Germination | Agriculture Optional Notes for UPSC PDF Download

Germination

Germination is the reactivation of the seed’s metabolism and the restoration of embryo growth.
There are two main reasons why seeds don’t germinate:

• They could still be quiescent because favorable external conditions do not yet exist. In particular, the environment could be too dry or too cold, or the oxygen levels could be too low to support embryo growth.
• They could be dormant — the characteristic that allows seeds to disperse in time.

External conditions

The external conditions required for germination to occur are:

• Water
• Oxygen
• Temperature
• Light (for some small or fine seeds, like lettuce)

Looking at these external conditions more closely:

• Moist conditions allow the seed to imbibe water. Seeds are typically very dry — somewhere in the 8–15% relative humidity range — so they will readily take up moisture from damp soil. Water moves through the pericarp and seed coat into cells and leads to reactivation of the metabolic processes. The seed’s nutritive reserves are metabolized for the embryonic cells to divide, enlarge, and differentiate.
• The breakdown of nutrient reserves to form energy for plant growth is called respiration, and it requires oxygen. The seed must have oxygen to respire. If you keep seeds in an oxygen-depleted atmosphere, they will not germinate. One all-too-common type of oxygen-depleted environment in which seeds are sometimes placed is waterlogged soil. If you over-water newly planted seeds, the water will keep oxygen from reaching the seeds, and although the seeds will imbibe water and swell as if everything is going well, they will not germinate, and will likely rot because there is not enough oxygen available to sustain respiration. Waterlogged soil also encourages growth of bacterial and fungal organisms that can infect and decompose the seed.
• Depending on the species, seeds have various temperature requirements for germination. Some spring flowers will germinate when soil temperatures are quite cool, even below 50ºF, while most of the seeds we plant in our gardens prefer temperatures in the 50–70ºF range. Again, respiration is the reason. Molecules move around faster when they are warmer, and this movement encourages the chemical reactions required for respiration. If the seeds are cold and the molecules aren’t moving around, the seeds won’t germinate.
• If light is required for seed germination, the species is said to be positively photoblastic. This characteristic allows the seed to remain dormant when buried deep underground, but to germinate when brought to the surface. As you might imagine, weeds that are successful in annually-tilled soils may be positively photoblastic. They remain dormant until tillage brings them to the soil surface.

A note about respiration

• Respiration refers to the set of reactions that take place in the plant cell to convert chemical energy stored in molecules into a form of energy that can be readily used by the cell to power other chemical reactions. Respiration converts the starch stored in the endosperm or cotyledon into ATP (Adenosine triphosphate) (optional reading), which is used in the apical meristems and the radical of the embryo to fuel cell division and the production of new cells.
• Starch is made up of a chain of glucose subunits. Glucose, shown in the three-dimensional model above, is a simple sugar that is made up of 6 carbon atoms, 6 oxygen atoms, and 12 hydrogen atoms. The shorthand formula for glucose is C₆H₁₂O₆. The shorthand formula for starch is [C₆H₁₂O₆]n, where the “n” indicates that there are “n” glucose molecules that, when linked together, make up a starch molecule.
• When we put a quiescent, but not dormant, seed in the ground and it has access to appropriate moisture, warmth, and oxygen (and light if positively photoblastic), it begins to respire. Enzymes are secreted by the cotyledon and, depending on the species, by other specialized cells surrounding the cotyledon and endosperm, which break down the stored starch into its glucose subunits. It is important that the starch be broken down to glucose because glucose is a sugar that is physically small enough to pass through the semipermeable cell membrane; starch is too large to get through. Starch can’t be moved from cell to cell, but glucose can. Starch can be stored in cotyledon or endosperm cells and be broken down to glucose, and that glucose then moves into the actively dividing meristem cells.
• Once in the cell cytoplasm, the glucose is broken in half by a process called glycolysis to form a 3-carbon compound known as pyruvate. Pyruvate first reacts with a carrier molecule and then moves into the mitochondria — the powerhouse organelles in the cell — where it is further metabolized to yield high-energy molecules of ATP. The processes in the mitochondria require the presence of oxygen. The ATP moves out of the mitochondria and to the parts of the cells where chemical reactions are taking place that need energy.
• Starch stored in the seed is a form of stored energy composed of glucose. Glucose is a transportable form of chemical energy that can move through cell membranes, so it helps surround the seed with chemical energy. Pyruvate is a compound formed from glucose that can move into the mitochondria and be broken down to yield ATP. ATP leaves the mitochondria and provides the cell with the energy needed for a wide range of chemical reactions.
• The inputs for respiration are glucose and oxygen. Respiration converts the energy stored in the glucose into ATP that will power reactions throughout the cell. Carbon dioxide and water are the two waste products.
• Seeds also store lipids and protein in the cotyledons. These too can be broken down during respiration. The lipids are first biochemically deconstructed into their components: glycerol and fatty acids. The glycerol molecule is made up of carbon, hydrogen, and oxygen — so it can also be converted to pyruvate and heads into the mitochondria for conversion to ATP. The fatty acids take a different biochemical route, but still end up yielding ATP. Protein respiration is even more complicated, and yields nitrogen-containing building blocks of protein and cells called amino acids that are used in construction of other molecules in the cell. Stored protein in the seed is better at providing amino acid building blocks than it is in providing ATP to energize the cell. Protein can provide energy if necessary, but starch and lipid are more efficient energy storage molecules.

Storing seeds

• Since germinating seeds require oxygen, moisture, and warmth, you can intentionally restrict germination by limiting one or more of these conditions. Why restrict germination? One reason is to store and save seeds for long periods of time. The most common method for storing seeds is to ensure that they remain dry. If you dry seeds in the sun on a low-humidity Minnesota day, you will get the seeds down to around 10% moisture, which is great for storage. (Don’t bake them in the oven; too much heat will kill the embryo.) At a moisture level of 10% or lower you can put them into a glass jar with a tight lid and put them on a shelf for a few years of storage. To store them longer, you can put them in your freezer, which of course means that you have drastically reduced the heat in the seed, which will stall respiration even further and extend the life of the seed. In extreme situations, such as that maintained at the National Seed Storage Laboratory in Fort Collins, Colorado, seeds are dried and placed in oxygen-depleted conditions and stored in a freezer, or put in vials and suspended in the vapor over liquid nitrogen for storage at about -150ºC.
• For home storage of most garden seeds, get them dry, put them in a tightly lidded glass jar, and, to make them to last 5–10 years, put the jar in the freezer. To dry small amounts of seeds, you can use a home food dehydrator set on a very low temperature. Don’t put your seeds in the refrigerator unless you have them very tightly sealed in an air-proof container, because refrigerators are damp. A refrigerator is a great place to store popcorn, because the humidity ensures that the kernels have enough moisture to pop strongly, but it’s a poor environment for seed storage.