NaCl effects on root plasma membrane ATPase of salt tolerant wheat

Wheat seedlings of a salt tolerant cultivar were grown hydroponically in pr esence and absence of 100 mM NaCl. Roots were harvested, and the plasma mem brane (PM) fraction was purified. PM ATPase required a divalent cations for activity (Mg > Mn > Ca > Co > Zn > Ni > Cu), and it was further stimulated by monovalent cations (K > Rb > NH4 > Li > choline > Cs). The pH optima we re 6.0 and 5.6 in absence and presence of 25 mM KCI, respectively. The enzy me was sensitive to vanadate and DCCD but insensitive to azide, oligomycine and nitrate. The enzyme displayed a high preference for ATP but was also a ble to hydrolyze other nucleotide tri- and diphosphates. The enzyme activit y showed a simple Michaelis-Menten kinetics for the substrate Mg2+-ATP in b oth control and salt exposed roots. The polypeptide patterns of control and salt stressed PM fractions, detected by SDS-PAGE, were very similar. NaCl substantially reduced the PM ATPase specific activity, whereas it had littl e effect on the apparent K-m for Mg2+-ATP. Since the root PM ATPase of salt sensitive and resistant genotypes responded similarly to salinity stress, it seems unlikely that the mechanism of salt tolerance in wheat is primaril y based on differences in PM ATPase characteristics.