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Facultative Crassulacean Acid in a C3 - C4 Intermediate

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Abstract

  • The Portulacaceae enable the study of the evolutionary relationship between C4 and CAM photosynthesis. Shoots of well-watered plants of the C3-C4 intermediate species, Portulaca cryptopetala Speg., exhibit net uptake of CO2 solely during the light. CO2 fixation is primarily via the C3 pathway as indicated by a strong stimulation of CO2 uptake when shoots were provided with air containing 2 % O2. When plants were subjected to water stress, day-time CO2 uptake was reduced and CAM-type net CO2 uptake in the dark occurred. This was accompanied by nocturnal accumulation of acid in both leaves and stems, also a defining characteristic of CAM. Following rewatering, net CO2 uptake in the dark ceased in shoots as did nocturnal acidification of the leaves and stems. With this unequivocal demonstration of stress-related reversible, i.e. facultative, induction of CAM, P.cryptopetala becomes the first C3-C4 intermediate species reported to exhibit CAM. P. molokiniensis Hobdy, a C4 species, also exhibited CAM only when subjected to water-stress. Facultative CAM has now been demonstrated in all investigated species of Portulaca, which are well sampled from across the phylogeny. This strongly suggests that in Portulaca, a lineage in which species engage predominately in C4 photosynthesis, facultative CAM is ancestral to C4. In a broader context, it has now been demonstrated that CAM can co-exist in leaves that exhibit any of the other types of photosynthesis known in terrestrial plants: C3, C4 and C3-C4 intermediate.

Publication Date

  • 2019

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