Preliminary Study On Mechanism Of Chitosan Treatment Delaying Cassava Postharvest Deterioration

Postharvest deterioration of cassava(Manihot esculenta Crantz)root is the major constraint to the development of the cassava industry.However,the main factors causing postharvest deterioration of cassava are still relatively unclear,which has resulted in slow progress in postharvest processing technology for cassava.Exploring the main causes of postharvest deterioration of cassava roots and feasible control methods is of paramount importance for the development of the cassava industry by addressing the problem of postharvest deterioration at the source.This study first analyzed the impact of single mechanical damage and subsequent bacterial infection on the induction of postharvest deterioration symptoms in cassava roots.Subsequently,by studying the changes in key genes and enzyme activity of phenylpropane metabolism pathway in postharvest cassava roots,observing the healing tissue structure of tuberous root wounds,and validating the functional healing ability of cassava tuberous root wound healing tissue,the self-healing ability of cassava tuberous root wounds after harvesting was systematically analyzed.Based on this study,this paper used chitosan as an exogenous additive to analyze the antibacterial activity of chitosan in vitro,the impact of chitosan on the healing of cassava tuberous root wounds,and the effect of chitosan on the induction of resistance in cassava tuberous roots,elucidating the preliminary mechanism of chitosan in delaying postharvest deterioration of cassava tuberous roots.This provides new research ideas for solving the problem of postharvest deterioration of cassava tuberous roots from the source.The main research results are as follows:1.This study isolated and purified three pathogenic fungi that can infect cassava roots and cause severe rot symptoms,namely L.Hormozganensis 6,P.palmivora 7,L.hormozganensis 8.The impact of mechanical damage and pathogenic fungi infection on the development of postharvest deterioration symptoms in cassava roots was systematically analyzed.The results showed that significant lesions appeared at the infection site of cassava roots 48 hours after pathogenic fungi infection,accompanied by obvious blue-black streaks around the lesion.Microscopic observations revealed that these blue-black streaks were deposited around the vascular bundle walls of the woody tissues.Localization of reactive oxygen species(ROS)showed that a large amount of ROS was deposited at the infection site 12 hours after pathogenic fungi infection.Scanning electron microscopy observation revealed that the membrane of the tissue cells in the lesion site of the cassava tubers was ruptured and perforated after being infected by pathogenic fungi.The deposition of blue-black streaks,burst of ROS,and membrane rupture were typical symptoms of postharvest deterioration in cassava roots.Single mechanical damage did not cause significant blue-black color changes in cassava tubers.In addition,there was no burst of ROS at the wound site of cassava roots after single damage,and the integrity of the membrane in cassava tuber tissues was not compromised.Based on the above analysis,this study concluded that the main symptom of postharvest deterioration in cassava roots is induced by pathogenic fungi infection.2.In this study,changes in key genes and enzyme activities of the phenylpropane metabolic pathway in postharvest cassava roots were analyzed,and the self-healing ability of the wound site in cassava roots was assessed by observing the deposition of lignin and suberin.The results showed that the phenylpropane metabolic pathway in cassava roots was activated after harvest,and the key genes PAL,C4 H,4CL,and CAD and their corresponding enzyme activities were upregulated during the self-healing process of the wound site.Microscopic observations showed that lignin gradually deposited at the wound site of cassava roots.Toluene blue penetration and pathogenic fungi infection experiments showed that the healing tissue of cassava root wounds could effectively prevent the loss of water and nutrients and the invasion of pathogenic fungi.Based on the above research results,it can be concluded that postharvest cassava roots have a complete self-healing ability,and wound healing can effectively prevent the rapid development of postharvest deterioration symptoms.3.This article analyzes the effect of chitosan treatment on the postharvest deterioration process of cassava roots.The results show that chitosan has a broadspectrum antibacterial effect on cassava root pathogens in vitro.Firstly,chitosan can directly inhibit the growth of pathogenic fungus mycelium,and the antibacterial effect becomes more significant with the increase of chitosan concentration.In addition to directly inhibiting the growth of pathogenic fungus mycelium,chitosan also destroys the structure of spores and inhibits spore germination.In vivo,chitosan can promote the upregulation of key genes and enzyme activities in the phenylpropane metabolic pathway at the wound site of cassava roots,accelerate lignin deposition,and ultimately accelerate the wound healing rate of cassava roots.Specifically,on the second day of wound healing,the chitosan-treated SC9 and SC6 cassava roots had already deposited a continuous and complete layer of red lignin at the wound site,while the lignin layer in the control roots was broken and discontinuous.On the 7th day of wound healing,the accumulated thickness of lignin layer in the chitosan-treated SC9 and SC6 roots was1.6 and 1.2 times that of the control group,respectively.Chitosan treatment also induced resistance in cassava roots.The results showed that chitosan treatment effectively inhibited the development rate of lesion area and depth in SC9 and SC6 cassava tuber varieties after inoculation with P.palmivora.48 hours after inoculation,the lesion area of the chitosan-treated SC9 and SC6 cassava tubers was 51.4% and 53.4%lower than that of the control group,respectively.24 hours after inoculation,the lesion depth of the chitosan-treated SC9 and SC6 cassava roots was 56.4% and 50.3% lower than that of the control group,respectively.Further analysis showed that chitosan significantly induced the upregulation of resistance genes CHI and GLU and corresponding enzyme activity in the SC9 and SC6 cassava varieties.Chitosan treatment also induced the accumulation levels of flavonoids and total phenols in cassava roots.In summary,chitosan directly inhibits bacteria,promotes the wound healing rate of cassava roots,and induces cassava tuber resistance,ultimately limiting the speed of pathogen invasion and delaying the rapid development of postharvest deterioration,thereby extending the shelf life of cassava tubers after harvest.