INTRODUCTION

A considerable portion of irrigated land resources in arid and semi-arid regions of the world including Pakistan where, a large area has been rendered agriculturally unproductive due to high concentration of salts in the root zone. According to Khan (1998), 6.67 million hectares (mha) area of Pakistan is salt-affected. Wheat is the source of almost 20% of total necessary calories for the world’s population. It is staple food of 43 countries including Pakistan, where it is grown on an area of 8.2 mha with total production of 16.6x 10⁶ Mg and an average yield of 2048 kg ha^-1 (Anonymous, 1995).

Salt-affected soils can be managed by reclamation, but due to less availability of good quality water, low soil permeability and high cost of management, this approach is not feasible on a large scale (Qureshi et al., 1990). Saline agriculture technology is an alternative approach for effective utilization of salt-affected soils, which involves the cultivation of salt-tolerant species/crop. This technology gives economic returns from salt-affected soils and provide vegetative cover to soil which reduces evaporation and hence the rate of salinization (Qureshi and Barret-Lennard, 1998). Study of response of plants/crops to salinity under natural saline conditions is not feasible due to extreme variability in soil salinity both spatially and temporally (Richards, 1983). To avoid this problem, comparative differences for salt-tolerance among crops/varieties can be study under artificially salinized controlled conditions.

Different physiological traits such as selectivity for potassium, exclusion and/or compartmentation of Na⁺ and Cl¯ ions, osmotic adjustment by accumulation of organic solutes have also been related to salt-tolerance of crop plants (Wyn Jones and Storey, 1981). In this study an attempt has been made to develop selection criteria for salt-tolerance based on different growth characteristics and comparative performance of different wheat cultivars in artificial sodium chloride salinity in solution culture.

Material and Methods

The screening experiment was conducted in Greenhouse of the Department of Soil Science and Agricultural Chemistry, Zhejiang Agricultural University Hangzhou, P. R. China, during September and October 1990 with natural day light, day/night temperature of 28/15 ⁰C. Seeds of seven cultivars (Yangmai No.5, Bao-119, Feng 86-4, Bao-22, cv.908, Bao-16 and cv.86-6) supplied by the Department of Soil Sciences and Agricultural Chemistry, Zejiang Agricultural University Hangzhou, P. R. China and seven (cv., 85276-2, cv. 86038, cv. 86299, cv. 86369, cv. 87094, FSD-85 and Pak-81) by the Ayub Agricultural Research Institute, Faisalabad, Pakistan were taken in a piece of muslin cloth separately and soaked in 0.2% fungicide solution for 18 hours, washed thrice with tap water and finally with distilled water. Then seeds were placed on filter paper in iron trays. The condition in trays kept moist with distilled water and trays remained covered until the sprout came out and waited for six days until the seedlings were transferred.

Six days old 20 seedlings of each cultivar were transferred on nylon mesh frames floating over half strength Hoagland nutrient solution in plastic containers having 50 liters capacity. Ten days old seedlings were subjected to incremental salt stress. Salt concentration was increased by 25mol m^-3 after every 12 hours by adding NaCl to nutrient solution until the required salinity levels (75, 150, 225mol m^-3) were obtained in the respective containers. Solutions were renewed after every 7 days. The pH of 6.0-6.5 was maintained daily and any loss of water was made regularly. Solutions were aerated for 9 hours every day with air pump by splitting into 3 equal parts and intervals. The plants were harvested 17 days after salinization. The plants were washed for five minutes in running tap water followed by a quick rinse in distilled water and plant were blotted with tissue paper. The fresh weights of shoot and root, plant height and root length, number of total and green leaves of 20 plants together were recorded and results presented on the basis of average of 20 plants. The dry weight was recorded after plant dried at 70 ⁰C. Then dried plant tissue were ground in mortar and pestle and stored in polyethylene bags and redried at the time of weighing for analysis. The ground plant materials were digested with 1N HCl for 24 hours at 40 ⁰C, then the volume was shaked for one and half-hours and filtered. In the digest, sodium and potassium were determined by “ICP”(ICP model, Jarrel-Ash, ICAP-9000). Means and correlation among various growth characteristics was subjected according to Steel and Torrie (1980). The relative shoot dry yield obtained at high salinity level has tentively been considered to be the standard (most rational) criteria of relative salt tolerance of wheat cultivars while the results based on other growth characteristics were evaluated against it.

Results

Seedling growth data and ionic composition on the basis of average of 14 wheat cultivars are presented in Table 1, correlation between shoot yield and other growth characteristics in Table 2, where as comparative salt tolerance of all the cultivars tested based on different growth characteristics at the highest salinity level in Table 3. It is obvious in Table 1 that, with the gradual increase of salinity, shoot and root yield/ length and potassium contents decreased whereas sodium contents increased. However, in case of root length some cultivars showed little increase at highest salinity level.

Table 1:	Effect of sodium chloride salinity on different growth characteristics of different wheat cultivars. (Average of 14 cultivars)
Figures in parenthesis are percentage of their respective control.

Table 2:	Relationship between shoot yield and other growth characteristics of wheat cultivars grown at 225 mol m-3 sodium chloride salinity.
* Significant ** Highly significant

Table 3:	Comparative salt tolerance of wheat cultivars based on different growth characteristics at 225-mol m-3 sodium chloride salinity.

The comparative salt tolerance of cultivars in Table 3 indicates that cv. 86369 and cv. 86-6 produced maximum and minimum relative fresh shoot yield where as cv. 908 and cv. 86-6 gave maximum and minimum relative dry shoot yield respectively. In case of root relative fresh and dry yield, cv.908 and cv. Bao-119 produced highest yield respectively, where as cv. Yangmai- 5 and cv.86-6 gave lowest relative fresh and dry yield, respectively. As regard shoot and root length, cv. 86299 and cv.908 produced maximum and minimum relative shoot length where as cv. Bao-119 and cv.87094 gave highest and lowest relative root length, respectively. Although salinity reduced the shoot and root length, but some cultivars showed little increase in root length at the highest salinity level as compared to lower salinity levels. From total number of leaves point of view, it was found that cv. 85276-2 and cv.86369 gave maximum and minimum relative leaves respectively, where as maximum and minimum green leaves were found in Feng 86-4 and Pak-81 respectively.

As regard sodium contents, cv.86-6 and Bao-22 accumulated maximum and minimum contents in their shoots, where as cv.86038 and cv.86369 in their roots respectively. From potassium contents point of view, Yangmai-5 and cv. 86038 accumulated maximum and minimum in their shoots where as cv. 87094 and cv. 86369 in their root respectively. In general all the cultivars accumulated different amount of sodium and potassium contents in their shoots and roots at all the salinity levels. There is no any apparent relationship between sodium and potassium contents and salt tolerance of cultivars tested was observed. Due to difference in growth habit of different cultivars, it was concluded that assessment of tolerance to salinity based on growth characteristics can not considered worthwhile. Relationship between shoot yield and other growth parameters in Table 2 indicated, that highly significant correlation exist among, absolute shoot yield and absolute plant height, relative shoot yield and relative plant height, absolute and relative shoot root yield, absolute and relative plant height and absolute and relative green leaves.