Photochemistry and xanthophyll cycle-dependent energy dissipation in differently oriented cladodes of Opuntia stricta during the winter

The photosynthetic and energy dissipation responses of four differently oriented photosynthetic surfaces (cladodes) from the cactus Opuntia stricta (Haw,) Haw, were studied in the field during the winter in Australia. Even under very low PFD (i.e. <80 mu mol m(-2) s(-1)) all surfaces experienced a dramatic decline in photosystem II (PSII) efficiency during the morning period when temperatures were below freezing. However, light energy absorbed during the warmer afternoon period was more efficiently utilised for photochemistry with less diversion through the thermal energy dissipation pathway. Low temperature presumably reduced the proportion of excitation energy that could be utilised photosynthetically, resulting in a high rate of energy dissipation with a concomitant decline in PSII efficiency. A lag in the diurnal de-acidification of malic acid, and therefore the availability of endogenous CO(2), may have also contributed to the low rate of photochemistry during the morning period. We interpret the increase in energy dissipation and decline in PSII efficiency as a controlled response of PSII that is dependent upon the de-epoxidised components of the xanthophyll cycle under conditions when the absorption of light exceeds the capacity of the photosynthetic apparatus to process the excitation energy through photochemistry.