Actinidia deliciosa A. Chev. and A. chinensis Planch. are dioecious species that have vegetative and compound buds, with flower clusters produced in the leaf axils of the first four to six nodes. Male and female flowers are perfect morphologically. The female flower contains some anthers, but only the stigma is functional, whereas the male vine typically produces 125 to 185 large anthers that surround a small, vestigial stigma (Thorp, 1994). Commercial kiwifruit production requires interplanting female and male plants for sufficient pollination to promote commercial fruit size (Grant et al., 1994).
Kiwi buds enter endodormancy during winter, which requires a minimum number of chilling hours for maximum budbreak and bloom (Lionakis and Schwabe, 1984; Snelgar et al., 1997). Floral uniformity and density in spring is directly related to the amount of chilling received during winter (Snelgar et al., 1997). Snelgar et al. (1997) proposed using hours below 7 °C to break the endodormancy of A. deliciosa. Because metabolism is so slow below 0 °C, a more accurate measure of chilling hours are Richardson units, which are defined as the accumulated hours between 0 °C and 7 °C (Grant et al., 1994; Samish and Levee, 1962). The site of growth inhibition of kiwi is apparently in the bud scales because their removal was shown to promote budbreak (Lionakis and Schwabe, 1984). Very little is known concerning the chilling requirement of different kiwi species and cultivars.
Actinidia deliciosa cv. Hayward is a green-fleshed kiwi that is the most widely planted commercial cultivar in the world. ‘Hayward’ has a chilling requirement of over 950 h for vegetative buds and 1150 h for optimum flowering (Caldwell, 1989). Some cultivars of A. deliciosa such as ‘Bruno’ may have a lower chilling requirement of 700 h (Caldwell, 1989). ‘AU Fitzgerald’ is an A. deliciosa female cultivar that is being released by Auburn University. ‘AU Fitzgerald’ was an open-pollinated seedling of ‘Hayward’ that has performed well on the northern coastal fringe of the Gulf of Mexico, which normally receives less than 700 chill hours during winter. Two yellow-fleshed A. chinensis cultivars that were developed by the Fruit and Tea Institute in Hubei Provence, China, ‘Golden Sunshine’ and ‘Golden Dragon’, have been observed to perform well in central Alabama, which has an average winter chilling of 800 to 1200 h. This study was conducted to determine the chilling requirement of ‘Golden Sunshine’, ‘Golden Dragon’, and ‘AU Fitzgerald’. Two male cultivars, ‘AU Authur’ (another cultivar that is being released by Auburn University) and ‘Matua’, were also tested to determine overlap in bloom with the female cultivars. Determining the chilling hours will aid decisions regarding recommendations in planting these cultivars in various climatic regions.
Caldwell, J. 1989 Kiwifruit performance in South Carolina and effect of winter chilling Proc. Ala. Fruit and Vegetable Growers Assoc 10 127 129
Grant, J.A., Polito, V.S. & Ryugo, K. 1994 Flower and fruit development: An overview Kiwifruit growing and handling Univ. CA. Div. Agr. Nat. Res., Publication 3344 CA
Lim, C.C., Arora, R. & Townsend, E.C. 1998 Comparing Gompertz and Richards functions to estimate freezing injury in Rhododendron using electrolyte leakage J. Amer. Soc. Hort. Sci. 123 246 252
Lindén, L., Palonen, P. & Lindén, M. 2000 Relating freeze-induced electrolyte leakage measurements to lethal temperature in red raspberry J. Amer. Soc. Hort. Sci. 125 429 435
Snelgar, W.P., Manson, P.J. & McPherson, H.G. 1997 Evaluating winter chilling of kiwifruit using excised canes J. Hort. Sci. 72 2 305 315
Snowball, A. & Considine, J. 1986 Flowering in kiwifruit (Actinidia deliciosa Liang et Ferguson): Positional effects and development Acta Hort. 175 85 89