ATPase Activities and Membrane Fine Structure of Rhizodermal Cells from Sorghum and Spartina Roots Grown Under Mild Salt Stress

The study aims at correlations between ultrastructure of rhizodermal membranes and ATPase activities of two gramineous C₄-species effected by salinization of the growth medium. Comparative investigations were done with drought-resistant Sorghum and drought-sensitive, salt tolerant Spartina plants grown under steady-state conditions without or with 40 mol m^?3 NaCl. Both species sustained this treatment and showed marked influences of salt on growth but not on root respiration. Generally, the intramembraneous particle (IMP) frequencies, found on freeze-fracture replicates of rhizodermal plasmic fracture faces of the plasmalemma (P_PF) and tonoplast (T_PF), were higher in Sorghum than in Spartina. NaCl leads in both species to an increase of the rhizodermal IMP frequency in the T_PF (～150%) and P_PF (～120%). The activities of T- and P-ATPase were determined for membrane vesicles from crude extracts and from isolated protoplasts of roots, respectively; the results from both preparations were the same. The vanadate-sensitive tissue ATPase activities increased under salt stress ～ 5 times in Sorghum and ～ 2 times in Spartina whilst the nitrate and azide-sensitive tissue ATPase activities increased ～ 6 times only in Sorghum. The combination of the results from the membrane fine structures with the biochemical tests points out that salt-effected increases of ATPase activities of Sorghum roots were ～4 times higher than the increases of IMP frequencies on rhizodermal membranes; in Spartina this effect is less intense. It is concluded that a salty environment leads to a higher ATPase activity per IMP in both species. The stronger reaction of Sorghum is explainable by the increase in K⁺/Na⁺ selectivity of the plasma membrane under salt stress. This is not the case for Spartina because the roots of this species include salt which subsequently is sequestered by the salt glands of the leaves.