Plant Pollination Requirements and Definitions

Plant Pollination Requirements and Definitions (6)

One of the most recent, controversial, and potentially revolutionary areas of agricultural research is the development of genetically engineered, or so-called transgenic crops. Genetically engineered organisms are ones in which genes from different species have been
inserted in such a way as to permit those genes to express their characteristics in the host organism. In crop plants the focus has been on inserted genes from the bacterium Bacillus thuringiensis that trigger production of insecticidal compounds in the tissues of the host
plant. The number of different transgenes that have been used to confer insect resistance in crops approaches, but only B. thuringiensis transgenes have been commercialized. Transgenes also have been used to confer herbicide tolerance, drought or salt tolerance, or to alter the nutritional qualities of the crop. The relevance of pollinators to transgenic crop technology rests on two issues – the potential harm to pollinators posed by transgenic crops, especially insecticidal ones, and the potential harm to the environment caused by pollinators spreading transgenes into wild plant populations.

Transgenic insect resistance is seen to have numerous advantages over conventional insecticides. It provides more targeted delivery of a toxin to the pest, greater resilience of the toxin to weather or other forms of biodegradation, reduced exposure risk to the applicator, and reduced use of conventional broad-spectrum insecticides and their associated risks to the environment. But there has been concern that engineered toxins, whether in the plant’s tissues or in its nectar and pollen, could be detrimental to bees. Fortunately, that risk seems small at this time. A sizeable research record has shown that B. thuringiensis toxins, whether conventional or engineered, are generally benign to bees. The risk from other candidate engineered toxins, namely the pesticidal proteins chitinase, glucanase, and cowpea trypsin inhibitor.

Another area of concern with this new technology has been the potential of transgenes ‘escaping’ from engineered crops into wild plant populations with unknown and perhaps detrimental effects. Chief among these concerns is the potential for transgenes for herbicide tolerance becoming incorporated into weedy species, thus making them more difficult to control. Bees, because of their pollen-collecting habit and catholic flower preferences, are seen as primary vehicles for the spread of such transgenes. One solution to the problem may be a buffer zone of conventional crop plants grown around the transgenic ones. Pollinators visiting the transgenic plants may subsequently deposit much of their plant-available pollen on to conventional crop flowers in the buffer zone before leaving the area to visit other plant species or to return to the nest. It is unlikely that buffer zones alone will solve the problem in all cases. In one study, honey bee colonies were placed at a distance of 250 m from a field of transgenic herbicide-tolerant maize. The transgenic field was surrounded by a 3 m buffer zone of conventional maize. Of the pollen samples collected from the colonies, 52% contained the transgene for herbicide tolerance; thus the 3 m buffer zone did not prevent the spread of the transgene. There is evidence that most of the pollen from a particular plant is deposited by a bee forager on to the next few subsequent flowers visited, but some pollen can persist for up to the 20th subsequent flower. Ongoing research is concentrating on gene flow from transgenic crops, the competitiveness of transgenic plants, the efficacy of isolation distances, and the interactions of bee foraging behaviour with pollen movement among plants.

Ecologic theory supports the notion that bee visitation, pollination and seed yield. Likewise, low nectar quality and associated low levels of bee visitation are limiting factors in fruit-set in avocado. There is a clear and positive relationship among nectar sugar concentration, frequency of bee visitation, and resulting seed number in watermelon. Thus, theoretical work and supporting field studies strongly suggest that it is in the best interest of farmers to grow crop plants that are attractive to bees.

Nectar production is affected by ambient conditions and culturing practices. Nectar production is relatively high under conditions of low nitrogen supply, moderate growth, and high levels of sugar in tissues. It is lower under conditions of abundant nitrogen supply, high vegetative growth, and low sugar levels. Nectar production is generally higher in sunny weather because sugars accumulate in plant tissues during photosynthesis. These sugars may reach a surplus and be excreted as nectar if plants are not growing maximally. However, if nitrogen is available it encourages plant growth which diverts stored sugars into proteins and other products necessary for producing tissues. Thus, nectar production tends to be lower in plants that are growing rapidly. (In perennial plants that bloom before leaves unfold in spring, nectar production relies on sugars stored in tissues from the previous season.) The model of nectar production presented above is not universally applicable; for example, nectar production in the Mediterranean herbaceous perennial Ballota acetabulosa is relatively unresponsive to measured changes in solar irradiation.

There is evidence that nectar production also may be affected by plant genetics. There are measurable differences in nectar production within crop genera or species, as in pepper, cranberry, and watermelon, and sometimes these differences are apparent even when environmental or cultural effects are controlled. These studies suggest that nectar production is at least partly under genetic control and could be increased by selective plant breeding.

