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 differentl y oriented photosynthetic surfaces (cladodes) from the cactus Opuntia stricta (Haw,) Haw, were studied in the field during the winter in Aus tralia. Even under very low PFD (i.e. <80 mu mol m(-2) s(-1)) all surf aces experienced a dramatic decline in photosystem II (PSII) efficienc y during the morning period when temperatures were below freezing. How ever, light energy absorbed during the warmer afternoon period was mor e efficiently utilised for photochemistry with less diversion through the thermal energy dissipation pathway. Low temperature presumably red uced the proportion of excitation energy that could be utilised photos ynthetically, resulting in a high rate of energy dissipation with a co ncomitant decline in PSII efficiency. A lag in the diurnal de-acidific ation of malic acid, and therefore the availability of endogenous CO2, may have also contributed to the low rate of photochemistry during th e morning period. We interpret the increase in energy dissipation and decline in PSII efficiency as a controlled response of PSII that is de pendent upon the de-epoxidised components of the xanthophyll cycle und er conditions when the absorption of light exceeds the capacity of the photosynthetic apparatus to process the excitation energy through pho tochemistry.