Not all photosynthesis is the same. For Crassulacean acid metabolism (CAM), the stomata in the leaves remain shut during the day. This reduces the loss of water during the day. At night the stomata open to collect CO2. The carbon dioxide is fixed by PEPcase as malic acid. During the day the malic acid breaks down and RuBisCO then photosynthesises the carbon dioxide. This way water use is reduced for CAM plants.
‘CAM plants’ isn’t a category with a fixed set of members. Plants can start using CAM when they exposed to salinity, which gives them higher water use efficiency. Talinum triangulare does not frequently encounter salt in its natural habitat but is cultivated in soils that may become salinized. Montero and colleagues examined whether plants of T. triangulare can grow in saline soils and show salt-induced CAM.
The authors grew adult plants of T. triangulare Jacq. Willd. [syn. Talinum fruticosum (L.) Juss.], in a transparent polythene-roofed shed at the Instituto de Biología Experimental in Caracas. They watered the plants daily. After 2 months, five control plants were watered every other day with tap water and weekly with nutrient solution, and three groups of five plants each watered every other day with 150, 300 and 400 mM NaCl and once a week with these solutions plus fertilizer.
They measured several features of the plants, leaf gas exchange, carbon isotopic ratio (δ13C), nocturnal acid accumulation (ΔH+), water relations, photosynthetic pigment and mineral contents, leaf anatomy and growth.
Montero et al. found T. triangulare tolerated salinity treatment up to 400 mM NaCl, the latter concentration decreasing growth and reproductive effort. Plants under 150 and 300 mM NaCl showed values of ΔH+ significantly higher than the control without nocturnal CO2 fixation, which is evidence of the operation of the CAM in the recycling mode.
They conclude: “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].”