Synthesis of organic osmolytes and salt tolerance mechanisms in Paspalum vaginatum

Synthesis of organic compounds in response to salinity stress and their contribution to organic osmotic adjustment were investigated in seashore paspalum (Paspalum vaginatum Swartz). Nine genotypes exhibiting the widest range of salt tolerance were grown in sea-salt amended nutrient solution in a greenhouse. Salinity ranges were 1.1 (EC_w0, control) to 49.7 dS m^?1 (EC_w50) based on electrical conductivity of the solution (EC_w). Organic osmolytes most important within seashore paspalum under salinity stress were proline, Gly-betaine, and trigonelline in terms of explaining intraspecific salt tolerance differences and, therefore, should be the focus of biotechnology approaches to enhance these traits. While these osmolytes differed in accumulation with increasing salinity and absolute concentrations among salt tolerant and intolerant genotypes, the magnitude of responses was not sufficiently large to suggest use for salt screening as physiological/biochemical markers. Fructose concentration increased with salinity, especially for salt sensitive ecotypes, and may have potential as a marker. Glucose, sucrose, and myo-inositol tended to increase with salinity, but changes did not relate to intraspecific salt tolerance, while mannitol and sorbitol were not affected by salinity. Proline demonstrated a 20.8-fold increase averaged across genotypes from EC_w0 to EC_w50 salinity. Proline was the primary organic osmolyte for osmotic adjustment accounting for an average of 9.3% to total solute potential (Ψ_s) at EC_w50 and 56% of the organic solute contribution to Ψ_s. In the salt tolerant genotype, SI 93-2, proline and Gly-betaine exhibited greater absolute concentration and accumulation rate relative to the least salt tolerant, Adalayd. The intraspecific role of Gly-betaine did not relate to osmotic adjustment differences, suggesting another role perhaps in protection of the thylakoid membrane.