Estimation of salinity tolerance threshold of licorice (Glycyrrhiza glabra L)

 Salinity is one of the most basic abiotic stresses in arid and semi-arid regions, so it limits plant vegetative and reproductive growth. Soil salinity spreads due to irrigation with salty water, improper drainage, and salt accumulation in desert and semi-desert lands. Licorice (Glycyrrhiza glabra L.; legume family) is one of the most valuable halophyte plants used worldwide as a medicinal-industrial plant. Considering that in the existing studies, the threshold of tolerance to licorice salinity has not been accurately reported, in the present study, the aim was to determine the threshold and investigate some quantitative characteristics of licorice in response to different salinity levels. 

To investigate the salinity tolerance threshold of licorice plants to different levels of salinity stress, an experiment was conducted in the form of a completely randomized design with three replications in the pot environment in the greenhouse of the National Salinity Research Center located in Yazd province. Salinity treatments for irrigation water included 0.6, 3, 6, 9, 12, 15, and 18 dS/m. At planting, all pots were irrigated with tap water until establishment and greening. After four days, salinity levels were increased to reach the desired salinity. In this study, various plant characteristics such as plant height, leaf area, aerial and root parts dry matter, relative water content, ion leakage, and potassium and sodium content were measured. Additionally, salinity tolerance thresholds and a 50% reduction in yield were calculated using the Mass-Hoffman model. Data variance analysis and determination of threshold values and salinity tolerance coefficients were performed using SAS Ver 9.2 statistical software. 

The analysis of variance indicated that all the studied traits in Glycyrrhiza glabra (licorice) were significantly affected by salinity. The increase in water salinity from the control treatment to 18 dS/m resulted in a 90% reduction in plant height, 88% in leaf area, 94% in aerial part dry weight, 73% in root dry weight, 22% in relative water content, 0.42% decrease in potassium content, and an increase of 13% and 2.63%, in ion leakage and sodium content in the plant respectively. The plant salinity tolerance threshold indicated that the plant salinity threshold was calculated based on irrigation water salinity. In addition, it was obtained for root dry matter equal to 2.89 ds m-1. The slope of root dry matter yield reduction per unit of salinity increase in terms of ds m-1 was obtained as 5.50%. Also, the salinity of the irrigation water, which caused a 50% reduction in yield and zero yield, was calculated as 10.98 and 21.06 ds m-1, respectively. Also, the salt tolerance index (ST-index) was estimated at 11.40 ds m-1.

Considering that the salinity level of irrigation water, which caused yield reduction to zero was equal to 21.06 ds m-1, this plant is classified as relatively resistant to salinity stress.