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Differential effects of drought and light levels on accumulation of citric and malic acids during CAM in Clusia
Author(s) -
FRANCO A. C.,
BALL E.,
LÜTTGE U.
Publication year - 1992
Publication title -
plant, cell and environment
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.646
H-Index - 200
eISSN - 1365-3040
pISSN - 0140-7791
DOI - 10.1111/j.1365-3040.1992.tb02149.x
Subject(s) - malic acid , citric acid , crassulacean acid metabolism , citrate synthase , chemistry , malate dehydrogenase , malic enzyme , organic acid , photosynthesis , botany , biochemistry , horticulture , biology , enzyme , dehydrogenase
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 CO 2 uptake from the atmosphere declined in all three species during prolonged drought periods, whereas citrate accumulation remained similar or increased. Recycling of respiratory CO 2 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 CO 2 for the formation of malate is respiratory CO 2 . However, the breakdown of citric acid to pyruvate in the light period releases three molecules of CO 2 , while the breakdown of malic acid releases only one CO 2 per pyruvate formed. Thus, citric acid should be more effective than malic acid as a mechanism to increase CO 2 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.