Germination is the process of a seed, spore, or other reproductive body emerging after it has been dormant for a length of time. Water absorption, time, chilling, warming, oxygen availability, and light exposure could all have a role in starting the process.
Water is taken by the embryo during seed germination, resulting in the rehydration and expansion of the cells. The rate of respiration increases shortly after the start of water uptake, or imbibition, and different metabolic activities that had been halted or greatly decreased during dormancy resume. These occurrences are linked to structural alterations in the embryonic cells’ organelles (membranous entities involved in metabolism).
Steps Involved in Germination of Pollen Grains
Pollen Attachment
Pollen grain attachment to the stigma is determined by the receptivity of the stigma as well as the shape and structure of the pollen grains. When pollen grains are expelled from the anther, they are either dry or wet. Wet pollen grains easily fall on the stigma, and the process is essentially mechanical. Depending on the layers of the stigma’s pellicle and pollen layers, dry pollen grains settle on the stigma. When pollen grains land, they absorb water and begin to germinate.
Pollen Tube Growth
- The hydration of the intine layer, which ruptures the exine, is the initial process linked with pollen tube growth.
- The stigmatic surface releases important elements such as boron and calcium, which are lacking in pollen but required for germination.
- The vegetative cell and exine components form a pollen tube inside the pistil, allowing germ cells to enter the ovule.
- The pollen tube arises from the germ pore on the pollen grain, which is covered in a very thin layer of the exine.
- The full-grain content is deposited into the tube.
- The entire cytoplasm is collected at the tip of the tube, with a vacuole filling the remainder of the tube.
- Callose plugs are generated at regular intervals throughout the tube to limit the cytoplasm at the tip.
- With an exterior pectin stratum and a central pectocellulosic stratum, the pollen tube is a three-layered structure.
- The pollen tube grows into the style’s intercellular gaps. The tube’s next path is determined by the style’s characteristics.
Entry of Pollen tube into the Ovule
Finally, the pollen tube enters the ovule. There are three types of fertilization based on the passage of the pollen tube into the ovary:
- Porogamy: The pollen tube reaches the ovary through the micropyle of this kind. Like Lily.
- Chalazogamy: The pollen tube enters the ovary by the chalazal end in chalazogamy. Like Casuarina.
- Mesogamy: The pollen tube reaches the ovary through the funicle or the integuments of this kind. Like Cucurbita.
Entry of Pollen tube into the embryo sac
Pollen tubes enter the ovary through the micropyle, regardless of how they enter the ovary.
Pollen tubes are guided into the cytoplasm of synergids by the filiform apparatus.
Male gametes are produced when the reproductive cells divide into two.
The egg in the embryo sac unites with one gamete to form a zygote, while the other gamete fuses with two polar nuclei to generate endosperm, a food store for the seed.
Angiosperms have a double fusion of cells, which is known as double fertilization.
Factors Affecting Pollen Tube Growth
- Carbohydrates: A small amount of sugar in the pollen grains’ hydration while landing on the stigma stops the pollen grains from exploding.
- Boron: Because pollen grains are lacking in boron, the stigma secretes enough boron to allow pollen tubes to develop.
- Calcium: The ‘population effect’ is aided by the presence of calcium ions in pollen grains. Pollen grains typically contain very little calcium, but when a high number of pollen grains clump together, a substantial amount of calcium is accumulated, promoting pollen tube production.
- Enzymes that promote tube elongation include pectinase and cellulase.
- Physical Factors: Temperature is the most important factor in pollen tube elongation. Temperatures between 20 and 30 degrees Celsius are ideal for pollen tube formation.
Germination rate and Germination capacity
The germination rate is a term used in agriculture and gardening to define how many seeds of a specific plant species, variety, or seed lot are likely to germinate over a given period. It is a metric for the length of time it takes for seeds to germinate, and it is commonly given as a percentage. Seed germination rate is influenced by genetic makeup, morphological characteristics, and environmental conditions. The germination rate can be used to determine how many seeds are required for a particular area or desired number of plants. “Germination rate” is defined by seed physiologists and seed scientists as the reciprocal of the time it takes for the germination process to complete from the moment of seeding. Germination capacity, on the other hand, refers to the number of seeds in a population that can complete germination.
CONCLUSION
The germination of a pollen grain following pollination is another germination event in the life cycle of gymnosperms and flowering plants. Pollen grains, like seeds, are extensively dehydrated before being released to aid the spread from one plant to the next. They are made up of a protective covering and numerous cells. A tube cell is one of these cells. The pollen grain takes up water and germinates when it rests on the stigma of a receptive flower (or a female cone in gymnosperms). Pollen germination is aided by hydration on the stigma, as well as the stigma’s and style’s structure and physiology. Pollen can also be made to germinate in a lab setting (petri dish or test tube). The tube cell elongates into a pollen tube during germination. The pollen tube next develops towards the ovule in the flower, where it discharges the sperm created in the pollen grain for fertilization.