Fruit growers always strive to obtain maximum yields of high grade quality produce from their orchards. There are many factors that can influence yields including climate, suitable varieties, soil and availability of water and nutrients. The fruit is the ultimate economic product of any fruit tree and is generally dependent upon successful pollination and fertilization. Therefore it is important for the fruit growers to ensure that conditions are favourable for pollination in the orchards.
Most horticultural fruit and nut species have outcrossing mechanisms and require insect pollination (McGregor 1976). Outcrossing mechanisms include dioecy where the unisexual flowers are separated on different plants, e. g. kiwi fruit; dichogamy where there is temporal separation of the sexes in hermaphrodite flowers, e.g. avocado; and self-incompatibility where physiological or biochemical barriers prevent self-fertilisation, e.g. macadamia. Fruits and nuts such as lychee, mango and cashew are all monoecious in that both male and female flowers are borne on the same plant. These individual flowers can mature at different times which can hinder self-fertilisation. Therefore in all these situations, insect vectors are needed to transfer pollen from one flower to another so that fertilisation and fruit development can occur.
Avocado is a native of Central America and the West Indies (Alexander 1978) which is being grown commercially in many parts of the world (Berg 1975).
The flowers show a marked protogynous dichogamy and complementary flower types (A and B) exist. In type A varieties the flowers first open in the female stage in the morning, then close completely to reopen in the male stage in the afternoon of the following day. In type B varieties, the flowers first open in the female stage in the afternoon, then close overnight and reopen in the male stage the following morning (Robinson and Savage 1926). Successful pollination of avocado occurs only during the female stage (Sedgley 1977).
Macadamia is native to Australia and is grown commercially for its edible nut. The flowers of macadamia are protandrous (Sedgley et al. 1985), borne in racemes and are ivory white (M. integrifolia) or light pink (M. tetraphylla). The anthers of macadamia flowers dehisce prior to anthesis and the pollen grains are deposited on the stylar tip (Urata 1954). At anthesis when the corolla splits, the extending style carries the pollen on its tip from where it is picked up by pollinators. The stigma papillae at the tip of the style are non-receptive at anthesis which prevents self-fertilisation.
Lychee is a major subtropical fruit crop which is widely cultivated in Asia. China is the major producer followed by India and Taiwan. Australia has an expanding industry at present.
Lychee flowers are functionally male or female and three types of flowers occur in succession on the same inflorescence (Mustard and Lynch 1959). The flowers are first male, with stamens but no visible ovary (functionally male) followed by hermaphrodite with small infertile stamens (functionally female) then hermaphrodites with a small infertile ovary (functionally male) (Shukla and Bajpai 1974, Scholefield 1982).
These floral mechanisms dictate the need for outside pollinators to bring about pollen transfer from one flower to another. In all these cases the agents of pollination are insects with the honey bee, Apis mellifera, playing a leading role (Vithanage and Ironside 1986, Vithanage 1986).
Field experimentation on pollination in avocado and macadamia have been carried out since 1983 in CSIRO orchards and private grower properties near Mildura, Victoria (Latitude 34°S). Those involving lychee have been carried out since 1987 in private grower properties near Nambour, Queensland (Latitude 26°S).
The experimentation on pollination biology on these crops can be approached in three ways.
Firstly, by evaluating visitation rates of honey bees and their foraging behaviour. Counting of pollen grains carried on the bodies of individual bees is a part of this study. Honey bee activity on avocado and macadamia compiled in 1985 is summarised in Table 1.
|Mean No. of insects/100 flowers/hour||Quantity of pollen carried/hour*|
|Avocado||15.97 + 2.03||25.6 x 103|
|Macadamia||8.7 + 0.94||9.6 x 105|
*Quantity of pollen carried/hour pollen carried/insect) x (mean visited/hour).
Secondly, by evaluating the success of pollen transfer as a result of bee visits. This is usually carried out by monitoring pollen tube growth through the style and from yield records of caged and uncaged inflorescences. Results of pollination success in terms of pollen tube growth observed in bagged and unbagged flowers are summarised in Table 2.
|Bagged flowers||Open flowers||t|
Thirdly, honey bee hives can be introduced to the orchards and the success of bee activity monitored as outlined in the other two approaches. Results of avocado yields in terms of fruit numbers and weights for orchards with and without bee hives are summarised in Table 3.
|Mean fruit weight||Mean no. of fruits/tree|
All these fruit and nut crops, namely, avocado, macadamia, lychee, cashew etc. are relatively recent introductions to Australian horticulture. Although some research data are available from countries where these crops have been grown for hundreds of years, often the results are not directly applicable to the Australian situation.
Overseas work has shown that provision of honey bee hives can improve crop production for all of these crops. Consequently growers are advised to place bee hives in orchards at flowering time. Yet in Australia, experiments along these lines have not shown the same success rate. There are two main factors that could influence these results. One is the relative attractiveness of the crop to honey bees in relation to competing flora in the vicinity. The other is the microclimate which can affect bee behaviour.
Current research on pollination biology of the above crops is geared towards resolving these two aspects. A better understanding of bee behaviour in terms of its preferences and the microclimate in the orchard will help us exploit bees for improved crop production.
Through plant breeding and selection, it may be possible to develop varieties that are self-compatible with less dependence on pollen vectors as has occurred with the more traditional fruit species. In the short term, however, the provision of bee hives at the time of flowering appears to be an essential management practice for these crops.
Financial support of the Honey Research Council for some of the work reported here is gratefully ackowledged.
Alexander, D. McE. (1978). Some avocado varieties for Australia. CSIRO publication pp 36. Bergh, B.D. (1975) . In: Advances in Fruit Breeding, eds. J. Janick and J. N. Moore, Purdue University Press, Indiana. 541-67.
McGregor, S.E. (1976). Insect pollination of cultivated crop plants. USDA Ag. Hndbk. 496.
Mustard, M.J. and Lynch, S.J. (1959). Notes on lychee panicle development. Proc.Fla.Sta.Hort. Soc, 72, 324-327.
Robinson, T.R. and Savage, E.M. (1926). Pollination of the avocado. USDA Circular 387, 1-16.
Scholefield, P.B. (1982). A scanning electron microscope study of flowers of avocado, litchi, macadamia and mango. Scientia Hort. 16, 263-272.
Sedgley, M. (1977). Reduced pollen tube growth and the presence of callose in the pistil of the male floral stage of the avocado. Scientia Hort. 7, 27-36.
Sedgley, M., Blessing, M. and Vithanage, V. (1985). A developmental study of the structure and pollen receptivity of the macadamia pistil in relation to protandry and self-incompatibility. Bot. Gaz. 146, 6-14.
Shukla, R.K. and Bajpai, P.N. (1974). Blossom bud differentiation and ontogeny in litchi (Litchi chinensis Sonn.). Ind. J. Hort. 31, 226-228.
Vithanage, V. andIronside, D.A. (1986). The natural insect pollinators of macadamia and their relative importance. J. Aust. Inst. Sci. 52, 155-160.
Urata, U. (1954). Pollination requirements of macadamia. Hawaii Agric. Exp. Stn. Tech. Bull. No.22. p 1-40
Q.D.P.!. Technical feature on "Banana Diseases - A continuing Threat." By Ken Pegg, David Jones, Peter Langdon Mark Ramsey, Plant Pathology Branch, and Mike Smith, Horticulture Branch. C.O.D. Fruit & Vegetable News Thursday March 22nd 1990.
DATE: May 1990
* * * * * * * * * * * * *