第二章分为四部分，1和2讲的是发芽和新梢（或者叶片）生长

1、germination

The germination process is usually initiated following the uniform distribution of seed over the planting site at the density needed to provide a satisfactory seedling stand. The new seedbed should be lightly raked as shown in the above picture, and lightly rolled to establish a firm seedbed to retain moisture in the immediate vicinity of the seed. For large plantings, tractor-drawn equipment may be used for applying the seed and rolling the seedbed. Regardless of the planting method employed, favorable seed-to-soil contact should be established to stabilize the seed and reduce the likelihood of desiccation of young seedling structures during germination. A seedling stand of sufficient density to ensure the development of a satisfactory turf within a reasonable period of time is the intended result of this process. Mature florets harvested from the inflorescence of flowering grass plants constitute what is commonly referred to as grass seed. The floret is composed, in part, of two floral bracts called the lemma (outer bract) and palea (inner bract). The bottom of the floret is the swollen base or callus and the top is the apex. Extending from the base of the palea is the rachilla, a short stemlike structure. When the lemma and palea are separated, the enclosed caryopsis is revealed. Caryopses vary in size, shape and color. The fescues and ryegrasses are the largest, bluegrasses are intermediate in size, and the bentgrasses are the smallest. The caryopsis, or "dried fruit" of the grass plant, contains the true seed surrounded by a pericarp, or ovary wall remnant. Just inside the seed coat is the aleurone layer: a thin, proteinaceous material that plays an important role in germination. Also contained within the seed are the embryo and the endosperm: the food supply for sustaining this plant during germination until it is capable of producing its own food through photosynthesis. The germination process begins when water is imbibed (absorbed) by the seed. Hydrolytic enzymes are produced that function in breaking down starch in the endosperm to simpler carbohydrates for nourishing the embryo. These enzymes are produced in the aleurone layer in response to gibberellins (hormones) produced in the scutellum. Carbohydrates from the endosperm are absorbed by the scutellum and transmitted to other parts of the embryo. The structure of the embryo is shown in the above illustration. The first morphological development evident during germination is the enlargement of the coleorhiza and the emergence of root-hairlike structures from the coleorhiza that anchor the embryo to the soil. This is followed by growth of the primary root through the coleorhiza and emergence of the coleoptile and its enclosed leaf above the soil surface. In some species, growth of the coleoptile may be associated with elongation of the mesocotyl, an internode located between the scutellar node and the coleoptile. The primary (seminal) root grows downward from the caryopsis and may branch and sprout root hairs soon after emergence. The coleoptile first appears as a translucent sheath growing upwards. The first true leaf enclosed within the coleoptile imparts its green color soon after emergence and initiation of photosynthetic activity. In observing the caryopsis during germination, the first indication that something is happening occurs when the caryopsis turns upright following the emergence of root-hairlike structures from the coleorhiza. Soon, the primary root and associated root hairs emerge followed by emergence of the coleoptile. Notice that the coleoptile has already turned green due to the presence of the first true leaf. Further growth of the coleoptile shows the enclosed leaf filling most of its internal volume. Additional growth of the root and associated root hairs and root-hairlike structures dramatically expands the new seedling's capacity to secure moisture and nutrients from the external environment. Notice the emergence of the first true leaf from a pore at the tip of the coleoptile. Notice also the presence of a second seedling; this indicates that two viable embryos occurred in this caryopsis. With further development of the seedling and the concurrent depletion of the endosperm's food reserves, the outer "skin" of the caryopsis (i.e., pericarp fused with the seed coat) collapses. At this point, the seedling should be producing its own food photosynthetically and entirely independent of the endosperm; this condition is called "autotrophic." When the developing seedling or embryonic plant is still dependent on the endosperm for its food supply, it is said to be "heterotrophic." To review, germination is the process by which a quiescent embryonic plant contained within a caryopsis is stimulated to develop into a seedling. During the germination process, the plant is initially dependent on the endosperm for its food supply (heterotrophic), but later becomes independent of the endosperm through its own photosynthetic activity (autotrophic). Factors influencing the success of the germination process are: (1) the amount of endosperm reserves available to support the embryo during its development; (2) the planting depth from which the coleoptile must emerge to reach the light; (3) the availability of sufficient light to support phtotsynthetic activity before endosperm reserves become exhausted, (4) a sufficient supply of moisture in the immediate vicinity of the embryonic plant to support early growth and development, and (5) a suitable temperature range within which growth and development can proceed satisfactorily. Once a seedling has developed, it has the capacity, through other morphogenetic activities, to not only mature but to expand into numerous shoots and roots. With a reasonable density of seedlings, these individual plants can, with proper care, form an excellent turf.

