THE XANTHOPHYLL CYCLE AND ENERGY-DISSIPATION IN DIFFERENTLY ORIENTED FACES OF THE CACTUS OPUNTIA-MACRORHIZA

Diurnal changes in titratable acidity, photosynthesis, energy dissipat ion activity, and the carotenoid composition of differently oriented c ladodes of the cactus Opuntia macrorhiza were characterized during exp osure to full sunlight in the field. Four cladode faces were chosen su ch that each was exposed to maximum photon flux densities (PFD) at dif ferent times of the day in addition to receiving different daily integ rated PFDs. The sum of all carotenoids per chlorophyll was found to in crease with increasing exposure to PFD, with the carotenoids of the xa nthophyll cycle present in the most exposed face at more than twice th e concentration found in the least exposed face. All faces exhibited l arge increases in xanthophyll cycle-dependent energy dissipation as th e sun rose in the morning, even those receiving only minimal levels of diffuse radiation. The transient high levels of energy dissipation in those faces that did not receive direct sunlight in the morning may h ave been due to low temperature inhibition of photosynthesis (predawn low of 2 degrees C). For the two faces receiving peak PFDs in the morn ing hours (north and east faces), the level of energy dissipation acti vity increased rapidly during exposure to direct sunlight in the early morning, gradually declining in the late morning under warm temperatu res, and was negligible during the afternoon low light conditions. Cha nges in the xanthophyll cycle paralleled the changes in energy dissipa tion with the majority of the cycle present as violaxanthin (V) prior to sunrise, largely de-epoxidized to zeaxanthin (Z) and antheraxanthin (A) during exposure to direct sunlight, and reconverted to V during t he afternoon. For the two faces receiving peak PFDs in the afternoon ( south and west faces), energy dissipation activity increased dramatica lly during the early morning low light period, subsequently decreasing during midday as decarboxylation of malic acid proceeded maximally (p roviding a high concentration of CO2 for photosynthesis), and then inc reased to the highest level in the late afternoon as the supply of mal ic acid was depleted and rates of photosynthetic electron transport de clined. The xanthophyll cycle, largely present as Z and A prior to sun rise in the south and west faces, was de-epoxidized to the greatest ex tent in the late afternoon, followed by epoxidation back to the predaw n level by sunset. In all cladode faces high levels of energy dissipat ion activity were accompanied by decreases in the intrinsic efficiency of photosystem II (PSII), indicative of a regulatory process that div erted the excess energy away from the reaction centers during periods of excess light. Furthermore, the overnight retention of Z and A by th e south and west faces was accompanied by a sustained reduction in PSI I efficiency (i.e., ''photoinhibition''). We suggest that this ''photo inhibition'' represents the sustained engagement of nocturnally retain ed Z and A in the photoprotective down-regulation of PSII.