Abstract
Supplemental calcium (Ca2+) is used in hydroponic studies on salinity to lessen the potential for Ca2+ deficiency. However, the Ca2+ concentration and the sodium (Na+): Ca2+ ratio used vary considerably. The implications of using a wide range of Na+: Ca2+ ratios for studies of salinity tolerance in wheat are not known. Also, despite the risk of development of Ca2+ deficiency under salinity stress, there are few reliable reports on the critical level of Ca2+ which can be used to diagnose Ca2+ deficiency in wheat. Two experiments were conducted to examine Ca2+ requirements of wheat under saline and non-saline conditions and to derive a critical level for Ca2+. Four bread wheat genotypes (Triticum aestivum L.) and a durum wheat genotype [Triticum turgidum subsp. durum) (Desf.) Husn.] with known differences in salinity tolerance were grown at 100 mM NaCl for four weeks with varying levels of external Ca2+ which resulted in Na+:Ca2+ ratios of 30, 20, 15, 5 and 2. The critical Ca2+ concentration was defined in a second experiment by growing the same wheat genotypes at seven levels of Ca2+ (0.05, 0.1, 0.2, 0.5, 1, 2 and 10 mM) under non-saline conditions. When grown at 100 mM NaCl salinity tolerance was greatest when the Na+:Ca2+ ratio ranged from 5 to 15. Growing plants at lower or higher Na+:Ca2+ ratios induced nutrient imbalances and additional osmotic stress which reduced the growth of plants. Transient Ca2+ deficiency occurred at high Na+:Ca2+ ratios and low Mg2+ occurred at the lowest Na+:Ca2+ ratio. Adding NaCl raised the tissue Na+ concentration and reduced the Ca2+ concentration and the most appropriate Na+:Ca2+ ratio in the solution was that which resulted in tissue Ca2+ concentrations similar to those of non-salinised plants. The critical level of Ca2+ in the youngest fully emerged leaf blades was 15-23 mmol kg(-1) DW (600-900 mg kg(-1) DW).