Salt effects on proline and glycine betaine levels and photosynthetic performance in Melilotus siculus, Tecticornia pergranulata and Thinopyrum ponticum measured in simulated saline conditions

We measured proline and glycine betaine levels and photosynthetic performance (net-photosynthetic rate (P_n), stomatal conductance (g_s), maximum quantum yield of PSII (F_v/F_m) and non-photochemical quenching (NPQ)) in relation to Na⁺ and Cl^– accumulation in Melilotus siculus (Turra) B.D.Jacks. (Fabaceae), Tecticornia pergranulata (J.M.Black) K.A.Sheph. & Paul G.Wilson (Amaranthaceae: Salicornioideae) and Thinopyrum ponticum (Podp.) Z.-W.Liu & R.-C.Wang (Poaceae) grown under saline conditions in the greenhouse. These plants were selected in this study because of their known salt-tolerance capacity and value as forage plants. Moreover, the pasture legume M. siculus is considered to have particular potential for saline land remediation because of its salinity and waterlogging tolerance. Maximum Na⁺ and Cl^– accumulation occurred in Te. pergranulata shoots. Minimum was in Th. ponticum shoots. Maximum Na⁺ accumulation occurred in the roots of Te. pergranulata, whereas that of Cl^– occurred in the roots of Th. ponticum. Accumulation of both Na⁺ and Cl^– was the least in M. siculus roots. Te. pergranulata metabolized high levels of glycine betaine (110 µmol g^–1 DW). M. siculus metabolized high levels of proline (6 µmol g^–1 DW). Th. ponticum accumulated intermediate levels of these organic osmolytes. No significant change occurred in F_v/F_m values. P_n value increased and NPQ value decreased in Te. pergranulata with increasing salinity and the reverse occurred in both M. siculus and Th. ponticum. A negative significant correlation occurred between P_n and glycine betaine in M. siculus and Th. ponticum. A positive significant correlation occurred between NPQ and glycine betaine in M. siculus. No correlation occurred between proline and P_n, proline and NPQ in the tested three plants. Te. pergranulata could maintain cell-osmotic balance by synthesising high levels of organic osmolytes especially glycine betaine and concurrently showing the most efficient photosynthetic performance. Compared with the levels of osmolytes in Te. pergranulata, the levels of osmolytes that occur in M. siculus and Th. ponticum were insufficient to maintain cell-osmotic balance and also that M. siculus and Th. ponticum showed a lower level of photosynthetic performance. We conclude that glycine betaine is potentially the vital organic osmolyte for Te. pergranulata and Th. ponticum enabling salinity stress tolerance. However, in M. siculus, proline appears to be the potential organic osmolyte in salinity stress tolerance. In terms of the potential of these species for stabilising saline soils in central-western New South Wales, Te. pergranulata would be the candidate of choice; however, for greater pasture value Th. ponticum would be the next.