Salinity induction of recycling Crassulacean acid metabolism and salt tolerance in plants of Talinum triangulare

BACKGROUND AND AIMS: Crassulacean acid metabolism (CAM) can be induced by salinity, thus conferring the plant higher water-use efficiency. Talinum triangulare does not frequently encounter salt in its natural habitat but is cultivated in soils that may become salinized. Here we examined whether plants of T. triangulare can grow in saline soils and show salt-induced CAM. METHODS: Leaf gas exchange, carbon isotopic ratio (δ(13)C), nocturnal acid accumulation (ΔH(+)), water relations, photosynthetic pigment and mineral contents, leaf anatomy and growth were determined in greenhouse in plants irrigated with 0, 150, 300 and 400 mm NaCl. KEY RESULTS: Salinity reduced gas exchange and induced CAM, ΔH(+) reaching 50.2 μmol H(+) g(−1) fresh mass under 300 mm NaCl. No nocturnal CO(2) uptake, but compensation, was observed. Values of δ(13)C were lowest under 0 and 400 mm NaCl, and highest under 150 and 300 mm. The difference in osmotic potential (ψ(s)) between control and treated plants averaged 0.45 MPa for the three [NaCl] values, the decrease in ψ(s) being accounted for by up to 63 % by Na(+) and K(+). Pigment contents were unaffected by treatment, suggesting lack of damage to the photosynthetic machinery. Changes in stomatal index with unchanged stomatal density in newly expanded leaves suggested inhibited differentiation of epidermal cells into stomata. Whole-leaf and parenchymata thickness increased under 150 and 300 mm NaCl. Only plants irrigated with 400 mm NaCl showed reductions in biomass (stems, 41 %; reproductive structures, 78 %). The K/Na molar ratio decreased with [NaCl] from 2.0 to 0.4. CONCLUSIONS: The operation of CAM in the recycling mode was evidenced by increased ΔH(+) with no nocturnal CO(2) uptake. Talinum triangulare can be classified as a halo-tolerant species based on its low K/Na molar ratio under salinity and the relatively small reduction in growth only at the highest [NaCl].