Apple pollination occurs when pollen is transferred, often by an insect vector, from the anthers of one blossom to the stigma of another. After pollen grains are deposited, they are rehydrated by stigmatic secretions, and pollen tube growth begins (Dennis, 2000; Jackson, 2003). Genetically compatible pollen tubes grow through the stigma and style and toward the ovaries. At the end of the style, the pollen tube enters the ovary through the micropyle, where two sperm cells are released into the ovule. Fertilization occurs when one nucleus fuses with the egg cell, and the second nucleus fuses with the embryo sac, initiating seed formation (Dennis, 2000). Investigations into apple pollen biology have been underway for over a century (Adams, 1916) and have recently been reviewed by Ramírez and Davenport (2013).
In commercial apple orchards, cross-pollination between two distinct genotypes results in greater fertilization rates and thus a greater fruit set, even in orchards containing self-fertile cultivars (Dennis, 2003). Various Malus species, colloquially referred to as crabapples, are preferred pollinizers in commercial orchards due to their long bloom periods, disease resistance, and abundant pollen production (Fitzgerald, 2005). In particular, the crabapple Malus baccata ‘Manchurian’ has been widely planted in commercial orchards as a pollinizer. However, it has fallen into disfavor in recent years due to its high susceptibility to canker diseases, which could limit apple exports into Asian markets (Hansen, 2014). Other popular crabapple pollinizers include ‘Snowdrift’, ‘Thunderchild’, ‘Indian Summer’, ‘Mt. Blanc’, ‘Wickson Crab’, and ‘Chestnut Crab’.
The pollen tube growth rate of ‘Snowdrift’ has been used to develop a temperature-driven pollen tube growth model that can predict the time between pollination and fertilization (Peck et al., 2016; Yoder et al., 2013). The model is now being used by commercial apple growers to precisely calculate when to apply bloom-thinning chemicals (Lehnert, 2014). The majority of bloom-thinning chemicals are caustic and prevent pollination or fertilization by destroying flower parts and inhibiting pollen germination and pollen tube growth. Liquid lime sulfur (calcium polysulfide) is the most studied and commercially used bloom-thinning chemical (McArtney et al., 2006).
Although Snowdrift pollen tube growth can differ by as much as 3-fold among maternal cultivars, published reports on the pollen tube growth rates of other crabapple species and cultivars are limited (Embree and Foster, 1999; McArtney et al., 2006; Stott, 1972; Williams, 1965). Successful application of blossom-thinning chemicals is highly dependent on understanding the rate of pollen tube growth among different pollen donor genotypes (Jackson, 2003). Analyzing the pollen tube growth rates of additional pollen donors could contribute to the selection of improved pollinizers for apple orchards.
The objectives of this study were to determine how pollen tube growth rates were affected by paternal (pollen source) and maternal cultivars, temperatures after pollination, and the interaction among these factors. We did this by comparing the pollen tube growth rates of five crabapple cultivars on three maternal culinary apple cultivars at four temperatures.
Berardi, A.E., Frey, F.M., Denton, E.M. & Wells, J.H. 2013 Betalain color morphs exhibit differential growth, defensive ability, and pollen tube growth rates in Miribilis jalapa (Nyctaginaceae) Intl. J. Plant Sci. 174 1229 1238
Chapman, P.J. & Catlin, G.A. 1976 Growth stages in fruit trees—From dormant to fruit set. N.Y.’s Food Life Sci. Bul. No. 58
Chen, C. & Gibson, P.B. 1973 Effect of temperature on pollen-tube growth in Trifolium repens after cross- and self-pollinations Crop Sci. 13 563 566
Dennis, F. Jr 2003 Flowering, pollination and fruit set and development, p. 153–166. In: D.C. Ferree (ed.). Apples botany production and uses. CAB Intl., Cambridge, UK
Embree, C.G. & Foster, A. Jr 1999 Effects of coatings and pollenicides on pollen tube growth through the stigma and style of ‘McIntosh’ apple blossoms J. Tree Fruit Production 2 19 32
Fitzgerald, T. 2005 Pollination in fruit trees. Washington State Spokane County Ext. Bul. C105
Hansen, M. 2014 Is there a better crab apple pollinizer? 19 Mar. 2014. <http://www.goodfruit.com/is-there-a-better-crab-apple-pollinizer/>.
Hedhly, A., Hormaza, J.I. & Herrero, M. 2005 The effect of temperature on pollen germination, pollen tube growth, and stigmatic receptivity in peach Plant Biol. 7 476 483
Jackson, J.E. 2003 Biology of apples and pears. Cambridge Univ. Press, Cambridge, MA
Kron, P. & Husband, B.C. 2006 The effects of pollen diversity on plant reproduction: Insights from apple Sex. Plant Reprod. 19 125 131
Lehnert, R. 2014 Pollen tube growth model makes thinning more precise. 19 Mar. 2014. <http://www.goodfruit.com/pollen-tube-growth-model-makes-thinning-more-precise/>.
McArtney, S., Palmer, J., Davies, S. & Seymour, S. 2006 Effects of lime sulfur and fish oil on pollen tube growth, leaf photosynthesis, and fruit set in apple HortScience 41 357 360
Peck, G.M., Combs, L.D., DeLong, C.N. & Yoder, K.S. 2016 Precision apple flower thinning using organically-approved chemicals Acta Hort. (In press)
Sorkheh, K., Shiran, B., Rouhi, V. & Khodambashi, M. 2011 Influence of temperature on the in vitro pollen germination and pollen tube growth of various native Iranian almonds (Prunus L. spp.) species Trees (Berl.) 25 809 822
Stösser, R., Hartmann, W. & Anvari, S.F. 1996 General aspects of pollination and fertilization of pome and stone fruit Acta Hort. 423 15 22
Yoder, K., Yuan, R., Combs, L., Byers, R., McFerson, J. & Schmidt, T. 2009 Effects of temperature and the combination of liquid lime sulfur and fish oil on pollen germination, pollen tube growth, and fruit set in apples HortScience 44 1277 1283
Yoder, K.S., Peck, G.M., Combs, L.D. & Byers, R.E. 2013 Using a pollen tube growth model to improve apple bloom thinning for organic production Acta Hort. 1001 207 214