Hello, dear readers. Today’s post will be a continuation from where I last stopped on REPRODUCTION IN FLOWERING PLANTS. Right now, I’ll embark on discussing the strategies of reproduction between plants.
PLANTS AND SEX: STRATEGIES IN REPRODUCTION
Sex in plants and animals is vital for two reasons: it creates new offspring and it shuffles the genes, giving variation. If plants remained genetically the same, they would never evolve. That said, sometimes a plant will remain unpollinated – perhaps it grows too far away from others of the same species. It is better to have some offspring instead of none at all, so many plants in this situation pollinate and fertilize themselves.
Self-fertilisation leads to less variation among the offspring, so this creates a dilemma. If self-pollination is too easy, a plant might end up doing this all the time, and so miss out on the chance to shuffle its genes with others. To ensure cross-fertilisation and the advantages that this brings, many plants have developed strategies that make self-fertilisation less likely than ‘mating’ – but not completely impossible, just in case.
Many plants have a definite flowering season. Within that season, individual plants produce the stamens and carpels of their flowers at slightly different times to increase the chance of cross-pollination. In the system called protandry, the stamens develop first and pollen is shed from the anthers before the stigma of the flowers on the same plant can accept it. When the female parts develop before the male, this is protogyny.
DIFFERENT FLOWER SHAPES
Primroses make two types of flower, the thrum and pin forms, though never together on the same plant: this is known as heterostyly. Thrum and pin flowers differ in the heights of the stamens and the stigma within the flower. Insect visitors to thrum flowers pick up pollen at a point on their proboscis that is level with the stigmas in pin flowers. Similarly, pollen from pin flowers is more likely to be transferred to stigmas in thrum flowers.
KEEPING THE SEXES SEPARATE
Some species have separate male and female flowers, but both may grow on the same plant. Other species are dioecious: they have separate sexes, so self-pollination is impossible.
Plants can be choosy about the pollen they accept
Even if a plant species does pollinate itself, it can make self-fertilisation more difficult. In some plants, special genes slow the rate at which the pollen tube can grow. Pollen from a genetically different parent may develop more rapidly and overtake a tube that is growing from its own pollen.
Recently, researchers have found that some plants can recognise the pollen of related plants, as well as their own pollen, and also prevent it from growing properly. This kin recognition enables them to avoid self-fertilisation and fertilisation by close relatives.source
Many flowering plants can reproduce asexually, as well as sexually. While sexual reproduction adds variety to the species, asexual reproduction allows plants to increase their numbers or to survive between growing seasons, without going through the complex processes of gamete formation, pollination, fertilisation and seed production. It gives them extra flexibility: they can still reproduce and spread, even outside the flowering season.
Many flowering plants can produce new plants without forming flowers and seeds. A single parent gives rise to offspring that are genetically identical, both to it and to each other. We call this type of reproduction vegetative propagation.
Along much of the UK coastline there are extensive sand dunes. Many of the dunes nearest the sea are held in place by marram grass. Only marram grass succeeds on the dunes where other plants become smothered by the shifting sand. Marram grass forms underground stems, rhizomes, which grow very rapidly and produce new roots and leaves at intervals. A single individual will quickly spread and colonise a newly formed dune.
Other plants reproduce asexually because they can regenerate and repair themselves: fragments of stems and leaves broken off by passing animals may fall to the ground, root and grow into new plants. Gardeners and farmers take advantage of this ability in the well-known practice of ‘taking cuttings’. Many modern varieties of apple are maintained by taking cuttings and then grafting the young plants onto a special rootstock that keeps the tree short. Vines are often grafted onto special roots that are resistant to Phylloxera, a small weevil that nearly ruined the wine industry in France in the nineteenth century, and that had for a while threatened to do the same in California.
Plants can reproduce asexually as a result of how they overwinter. During the growing season they develop specialised underground stems, or rhizomes, which they pack with stored food. When cold winter temperatures make the plant die back, the rhizomes remain alive and then use the stored food to produce new shoots in the next growing season. Corms, bulbs and tubers are other modified shoots adapted for food storage over winter.
