Assessment of internal quality attributes of mandarin fruit. 1. NIR calibration model development

The utility of near infrared spectroscopy as a non-invasive technique for the assessment of internal eating quality parameters of mandarin fruit (Citrus reticulata cv. Imperial) was assessed. The calibration procedure for the attributes of TSS (total soluble solids) and DM (dry matter) was optimised with respect to a reference sampling technique, scan averaging, spectral window, data pre-treatment (in terms of derivative treatment and scatter correction routine) and regression procedure. The recommended procedure involved sampling of an equatorial position on the fruit with 1 scan per spectrum, and modified partial least squares model development on a 720–950-nm window, pre-treated as first derivative absorbance data (gap size of 4 data points) with standard normal variance and detrend scatter correction. Calibration model performance for the attributes of TSS and DM content was encouraging (typical Rc2 of >0.75 and 0.90, respectively; typical root mean squared standard error of calibration of <0.4 and 0.6%, respectively), whereas that for juiciness and total acidity was unacceptable. The robustness of the TSS and DM calibrations across new populations of fruit is documented in a companion study.

Assessment of internal quality attributes of mandarin fruit. 2. NIR calibration model robustness

The robustness of multivariate calibration models, based on near infrared spectroscopy, for the assessment of total soluble solids (TSS) and dry matter (DM) of intact mandarin fruit (Citrus reticulata cv. Imperial) was assessed. TSS calibration model performance was validated in terms of prediction of populations of fruit not in the original population (different harvest days from a single tree, different harvest localities, different harvest seasons). Of these, calibration performance was most affected by validation across seasons (signal to noise statistic on root mean squared error of prediction of 3.8, compared with 20 and 13 for locality and harvest day, respectively). Procedures for sample selection from the validation population for addition to the calibration population (‘model updating’) were considered for both TSS and DM models. Random selection from the validation group worked as well as more sophisticated selection procedures, with approximately 20 samples required. Models that were developed using samples at a range of temperatures were robust in validation for TSS and DM.

Host Susceptibility of Citrus Cultivars to Queensland Fruit Fly (Diptera: Tephritidae)

Citrus crops are considered to be relatively poor hosts for Queensland fruit fly, Bactrocera tryoni (Froggatt), as for other tephritid species. Australian citrus growers and crop consultants have reported observable differences in susceptibility of different citrus cultivars under commercial growing conditions. In this study we conducted laboratory tests and field surveys to determine susceptibility to B. tryoni of six citrus cultivars [(Eureka lemon (Citrus limon (L.) Osbeck); Navel and Valencia oranges (C. sinensis (L.) Osbeck); and Imperial, Ellendale, and Murcott mandarins (C. reticulata Blanco)]. The host susceptibility of these citrus cultivars was quantified by a Host Susceptibility Index, which is defined as the number of adult flies produced per gram of fruit infested at a calculated rate of one egg per gram of fruit. The HSI was ranked as Murcott (0.083) > Imperial (0.052) ≥ Navel (0.026) ≥ Ellendale (0.020) > Valencia (0.008) ≥ Eureka (yellow) (0.002) > Eureka (green) (0). Results of the laboratory study were in agreement with the level of field infestation in the four citrus cultivars (Eureka lemon, Imperial, Ellendale, and Murcott mandarins) that were surveyed from commercial orchards under baiting treatments against fruit flies in the Central Burnett district of Queensland. Field surveys of citrus hosts from the habitats not subject to fruit fly management showed that the numbers of fruit flies produced per gram of fruit were much lower, compared with the more susceptible noncitrus hosts, such as guava (Psidium guajava L.), cherry guava (P. littorale Raddi), mulberry (Morus nigra L.), loquat (Eriobotrya japonica (Thunb.) Lindl.), and pear (Pyrus communis L.). Therefore, the major citrus crops commercially cultivated in Australia have a relatively low susceptibility to B. tryoni, with Eureka lemons being a particularly poor host for this tephritid fruit fly.