Germination & Early Seedling Growth
The kernel (seed), or caryopsis, consists of a seed coat surrounding an embryo and endosperm. The embryo con- tains the seedling root (radicle), stem, and growing points of the new grain plant. The endosperm provides nutrients for growth until the first true leaves emerge and the root system is established. When moisture conditions are favor- able, the seed germinates with the emergence of the radi- cle and the coleoptile, the first leaf that forms a protective sheath around the first four leaves.
The primary root system includes the radicle and roots that develop from stem tissue near the kernel (see fig. 1). It may penetrate the soil up to 12 inches (30?cm) and provides the developing seedling with water?and nutrients. The primary root system supports plant growth until tillering, when the secondary root system becomes the main root system of the plant. The primary roots may persist for the life of the plant and can sup- port some plant growth through the heading stage.
The first secondary roots appear at the tillering node about 1 inch (2.5 cm) below the soil line at the two- or three-leaf stage.These roots are always produced at about the same distance below the soil’s surface, regardless of the depth at which the seed is planted.
The secondary root system makes up the major part of the fully developed plant’s root system. Root growth may continue at 1⁄2 inch (12.5 mm) per day for 60 to 70 days. With no competition, roots may spread 4 feet (1.2 m) hori- zontally and 6 feet (1.8 m) vertically, with most roots in the top 2 feet (60 cm) of soil. Horizontal root growth is less than 1 foot (30 cm) in wheat fields with normal plant population density. Root development approaches the maximum at about the boot stage. The “boot” represents the swollen flag leaf sheath within which the developing spike is located after being pushed up as the stem has elongated.
As the seedling root system is forming, the coleoptile grows upward and rup- tures, allowing the first leaf to begin unfolding as soon as the coleoptile tip breaks the soil surface. Emergence usually occurs 6 to 20 days after sowing, depending on temperature and moisture. Emergence can be later than 20 days after sowing under prolonged cold or dry conditions. Initial formation of leaves and stems occurs at the shoot apex, which is located just below the soil surface.