Differential effects of drought and light levels on accumulation of citric and malic acids during CAM in Clusia

Night-time citrate accumulation has been proposed as a response to stress in CAM plants. To address this hypothesis, gas exchange patterns and nocturnal acid accumulation in three species of Clusia were investigated under controlled conditions with regard to water stress and responses to low and high photosynthetic photon flux density (PPFD). Under high PPFD, leaves of Clusia nocturnally accumulated large amounts of both malic and citric acids. Under low PPFD and well-watered conditions, substantial night-time citrate accumulation persisted, whereas malate accumulation was close to zero. Malate accumulation and night-time CO₂ uptake from the atmosphere declined in all three species during prolonged drought periods, whereas citrate accumulation remained similar or increased. Recycling of respiratory CO₂ was substantial for both well-watered and water-stressed plants. The suggestion that citrate accumulation is energetically more favourable than malate accumulation is not supported if the source of CO₂ for the formation of malate is respiratory CO₂. However, the breakdown of citric acid to pyruvate in the light period releases three molecules of CO₂, while the breakdown of malic acid releases only one CO₂ per pyruvate formed. Thus, citric acid should be more effective than malic acid as a mechanism to increase CO₂ concentration in the mesophyll and may help to prevent photoinhibition. Organic acid accumulation also affected the vacuolar pH, which reached values of 2·6–3·0 at dawn. At these pH values, the transport of 2H⁺/ATP is still feasible, suggesting that it is the divalent form of citrate which is being transported in the vacuoles. Since citrate is a well-known buffer, and Clusia spp. show the largest day-night changes in organic acid levels measured in any CAM plant, it is possible that citrate increases the buffer capacity of the vacuoles. Indeed, malate and titratable acidity levels are positively related to citrate levels. Moreover, Clusia species that show the highest nocturnal accumulation of organic acids are also the ones that show the greatest changes in citric acid levels.