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Sodium and cultivar effects on potassium nutrition of wheat

Krishnasamy, Karthika (2015) Sodium and cultivar effects on potassium nutrition of wheat. PhD thesis, Murdoch University.

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In arid and semiarid regions, soil salinity is largely due to excessive sodium chloride (NaCl) which, apart from osmotic and specific Na+ and Cl- ion effects, has a detrimental effect on potassium (K) uptake and nutrition of most crops. However, in K deficient soils, Na+ can substitute for some functions of K+, provided that plants have the ability to take up, translocate, and compartmentalise Na+ into the vacuoles where it mainly replaces the biophysical functions of K+ in maintaining cell turgor, ionic balance, regulating osmotic potential and improving water balance via stomatal conductance. Potassium deficiency and soil salinity stress have become increasingly common in agricultural lands of Western Australia (WA) and many parts of the world, but the role of Na in K nutrition of wheat (Triticum aestivum L.) is not well understood. The interaction between K and Na in wheat genotypes differing in K-use efficiency has not been researched previously. This research focussed mainly on low to moderate concentrations of Na in wheat K nutrition and less emphasis is placed on Na toxicity effects as there is a large body of research available on Na toxicity effects. A series of glasshouse experiments were designed for both soil and solution culture where Na was supplied at a range of concentrations at low and adequate K levels. The responses of K-efficient and K-inefficient Australian wheat cultivars were examined. Plant responses were assessed by measuring plant growth, leaf gas exchange, shoot and root K and Na concentrations and their contents. High soil Na levels (100 and 200 mg Na/kg) greatly reduced the plant growth in wheat cultivars predominantly at low soil K (40 mg K/kg). By contrast, low to moderate Na levels (25 and 50 mg Na/kg in soil culture and 2 mM Na in solution culture) stimulated wheat growth at low K supply, particularly in K-efficient cultivars compared with K-inefficient cultivars. Roots were more responsive to low concentrations of Na than shoots in experiments where growth stimulation was observed. Low to moderate Na supply also increased leaf net photosynthesis and stomatal conductance at low K supply, with the measured values similar to those observed under adequate K condition both in soil and solution culture. In the split-root experiment, the positive effects of moderate soil Na on growth and K uptake of low K plants were evident when K and Na were supplied in the same or different parts of the root system. In low K soil, low to moderate Na levels increased plant K content, particularly shoot K content, which may account for the increased leaf net photosynthesis rate, stomatal conductance, and plant dry weight. In contrast to previous reports, which attributed Na stimulation of plant growth at low K to increased Na+ uptake and utilisation in place of K+, in wheat, Na+ increased K+ uptake in soil culture, and it increased Rb+ uptake (as a tracer of K+) in solution culture experiments. Hence we attribute most of the benefits of low to moderate Na application in wheat to improved K uptake and K nutrition. The main mechanism for Na+-stimulated K+ uptake under limited K availability with low external Na supply in wheat is likely the effect of Na+ on K+ transporters, both on high-affinity and low-affinity K+ uptake transport systems. In this study, K-use efficiency among wheat cultivars showed varied responses to Na supply at low K, with increased stimulation in root growth, shoot K concentrations, K uptake and leaf photosynthesis in K-efficient cultivar relative to K-inefficient cultivar. Genotypic differences in K-use efficiency also influenced Na uptake and salt tolerance with K-efficient cultivars being more salt tolerant than K-inefficient cultivars. The current research on K+ substitution by Na+ in wheat physiological processes is of great importance in fertiliser management strategies. The application of expensive K fertilisers is limited by poor farmers especially in developing countries, and partial substitution of K by Na in plant nutrition can decrease the cost of production. Based on this study, when K-efficient wheat cultivars are grown under low to moderately saline conditions, the substitution of K by Na was not strong enough to recommend Na-based fertilisers in place of K in wheat. Nevertheless, the alleviation of K deficiency symptoms in wheat by addition of moderate Na provides a trigger for conducting further studies. The present research based on glasshouse experiments needs to be evaluated under field conditions with further studies under varying soil and agro-climatic conditions to define critical soil levels of Na that stimulate wheat growth.

Publication Type: Thesis (PhD)
Murdoch Affiliation: School of Veterinary and Life Sciences
Supervisor: Bell, Richard and Ma, Qifu
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