Pollination is a major yield-limiting constraint, due to the large number of flowers on trees and low fruit set. Unlike polyembryonic cultivars, which produce nucellar embryos, pollination is necessary for fruit set with mo-noembryonic cultivars (Popenoe, 1917; Young, 1942; Spencer and Kennard, 1955; Gunjate et al., 1983; Pimentel et al., 1984; Robbertse et al., 1994). Pollen compatibility within and between cultivars has been widely investigated. Complete or partial self-incompatibility has been reported (Mukherjee et al., 1961; Singh et al., 1962b; Sharma and Singh, 1970, 1972; Ram et al., 1976; De Wet et al., 1989; Robbertse et al., 1993). Incompatibility is evident by degeneration of embryonic and nucellar tissues and excessive loss of fruitlets. Cross incompatibility between some cultivars has also been noted (Saha and Chhonkar, 1972; Ram et al., 1976; Robbertse et al., 1993).
Early investigators concluded that the species is wind pollinated (Hartless, 1914). Initially wet pollen dries to a powdery consistency on anthers soon after anthesis in dry conditions (Pimentel et al., 1984), whence it is likely to be liberated in moving air or via gravity to adjacent stigmas on the same and nearby flowers (Naik and Mohan Rao, 1943; Mallik, 1957). Singh (1954a) and S.N. Singh (1961) suggested, however, that the amount of pollen moving in air streams was too low for wind to be a pollination vector. They did not report the location of pollen-collecting slides or take into account the close proximity of flowers within inflorescences or numbers of open flowers in the canopy. Panicles bagged to exclude pollinating insects were reported to set fruit (Free and Williams, 1976), which were retained to maturity, thereby confirming that mango pollen can be transferred by air movement or gravity (Bijhouwer, 1937; Mallik 1957). The tacit assumption that open-pollinated flowers are exclusively crossed is likely to be incorrect, although mango may favour cross-pollination.
Popenoe (1917) reasoned that pollen transfer occurs primarily within flowers by insects. Panicles bagged to exclude insect visitation generally result in less fruit set than on panicles in the open (Popenoe, 1917; Musahib-ud-din and Dinsa, 1946; Mallik, 1957; Free and Williams, 1976; Jiron and Hedstrom, 1985). Insects working mango flowers include Diptera, Hymenoptera, Lepidoptera and Coleoptera (Popenoe, 1917; Simao and Maranhao, 1959; Randhawa and Damodaran, 1961b; McGregor, 1974; Anderson et al., 1982; Jiron and Hedstrom, 1985). Flies of various genera are common on mango flowers (Pope-noe, 1917; Burns and Prayag, 1921; Bijhouwer, 1937; Singh, 1954a; Spencer and Kennard, 1955; Eardley and Mansell, 1993). Polistes wasps are observed on mango flowers but are considered to be ineffectual for pollen transfer (Spencer and Kennard, 1955; Free and Williams, 1976; Wolfenbarger, 1977). Honeybees (Hymenoptera) are occasional visitors (Young, 1942; Simao and Maranhao, 1959; Smith, 1960; Morton, 1964; Jiron and Hedstrom, 1985; Mac-Millan, 1991; Du Toit and Swart, 1993, 1994; Eardley and Mansell, 1993, 1994), but only if other more inviting flowers are not present (Spencer and Kennard, 1955; Free and Williams, 1976; McGregor, 1976). They are assumed to be the most effective pollinators of mango and may be more effective if hives are placed in orchards during flowering (Du Toit and Swart, 1993, 1994). Anderson et al. (1982) recorded actual pollen transfer on mango flowers by insects and found, in order of importance, the most efficient pollinators to be wasps, bees, large ants and large flies.
With few exceptions (Mallik, 1957), pollen deposition rates are generally low (Naik and Mohan Rao, 1943; Mukherjee, 1951). Differences in pollination rates can be attributed to environmental conditions during flowering, differing attraction of insects to specific cultivars, proximity of more attractive flowering species or a combination of the above. Young (1942) observed that insects visit only 10-12% of available flowers. Depending on weather conditions, insect activity on mango flowers is usually continuous from early morning to late afternoon, but nocturnal activity of some species has also been reported (Jiron and Hedstrom, 1985).
The role of insects in cross-pollination is not understood. Anderson et al. (1982) observed various insects carrying pollen to and from flowers and noted pollination subsequent to those visits; however, they made no distinction between actual pollen depositions by visiting insects and pollen transferred by other means. Wester (1920) considered that pollination is facilitated by wind and to a lesser extent by insects, and this conclusion is probably correct in most environments. Self-pollination within flowers by insects while the pollen is still damp is likely to occur. Use of isozyme (Degani et al., 1990; Rob-bertse et al., 1993) and microsatellite DNA markers (Adato et al., 1995; Schnell et al., 2005) to discern ratios of cross- versus self-pollinated fruitlets and offspring is the most accurate procedure to confirm self- and cross-pollination. Initial fruit set of pollinated flowers is inconsequential since most of these fruitlets abscise before reaching maturity (Lynch and Mustard, 1950; Singh, 1954a; Randhawa and Demodaran, 1961a).
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