Mechanism Of Nitric Oxide Treatment In Inducing Chilling Tolerance Of Postharvest Banana Fruit

Banana fruit are susceptible to chilling injury during storage at low temperature, which result in decrease in storage period and fruit quality extremely. Thus, it is very important to alleviate chilling injury (CI) in these fruit for theirpreservation and quality maintaining during storage at low temperature. Nitric oxide (NO) is a signal molecule existing in various organisms. Recently, pre-storage of NO for postharvest fruit has received more attention, and it has been found that oxalic acid plays important roles such as delaying the ripening process, and alleviating browning and CI symptoms in postharvest fruit. In order to further understanding the chilling injury effect of NO on alleviating the chilling injury in chilling-sensitive fruit, influences of NO in chilling injury, fruit quality, antioxidant capacity, proline acumulation, energy metabolism, polyamine and GABA accumulation in banana fruit under chilling stress were investigated in this paper. The main results were as follows:1. CI index in banana fruit gradually increased along with storage time under chilling stress. On day 10, the CI index in NO-treated group was 26.9% lower than the control. The results indicated that NO treatment could effectively alleviate chilling injuries in cold stored banana fruit. NO treatment obviously increased activities of superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), ascorbate peroxidase (APX) and glutathione reductase (GR), promoting the accumulation of reduced glutathione (GSH) and ascorbic acid (AsA), which resulted in improvement on scavenging capacity for reactive oxygen species such as hydrogen peroxide and the production rate of superoxide anion in banana fruit, and thus contributed to maintaining membrane integrity associated with alleviating the chilling injury in banana fruit under chilling stress. Besides, proline content increased first and then declined until the end of storage. NO treated fruit had the higher proline content than control fruit. NO treatment promoted P5CS activity, and inhibited PDH activity compared to control fruit. The P5CS activity in treated fruit was 84.7% higher than control fruit on day 10. At the end of storage, PDH activity in control fruit was 1.7-fold compared to NO treated fruit. These results indicated that proline accumulation was closely related to activities of P5CS and PDH. The higher proline in banana fruit with NO treatment may be due to enhanced proline synthesis and reduced proline degradation. NO may play an important role in inducing chilling tolerance of banana fruit through mediating the catabolism of proline.2. NO treatment significantly retarded the decrease in firmnes and titratable acid (TA) and the increase in total soluble sugar (TSS) in banana fruit. These effects of NO might collectively contribute to delaying the ripening and senescent process and thus to maintaining fruit quality for chilling sensitive fruit under cold storage. NO treatment significantly retarded the decrease of chlorophyll. Total chlorophyll content in group NO+cold was 10.7% higher than that in group CK+cold at the end of shelf life. Chlorophyll, as an important indicator for ripening and a primary antioxidant in banana fruit, is closely related with stress resistance for fruit. NO treatment significantly inhibited the chlorophyllase activity. Chlorophyllase activity in CK group was 16.3% higher than that in NO group on day 4. NO could also protect biomembranes against oxidative stress through promoting antioxidant defense systems, thus contributed to the chlorophyll retention in banana fruits.3. The enhancement of chilling tolerance in NO-treated banana fruit could be a result of an elevated energy metabolism induced by NO treatment. NO treatment enhanced the content of adenosine triphosphate (ATP) and adenosine diphosphate (ADP) in banana fruit, and inhibited the increase in adenosine monophosphate (AMP) and the decrease in energy charge during cold storage. Furthermore, NO treatment increased activities of succinodehydrogenase (SDH), cytochrome oxidase (CCO), H+-ATPase and Ca2+-ATPase in banana fruit. CCO activity in NO-treated fruit was 32.2% higher than that in control fruit at the end of the storage, meanwhile SDH activity is 2.7-fold in treated group compared to control. These results suggested that NO treatment could regulate energy metabolism and related enzyme activities for maintaining high energy level in banana fruit. Besides, NO treatment significantly promoted the activities of key enzymes involved in glycolysis, the oxidative pentose phosphate pathway, the mitochondrial tricarboxylic acid cycle. On day 5, activities of Glucose-6-phosphate dehydrogenase (G6PDH) and 6-phosphaogluconate dehydrogenase (6PGDH) in NO-treated fruit were 79.6% and 23.2% higher than that in control fruit, respectively. The results suggested that NO could enhance the energy related catabolism, providing sufficient energy and metabolites for the formation of banana chilling tolerance or restoration of chilling damage, thus alleviating the chilling injury of banana fruit under cold storage.4. NO treatment could play an important part in alleviating chilling injury of banana fruit, through mediating the accumulation of secondary metabolites such as polyamines and GABA. NO treatment increased the activities of key enzymes involved in polyamine biosynthesis, ornithine decarboxylase (ODC) and arginine decarboxylase (ADC), which led to the accumulation of polyamines. The putrescine content in treated fruit was 23.3% higher than control fruit on day 10. NO treatment increased the activity of glutamate decarboxylase (GAD), but repressed activity of GABA transaminase (GABA-T), which led to the accumulation of GABA. The putrescine content in treated fruit was 1.4-fold compared to control fruit at the end of the storage. Through the enhancement of diamine oxidase (DAO) and polyamine oxidase (PAO) activities, NO promoted the degradation of polyamine to GABA. At the end of the storage, activities of DAO and PAO in NO-treated fruit were 33.9% and 15.6% higher than that in control fruit, respectively. The results suggest that NO plays an important role in alleviating banana fruit chilling injury through enhancement of polyamines and GABA accumulation.5. NO treatment promoted the accumulation of GABA in banana fruit under cold storage, which may be one of the main mechanisms of inducing chilling tolerance. With the application of exogenous GABA, the accumulation of proline and total phenolics, as well as the total antioxidant capacity changes was determined in order to verify the role of GABA in inducing chilling tolerance of banana fruit. GABA treatment increased the activity of P5CS but repressed activity of PDH, which led to the accumulation of proline. Higher content of total phenolics was also observed in the peel of GABA-treated fruit which accompanied by an increase in PAL activity. As indicated by Ferric reducing antioxidant power (FRAP) assay and 1,1-Diphenyl-2-picrylhydrazyl (DPPH) radical scavenging activity, of GABA-treated banana fruit displayed a relatively high level of total antioxidant capacity. The results suggest that application of GABA plays an important role in alleviating banana fruit chilling injury through proline accumulation and enhancement of antioxidant defense system.