¹M. U. Shirazi, ^2*M. Y. Ashraf, ¹M. A. Khan and ¹M. H. Naqvi

¹Nuclear Institute of Agriculture, Tando Jam, Sindh, Pakistan
²Stress Physiology Liboratory, Nuclear Institute for Agriculture and Biology (NIAB), P. O. Box 128, Jhang Road, Faisalabad, Pakistan
*Corresponding Author, E-mail: niabmyashraf@yahoo.co.uk

Received 7 May 2005; revised 10 June 2005; accepted 17 July 2005; onlined 30 September 2005

Code Number: st05032

Abstract

Water culture experiments were conducted to study the response of ten wheat genotypes to external K application (10 mmol KCI dm^-3) at seedling stage under saline condition (0 and 100 mmol NaCl dm^-3). The data showed that there was an increase in the shoot and root length with the application of external K. The increase was more pronounced under control than under saline conditions. The better performing genotypes under two treatments were Bhitai, NIAB-41, NIAB-I076 and Khirman. The enhanced growth of these genotypes under saline condition might be due to the quick response to external K application, resulting in high K/Na ratio. The results indicated that the genotypes, which have the ability of enhanced K/Na discrimination, might perform better under saline conditions when sufficient potassium is applied in the rooting medium.

Key words: Potassium, salt tolerance, wheat, K⁺/Na⁺ ratio

Introduction

Wheat is commonly classified as a moderately salt tolerant crop (Mass and Hoffman, 1977). The threshold value for wheat is about 7 dS/m corresponding to 4480 mg/l. Salt tolerance in wheat is mostly related to its enhanced ability to discriminate between K and Na during transport of these ions to the shoot (Gorham, 1990). It has been reported that in wheat (hexaploid), the 4D chromosome, derived from the wild grass Aegilops squarrosa is responsible for salt tolerance and K⁺/ Na⁺ discrimination character (Shah et al., 1987). Potassium contributes more than Na⁺, Cl^_ and glycinebetaine in osmotic adjustment under saline conditions (Ashraf and Sarwar, 2002).

Addition of external K⁺ alone or in combination with P and Ca to saline medium significantly increases K⁺/Na⁺ ratio in the leaf, stem and root (Epstein, 1966). This increased uptake of K could result a direct competition between K⁺ and Na⁺ at sites of uptake in plasmalemma (Epstein, 1966), an effect of K⁺ on Na⁺ transport into xylem and/or a K⁺ induces net extrusion from roots (Jeschke and Nassery, 1981 and Munns et al., 1983). However, there exists some variation within the species/ genotypes for Na⁺ accumulation and for discrimination in favour of K⁺ transport to shoot (Gorham, 1990). Therefore, a study was planned to investigate the response of some local advanced wheat genotypes to K⁺ application and to explore a suitable K⁺ dose for enhancing salt tolerance of wheat.

Salt tolerance in the Triticeae is associated with enhanced ability to discriminate between Na⁺ and K⁺ in the soil solution and to preferentially accumulate K⁺ and exclude Na⁺ (Omielan, et aI., 1991 and Ali, et al., 2004). In the present study, almost all the wheat genotypes responded varyingly to external potassium application. Wheat genotypes Bhitai, NIAB-I076, NIAB-41 and Khirman showed better performance in terms of shoot and roots growth.

Better performance of these genotypes might be due to quick response to K⁺ application, especially in case of NIAB-41, NIAB-I076 and Khirman which were showing high Na⁺ at 100 mM NaCl, but due to the application of external potassium its concentrations decreased significantly. The lower concentrations of Na⁺ by these genotypes might be due to the competition between Na⁺ and K⁺ at the uptake sites of plasmalemma, resulting in high K⁺/Na⁺ ratios i.e. 10.06, 8.34, 6.26 and 5.59 in Bhitai, NIAB-1O76, NIAB-41 and Khirman, respectively. Gorham et al, (1987), Sarwar, et al. (2003) reported that in wheat genetic variation in salt tolerance is associated with low rates of salt transport to shoot [especially low rates of Na⁺ transport and high selectivity for K⁺ over Na⁺. On the other hand, the genotype NIAB886, RWM-9313, HT-37 and Sarsabz although have low Na accumulation under salinity (l00 mmol NaCl dm^-3), responded poorly to external K application. This indicates their less discriminating ability between Na⁺ and K⁺. The K⁺/Na⁺ ratio in these genotypes ranges from 5.34 to 4.16. The genotype HT-29 has also showed high K⁺/Na⁺ ratio, but was showing low growth performance, which indicates that the mechanism responsible for enhanced K⁺/Na⁺ discrimination is not activated even at high K⁺/Na⁺ ratio (7.28). It has been reported that the K⁺/Na⁺ discrimination character enhances a process that is already operating in cereal roots and to a greater or lesser extent in the plant roots of all plants (Ashraf and Sarwar, 2002, Gorham et al., 1985 and Jeschke, 1984). Physiologically it is a quantitative rather than a qualitative phenomenon, although weather it affects an existing transport process or superimposes a new mechanism is yet to be established (Gorham, 1990; Kamal et al., 2003).