Oligochitosan (OC) is effective biostimulant on growth promotion and elicitation against disease infection for plants. However, the range of OC molecular weight that exhibits the most effective activity is not fully understood and requires further investigation. In this study, OCs with different weight average molecular weight (Mw) were prepared by gamma Co-60 irradiation degradation of chitosan in solution and the effect on growth promotion and enhancement of fruit yield of chili plant (Capsicum frutescens L.) by foliar application of OCs particularly with Mw of 7.8, 5.0, and 2.5 kDa was investigated. Chili plants, cultivated in a greenhouse were sprayed with OC concentration of 50 mg/L for three times. Results indicated that among treatments, OC with 2.5 kDa proved to be the best, which increased the shoot fresh weight by 71.5%, shoot dry weight by 184%, total chlorophyll content by 12%, and fruit fresh weight by 49.8% for the control. Thus, OC with low Mw (2.5 kDa) that can be suitably produced on large scale by gamma Co-60 ray irradiation degradation of chitosan solution is potentially promising to apply as a biostimulant to enhance chili fruit yield significantly.

Oligochitosan (OC) with molecular weight Mw of 5000 g/mol was prepared by gamma Co-60 ray irradiation of chitosan solution. Nanosilica (nSiO 2 ) with the size of 10 -30 nm was synthesized by calcination of acid treated rice husk. The mixture of 1% OC -1% nSiO 2 was prepared by dispersion of nSiO 2 in OC solution. The morphology of nSiO 2 in the mixture of OC-nSiO 2 was measured from images of transmission electron microscopy (TEM). The effect of foliar application of the mixture of OC-nSiO 2 on the induction of resistance against anthracnose disease caused by Colletotrichum gloeosporioides fungus on chili fruits was investigated. Results indicated that foliar application of OC-nSiO 2 with the concentration of 60 mg/l -60 mg/l was found to be as the optimal treatment that reduced the disease severity on chili fruits to 22.2% compared with 90.0% of the control. Thus, OC-nSiO 2 hybrid material could be considered as an effective biotic elicitor to prevent anthracnose disease infection for chili fruits. Furthermore, the prepared OC-nSiO 2 hybrid material can also be used as an environmentally friendly agrochemical product for sustainable development of agriculture.

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This study presents the structural characteristics and the antifungal efficiency of the oligochitosan-Zn2+ complexes. Oligochitosan with the average molecular weight of ~5 kDa was effectively prepared by gamma-ray irradiation degradation of chitosan in a solution containing H2O2. The oligochitosan-Zn2+ complexes with the different molar ratios of -NH2/Zn2+ were prepared by mixing Zn(NO3)2 into oligochitosan solution. The resultant complexes were characterized by FTIR, XRD, UV-Vis, and ICP-AES. The obtained results demonstrated that Zn2+ ions were stably bound with oligochitosan molecules through interacting with -OH and -NH2 groups. The in vitro antifungal effect of oligochitosan-Zn2+ complexes was assessed against Colletotrichum truncatum, a fungus species causing anthracnose on crops. The antifungal activity was significantly improved as the increase of Zn2+ content in the complexes. Particularly, the antifungal efficiency of the complexes reached to 75–100% compared to that of about 12% for oligochitosan. Thus, the addition of Zn2+ into oligochitosan strengthens its performance towards antifungal property and bring forward a new approach for progressing biobased materials for controlling plant diseases.