Why Seeds Need Warmth to Germinate

Seeds need warmth to germinate for several critical reasons. One of the main reasons is the role of warmth in enhancing metabolic activity, water absorption, cell growth and division, breaking seed dormancy, and microbial activity.

Metabolic Activity

As temperatures rise, the metabolic processes within the seed increase. Enzymes, which play a crucial role in cell division and respiration, become more active. This activation facilitates the biochemical reactions necessary for the germination process.

Water Absorption

Higher temperatures often enhance the ability of seeds to absorb water. Water is essential for activating enzymes and initiating the germination process. Seeds need to imbibe water through a process called imbibition, which is a special type of diffusion.

Cell Growth and Division

Warmth promotes cell division and growth, which are essential for the seed to develop into a seedling. Cooler temperatures can significantly slow down these processes. This is why optimum temperature for seed germination is generally considered to range between 25°C to 35°C.

Seed Dormancy

Some seeds have built-in dormancy mechanisms that are broken by specific temperature ranges. Warmth can signal to the seed that conditions are favorable for growth, thereby breaking the dormancy. The dormancy can be further influenced by external factors such as microbial activity in the soil, which can help break down organic matter and provide nutrients to the germinating seed.

Microbial Activity

Higher temperatures can enhance microbial activity in the soil. Microorganisms play a vital role in the breakdown of organic matter, releasing nutrients that are beneficial for the germinating seed.

The Importance of Temperature in Germination

Temperature is energy for the process of germination. It affects germination in three primary ways: moisture absorption, hormone production, and enzyme activity. For seeds to germinate, they need to imbibe water—an essential step known as imbibition.

The relationship between temperature and seed germination is complex. The germination percentage usually increases linearly with temperature up to an optimal temperature after which the germination percentage decreases sharply. Increased temperatures not only affect seedling growth after seed germination but also directly affect the germination process.

Temperature and Seed Deterioration

Seeds continuously deteriorate, and unless they are germinated, they will ultimately die. The rate of deterioration depends mainly on moisture content and temperature. The Q10 for the rate of loss of viability in orthodox seeds consistently increases from about 2 at -10°C to about 10 at 70°C.

Seed Dormancy and Temperature

Many seeds are initially dormant. Relatively dry seeds continuously lose dormancy at a rate that is dependent on temperature. Unlike enzyme reactions, the Q10 remains constant over a wide range of temperatures, typically having a value around 2.5-3.8. Hydrated seeds respond quite differently:

High temperatures generally reinforce dormancy or may even induce it. Low temperatures may also induce dormancy in some circumstances but in many species they are stimulatory. Small dormant hydrated seeds are typically also stimulated to germinate by alternating temperatures.

Germination Rate and Temperature

The rate of germination shows a positive linear relation between the base temperature (below which the rate is zero) and the optimum temperature (at which the rate is maximal) and a negative linear relation between the optimal temperature and the ceiling temperature (above which the rate is again zero).

The optimum temperature for germination rate is typically higher than that required to achieve maximum percentage germination in partially dormant or partially deteriorated seed populations. Understanding these temperature responses is fundamental to designing seed stores, implementing germination tests, and understanding seed ecology.