Soil salinity in arid and semi-arid regions is one of the most important factors limiting agricultural production Neumann (1995). Salinity levels higher than 2.0 and 2.6 dS m-1 reduce fresh yield and plant growth of lettuce De Pascale and Barbieri (1995).
Lettuce is the most important and useful horticultural crop in the World. Lettuce is considered to be a moderately salt-sensitive crop, with a threshold electrical conductivity of 1.3 dS m Ayers, R.S. and Westcot, D.W. (1985). Lettuce (Lactuca sativa L.) is categorized as being moderately salt tolerant.
1. Effect of salt stress on lettuce plant
1.1. Growth and yield parameters
Salinity levels of more than 2.0 and 2.6 dS m¯¹ reduce lettuce fresh yield and plant growth, respectively (De Pascale and S.; Barbieri, G., 1995). They reported that lettuce has a threshold value of 1.1 dS m and the relative yield decrease in slope about 9.3% after this threshold. The results further showed that the variables of leaf number per plant, plant height and water use efficiency. Ünlükara et al (2008). Andriolo et al. (2005) reported that a salinity level above 2.0 dS m-1 reduced lettuce fresh yield (‘Vera’); therefore, for commercial purposes. Turhan et al (2013) fund that fresh yield, marketable yield, and dry matter of lettuce (Lactuca sativa L. cv. Funly) decreased in response to 10% seawater, and the lowest values were obtained in response to 20% seawater. Qin et al (2013) results showed that the lettuce is sensitive to NaCl which significantly decreases those indices of growth and physiology. Notably, the lettuce remains high productivity of edible biomass in low NaCl stress, although its salt-tolerant limitation is lower than amaranth. Therefore, we recommended that amaranth could be cultivated under a higher NaCl stress condition (<5000 ppm) for NaCl recycle while lettuce should be under a lower NaCl stress (<1000 ppm) for water cleaning in future BLSS. Cemek et al (2011) found that The highest yield was obtained in low soil salinity (T 0 = 0.75 dS m-1, T 1.5 = 1.5 dS m-1) and high evapotranspiration, whereas the lowest yield was obtained in high soil salinity (T 2.5 = 2.5 dS m-1, T 3.5 = 3.5 dS m-1, T 5 = 5.0 dS m-1, and T 7 = 7.0 dS m-1) and low evapotranspiration cases. The relationships between evapotranspiration soil salinity and yield some growth parameters were generally linear with increasing or decreasing tendencies. Unlukara et al (2009) The results further showed that the variables of leaf number per plant, plant height, water use efficiency, and accumulation of calcium (Ca) and iron (Fe) in the leaves decreased with salinity, whereas accumulation of potassium (K), nitrogen (N), copper (Cu), manganese (Mn), and zinc (Zn) in plant leaves was unaffected.
Kim, H. J. et al (2009) found that the dry weight, height, and color of the lettuce plants were significantly changed by long-term irrigation (15 days) with higher NaCl concentration (i.e., 100 mM). However, no significant differences were observed in the growth and appearance among the control, all short-term treatments (2 days; 50, 100, 500, and 1000 mM), and long-term irrigation with low salt concentration. Moreover, in romaine lettuce treated with long-term irrigation with 5 mM NaCl, the total carotenoid content increased without color change, and the contents of major carotenoids in romaine lettuce, lutein and beta -carotene, increased 37 and 80%, respectively. No differences were observed in lutein and beta -carotene contents in short-term-treated lettuce. The phenolic content of the romaine lettuce declined with short-term salt irrigation, whereas there were no significant differences among treatments exposed to long-term irrigation. This research indicates that long-term irrigation with relatively low salt concentration, rather than short-term irrigation with high salt concentration, can increase carotenoid content in romaine lettuce without causing a tradeoff in yield or visual quality.
Soares et al (2007) reported that from 2 October to 3 November 2006, the effect of saline water on crisp head lettuce production under hydroponic conditions was investigated. Different water salinity levels (0.43, 1.40, 2.23, 3.08 and 3.93 dS/m) were prepared with NaCl and CaCl2 (1:1, weight basis). Increase in water salinity significantly decreased the shoot dry mass and water consumption. For each unit increase in water salinity, the lettuce production decreased by 4.08%.
Zheng YiYan and Feng Gu (2007) In order to test the hypothesis that arbuscular mycorrhizal fungi (AM fungi) can improve growth of vegetables cultivated in protected horticultural soils which was affected by secondary salinization as a result of over-fertilization, Lactuca sativa plants growing at three soil electrical conductivity (EC) levels (607, 1236 and 1866 microS cm). Results showed that with the increase in soil EC value, the lettuce decreased in biomass.
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