Pollination occurs when pollen grains move from anthers to a reproductively mature stigma. After pollen lands on the stigma, it initiates pollen tube formation. Pollen tubes are channels which grow down the length of the style to the ovules. After a pollen grain travels down a pollen tube it intercepts an ovule, fertilizes the female germ cells and initiates seed development.
Pollen doesn’t have legs! How does it move from anthers to stigma?
Pollen can move from anthers to stigma with the help of wind or animals. Walnut, chestnut, pistachio, and olive pollen is blown by the wind from anthers to receptive stigmas. Wind pollination is also common in pine trees and many grasses present in California. Video 1 shows pollen released by juniper trees.
All fruit tree crops in the Prunus genus (apricot, peach, cherry, nectarine, almond, plum and prune) require insects for pollination, however the extent to which they rely on insects for adequate fruit set varies among species. Video 2 shows the bee collecting pollen to consume and store in pollen sacs on its legs. While collecting pollen some lands on the body of the bee and is transferred to the stigma of the next flower it visits.
The relationship between pollination and fruit development will be discussed in the section on Seed & Fruit Development.
Pollination on Video...
Video 1. Wind dispersed pollen shedding from Juniper trees over the course of several days. Pollen in this wind pollinated species is abundant and travels from male flowers to female flowers downwind. Source: Mountain Cedar Tree Exploding with Pollen (YouTube)
Video 2. Time lapse images of pollination by a bee. Source: Bees and Plant Pollination (YouTube)
The Relationship between Flower Morphology & Pollination
Wind and animal pollination strategies are associated with consistent differences in flower morphology which maximize the likelihood of successful pollination:
Wind pollinated flowers of tree fruit and nut crop species are typically imperfect (separate male and female flowers). Male flowers have exerted anthers, small light-weight pollen grains to facilitate wind dispersal, and lack petals and nectaries. Female flowers lack petals and nectaries, and contain exerted stigmas with a large surface area to capture pollen blowing in the wind (Figure 5).
Figure 5 (below) 5a: Walnut female flower with small pollen grains (orange), enlarged stigma (purple), sepals (light green) and a bract (green). 5b: Walnut male flower containing only sepals (light green), a bract (dark green), and anthers (blue). 5c: In walnut, as in many other wind pollinated crop species, the male flowers are arranged in groups along long “catkins”.
Figure 6 (right). Illustration of an insect pollinated peach flower with multiple reduced anthers (dark blue), large pollen grains (orange), smaller stigma (dark purple), long exerted style (light purple), showy petals (pink), and nectaries (yellow). Image source: USDA Handbook 495.
Does pollination always result in fertilization and seed development?
No! Many flowering plant species have evolved mechanisms which prevent pollen from successfully fertilizing ovules of the same individual (self-fertilization). Inbreeding (mating and reproduction between closely related individuals) is generally associated with an increased risk of producing malformed offspring. As a result of the negative impacts of inbreeding several strategies have evolved in plants to encourage mating and reproduction among unrelated individuals (outcrossing ), including:
Temporal separation of pollen and ovules
In some species the female and male reproductive structures within an individual mature at different times. For example, stigmas may become receptive before anthers release pollen on an individual tree. This temporal separation will increase the likelihood that pollen arriving at stigmas was transported from another individual, not from different flowers on the same tree or the anthers within the same flower. If there is very little overlap between pistil receptivity and pollen dispersal within a cultivar, (for example, in Chandler walnuts), growers should plant 1-2 individuals of at least one additional cultivar that disperses pollen in synchrony with pistil receptivity of the primary cultivar, to ensure adequate fruit set.
Spatial separation of pollen and ovules
Some plants have imperfect flowers with male and female structures separated in different flowers. Male and female flowers may be located on the same tree (monoecious) or on separate individuals (dioecious). Spatial separation between pistils and stamen among imperfect flowers eliminates the risk of pollen from one flower landing on the stigma of the same flower, a potential problem in perfect flowers. Similarly, increasing the physical separation between male and female flowers (as in dioecious species) lowers the likelihood of inbreeding. The ratio of male and female individuals planted should maximize yield by planting the minimum number of males (which do not produce fruit) needed to ensure adequate pollination of female trees which produce fruits or nuts.
Stigma-pollen interactions and self-incompatibility genes
Self-incompatibility (SI) genes prevent the formation of pollen tubes and fertilization by related individuals within the same species.If a pollen grain contains the same version of the SI gene as the stigma it lands on, because it comes from a closely related individual, pollen tube growth is inhibited and fertilization cannot occur. However if a pollen grain contains a different version of the SI gene, because it comes from an unrelated individual, pollen tube growth is initiated, the ovule will be fertilized and a seed will develop. SI genes are common in tree fruit and nut crops, including almond and sweet cherry, and necessitate planting several cultivars in an orchard to ensure successful fertilization and fruit set.