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Chitooligosaccharides (COS) were prepared by hydrogen peroxide (1%) degradation of chitosan solution (3%) in dilute lactic acid at ambient temperature. The molecular weights of degraded chitosan were measured by gel permeation chromatography, and the chemical structure of resultant COS were characterized by Fourier transform infrared spectra. The decrease of chitosan Mw followed the first-order kinetics with the rate constant of 5.71 × 10−7 mol/g/h. As a result, the COS with Mw of 4500 g/mol was obtained at 360 h of the reaction time and its chemical structure almost remained the same as initial chitosan. Germination ability of soybean seeds pre-soaking in COS (4500 g/mol) solution with different concentrations, particularly 0 (control), 50, 100 and 200 mg/L was investigated. Results indicated that germination rate increased significantly for all soybean seeds treated with COS compared to the control. Thus, COS prepared by this method can be potentially applied to large-scale production with reasonably low cost and used as an effective biostimulant for sustainable agriculture.

The brown spot disease caused by Neoscytalidium dimidiatum fungus on dragon fruit plants (Hylocereus undatus) is an extremely serious disease. Oligochitosan (OC) and nanosilica (nSiO2) have been considered as effective plant elicitors. In this study, OC with Mw of 3000, 5000, and 7000 g/mol was prepared by γ-irradiation and nSiO2 with size ~ 45 nm was synthesized from rice husk. Mixture of nSiO2–OC was prepared by dispersing nSiO2 in OC solution. The resistant effect against brown spot disease on dragon fruit plants was assessed by measuring the induced chitinase enzyme activity and the disease severity after spraying OC, nSiO2–OC and inoculating fungus. The results showed that all OC enhanced chitinase induction and reduced disease severity compared with control, and the OC ~ 3000 g/mol exhibited the highest activity. The nSiO2–OC also exhibited similar effect as OC and nSiO2, but it was more effective than individual OC or nSiO2 from 120 h onward.