All terrestrial plants are thought to have descended from a single-celled green algal progenitor. Water was extremely important to the progenitors of early plants, not just for maintaining moisture levels but also for structural support. The surrounding medium in a terrestrial setting is air rather than water. The absence of air provides no support for upright development. Because drying out or desiccation is a persistent threat to organisms exposed to air. Plant-dominated ecosystems have steadily grown as a result of the evolution of significant morphological novelties, leading to the establishment of major biomes. The development of increasingly complex and differentiated multicellular architecture, as well as the invention of novel reproductive structures and techniques, were critical to the success of plant colonisation of the earth. Plants have a variety of reproductive adaptations.
Pollination and Fertilisation
The transfer of pollen from the anther to the stigma after it has germinated is known as pollination. The tube nuclei and the generative cell are the two cells that make up the pollen grain. The generative cell splits and creates two sperm nuclei during mitosis. The stigma secretes a sugary solution as the female portion of the flower develops. If viable, suitable pollen comes into touch with this wet stigma, this enhances pollen grain germination. Flowers are the primary site of sexual reproduction in plants. Around 225 million years ago, blooming plants and their pollinators began to co-evolve. For angiosperms, the sexual cycle comprises the development of the male (pollen) and female (embryo sac) flower components. Pollen grains and an eight-celled embryo sac contain haploid (1n) male and female gametes, respectively. During this portion of the cycle, chromosomal division occurs to form the haploid (n) chromosome number. The stigma’s germinated pollen grain forms a tube (pollen tube) that transports two sperm nuclei (n) down the style into the ovary until they reach the embryo sac. Fertilisation takes place in the embryo sac, and one sperm fertilises the egg to generate a zygote (2n). The other sperm merges with the embryo sac’s big centre cell to form a triploid (3n) cell, a process known as triple fusion.
Ovule development
Following fertilisation, the ovule (which contains the zygote and the 3n central cell) develops into a seed. Seeds emerge from ovules in gymnosperms and angiosperms. The nucleolus is carried by a stalk in ovules (equivalent to the megasporangium; diploid maternal tissue). The nucleus is surrounded by one (gymnosperms) or two (angiosperms) layers of integuments (diploid maternal tissue). In a developmental sense, an ovule is an unfertilised, immature seed precursor and a giant sporangium surrounded by integuments in a morphological and evolutionary sense. These integuments comprise the testa (seed coat), with the outer cell layers of the outer integument forming a dead covering layer in mature seeds, while the inner cell layers may stay alive. A megaspore develops into a haploid megagametophyte within the nucleolus (female gametophyte). Gymnosperm and angiosperm mega gametophytes differ significantly.
Seed Maturation and Harvestation
- Seed moisture content: Both monocots and dicots have an ovule moisture content of around 80% at the time of fertilisation. Because water is the channel for delivering nutrients from the parent plant to the developing seeds, its value declines throughout maturation but remains reasonably high for the majority of the maturation period. The initial phase of drying is sluggish, but it speeds up after the seeds reach their maximum dry weight; at that point, conventional monocot and dicot seeds produced in dry fruits have 35 per cent to 55 percent moisture content, respectively.
- Seed dry weight: As a result of nutrient accumulation and water intake, the growing seeds begin to gain weight after the sexual union. Because cell division and elongation are happening during this stage, seed fill is initially delayed. Dry mass buildup rises rapidly after that until seeds achieve their maximum dry weight.
Seed Germination
Plant propagation is the sexual and asexual multiplication of plants and has some aspects-
-Know about Chemical, physical and environmental manipulations are all covered, as well as the necessary technical abilities.
-Specialised knowledge of certain plant species
Sexual propagation/seed germination involve the combination of pollen from a male floral part with the egg of a female ovary to generate a seed is known as sexual propagation. Intermediator orthodox seed can be resistive. By lowering the storage temperature and seed water content without causing substantial harm to seed metabolism, the lifetime of orthodox seeds can be extended throughout a wide variety of climatic circumstances. As a result, orthodox seeds are considered desiccation resistant. Recalcitrant seeds, on the other hand, lose viability when their water content falls below a certain threshold.
As a result, they are classified as desiccation intolerant. The seed has three parts: an exterior seed coat that covers the seed, an endosperm that serves as a food reserve, and an embryo that is the new plant itself. A seed germinates when it is ripe and placed in a favourable environment.
Stages of seed germination
- 1st Stage- Imbibition is the initial absorption of water by the seed to hydrate it. Increased respiration and protein synthesis due to metabolism activation
- 2nd stage- Storage food digestion, such as starch to sugars in the cotyledon or endosperm. Sugars are translocated to the embryo for development
- 3rd stage- Seedling cell division and growth development-
This phase of development is characterised by a relatively minimal change in seed water content after initial water intake until it is terminated by the commencement of embryo growth. Energy metabolism restarts, respiration activities are engaged and the cell cycle may begin at this period, but seed maturation actions are repressed. Even when the growing seed is separated from its surrounding fruit tissue, germination is not possible until the seed is close to maturity. Seeds’ capacity to germinate rapidly when conditions are favourable for effective growth, as well as their ability to prevent germination at inopportune periods, are thus critical to a species’ survival. The duration of germination in the precise sense is the interval between seed imbibitions and radical emergence.
Conclusion
For photosynthetic creatures, the development of the seed constitutes a tremendous change. Furthermore, seeds stay dormant until the circumstances for development become suitable, which is produced by desiccation and the hormone abscisic acid. Seeds are dispersed in a growing geographic area, whether blown by the wind, floating on water or transported away by animals, avoiding competition with the parent plant. Seed evolution reduces the need for water during sexual reproduction and allows for long-distance fertilisation. To summarise, various plants require different pretreatments in order to produce vigorous seedlings and even for production. As a result, special attention should be paid to plant propagation, especially for indigenous tree species and seedling multiplication should be regarded as a part of our culture in order to create a suitable habitat.