Growth response and ion homeostasis in two bermudagrass (Cynodon dactylon) cultivars differing in salinity tolerance under salinity stress

Bermudagrass (Cynodon dactylon) is a salinity-tolerant turfgrass that has good use potential in the saline-alkali lands of warm regions. However, the systematic Nasup(+) and Ksup(+) regulation mechanisms under salinity stress remain unclear at the whole plant level. Two bermudagrass cultivars differing in salinity tolerance were exposed to 0, 50, 100, 200, or 300 mM NaCl in a hydroponic system. Growth, absorption, transportation, and secretion of Nasup(+) and Ksup(+), and gas exchange parameters were determined in both cultivars. Ksup(+) contents were decreased and Nasup(+) contents and Nasup(+)/Ksup(+) ratios were increased in both bermudagrasses with increased salinity; however, lower Nasup(+) content and Nasup(+)/Ksup(+) ratio and more stable Ksup(+) content were found in the leaves of the salinity-tolerant 'Yangjiang' han the salinity-sensitive 'Nanjing'. Higher Nasup(+) contents in root cortical cells were found than in the stele cells of 'Yangjiang', but the opposite was observed in 'Nanjing'. Lower Nasup(+) contents and higher Ksup(+) contents were found in vessels for 'Yangjiang' than for 'Nanjing'. Salinity stress increased the selective transport of Ksup(+) over Nasup(+) from roots to leaves and the Nasup(+)-selective secretion via salt glands, which were stronger in 'Yangjiang' than' Nanjing'. Net photosynthetic rate and stomatal conductance decreased in the two bermudagrasses with increased salinity; however, they were more stable in 'Yangjiang'. The results suggested that bermudagrass could reduce Nasup(+) accumulation and maintain Ksup(+) stability in leaves under salinity stress by restricting Nasup(+) into vessels in roots, selectively transporting Ksup(+) over Nasup(+) from roots to leaves, selectively secreting Nasup(+) via leaf salt glands, and maintaining suitable stomatal conductance.