| Strawberry(Fragaria×ananassa Duch.)is a perennial herb belonging to the genus of fragaria and the family of rosaceae.Strawberry is regarded as important sources of dietary antioxidants cultivated in the country and world-wide.However,a limited number of cultivars have been characterized as having good performance,moreover,there are existed the problem of cultivars degradation.Therefore,there is a great deal of interest in better exploiting new strawberry cultivars.Strawberry industry is restricted to some extend due to the fruit with short shelf life.Ultraviolet-C(UV-C)irradiation is considered as a viable and environmentally friendly physical method to delay ripening and to enhance antioxidant capacity and to control disease of fruit and vegetables,which is consistent with the trend of consumers' pursuit on the safety and nutrition of food.However,most researches were made by postharvest stage.Low UV-C doses applied at appropriate intervals were shown to effectively control the incidence of fungal disease on strawberry plants.Strawberries are characterized by their physical fragility due to the absence of a strong pericarp.Hence,postharvest UV-C treatment requiring that the fruits be rotated in order to expose all sides to the beneficial rays might be inadvisable.Therefore,to circumvent the lack of systemicity of UV-C action,the preharvest treatment of strawberry plants may be an interesting alternative.The experiments were carried out in the greenhouse and field located in L'Acadie in Agriculture and Agri-Food Canada's horticulture research and development center from Dec.2011 to Dec.2013.UV-C treatment was applied with different doses(0.25,0.5,0.75 and 1kJ/m~2)from flowering until the fruit reached commercial maturity on strawberry plants(cv.‘Seascape')grown under greenhouse conditions.The aim of this research was to investigate the effect of preharvest UV-C irradiation on the strawberry plant growth and fruit qualities,the physiological and biochemical mechanism of relevant secondary metabolism were also studied.From the effect of different doses of UV-C irradiation,three strawberry cultivars(cv.‘Albion',‘Charlotte' and ‘Seascape')were subjected to preharvest UV-C light(0.6 kJ/m~2)during growth for two consecutive seasons.The aim was to investigate the influences ofUV-C,cultivar and environment(light intensity)on fruit quality and secondary metabolism and disease resistance,and the influences of cultivar and environment on the effect of UV-C dose.In addition,11 advanced strawberry selections were used to analyze for their agronomic attributes and phytochemical profiles and disease resistance and compare them with those of one commercial cultivar and to attempt to provide the technical basis for the evaluation of breeding new varieties.The main results were as follows:1.UV-C irradiation with different doses applied at appropriate intervals on strawberry plants had no significant effect on leaves soluble protein,superoxide dismutase(SOD)and peroxidase(POD)activity.After the last time of UV-C treatment,UV-C doses at 0.75 and 1kJ/m~2 significant increased catalase(CAT)activity,soluble sugar was also enhanced by UV-C(0.5 and 0.75 kJ/m~2).UV-C(1 kJ/m~2)significantly reduced TPC and a significant increase in the malondialdehyde(MDA)and the relative conductivity.After two weeks of UV-C treatment,the carotenoids content was significant increased exposure to all of UV-C doses.UV-C at 1 kJ/m~2 significantly decreased chlorophyll content.UV-C irradiation(0.5,0.75 and 1kJ/m~2)increased the biomass and leaf number and leaf area and 1 kJ/m~2 with lower rate of increase.The results showed that 0.5 and 0.75 kJ/m~2 of UV-C light promote plant growth to a certain extent,the effect of 0.25 and 1 kJ/m~2 of UV-C treatments are not obvious.2.Preharvest UV-C irradiation with different doses reduced soluble solids content(SSC),simple sugar,total sugars,titratable acidity(TA),organic acid and total acids,indicating the delay of the ripe fruits.UV-C treatment(0.5 and 0.75 kJ/m~2)enhanced fruit firmness and improved individual phenolic compounds,in particular 0.5 kJ/m~2.In addition,UV-C at 0.5kJ/m~2 significantly changed fruit color with higher Hue(ho)value.Compared with the control,all of UV-C doses did not significantly affect total antioxidant capacity(TAC)of fruit.Therefore,0.5 and 0.75 kJ/m~2 or doses between them were selected for further study.3.Preharvest UV-C irradiation at 0.6 kJ/m~2 significantly increased fruit hardness,improved disease resistance and prolonged shelf life when applied on three strawberry varieties for two consecutive seasons.Moreover,UV-C irradiation also significantly improved fruit color with higher a* value,indicating the color of fruit from preharvest UV-C treatment more red compared with control.In addition,UV-C increased L*,b*,C(Chroma)and h o value,however,significant increase only was found in a* and C values in cv.‘Charlotte' in the second growing season.The content of the ellagic acid(EA),kaempferol-3-glucuronide(K3Gr),Quercetin-3-glucuronide,(Q3Gr)and Quercetin-3-glucoside(Q3G)was promoted in the fruits from UV-C treated plants in the first season.This observation was particularly evident in ‘Albion'.UV-C treatment did not consistently influence the other phenolic compounds(Anthocyanins,ACY)considered.However,the TAC of three strawberry cultivars were not affected by UV-C treated plants,neither were organic acids,simple sugars,SSC,TA and pH,although in most cases slight decreases were noticed.The results show the bioactives content in strawberry fruits exposure to preharvest UV-C light were cultivar dependent with a strong influence of season in most cases.4.The significant variations were existed among selections tested in terms of agronomic attributes and antioxidant capacity and disease resistance.Selection ‘SJ069-3' had higher production yield and average fruit weight,excellent SSC,mid-level TA,low weight loss,and the best firmness and longer shelf life in all genotypes.In addition,higher TAC was also found in ‘SJ069-3',an indication that ‘SJ069-3' is a promising new selection for the fresh market and breeding material.In addition to rich individual phenolics,‘SJ061-8' had the highest TAC as determined by three assays which was significantly different from the other genotypes,suggesting the potential use of this line as parent material in breeding or as a functional food ingredient.There was a strong relationship between TAC and disease resistance,indicating higher TAC led to higher disease resistance,thus improving shelf life of the fruits... |
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