Soil temperature governs how quickly a seed will germinate and send out the radical, or first root, and the coleoptile, the shoot. When planted in cool soils, seeds will absorb water, but the germination process is halted. Warm soil releases needed nutrients and encourages root growth.
Using corn as an example, when seeds are planted when the soil temperature is below 50 degrees Fahrenheit, they remain dormant. Since the seeds absorb water, the hulls are soft and susceptible to insect predation and infection. When the soil does warm and germination begins, the roots may grow unevenly or experience a lack of development.
If corn seeds are planted in warm soils above 50 degrees F, there is no disruption in the germination process. Water is absorbed, nutrients are readily available and the seeds germinate quickly. The result is a healthier crop.
The root emerges from the seed first because it absorbs water and nutrients from the soil to support the rest of the plant. It is the only source of nutrition available until the plant grows large enough to begin photosynthesis. Maturing plants start to produce chlorophyll, the green substance found in the leaves and stems. The heat of the sun helps chlorophyll convert water and carbon dioxide to sugar, which the plant uses for food, and oxygen.