The objective of this study was to evaluate the effect of Si supply on osmotic adjustment and solutes allocation of two sorghum cultivars either exposed or not exposed to alkalinity stress. The present study was carried out in a greenhouse at the Faculty of Science, Mansoura University, Egypt, through the two successive seasons (2016/2017) and (2017/2018). The experiment was conducted in pots to evaluate the beneficial effect of grain presoaking in sodium meta-silicate (Na2Sio3.5H2o at 1.5 mM) on two sorghum (Sorghum bicolor (L.) Moench) cultivars (alkalinity sensitive cultivar Giza 15 and alkalinity tolerant one ICSR 92003). Alkalinity stress was imposed by various alkaline salt concentrations of 0 (control), 25, 50, and 75 mM Na2CO3, with fifteen replications. Osmotic adjustment (OA), osmotic pressure (OP) and solutes accumulation (TSS, TSN, proline, organic acids and inorganic ions (Na+, K+, P3+, Ca2+, Mg2+ and Cl-) were quantified in flag leaf of both sorghum cultivars during grain-filling. Alkalinity induced significant increase in osmotic adjustment and osmotic pressure. Furthermore, Silicon caused additional increase in osmotic adjustment, organic solutes (TSS, TSN, proline and organic acids) and inorganic ions (Na+, K+, P3+, Ca2+, Mg2+ and Cl-). On the other hand, clear reduction in K+/ Na+ ratio in the flag leaf of both cultivars was observed. The capacity of osmotic adjustment was greater with higher concentration in both cultivars. Moreover, the production of both organic and inorganic ions tended to be higher in ICSR 92003 than in Giza 15. Osmotic pressure of flag leaf sap correlated positively with proline, TSN, TSS, organic acids and ions content.

Keywords: Sorghum, Alkalinity, osmotic adjustment, Compatible solutes, proline