Seed encapsulation in chitosan and its derivatives restores levels of chlorophyll and photosynthesis in wilt-affected cotton (Gossypium L., 1753) plants

Chitosan-based biologicals and chemicals have been proved to possess antiviral, antibacterial and antimycotic activity, and be able to stimulate plant immunity. In field trials (Tashkent region, Uzbekistan, 2015-2016) we first evaluated an impact of seed encapsulation with UzChitan, chitosan ascorbate and Cu2+-chelating chitosan synthesized by the authors' method in the Institute of Chemistry and Polymer Physics of the National University of Uzbekistan (NUUz, Tashkent) from waste of silkworm cocoon processing on photosynthetic activity at wilt ( Verticillium dahliae ) artificial infection and in wilt-free (healthy) crops of cotton ( Gossypium L., 1753) variety Sultan. It was found that wilt-affected control plants (non-treated with chitosan derivatives) had higher specific leaf weight (SLF index), their chlorophyll content was reduced and rates of respiration and apparent photosynthesis were depressed in comparison with healthy control plants. Chlorophyll content in infected control plants was decreased by 27-30 % in terms of mg/g. Meanwhile, all plants from pre-treated seeds contained more chlorophyll compared to control: the pigment content (mg/g of dry matter) was 25.5 and 17.7 % higher when ascorbate and Cu2+-chelating chitosan used. Chlorophyll content in leaves did not vary significantly in absence of the pathogen. The apparent photosynthesis rate in control under wilt was reduced by 33 % (0.161±0.027 against 0.245±0.028 мmol O2 · m-2 · sec-1). That corresponds to toxic effect of pathogen which penetrates through roots and goes up into top parts of plants, damages their phloem system, disturbs water transport, and destroys chlorophyll and leaf tissues. Besides, some tendency to respiration rate inhibition was observed for non-treated cotton plants under wilt infection (by 22 % compared with control plants from the healthy field). Netto-oxygen production (apparent photosynthesis) rates in the treated groups were higher as compared to control plants under wilt; the best results were shown by ascorbate and Cu2+-chelating chitosan (54 and 46 % higher, respectively). Respiration rates did not differ significantly in all groups under wilt. Oxygen balance (OB) index, estimated as ratio between measured oxygen netto-production and oxygen consumption rates, which reflects physiological status of plants under biogenic stress (including pathogen fungi), did not change significantly in a healthy environment when chitosan derivatives used. Under wilt, the plants from pre-treated seeds possessed higher OB indices than the corresponding control plant from the same field. Thus, the OB values with ascorbate and Cu2+-chelating chitosan were higher compared to control by 36 and 52 %, respectively. Our findings indicate that in case the cotton seeds were pre-treated with chitosan derivatives, the oxygen production, oxygen consumption and OB indices were not significantly different in the pathogen-affected and unaffected plants. Thus we can conclude about a nonspecific resistance of cotton plants to wilt ( Verticillium dahliae ) induced by chitosan derivatives, and reduction of toxic effects of the pathogen which manifests itself in decreasing respiration and photosynthesis rates. Possibly, it is a result of non-specific elicitor activity of chitosan and its derivatives, or their ability, due to natural polycation properties, to stimulate plants to produce specific antifungal metabolites - reactive oxygen species and phytoalexins triggering subsequent defense mechanisms. Currently, using elicitors for plant immunity correction are considered promising as they contribute to formation sustainable agrophytocenosis with high adaptiveness and yield production.