Effect of salt stress on some morphological traits of fenugreek and determination of the salt tolerance threshold at vegetative stage using some experimental models

Iran, the second largest country in the Middle East, has an area of 165 million ha. Approximately, 90% of the country is classified as arid and semi-arid region, most of which is faced with low rainfall, high evapotranspiration, salinization, shortage of fresh water, erosion, excessive heat and desertification. Fresh water resources are declining in the central plateau of the country as a result of overusing underground water and severe drought in recent years. Land salinization is a major limiting factor for conventional crop production in the country. Continuous cropping together with an excessive use of chemical fertilizers and ill-managed irrigation has turned hundreds of cultivated fertile fields into saline ones. These limitations have great impacts on the welfare of the farmers whose income is solely dependent to agriculture. Regarding the increasing trend in the salinity of soil and water resources, cultivation of salt tolerant medicinal plants has been suggested as one of the strategy for utilizing saline soil and water resources. One of the medicinal plants that has a long history of use in traditional medicine and has also many therapeutic properties is fenugreek (Trigonella foenum-graecum L.). This research was conducted to determine the salt tolerance threshold, yield reduction slope and to evaluate effects of utilizing saline water on yield (shoot dry weight) at vegetative stage under greenhouse conditions.

In this experiment, treatments were included seven levels of salinity (0.5, 2, 4, 6, 8, 10 and 12 dS/m) obtained by mixing a saline groundwater resource (with electrical conductivity of 14 dS/m) and a fresh water resources (tap water). A leaching fraction of 30% was considered to wash out some excess salts from soil profile and preventing their accumulation in the root zone. In order to control soil salinity, the amount and electrical conductivity of both irrigation and drainage water was measured in all irrigation practices. Furthermore, athe soil salinity was monitored using a soil salinity bridge instrument. The statistical design was arranged as a complete randomized block design with three replications. In this study, different experimental models were used to determine the salt tolerance threshold, the slope of yield (shoot dry weight) reduction, the amount of salinity at which yield was reduced by 50% (EC50) and the salt tolerance index, as well.

 Results showed that there was a statistically significant difference among different salinity levels. Based on the results, salinity reduced shoot height (27.66%), number of leaves (18.03%), number of branches (5.14%), number of nodes (8.77%), stem diameter (27.04%), internodes length (54.21%), mean of expanded leaves area (46.91%), root to shoot ratio (16.97%), water content (14.62%), water use efficiency (14.70%) and increased leaf thickness (73.55%) and greenness index (47.58%), however, salinity had no significant effect on special leaf area. Although salinity stress had an adverse effect on most studied traits, the trend of this effect was varied depending on the trait. Based on the linear model, the salt tolerance threshold of fenugreek and the slope of yield reduction was estimated 1.28 dS/m and 4.91 percent, respectively. However, according to non-linear models, a reduction of 10 and 25 percent in relative grain yield was occurred at 3.38 and 6.28 dS/m, respectively. Based on the results of this research, the salinity at which the relative yield decreased by 50% percent was observed at soil salinity of 11.67 dS/m.

In this research, the fenugreek salt tolerance index was calculated as 12.24. Therefore, based on both the salinity tolerance threshold, the slope of yield reduction and salinity tolerance index, fenugreek can be classified into the group of moderately sensitive to salinity stress at the vegetative growth stage.