RESPONSES OF 3 ARID ZONE GRASS SPECIES TO VARYING NA/CA RATIOS IN SALINE SAND CULTURE

The effects on three grass species, Cenchrus pennisetiformis Hochst. & Steud., Leptochloa fusca (L.) Kunth. and Panicum turgidum Forssk., of decreasing Ca2+ concentration in a saline growth medium were assessed after 7 weeks growth in sand culture. The different Na/Ca ratios of the salt treatment were 24, 49, 99, and 199, at a constant concentration of 200 mol m-3. Leptochloa fusca produced the highest fresh and dry biomass, and was able to maintain Na+ and Cl- concentrations in the shoots and roots almost constant at varying external Na/Ca ratios, except that the shoot Na+ concentration increased significantly at the highest Na/Ca ratio. In C. pennisetiformis the shoot Na+ decreased, whereas the shoot Cl- concentration increased at the highest external Na/Ca ratio. But the root Na+ and Cl- concentrations in this species remained unchanged at varying Na/Ca ratios. C. pennisetiformis and L. fusca maintained almost constant K+ and Ca2+ concentrations in both shoots and roots at varying Na/Ca ratios. In P. turgidum the shoot K+ and shoot and root Ca2+ remained almost unchanged at all Na/Ca ratios, whereas the root K+ concentration decreased significantly but uniformly at the three higher Na/Ca ratios. No consistent pattern of increase or decrease was observed in the shoot and root Na/K and Na/Ca ratios of all the species. The shoot selectivity (S(K,Na)) increased consistently in C. pennisetiformis with the increase in Na/Ca ratios and it decreased in L. fusca only at the highest Na/Ca ratio in the growth medium. It was established that L. fusca was tolerant, C. penni-setiformis intermediate and P. turgidum relatively sensitive to low Ca2+ concentrations of the saline growth medium.