Factors Affecting Pollen Exposure Dispersal and Success

The timing for anther opening and pollen release varies with geographical area and season. There are few reports of anthers closing when the weather is wet or during rain. Many flowers open when insects start to fly. In an anemophilous temperate-zone species such as Mercurialis annua, that blooms all year around, anthers open around 7 a.m. in summer and 11-12 a.m. in winter. In temperate zones, anthers of anemophilous and entomophilous flowers generally open from 8 a.m. to 2 p.m., night pollination is restricted to dry summer periods, and night mists purge the air of pollen. In tropical countries, pollination occurs around the clock, but night pollination is always zoophi-lous. In temperate countries, pollen vectors vary with the seasons, anemophilous trees pollinating in late autumn and late winter-early spring, when many have shed their leaves. The period when entomophilous pollination may occur progressively reduces from the tropics to the poles. Table 1 shows the main environmental parameters affecting pollination.

Pollen may have different probabilities of effecting legitimate pollination (Figure 1), depending on the dispersing vector, and the distances it may travel vary (Table 2). Pollen vectors have different specificities, that is, possibility of transporting pollen to the right landing site. Table 2 shows the specificity, efficiency, and distances reached with different pollination vectors. The main features of anemophilous and entomophilous pollination are shown in Table 3.

Abiotic pollination appears random since there are no mechanisms to ensure cross-pollination. Pollen is released into air or water, the movements of which disperse and transport it. When air or water speed is high, pollen may be dispersed long distances, to places where no individuals of the species grow. The further the pollen is dispersed, the less its chance of finding the right female counterpart. Pollen flight is quick and short in farmed anemophilous species, such as wheat, oats, rice, and corn, all of which have PHP.

Although anemophily may seem random, in some gym-nosperms and angiosperms at least, anemphilous pollen dispersal ceases being random when pollen grains approach the tip of a cone or flower. By virtue of the shape of the grains and female parts, pollen is conveyed by air currents to the right landing site. Pollen lands on the stigma by gravity (anemophily), or because a pollen-dusted insect incidentally touches the stigma. Electrostatic forces are invoked to explain pollen uptake by insects and release on the stigma.

Biotic pollination first occurred when an animal touched an anther and incidentally delivered pollen to the female

Table 3 Features of common pollination syndromes

Features

Entomophily

Anemophily

Habitus

Isolated herbs, shrubs, and trees

Trees and social herbs such as grasses

Environment and

Tropical, all year around

Tropical, only dry periods

season

Temperate, only spring and summer

Temperate, mainly late autumn and late winter

Inflorescence

Different types or solitary flowers

Often pendulous and monoecious

Flowers

Hemaphrodite, rarely monoecious or

Monoecious, dioecious, rarely hermaphrodite

dioecious

Large

Inconspicuous, not attractive

Stamen and stigma often inside corolla

Pendulous stamen with long filaments and stigma often outside

corolla

Pollen

With abundant ornamentation

With reduced ornamentation

With pollenkitt or devices for mass

Free, independent grains

transport

Ovules/ovary

Generally many

Generally one

counterpart. Interactions between partners began in this way, sometimes leading to species-specific relationships, which are dangerous because if the pollinator becomes extinct or rarefied, the plant can no longer reproduce sexually and will die out if unable to reproduce vegetatively. While pollination is important for plants, being one of the first steps of plant sexual reproduction, for the animal counterpart it represents a source of food: pollen and/or nectar. Abiotic pollination is less expensive for plants because investment in rewards is not necessary; however, investment in an excess of pollen is necessary for random dispersal.

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