2. Plant pollination requirements
The impact of any pollinator, whether in terms of agricultural production or plant fitness, is an interaction between at least two dynamics: the pollen vectoring capacity of the flower visitor and the genetic obligation, or responsiveness, of the plant to pollen deposition on its stigmas (Delaplane, 2011). Most of this chapter is devoted to appraising pollen vectoring capacity, but in this section we begin with the underlying demands of the plant because this is the necessary starting point for understanding and contextualizing any pollination syndrome; the suite of flower characters derived by natural selection in response to pollinating agents, whether biotic or abiotic (see Faegri and Pijl, 1979).
To begin, pollination is the transfer of pollen from the anthers to the stigma of flowers of the same species and is essential to the reproduction of most angiosperms (flowering plants). Pollination success is often measured in terms of percentage fruit- or seed set. Fruit- or seed-set is the ratio of ripe fruit or seeds relative to initial number of available flowers or ovules, respectively. This ratio is rarely 100% owing to such factors as normal levels of fruit abortion, suboptimal pollination conditions, herbivory, or cultural problems.
The degree to which a plant species depends on a particular pollinator is determined in part by the mating and breeding system of the plant (Fig. 1). Some plants can produce seeds or fruits without pollination, and understanding this process is important for understanding when the honey bee can or cannot contribute to fruit- or seed-set and yield enhancement. Asexual reproduction through non-fertilized seeds is called apomixis or agamospermy. Apomixis happens when an embryo is formed either from an unfertilized egg within a diploid embryo sac that was formed without completing meiosis (blackberries, dandelions) or from the diploid nucleus tissue surrounding the embryo sac (some Citrus species, some mango varieties). When fruit forms without fertilization of ovules, either naturally or chemically-induced, this is called vegetative parthenocarpy (banana, pineapple, seedless cucumber). In either apomixis or parthenocarpy no fertilization occurs, and pollination is not required. However, in some plant species, pollination or some other stimulation is required to produce parthenocarpic fruits, a chief example being seedless watermelon, a type of stimulative parthenocarpy. Also, in many apomitic plants apomixis does not always occur, or occurs only partially, and sexual reproduction can also take place (Citrus and mango).
Most angiosperms, however, need pollination to set seeds and fruits, and with the exception of those whose flowers are capable of autopollinating (ex. many beans, soybean, peach, peanuts), they rely on agents to vector the pollen. Angiosperms have basically two mating systems: outcrossing (xenogamy) in which pollination occurs between plants with different genetic constitutions, and selfing (autogamy) in which no mixing of different genetic material occurs other than through recombination. Outcrossing is achieved by cross pollination, resulting from the transfer of pollen between different flowers of different plants of the same species, while selfing is the outcome of pollen transfer within the same flower (self-pollination) or between different flowers of the same plant (geitonogamy). Some plant species are strictly xenogamous while others are autogamous, but mixed mating systems in which plants use outcrossing and autogamy or even outcrossing, autogamy and agamospermy are not uncommon (Rizzardo et al., 2012).
The extent to which an angiosperm responds to pollination and the fraction of that pollination that is selfed or out-crossed vary greatly by plant species or variety, and in any particular case a flower visitor must meet specific needs to qualify as a legitimate pollinator. We describe below some field methods for determining the mating system and pollination requirements of plants and the potential pollination role of abiotic and biotic agents, focusing on the level of the individual plant rather than the plant population and drawing heavily upon the following published works (Spears, 1983; Mesquida et al., 1988; Freitas and Paxton, 1996, 1998; Sampson and Cane, 2000; Dafni et al., 2005; Pierre et al., 2010, Vaissière et al., 2011).
Fig. 1. Plant mating systems and pollination requirements.