As a recently domesticated crop, kiwifruit has grown from a small, specialized commodity for one country (New Zealand) to a vital commercial crop grown worldwide (Ferguson, 1999). The vast majority of production in the United States (98%) is in California, where the main cultivar grown is Actinidia deliciosa (A. Chev.) C.F. Liang et A.R. Ferguson ‘Hayward’ (California Kiwifruit Commission, 2016). Emergence of the international kiwifruit industry was led by one cultivar in particular, ‘Hayward’ (Ferguson, 1999). ‘Hayward’ has dominated the green-fleshed varieties (Morley-Bunker and Lyford, 1999). Fruit quality, appeal, and storage life of this cultivar has contributed to the success of the kiwifruit industry. Until recently, the only other cultivar to be traded globally was Actinidia chinensis Planch. ‘Hort16A’ (Patterson et al., 2003). This gold-fleshed cultivar has the closest resemblance to A. deliciosa and has had much success commercially. ‘Hort16A’ was noted as being more productive than ‘Hayward’, having larger, sweeter fruit and thought by many to be superior in flavor (Ferguson, 1999; Patterson et al., 2003). However, ‘Hort16A’ was observed to be highly susceptible to Pseudomonas syringae pv. actinidiae (Psa), and 2 years after the introduction of Psa to New Zealand, a new gold-fleshed kiwifruit cultivar was released, ‘Gold3’ (Peacock, 2014). This cultivar was observed to be less susceptible to the disease than ‘Hort16A’ and was commercialized by Zespri (Zespri International Ltd., Mount Maunganui, NZ) in 2010.
In the southeastern United States, commercial kiwifruit production was first introduced in the mid to late 1980s (Powell et al., 2000). In 1987, commercial and experimental kiwifruit plantings were established in central and southern Alabama. The vegetative growth of these plantings was acceptable, but it soon became evident that the warm climate of southern Alabama was restrictive for flower production of ‘Hayward’. Fruiting was acceptable in the central part of the state, where chilling hours ranged from 1000 to 1300 hr, thus it appeared that the lack of chilling hours in south Alabama obstructed floral and fruit development for this cultivar (Wall et al., 2008).
‘AU Fitzgerald’ (A. deliciosa) originated in southern Alabama (Summerdale, AL; lat. 30°29' N, long. 87°42' W) from seeds sown by Mrs. A. A. Fitzgerald from fruit purchased at a local store, probably ‘Hayward’ (Dozier et al., 2010). From these seeds, female (‘AU Fitzgerald’) and male plants (‘AU Authur’) emerged, bloomed, and produced a quality crop. The fruit were cylindrical in shape with brown skin that had medium-length hairs and green flesh. ‘AU Fitzgerald’ plants have performed well in Summerdale, AL, where chilling hours average less than 700 per growing season, indicating that the chilling hour requirement is markedly lower than ‘Hayward’ (Wall et al., 2008).
Auburn University has worked with The Fruit and Tea Institute of Hubei province, P.R. China, to patent A. chinensis ‘AU Golden Sunshine’ (Dozier et al., 2011a). The fruit produced by this cultivar are cylindrical, with brown skin that has short soft hairs and a golden yellow flesh. In Alabama, ‘AU Golden Sunshine’ has performed well and was paired with a pollinizer, A. chinensis ‘AU Golden Tiger’ (Dozier et al., 2011b). ‘AU Golden Sunshine’ has a low vegetative chilling requirement with 700 h needed for budbreak and 900 h needed for optimal floral development (Wall et al., 2008).
A. deliciosa and A. chinensis are functionally dioecious species that require interplanting of female and male plants for sufficient pollination to promote commercial fruit size (Grant et al., 1994). Pollination is the most influential factor affecting fruit size and yield, as kiwifruit size is positively correlated with seed number (Ferguson, 1991). A. deliciosa fruit can have more than 1200 seeds per fruit, whereas A. chinensis ‘Hort16A’ was reported to contain up to ≈700 seeds per fruit (Goodwin et al., 2013; Hopping, 1976). For adequate pollination, an 8:1 or 6:1 female:male vine ratio is suggested (Reil, 1994).
Alone, wind pollination is ineffective in producing fruit of marketable size (Morley-Bunker and Lyford, 1999). For effective pollination, insects must be involved; the honeybee is the primary insect used. However, bees are not typically attracted to kiwifruit flowers compared with other flowers such as citrus and clover (Clinch, 1984; Ferguson, 1991). Kiwifruit flowers naturally lack nectar, which can make attracting pollinators difficult (Clinch, 1984). Growers go to great lengths to ensure successful pollination and often use supplemental pollen applied by hand or mechanically to increase pollination effectiveness and productivity.
To optimize pollination of female plants, it is important to know the length of time that flowers can be successfully pollinated. The EPP has been defined as the period following anthesis in which pollination can effectively produce a fruit (Sanzol and Herrero, 2001). This concept was developed by R.R. Williams (1970b) as a means of evaluating flower receptivity for fruit crops. The EPP for the commercial green kiwifruit standard, ‘Hayward’, was determined to be 4 DAA (Gonzalez et al., 1995). Fruit set during this 4-d period was 80% or greater. Stigmatic receptivity followed the same pattern as the EPP, as they both remained high for the first 4 DAA and then dropped on day 5. Hence, stigmatic receptivity was suggested to be the limiting factor. Stigmatic receptivity was also studied by Goodwin et al. (2013) for ‘Hort16A’ and was found to be the highest at 2 DAA. The EPP for ‘Hort16A’ was not defined.
With the development of AU kiwifruit cultivars that perform well in the southeastern United States, determining best management practices that optimize production of marketable fruit is important for the emerging kiwifruit industry. Enhancing pollination of this newly introduced crop will be necessary for producers to increase production of marketable fruit to increase associated returns on investment. The EPP has not previously been determined for the AU cultivars, and to our knowledge, has not been determined for any A. chinensis cultivars. By determining the EPP, growers will be able to concentrate their efforts during this crucial time to increase pollination and, in turn, improve orchard success. Hence, the objective of this study was to determine the EPP for the AU kiwifruit cultivars: A. chinensis ‘AU Golden Sunshine’ and A. deliciosa ‘AU Fitzgerald’.
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