Productivity, photosynthetic performance and nutritional values of two halophyte vegetable crops, mesembryanthemum crystallinum and portulaca oleracea under saline conditions

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Food security maintenance, in terms of quantity and quality, has always been a challenge for Singapore where land and water are limited. In land-scarce Singapore, vertical farming is growing to strengthen food supply. However, being one of the water-stressed nations meant that food production can be threatened anytime. Growing halophytes using saline water would be a feasible solution to the vulnerable water supply in Singapore. In this study, two nutritious halophytes, Mesembryanthemum crystallinum and Portulaca oleracea, were grown under increasing saline conditions at a PPFD of 200 μmol m-2 s-1 (12 h). Productivity and leaf growth of both species grown in nutrient solution with 100 mM NaCl were elevated than without NaCl, indicating that both halophytes require some salt to perform optimal growth. However, productivity decreased with increasing NaCl concentration >100 mM in both species. Grown with 500 mM NaCl, M. crystallinum appeared healthy albeit smaller while P. oleracea, was only able to survive at 300 mM NaCl, indicating that P. oleracea is more sensitive to salt stress. Those high NaCl salinities negatively affected root morphology. In both species grown with high salinities, the lower concentrations of nitrate, dietary minerals (K, Ca and Mg) could be attributed to the stunted roots which limited water and mineral uptake. However, the correlation between root morphology and Fe uptake was not clear in P. oleracea. Lower specific leaf area of both species grown with high NaCl concentrations were due to their high dry matter content, lower water content and lower leaf succulence. Grown with 300 and 500 mM NaCl, P. oleracea and M. crystallinum respectively had the highest concentrations of photosynthetic pigments and Chlorophyll a/b ratios than with lower NaCl concentrations. All plants had Fv/Fm ratios ~0.8, implying that salinity did not damage PS II in any plants. However, electron transport rate and photochemical quenching were negatively affected by high NaCl salinity in both species. CAM was induced from C3 and C4 photosynthesis in M. crystallinum and P. oleracea respectively, grown with high NaCl salinity coupled with higher non-photochemical quenching. Reduced Cytb6f concentration was found in P. oleracea grown with 300 mM NaCl. All P. oleracea had comparable PS II concentrations. However, salinity did not affect Cytb6f concentration for M. crystallinum. CO2 assimilation rate (A), stomatal conductance (gs) and transpiration rate (Tr) reduced as salinity in M. crystallinum increased. For both species, although high salinity reduced leaf NO3− concentration, all plants had similar amounts of Rubisco protein as nitrogen deficiency did not occur in any plants. Low absorption of dietary minerals may result from the excessive accumulation of Na in both species. For M. crystallinum, high salinity enhanced proline, total soluble sugar, total ascorbic acid and total phenolic compounds. However, only proline content was increased in P. oleracea grown with high salinity. In conclusion, it is feasible to grow both halophyte species nutrient solution with 100 mM NaCl to obtain greater productivity and better nutritional values. However, accumulation of phytochemicals with increased salinity may depend on not only salinity but also species.