Abstract

Plant carbon assimilation via photosynthesis can be estimated as the minimum rate allowed by three factors: (i) CO2 concentration in the carboxylation sites, (ii) the regeneration of Rub-BP in the Calvin cycle which depends on the reducing power and ATP derived from light reactions and (iii) the triose-phosphate use efficiency driven by sink demand for carbohydrates. In the case of cactus pears (Opuntia ficus-indica: Ofi), a CAM plant, CO2 absorption and light reactions are chronologically separated: the former occurring during the night, when stomata open and, the latter, occurring during the day, when light is available. This chronological pattern confers CAM plants a high water use efficiency because stomatal opening happens under the low atmospheric water demand conditions of the night when relative humidity is higher and temperature is lower than during daytime. During the night, CO2 is stored in the vacuoles in the form of malate. While CAM plants can sense the amount of malate stored during the night, they cannot predict how much light they are going to intercept during the following day. We measured predawn and dusk titratable acidity along with carbon exchange (IRGA: LiCor 8100) in growing Ofi cladodes under full light and 50% shading during three subsequent days; half of the plants were switched to the other treatment during the third day. We also estimated potential photosynthetic light use by performing light response curves of electron transport rate (chlorophyll fluorometer: Hansatech FMS2). Our results showed that carbon assimilation was tightly associated to dusk carbon availability (i.e. malate content) of the preceding night and not to light availability, even in plants switched from light to shade. This result indicates that Ofi photosynthesis is primarily adapted to optimize water use rather than light use which matches the arid climatic conditions it evolved in: low water and high light availability. This result indicates that, if Ofi is grown under irrigation, yield could be enhance by management practices and breeding strategies which increase nighttime CO2 absorption.