2.1 POLLINATION
2.1.11 The potential role of pollination in crop improvement
Crop pollination is perhaps the most interdisciplinary field of study in all the agricultural sciences. It involves botany, entomology, plant breeding, horticulture, agronomy, genetics, bee breeding, ecology, agricultural economics, and pheromone biology. For many crops there is still paucity of information with which one can make good management decisions (DELAPLANE and MAYER, 2000). Fruits and seeds are the economic products of most crop plants. Effective pollination is a prerequisite for fruit- and seed-set. Therefore, successful pollination is of vital importance to realize optimal yield. In self-incompatible species, pollination is largely dependent on adequate cross-pollination. Even in self-compatible species, pollination is largely dependent on pollinating agents as automatic selfing seldom occurs or is insufficient in most of the self-compatible species. The majority of the crop plants, except cereals, are pollinated by insects, particularly bees. Adequate pollination is often a major constraint in many crop species due one or more of the following reasons (SHIVANNA, 2003):
• Drastic reduction in native pollinator populations because of the steady disappearance of natural habitats of insects, a marked increase in levels of pollutants and extensive use of environment-unfriendly chemicals, in particular herbicides and pesticides;
• Lack of a sufficient number of native pollinators due to the enormous increase in area covered by the same crop species (monoculture cropping); and
• Absence of natural pollinators for crops introduced from other regions.
Production in many of the fruit, seed and nut crops could be increased substantially by careful management of pollination (ROUBIK, 1995). Increased pollination efficiency can lead to an increase in crop value by increasing crop yield, uniformity, quality and decreasing the time of crop maturity. FREE (1993) and CURRIE (1997) reported several effective approaches to overcome pollination constraints. Some of these approaches are:
2.1.11.1 Habitat management
Increasing local populations of native pollinator species through habitat management.
This is particularly useful when availability of nest sites is a limiting factor. Habitat management can be achieved by maintaining uncultivated strips along field margins and providing permanent nest boxes (POMEROY, 1981). Such strategies should also ensure availability of forage sources when the target crop is not in bloom. This approach also requires management of cropping practices in such way that the flowering period of the target crop coincides with the peak populations of the pollinator (FREE, 1993). However, management of habitat is more expensive, particularly in areas of intensive agronomic practices (TORCHIO, 1990).
2.1.11.2 Use of commercially managed pollinators
The most economically viable and effective approach to overcome pollination constraints has been use of commercially managed pollinators, in particular honeybees (Apis spp), for pollination services. Honeybees are the most effective pollinators in a range of crop species (ROBINSON, 1979). Management of honey bees is convenient because of their large foraging populations, year-round availability and easy transportation (FREE, 1993). Increasing pollination efficiency through management of pollinators is warranted only when crops are pollen-limited and the cost involved is lower than the value realized through increases in crop production (CURRIE, 1997).
2.1.11.3 Spraying pollinator attractants on target crop
A number of studies have shown the potential of sprays with various substance on the target crop to attract pollinators. Sprays with dilute solutions of pheromones have shown considerable potential. Secretions from the Nasnov gland located on the dorsal side of the abdomen of worker bees consist of seven terenoids. Of these, geraniol and citral increase honey-bee foraging activity. Similarly, the queen honey- bee secretes a five component pheromone from its madibular glands. Sprays of
synthetic mandibular pheromones do increase honey-bee foraging activity and crop yield under a wide range of conditions (CURRIE et al., 1992; WINSTON and SLESSOR, 1993). Application of pheromones seems to be particularly effective on crops with flowers relatively unattractive to bees or during inclement weather conditions (SHIVANNA, 2003). Apart from pheromones, sprays of synthetic plant volatiles isolated from nectar or pollen are also effective in attracting honey-bees (DOBSON, 1994). Sprays containg food supplement such as Beeline® have also been reported to act as bee attractant in some crop species (MARGALITH et al., 1984).
