الفهرس 
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Abstract
A pot experiment was conducted to evaluate the impact of different water regimes on alfalfa plants grown in different soil types. Three types of soil with 33, 67 and 100% sand were used. Drought began 45 days after sowing so that the plants were subjected to three irrigation systems; control, moderate drought and severe drought. Samples were taken 75 days post-cultivation. The obtained results showed that stress increased the amount of phenolics (saponins, tannins, flavonoids, alkaloids and total phenols), non-photosynthetic pigments (anthocyanins, lycopene and β-carotene), vitamins (ascorbic acid, thiamine and riboflavin) and minerals (Na, K and Ca) but decreased the amount of P and Mg as well as the activity of amylases, invertase, pectinase, proteases and lipase. Stress also increased diphenyl-picrylhydrazyl-scavenging activity, antiradical power, H2O2-scavenging activity, total antioxidant capacity and reducing power. Stress also increased the activity of peroxidase, ascorbic peroxidase, polyphenol oxidase, catalase, superoxide dismutase, phenylalanine ammonia lyase and glutathione reductase. Moreover, stress inhibited the growth of Escherichia coli, Klebsiella pneumoniae, Proteus vulgaris, Pseudomonas aerugenosa, Salmonella typhi, Shigella flexneri, Staphylococcus epidermidis, Streptococcus pyrogenes, Candida albicans, Rhizopus azygosporus and R. microsporus.
The results also showed that stress reduced most growth parameters of leaves, shoot and root. Stress also reduced water use efficiency, transpiration rate and relative water content with an increase in saturation water deficit and transpiration efficiency. Both leaf thickness and ground tissue thickness increased in response to stress. On contrary, stress reduced phloem, xylem and the whole vascular bundle areas. Scanning electron microscopy revealed that water stress reduced both stomatal and stomatal opening areas but increased epidermal and hair frequency. In addition, transmission electron microscopy cleared that stress reduced the number of both chloroplasts per cell and starch grains per chloroplast as well as the area of chloroplasts and starch grains. Moreover, severe drought resulted in the appearance of plastoglobuli within the chloroplasts.
Stress could also increase lipid peroxidation and membrane leakage but decreased membrane stability index. Stress reduced the amount of phospholipids, organic and total phosphorus with an increase in inorganic phosphorus content. With respect to Chl a, Chl b, Chl (a+b), total pigments and photosystem II activity, these were all declined because of stress. Meanwhile, stress enhanced the production of carotenoids. Furthermore, stress decreased total protein and nucleic acids contents. Stress also increased glucose, fructose, trehalose, sucrose and total soluble sugars; but reduced the amount of polysaccharides and total carbohydrates. In addition, ammonia, amino, amide, nitrite and total soluble nitrogen were all increased in response to stress. Meanwhile, nitrate, peptide, protein as well as total nitrogen were declined. Stress also increased the osmotic pressure as well as the amount of organic and inorganic osmoprotectants.
Soil ionic contents and ratios as well as soil pH, electric conductivity, organic carbon and matter, CaCO3, HCO3-, SO4-- and total soluble salts were all fluctuated in a random fashion. Meanwhile, the amounts of soil ammonia nitrogen before cultivation were lower than those after cultivation with well-watered or moderately-droughted plants. The amounts of soil amino, amide, nitrite and protein nitrogen before cultivation were higher than those after cultivation. In a reversed trend, the amounts of soil nitrate, peptide, total soluble and total nitrogen before soil cultivation were significantly lower than those after cultivation.