Maturation and Ripening of "Midknight" Valencia Sweet Orange Fruit: Dynamics in Ethylene Production, Respiration Rate, and Quality Parameters

Midknight Valencia is a late maturing cultivar of sweet orange (Citrus sinensis L. Osbeck) which is harvested in Western Australia during October to mid-December. Changes in ethylene production, respiration rate, and quality parameters viz. fruit colour, juice percentage, SSC: TA ratio, individual sugars, vitamin C and total antioxidants during fruit maturation and ripening (314 to 389 days after fruit set, DAFS) at 15 days interval were investigated. Thirty fruit were randomly sampled in each replication from five trees ( nine years old), grown on Carrizo citrange rootstock, spaced  at 7.5 m x 2.7m, with north-south row orientation at Moora (latitude 31^° 21' South, longitude 155^º 55' East), Western Australia. The experimental design was completely randomised design with four replications. Ethylene production was significantly higher (2.5 and 2.6 pmoles kg^-1s^-1) in both 314 and 389 DAFS as compared to all other sampling times. Respiration rate was increased from (0.1 to 0.2 μmole kg^-1 s^-1) from 314 to 344 DAFS and decreased from 359 to 374 DAFS and was significantly highest (0.3 μmole kg^-1s^-1) on 389 DAFS. Citrus color index (CCI) of the fruit was significantly higher (6.9 and 7.1) on 314 and 329 DAFS and declined to minimum (5.4) on 389 DAFS due to re-greening of the rind. The concentration of glucose (11.5 g L^-1) and fructose (20.1 g L^-1) in the juice were found higher on 374 DAFS, while sucrose (45.2 g L^-1) and total sugars (73.6 g L^-1) was found maximum on 329 DAFS. The highest levels of vitamin C in the juice was (429.40 mg L^-1) on 344 DAFS. Total antioxidants declined in the juice from 314 -359 DAFS followed by an increase from 374-389 DAFS. Juice percentage increased (40 - 54%) from 359 to 389 DAFS and SSC: TA ratio steadily increased with the advancement of fruit maturation and ripening from 314 to 389 DAFS. TA of the juice was lowest and SSC was found maximum on 389 DAFS which indicates the maximum sweetness of fruit. In conclusion, Midknight Valencia fruit harvest from 314 to 389 DAFS will be beneficial for juice industry due to higher SSC: TA ratio and per cent juice but lower CCI due to slight re-greening of fruit under Western Australian conditions.