Abstract
Whilst high Na is known to be detrimental to plant growth and K nutrition, low to moderate Na can be beneficial to plant growth especially when soil available K is deficient, since Na can substitute for specific biophysical functions of K. Potassium deficiency are common in agricultural soils but the role of Na in K nutrition of wheat genotypes is not well understood. We hypothesised that K-efficient cultivars would be less affected by excess Na than K-inefficient cultivars under high saline conditions, and less responsive to Na substitution under low to moderate Na and low K supply. In a pot experiment, Triticum aestivum L. cvv Wyalkatchem and Cranbrook (K-efficient cultivars), and cvv Gutha and Gamenya (K-inefficient cultivars) were grown in soil containing 40 or 100 mg K/kg in combination with nil, 25, 50, 100 or 200 mg Na/kg for 8 weeks. High soil Na levels (100 and 200 mg Na/kg) greatly reduced plant growth in all four cultivars especially at low soil K (40 mg K/kg). Low to moderate soil Na levels (25 and 50 mg/kg) stimulated wheat growth when soil K was deficient, particularly with a significant increase in root growth in K-efficient cultivars compared with K-inefficient cultivars. The beneficial effect on photosynthesis and stomatal conductance of wheat with the addition of low soil Na levels at K deficiency was similar to that under K adequate supply. Further research is under way to determine whether moderate Na levels stimulated growth by promoting K uptake or by substituting for biophysical functions of K in wheat.