produce copious, thick nectar
On the basis of pollination mechanisms, plants may be grouped into two mating systems: self‐pollinated or cross‐pollinated. Self‐pollinated species accept pollen primarily from the anthers of the same flower (autogamy). The flowers, of necessity, must be bisexual. Cross‐pollinated species accept pollen from different sources. In actuality, species express varying degrees of cross‐pollination, ranging from lack of cross‐pollination to complete cross‐pollination.
5.5 What is autogamy?
Self‐pollination or autogamy occurs in a wide variety of plant species – vegetables (lettuce, tomatoes, snap beans, endive), legumes (soybean, peas, lima beans), and grasses (barley, wheat, oats). Certain natural mechanisms promote or ensure self‐pollination, specifically cleistogamy and chasmogamy, while other mechanisms prevent self‐pollination (e.g. self‐incompatibility, male sterility).
5.5.1 Mechanisms that promote autogamy
Cleistogamy is the condition in which the flower fails to open. The term is sometimes extended to mean a condition in which the flower opens only after it has been pollinated (as occurs in wheat, barley, and lettuce), a condition called chasmogamy. Some floral structures, such as those found in legumes, favor self‐pollination. Sometimes, the stigma of the flower is closely surrounded by anthers, making it prone to selfing.
Very few species are completely self‐pollinated. The level of self‐pollination is affected by factors including the nature and amount of insect pollination, air current, and temperature. In certain species, pollen may become sterilized when the temperature dips below freezing. Any flower that opens prior to self‐pollination is susceptible to some cross‐pollination. A list of predominantly self‐pollinated species in presented is Table 5.2.
Table 5.2 Examples of predominantly self‐pollinated species.
| Common name | Scientific name |
| Barley | Hordeum vulgare |
| Chickpea | Cicer arietinum |
| Clover | Trifolium spp. |
| Common bean | Phaseolus vulgaris |
| Cotton | Gossypium spp. |
| Cowpea | Vigna unguiculata |
| Eggplant | Solanum melongena |
| Flax | Linum usitatissimum |
| Jute | Corchorus espularis |
| Lettuce | Letuca sativa |
| Oat | Avena sativa |
| Pea | Pisum sativum |
| Peach | Prunus persica |
| Peanut | Arachis hypogaea |
| Rice | Oryza sativa |
| Sorghum | Sorghum bicolor |
| Soybean | Glycine max |
| Tobacco | Nicotiana tabacum |
| Tomato | Solanum lycopersicum |
| Wheat | Triticum aestivum |
5.5.2 Mechanisms that prevent autogamy
There are several mechanisms in nature that work to prevent self‐pollination in species that otherwise would be self‐pollinated. These include self‐incompatibility, male sterility, and dichogamy.
Self‐incompatibility
Self‐incompatibility (or lack of self‐fruitfulness) is a condition in which the pollen from a flower is not receptive on the stigma of the same flower and hence incapable of setting seed. This happens in spite of the fact that both pollen and ovule development are normal and viable. It is caused by a genetically controlled physiological hindrance to self‐fertilization. Self‐incompatibility is widespread in nature, occurring in families such as Poaceae, Cruciferae, Compositae, and Rosaceae. The incompatibility reaction is genetically conditioned by a locus designated S, with multiple alleles that can number over 100 in some species such as Trifolium pretense. However, unlike monoecy and dioecy, all plants produce seed in self‐incompatible species.
Self‐incompatibility systems
Self‐incompatibility systems may be classified into two basic types: heteromorphic and homomorphic.
Heteromorphic incompatibilityThis is caused by differences in the lengths of stamens and style (called heterostyly) (Figure 5.5). In one flower type called the pin, the styles are long while the anthers are short. In the other flower type, thrum, the reverse is true (e.g. in Primula). The pin trait is conditioned by the genotype ss while thrum is conditioned by the genotype Ss. A cross of pin (ss) × pin (ss) as well as thrum (Ss) × thrum (Ss) are incompatible. However, pin (ss) × thrum (Ss), or vice versa, is compatible. The condition described is distyly because of the two different types of style lengths of the flowers. In Lythrum three different relative positions occur (called tristyly).
Homomorphic incompatibilityThere are two kinds of homomorphic incompatibility: gametophytic and sporophytic (Figure 5.6).Gametophytic incompatibilityIn gametophytic incompatibility (originally called the oppositional factor system), the ability of the pollen to function is determined by its own genotype and not the plant that produces it. Gametophytic incompatibility is more widespread than sporophytic incompatibility. Gametophytic incompatibility occurs in species such as red