The time is right to give renewed attention to increasing nectar production in the world’s most important bee-pollinated crops. This is especially justified given the evidence for a generally decreasing pool of available bee pollinators. Nectar production has received comparatively little attention from crop breeders, agronomists, and horticulturists. Low nectar production may not be a problem in areas with abundant bee populations, but where bees are scarce a nectar-poor crop will have trouble competing with weeds for the
limited number of pollinators.

Bees use signals from plants to identify worthwhile visits. In some species the flowers remain open, intact and turgid until they are pollinated, after which they are no longer attractive to pollinators. The negative cues involved in this include cessation of nectar and scent production, change in colour, wilting, permanent flower closure, and petal drop. Even receptive inflorescences can vary in their attraction to bees. In general, inflorescences with a larger number of open flowers have higher nectar rewards, and bees preferentially land on those inflorescences. Once landed on an inflorescence, bees prefer wide, relatively shallow flowers, presumably because the nectar is more accessible to evaporation which concentrates it and increases the energetic profit of the visit.

These observations from ecological studies form the basis of a practical crop pollination recommendation. It is advisable for growers to delay the introduction of bee hives in an orchard until the crop has already begun a modest amount of flowering. This practice will provide bees with an abundance of floral signals that will encourage them to concentrate on the crop instead of non-target plants in the area. It is customary in apple to delay hive introduction until the crop is at about 5% bloom.

Bees, having encountered a patch of profitable flowers, tend to forage in a more-or-less straight line. This behaviour limits the chance of a bee revisiting a flower recently emptied of its nectar. The most straightforward implication for crop pollination involves those crops in which a main variety is inter-planted with one or more pollenizer varieties to ensure cross-pollination. Optimal foraging theory would suggest that the main varieties and pollenizers should be planted in the same orchard row to increase chances of bees cross-pollinating them.

From a pollination perspective, bee foraging activity is generally more efficient in flower patches that are rich in nectar and pollen. It has been shown that animals forced to forage in resource-poor habitats tend to spend more time at each food site than do animals in rich habitats (Pyke et al., 1977). It is advantageous for insects to be moving rapidly between flowers, accomplishing a high rate of pollination, rather than lingering for relatively long periods on the few flowers in a patch that are yielding nectar. Southwick et al. (1981) demonstrated that bee visitation rates increased in flower patches with increasing number of nectar-bearing flowers, nectar volume, and sugar concentration of nectar.

Not only do resource-rich plantings encourage rapid bee visitation between flowers, but they encourage pollinators to stay in that patch. It was shown that bumble bees and honey bees that have just visited highly-rewarding flowers fly shorter distances before visiting another flower than do bees that have just visited less rewarding flowers (Pyke, 1978; Waddington, 1980). This behaviour increases the likelihood of the bee encountering another rewarding flower in a site which is shown to be profitable.

Collectively, these studies make a strong argument for improving the nectar and pollen production output of our important bee-pollinated crops. Optimal foraging theory predicts that if nectar output of a crop is relatively high, bees pollinate more efficiently because they visit more flowers in a given period of time. Conversely, if the crop is nectar-poor, bees forage more slowly and visit fewer flowers.

Not all flowering plants have the same pollination requirements.

Cross-pollination is the transfer of pollen from flowers of one plant to the flowers of a different plant or different variety. Many crops require or benefit from cross-pollination.

Self-fertile plants can develop seeds and fruit when pollen is transferred from anthers of a flower to the stigma of the same flower or different flower on the same plant. However, such plants are not necessarily self-pollinating. Insects still may be necessary or helpful in moving pollen to the stigmas. Inter-planting of varieties is not necessary but may be helpful; for example, many self-fertile crops, such as Swede rape (canola) and high-bush blueberry respond well to cross-pollination.

Self-sterile plants require pollen from a different plant or even a different variety. If the plant requires different varieties, the grower must interplant pollenizer varieties with the main variety. Cross-compatible varieties are receptive to each other’s pollen, whereas crossincompatible varieties are not. Seed and nursery stock catalogues usually provide tables that cross-list compatible varieties.

Monoecious plants have both male and female flowers on the same plant. Dioecious plants have only one sex of flower on the same plant, rendering cross-pollination obligatory.

Parthenocarpic plants develop fruit without requiring the pollination process, and that being the case, parthenocarpic fruits can be partially or completely seedless. There are plant growth regulators that can be applied to induce plants, even plants normally cross-pollinated, to develop fruit parthenocarpically. This is the case in rabbiteye blueberry with the growth regulator gibberellic acid which is applied in early spring to augment natural pollination. It is important to treat these chemicals as supplements for pollination, not its replacement.