Productivity and photosynthetic characteristics of the facultative halophyte Mesembryanthemum crystallinum grown indoors with LED lighting under different salinities

The facultative halophyte Mesembryanthemum crystallinum (ice plant) has high nutritional values. In Singapore, we have successfully grown M. crystallinum indoors under different LED-lightings using freshwater. Since halophytes are capable of tolerating a wide range of salinities, it provides a basis for cultivating them as vegetables using saline water. This study aimed to investigate the impacts of different NaCl concentrations on productivity and photosynthesis of M. crystallinum. All plants were grown under different salinities by adding 0, 100, 250 and 500 mM NaCl, respectively, to a full strength Netherlands Standard Composition. Photosynthetic photo flux density at 156 μmol m^‑2 s^‑1 (12 h photoperiod) was provided to all plants by LED with red:blue ratio of 2.2. Twenty days after transplanting, plants grown under 100 mM NaCl had the highest shoot fresh weight and largest total leaf area followed by plants grown at 0, 250 and 500 mM NaCl. Plants grown under 500 mM NaCl had the lowest specific leaf area due to its highest leaf dry matter content and lowest water content. However, all plants had similar values of leaf succulence (g FW cm^‑2). Although all plants had chlorophyll fluorescence F_v/F_m ratios >0.8, photochemical quenching and electron transport rate were significantly higher in plants grown under 0 mM NaCl than under 100, 250 and 500 mM NaCl measured under the growth irradiance. CAM acidity of plants grown at 500 mM NaCl was induced while CAM acidity of plants grown with freshwater was undetectable. In conclusion, M. crystallinum requires some salt (100 mM NaCl) to perform healthy growth compared to those grown with freshwater. Low productivity of M. crystallinum under high salinity could mainly be due to its slow leaf growth resulting from the switch of C₃ to CAM photosynthesis.