2.1.11.4 Introduction of pollinators
Introduction of pollinators is one of effective approaches when crops are grown in areas where natural pollinators are absent, as often happens when a crop is introduced from one country to another. This approach involves detailed studies on the biology of the pollinator and monitoring its establishment in the new area (SHIVANNA, 2003). Oil palm (Elaeis guineesis) is native to Africa and Central South America. It was introduced to Malaysia and Indonesia and is grown extensively. In its native habitat, oil palm is pollinated by wind as well as many insects, in particular weevils. In many parts of Malaysia, where pollinating insects are absent, natural pollination was inadequate. Introduction of the weevil Elaeidobius kamerunicus, an important pollinator of oil palm from Cameroon is a successful example of such an approach. Introduction of the weevils has markedly increased the yield. Over the first seven months after weevil introduction, oil yield increased 20-53% (SYED, 1979).
2.1.11.5 Supplementary pollination
Assisted/supplementary pollination through pollen sprays or other methods such as manual hand pollination is the most effective technique for sustaining crop yield (WILLIAMS and LEGGE, 1979; HOPPING and JERRAM, 1980b). This is routinely carried out for small-scale production of a few crops, such as passion-fruit (ROUBIK, 1995). In high value plantation crops such as oil palm, pollination is a major constraint even in the presence of weevils, especially in younger plantations.
Although oil palm is monoecious, the male and female phases alternate, each extending for many months, thus at any given time the plant will be either male or female phase dominated. Insufficient number of plants in the male phase in the plantation and unfavourable weather conditions reduce pollination efficiency.
Assisted pollination is a common practice in oil-palm plantations particularly in younger plantations. Assisted pollination has been reported to increase yield 20- 150%, depending on age of the plants and weather conditions (SHIVANNA, 2003).
Assisted pollination requires standardization of protocols for pollen collection.
Different methods have been assayed for assisting pollination in oil palm. Hand pollination is regularly practiced for the Vanilla orchid. This plant is a native of southern Mexico and Central America where it is pollinated by the euglossine bee Eulaema (ROUBIK, 1995). Vanilla is grown extensively in many parts of tropical Asia where the pollinator is absent, hand pollination is routinely carried out to induce fruit- set (SHIVANNA, 2003).
Floral biology and pollination ecology of J. curcas were studied by RAJU and EZRADANAM (2002); BHATTACHARYA et al. (2005); CHANG-WEI et al. (2007).
However, these studies did not cover the implementation of pollination management for yield improvement in this plant. Therefore, Table 2.8 summarieze some pollination improvement done on other crop species.
Table 2.8 Improvement done on different crop species by pollination management.
Crop Aspects Reference
Avocado (Persea americana) Pollinators; pollination requirements pollinator behaviour;
and self- and cross-pollination
VITHANAGE (1990); ISH-AM et al. (1999);
DELAPLANE and MAYER (2000);
DEGANI et al. (2003).
Apple (Malus domestica) Pollinators and pollination requirements DELAPLANE and MAYER (2000).
Almond (Prunus dulcis) Pollinators and pollination requirements DELAPLANE and MAYER (2000).
Cotton (Gossipium hirsutum) Pollinators and pollination requirements DELAPLANE and MAYER (2000).
Cranberry (Vaccinium macrocarpon) Pollinators and pollination requirements DELAPLANE and MAYER (2000).
Kiwifruit (Actinidia deliciosa) Pollinators and pollination requirements; influence of honey bee on pollination and fruit quality
DELAPLANE and MAYER (2000);
HOWPAGE and SPOONER-HART (2001).
Peach and nectarine (Prunus persica) Pollinators and pollination requirements DELAPLANE and MAYER (2000) Pear (Pyrus communis) Pollinators and pollination requirements DELAPLANE and MAYER (2000) Plum and prune (Prunus domestica) Pollinators and pollination requirements DELAPLANE and MAYER (2000) Raspberry (Rubus idaeus Pollinators and pollination requirements DELAPLANE and MAYER (2000) Guava (Psidium guajava L.) Visitation effectiveness by honey bees FREITAS and ALVES (2008)
Mango (Mangifera indica) Pollinators DAG and GAZIT (2000)
Coffee (Coffea canephora) Pollinator effectiveness ROUBIK (2002); KLEIN et al. (2003).
Sunflower (Helianthus annuus) Pollinators effectivness and pollination requirements DELAPLANE and MAYER (2000); NDERITU et al.
(2008).