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Jonathan Dixon and Errol W. Hewett

Loss of green color (yellowing) in apples (Malus ×domestica Borkh.) is due to chlorophyll breakdown, an enzyme-mediated reaction conforming to first-order kinetics. Nondestructive measurements of yellowing, using a tristimulus chromameter, were related to chlorophyll content for `Cox's Orange Pippin' and `Granny Smith' apples stored at temperatures of 0 to 35 °C. Yellowing determined by rate constant (k) of total chlorophyll and changes in hue angle, as a function of temperature, increased slowly from 0 to 5 °C, increased exponentially from 5 to 20 °C, reached a maximum from 20 to 24 °C, then declined at higher temperatures. Values of k and the rate of change of hue angle were characterized by a modified Arrhenius equation. `Cox's Orange Pippin' apples harvested early had lower k and lower rates of hue angle change than late-harvested fruit. Values of k and hue angle change of `Granny Smith' apples differed between years but not between harvests. The method used to characterize the relationship between yellowing and temperature may describe changes in other important quality attributes of apple fruit during storage and shelf life and help evaluate the efficacy of cool-chain handling and storage systems.

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Errol W. Hewett and Christopher B. Watkins

The incidence of external and internal bitter pit in `Cox's Orange Pippin' apple (Malus domestics Borkh.) fruit sprayed with normal therapeutic sprays either with or without Ca salts at 2-week intervals during the growing season was determined after 6 weeks of storage over 7 consecutive years. Following harvest, fruit was either vacuum-infiltrated with CaCI2 or received no further treatment. Although there was a tendency for fruit that had been sprayed and vacuum-infiltrated with Ca to exhibit the greatest degree of bitter pit control, this treatment was not significantly superior to Ca sprays alone. Vacuum infiltration alone reduced the disorder to a lesser extent than Ca sprays and was more effective in reducing external than internal bitter bit. The results suggest that Ca applications over the growing season are superior to postharvest vacuum-infiltration with Ca in the prevention of bitter pit.

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Craig M Mowatt, Nigel H Banks, and Errol W Hewett

Positions of every individual kiwifruit were mapped on each of five eight-year-old vines on a T-bar training system before harvest. The proportion of excessively soft kiwifruit (< 1.0 kgf penetrometer reading at 20C) after 130 days coolstorage at 0C on individual vines ranged from 7 to 45%. Fruit at the distal ends of fruiting canes were significantly heavier and firmer (mean wgt 108.5 gms, mean firmness 1.22 kgf) than fruit closest to the main leader (105.6 gms, 1.18 kgf). Conversely, for multiple clusters, fruit on spurs adjacent to the fruiting cane were heavier than those at the terminal end (109.9 and 103.8 gms), respectively, though firmness of these fruit did not differ significantly. The firmest fruit had less nitrogen, less potassium, less phosphorus and more calcium than the soft fruit. Potential means by which this information could be used to improve fruit storage quality will be discussed.

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Keith A. Funnell, Errol W. Hewett, Ian J. Warrington, and Julie A. Plummer

Dry matter accumulation and partitioning in plants of Zantedeschia Spreng. `Best Gold' aff. Z. pentlandii (Wats.) Wittm. (syn. Richardia pentlandii Wats.) were quantified under a range of temperature and photosynthetic photon flux (PPF) regimes using plant growth analysis. The relative rate of dry matter accumulation [relative growth rate (RGRM), g·g-1·d-1] was highly correlated with the partitioning of the daily increment of dry matter into leaf tissue [leaf matter partitioning (LMP), g·d-1 per g·d-1]. In contrast, a poor correlation existed between RGRM and net assimilation rate (NAR, g·m-2·d-1). Maximum values of RGRM increased linearly with increasing temperature (from 13 to 28 °C), with a base temperature of 2.1 ± 2.7 °C. The optimum temperature for growth was PPF dependent with maximum total plant dry mass occurring under high PPF (694 μmol·m-2·s-1) at 25 °C. However, as the plant responded to PPF by altering LMP, final total plant dry mass was actually greater under the low PPF regime (348 μmol·m-2·s-1) at temperatures <22 °C. The optimum temperature for dry matter accumulation was close to the average daily air temperature during the growing season for the natural habitat of the parent species. Similarly, the greater dry matter accumulation under the combination of either low PPF and cooler temperatures or high PPF and warmer temperatures was paralleled by the diversity of PPF habitats in the natural open grassland and forest margin the parent species occupies. It is therefore suggested that Zantedeschia `Best Gold' is well adapted to optimize growth under these environmental conditions.

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Paul T. Austin, Errol W. Hewett, Dominique Noiton, and Julie A. Plummer

Integer values used to represent apricot (Prunus armeniaca L.) flower bud growth stages in a phenological scale were adjusted by a simple technique based on cumulative counts of bud observations. Adjusted stage values on a new continuous scale were calculated so that differences between consecutive values were proportional to the frequency with which buds were observed in each growth stage class during the entire assessment period. This meant that adjusted scale values were linearly related to bud development rate at 20 °C. The method was applied to a scale describing flower development from budbreak to petal fall for three cultivars of apricot growing under orchard conditions.

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Nihal C. Rajapakse, Nigel H. Banks, Errol W. Hewett, and Donald J. Cleland

Steady-state oxygen diffusion in flesh of apples (Malus domestics Borkh. cvs. Braeburn and Cox's Orange Pippin), Asian pears (Pyrus serotina Rehder. cvs. Hosui and Kosui), and nectarines [Prunus persica (L.) Batsch. cvs. Red Gold and Sunglo] was studied using a nondestructive method at 20C. Fruit flesh was found to exert a significant resistance to O2 diffusion resulting in measurable O2 gradients between tissues immediately beneath the skin and those at the fruit center for all these fruits. The magnitude of these O2 gradients varied between crops and cultivars and depended on the respiration rate and on effective O2 diffusivity in fruit flesh (De). Values of Dc varied with the cultivar and were broadly consistent with intercellular space volume. The range of De values obtained suggested that 02 diffusion in fruit flesh takes place in a combination of series and parallel modes in the intercellular space and fluid/solid matrix of the flesh. The results imply that O2 diffusivity in flesh tissues must be taken into consideration in the determination of critical external O2 level in controlled/modified atmosphere (CA/MA) storage.

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Keith A. Funnell, Errol W. Hewett, Julie A. Plummer, and Ian J. Warrington

Photosynthetic activity of individual leaves of Zantedeschia Spreng. `Best Gold' aff. Z. pentlandii (Wats.) Wittm. [syn. Richardia pentlandii Wats.] (`Best Gold'), were quantified with leaf expansion and diurnally, under a range of temperature and photosynthetic photon flux (PPF) regimes. Predictive models incorporating PPF, day temperature, and percentage leaf area expansion accounted for 78% and 81% of variation in net photosynthetic rate (Pn) before, and postattainment of, 75% maximum leaf area, respectively. Minimal changes in Pn occurred during the photoperiod when environmental conditions were stable. Maximum Pn (10.9μmol·m-2·s-1 or 13.3 μmol·g-1·s-1) occurred for plants grown under high PPF (694 μmol·m-2·s-1) and day temperature (28 °C). Acclimation of Pn was less than complete, with any gain through a greater light-saturated photosynthetic rate (Pmax) at high PPF also resulting in a reduction in quantum yield. Similarly, any gain in acclimation through increased quantum yield under low PPF occurred concurrently with reduced Pmax. It was concluded that Zantedeschia `Best Gold' is a shade tolerant selection, adapted to optimize photosynthetic rate under the climate of its natural habitat, by not having obligate adaptation to sun or shade habitats.