All these structures branch from buds (the ‘eyes’ on potatoes are buds). Each bud can form a new individual the following season.
A COMMERCIAL PLANT BREEDING
Most of our food crops are flowering plant species that have been selectively bred over many generations. When you have got a crop with exactly the characteristics you want, why risk losing it in the lottery of sexual reproduction?
Many of our crops naturally reproduce asexually. Potatoes make tubers, strawberries make runners and onions make bulbs – all are organs of vegetative reproduction. Once selective breeding has produced the desired characteristics, such as large, sweet strawberries that have good frost resistance, the genotype can be maintained generation after generation by vegetative propagation in which we take advantage of the plant’s natural methods of asexual reproduction.
For crops that don’t reproduce asexually, it is necessary to be a little more devious. Some plants will grow from cuttings: you simply take a piece of the parent plant, separate it, put it into soil and it will grow roots. For more reluctant species, rooting powder containing an auxin can be used.
In plants that won’t take root so readily, it is often easier to graft. This is especially common in fruit trees. A shoot from the required crop, called the scion, is inserted into a slit in the bark of the rootstock plant so that the cambium tissues match. There is not the same problem of rejection that you get in animals, and the cambium is able to differentiate into new xylem and phloem. Once the ‘tubes’ are connected, the rootstock provides the graft with water and minerals, while the graft provides the stock with sugars and other products of photosynthesis. You can even have two (or more) different grafts on the same rootstock so, for example, you could have Granny Smiths and Cox’s Orange Pippins growing on the same apple tree. This works with apples and pears. The fruit produced is a result of the genes of the graft, not of the rootstock.
A good example of this is seen in the growth of apricots. Apricot branches are often grafted on to a plum tree rootstock – this will produce more apricots, and be more disease resistant, than a ‘pure’ apricot plant.
|Growth is faster because the plants often have food reserves, e.g. in a tuber||Vegetative structures (tubers, bulbs, runners) are bulky – difficult to transport and store compared with seeds.|
|It is easier and cheaper than collecting and growing seeds.||Tubers, etc. have a high water content- more likely to rot, i.e, shorter shelf life than seeds.|
|Genetic stability is maintained – all plants are clones of the parents. This is the key advantage.||Tubers, etc. are more disease prone. If original stock is contaminated, the whole crop may be Infected.|
|Plants grow at the same rate and can be harvested at the same time||No variation – being clones, if one individual gets a disease they are all likely to be susceptible.|
The techniques of vegetative propagation described above have been practised for decades or centuries, but now valuable crops can be cloned by micropropagation. This technique involves tissue culture in which new plants are grown from tiny samples taken from virtually anywhere on the parent plant. The samples are placed on a sterile nutrient medium that also contains a carefully balanced mixture of hormones (auxin and cytokinins).
This technique works well with crops that are difficult to reproduce by more traditional methods, and can be used to produce virus-free crops. But perhaps the biggest advantage is that many generations of plants can be reared quickly and in a minute space.
In this post and the previous ones on Reproduction in flowers, I explained the following:
- Sexual reproduction enables plants to pass their genes on, add to the number of the species, increase variation and evolve.
- Flowering plants produce spores before they produce gametes. There are two types, microspores and megaspores.
- Microspores or pollen grains are transferred by wind or an animal vector to the stigma of another flower, often on a different plant of the same species.
- The pollen grain grows into a tube that penetrates the female part of the flower. Sperm nuclei travel along this tube to meet the egg.
- Double fertilisation occurs in plants: this leads to a zygote and a triploid cell. The zygote grows into an embryo, the triploid cell develops into a food store, the endosperm.
- The ovule develops into a seed, which protects the embryo for a while. The ovary develops into a fruit.
- There are a variety of means to promote cross-pollination.
- Many plants can self-fertilise, but many have strategies to make sure that they cross-pollinate.
- Many plants reproduce vegetatively, by asexual reproduction.
- Commercial plant growers can clone plants, and keep all desirable characteristics, by taking cuttings, grafting or by micropropagation.
Thanks for coming.