The effect of six SiO2 nanosized concentrations (0, 5, 20, 40, 60 and 80 mg L-1) and three seed prechilling treatments (control, seed prechilling before nano SiO2 treatments, treatments of seed with nano SiO2 and then prechilling) and their interactions on tall wheatgrass (Agropyron elongatum L.) were studied. Results indicated that application of SiO2 nanoparticles significantly increased seed germination of tall wheatgrass from 58 in control group to 86.3 and 85.7% in 40 and 60 mg L-1, respectively. Applying SiO2 nanoparticles increased dry weight of shoot, root and seedling of tall wheatgrass. Increasing concentration of nanoparticle from 0 up to 40 mg L-1 increased seedling weight around 49% compared to the control, nevertheless decreased under 60 and 80 mg L-1 treatments. SiO2 at 40 mg L-1 treatment improved Vigor Index I by 181% and vigor index II by 120% as compared to the control. Seed germination was 77.8% under prechilling treatment while under no prechilling and seed prechilling simultaneously with nano SiO2 were 52.8% and 80.2%, respectively. Treatments under seed prechilling before nano treatment, exposing of seeds to 40 and 60 mg L-1 improved germination rates by 40% and germination speed by 123% in compared with control. Seed prechilling together with SiO2 nanoparticles largely broke the seed dormancy for A. elongatum.

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