How2Day: The Factors of Bean Seed Germination

Inside the bean pod rests multiple seeds that mature with a hard, dry shape and texture. Inside each bean lies dormant a living small plant embryo. Gardeners sow the seeds of beans when conditions favor germination. These factors need to remain constant, otherwise disruptions kill the tender emerging root and leaves from the sprouting bean seed. Hundreds of different species of beans exist worldwide, each with specific thresholds of factors that initiate germination.

Bean seeds germinate only when temperatures above freezing permit liquid water to help soften the bean seed's hard outer layer, called the testa, and permit the internal seed embryo to swell and grow. Each bean species may need different soil temperatures for germination, based on adaptations to where they are native. For example, beans native to tropical regions need warmer soil temperatures to grow, while a bean hailing from a temperate zone may sprout when soil temperatures are much cooler but still above freezing. A standard, general seed germination temperature is 70 degrees Fahrenheit.

Even though a bean seed feels and looks dry, minute amounts of moisture within the seed keep the dormant embryo alive. For the embryo to grow, some external water source causes the hard testa to soften and split and trigger metabolism in the dormant embryo. Water enters the seed in a tiny hole in the testa called the micropyle. According to The Open Door Web Site, a delicate balance exists in the appropriate amount of water necessary for bean seed germination. Too little moisture and the seed begins to sprout but then dries up and dies; too much water and the embryo dies and rots.

Plants absorb carbon dioxide from the atmosphere, but plants also need oxygen to metabolize food and to grow. This is particularly true for seed germination. Air must reach the embryo and that's why a moist but crumbly, loose soil environment promotes best germination. Oxygen helps the swelling bean embryo's two leaves and roots to expand and cellular growth to continue. The self-contained metabolism in the bean seed continues until the young seedling sustains itself with moisture from its own root, and its leaves green up and reach the soil surface and basks in light.

With the large diversity of bean species and natural habitats, other factors exist that trigger or are required before germination. It depends on the bean species. For example, some species' seeds need exposure to winter cold to crack the hard seed coat or pod, or to trigger production of hormones within the bean seed. Or, hormones for germination end dormancy only when photo period dictates --- such as the day length being long enough in spring or summer to promote seed sprouting.

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