACAO L) .2. GAS-EXCHANGE AND CHLOROPHYLL FLUORESCENCE, Tropical agriculture, 73(1), 1996, pp. 29-33
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 re
duced 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 reduct
ion in the photochemical efficiency of photosystem II (PSII) and the r
ate of electron transport under full sunlight. Net photosynthesis (P-n
) was increased in 100% sunlight even though the half time for fluores
cence rise and the variable:maximum fluorescence ratio were reduced. S
tomatal conductance and transpiration rate also were greater in plants
grown in full sunlight. There was variability in P-n between cultivar
s, and the results suggest that differences in leaf morphology may inf
luence gas-exchange rates under high irradiance.