Physiology of salt tolerance in Salicornia bigelovii Torr.

Growth of most crop plants (glycophytes) is reduced in saline environments. A few plant species (euhalophytes) not only tolerate, but require salt, and grow optimally in salinities between 100 and 200 mM NaCl. The halophyte Salicornia bigelovii Torr. shows optimal growth in 200 mM NaCl and reduced growth in low saline conditions. In spite of years of research, mechanisms that confer salt tolerance to some plants and sensitivity to others are poorly understood. This research was undertaken to obtain physiological information in an attempt to determine why S. bigelovii requires salt to reach maximum growth. Salicornia seedlings were grown in the greenhouse in aerated nutrient solutions with 5, 200 or 600 mM NaCl. Plants grown in 200 mM NaCl showed optimal growth. Fresh and dry weight of the plants were reduced when grown in 5 and 600 mM NaCl. The main differences in plants grown in 5 and 600 mM NaCl had to do with ion accumulation. These differences in ion accumulation suggested that salt tolerance in Salicornia was established by regulation of ion transport. This was confirmed by studying two primary transport systems in plants grown in 5 or 200 mM NaCl. These transport systems are the H⁺-ATPases on the plasma membrane (PM-ATPase) and the tonoplast (V-ATPase). Higher PM-ATPase (55%) activities were observed in 200 mM NaCl grown plants. Increases in growth and in PM-ATPase activity in Salicornia shoots after exposure to salinity were highly correlated. V-ATPase activity was significantly stimulated in vivo and in vitro (26 and 46%) after exposure to 200 mM NaCl, and this stimulation was Na⁺-specific. Increased V-ATPase activity was consistent with an increased Na⁺ accumulation (45%) compared to plants grown in 5 mM NaCl. Na⁺-stimulation of ATPases may confer salt tolerance in Salicornia by providing the driving force for regulation of intracellular Na⁺ levels. The ATPases provide an increased H⁺ electrochemical gradient across membranes that may be used by the Na⁺/H⁺ exchangers on the plasma membrane and tonoplast. In addition, H⁺ transport across the plasma membrane leads to acidification of the apoplast that is required for cell wall extension and growth. These transport systems need to work in concert for optimal growth and salt tolerance