الفهرس 
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Abstract
A pot experiment was conducted to evaluate the beneficial effect of grain presoaking in sodium meta-silicate (Na2Sio3.5H2o at 1.5 mM) on two sorghum (Sorghum bicolor L.) cultivars Giza 15 (alkalinity sensitive cultivar) and ICSR 92003 (alkalinity tolerant cultivar). The present study was carried out in a greenhouse at the Faculty of Science, Mansoura University, Egypt, through the two successive seasons (2016/2017). Alkalinity stress was imposed by various alkaline salt concentrations of 0 (control), 25, 50, and 75 mM Na2CO3.Samples were taken for measurements of growth vigor as well as physiological and anatomical criteria during grain-filling (60 days from sowing) as well as at harvest (90 days from sowing) for estimating yield and yield attributes as well as the biochemical aspects of yielded sorghum grains. In general, the obtained results showed that alkalinity stress caused general decrease in the growth vigor of root, shoot and flag leaf, photosynthetic pigment contents as well as total protein and nucleic acids content in flag leaf of both sorghum plants. Exogenous application of silicon alleviated the adverse effects of alkalinity stress by improving growth vigor of root and shoot, retention of pigment contents and increasing the content of total proteins and nucleic acids. Among the cultivars, tolerant showed a better performance and produced more biomass under alkalinity stress when compared with sensitive one. Alkalinity stress induces marked decrease in relative water content but led to significant increase in saturation water deficit in both flag leaves of treated sorghum cultivars. Grain priming with silicon counteracted the stress induced by alkalinity stress by recovering the turgidity in flag leaf of alkali sorghum plants. The application of silicon as sodium meta-silicate induced some modifications in the anatomical features of the flag leaf of main shoot which appeared to be an adaptive response to alkalinity stress. Silicon induced marked increase in conductive canals between flag leaf (i.e. source) and peduncle (i.e. sink) that help manipulation of photo-assimilates towards developing grains and consequently improved yield quality and quantity. Alkalinity stress reduced bio-membranes stability through inducing its lipid peroxidation resulting in an increment in membrane leakage (ML) with marked decrease in membrane stability index (MSI) in flag leaf of both sorghum cultivars during grain-filling. This effect was more conspicuous with sensitive cultivars. Application of silicon markedly increased MSI and significantly decreased lipid peroxidation and ML in flag leaf of alkalinity stressed sorghum plants.