Kiwifruit growers need to consistently produce high yields of kiwifruit with high DMC to meet consumer preferences for fruit with intense flavor and sweetness (Harker et al., 2009) without compromising storage quality. Mature kiwifruit vines allocate ≈50% of annual newly fixed biomass to fruit each season (Boyd et al., 2010; Clark and Smith, 1992). This figure is relatively low compared with some perennial fruit crops; in apples, for example, 70% of annually fixed biomass is allocated to fruit (Heim et al., 1979). The kiwifruit vine is a climbing or straggling plant (Ferguson, 1990) and requires careful canopy management to maintain yields and fruit DMC while controlling canopy vigor. Vine management practices routinely used include trunk girdling, attention to summer pruning, and growing fruit on older, less vigorous wood (Cooper and Marshall, 1991; Davison, 1990; Goren et al., 2004; Miller et al., 2001). A primary aim of these techniques is to minimize competition for resources between fruit and rapidly growing shoots.
Perennial plant responses to excessive defoliation, whether caused by harsh pruning, herbivory, or frost, include reallocation of resources toward shoots at the expense of root or reproductive growth. Badly timed or excessive pruning can reduce fruit DMC, delay maturity, and reduce concentrations of inorganic nutrients in fruit (Candolfi-Vasconcelos and Koblet, 1990; Siham et al., 2005; Valladares et al., 2007). Trunk girdling interrupts the phloem connection between the canopy and roots affecting the transport of carbohydrates and plant growth regulators from canopy to the roots (Goren et al., 2004). There is little published information on the effects of trunk girdling in kiwifruit vines, although Davison (1990) reported that girdling advanced fruit maturity in the year of girdle application and enhanced flower numbers in the season after girdle application. The effects of girdling on fruit production in other fruit crops include increased fruit set, increased fruit size, and advanced fruit maturity (Goren et al., 2004). It is not clear exactly how girdling affects fruit set and development, but the increased availability of carbohydrates in the canopy and the altered supply of root-generated hormones are both likely to be involved (Cutting and Lyne, 1993; Harrell and Williams, 1987).
Kiwifruit can be cool-stored for several months after harvest but can be affected by storage disorders such as low temperature breakdown. Both fruit maturity at harvest and fruit concentrations of inorganic nutrients can affect storage quality in kiwifruit (Clark et al., 2004; Ferguson et al., 2003). However, there is little information about whether vine management techniques implemented to improve kiwifruit DMC also affect fruit maturity and composition and, ultimately, storage performance.
This long-term study was established on mature, field-grown ‘Hort16A’ kiwifruit vines using different techniques (e.g., pruning, fruit thinning, and girdling) to modify whole-vine resource allocation. High croploads combined with badly timed and excessive pruning were used to deplete whole-vine carbohydrate status (the famine treatment), low croploads with high leaf to fruit numbers were used to provide abundant carbohydrates to fruit (feast treatment), and trunk girdling was used to isolate the canopy from the roots for extended periods of time. The famine and ETG treatments were designed to identify any problems that may occur if growers channel too much assimilate into fruit production. The aim of the project was to determine how such treatments affected vine productivity, fruit DMC, and storage performance over several seasons.
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