 

2、 shoot growth

The grass shoot is composed of leaves that differentiate into flattened blades and folded or rolled sheaths, and stems that may be hidden within a series of enclosing leaf sheaths. In a vegetative aerial shoot, the only stem present occurs at the base of the leaves and is hidden from view. This highly compressed stem is called a crown. The grass crown is an unelongated stem with a growing point situated at the top, axillary buds along the sides, and adventitious roots that may emerge from its lower portions. The growing point continually forms leaf primordia which eventually develop into fully expanded leaves. Leaf primordia arise due the cell devision below the apical meristem. Rapid division of cells at the midpoint of each leaf primordium results in the formation of the leaf tip. Subsequent meristematic activity is restricted to the basal portion of the leaf primordium, establishing the intercalary meristem. The number of leaf primordia visible at any time varies from a few to as many as twenty or more, depending on species, plant age, and environmental conditions. The entire length of the growing point is usually less than one millimeter. New leaves emerge from the lowermost leaf primordia, as shown in this illustration. With intercalary meristematic activity (i.e., cell division) and subsequent cell elongation, each developing leaf primordium quickly enshrouds the entire growing point. For a while, further development of the emerging leaf is completely hidden within several mature, subtending leaf sheaths. With further growth and its eventual exposure to sunlight, the new leaf initiates photosynthetic activity. The leaves of a turfgrass shoot vary in age; the oldest is the most exposed and situated at the base of the shoot, while the youngest is the least exposed and situated at the uppermost position. Under a specific set of environmental conditions, the shoot maintains a constant number of leaves; as the oldest senesces and dies off, it is replaced by a new one from the growing point. While emerging leaves are photosynthetically active, their requirements for photosynthates to support growth exceed their own production; consequently, they are net importers of photosynthates. A young, fully expanded leaf typically has the highest level of photosynthetic activity and exports much of what it produces to other portions of the plant. As the leaf ages, it produces less and, thus, exports less until, near senescence, its contribution of photosynthates ceases altogether. In addition to producing new leaves, a shoot may also produce new shoots from its axillary buds. When these develop within the subtending leaf sheaths and emerge intravaginally, the new shoots form tillers; when they break through the subtending leaf sheaths (if still present) and emerge extravaginally, they form stolons or rhizomes, depending on their position above or below ground, respectively. While intravaginal growth builds shoot density in the immediate vicinity of the parent shoots, aggressive extravaginal growth can extend the plant population well beyond the parent shoots and contribute to turfgrass coverage over a large area. Considering the potential impact of these two growth processes, one can visualize how an entire lawn could eventually develop from a single seedling. Let's explore the process of rhizome development. A rhizome is a lateral shoot that grows beneath the surface of the ground. The two types of rhizomes are: determinate and indeterminate. Determinate rhizomes are relatively short; their growth occurs in three phases: initially downward, then horizontal, and finally upward. When they reach the surface of the ground, light interception results in the cessation of internode elongation and the formation of a new aerial shoot. Turfgrasses having determinate rhizomes are: Kentucky bluegrass, creeping red fescue and redtop. Indeterminate rhizomes are long and tend to branch at the nodes. Aerial shoots arise from axillary buds along these submerged shoots. Bermudagrass is a turfgrass with indeterminate rhizomes. Unlike roots, which grow by adding cells at the tip, a rhizome grows by intercalary meristematic activity in the vicinity of the stem nodes and subsequent expansion and differentiation of the newly created cells; the resulting elongation of the stem internodes is partly responsible for pushing the rhizome tip through the soil. The conical tip of the rhizome is actually formed by bladeless leaves, called cataphylls, that are produced at the growing point in somewhat the same fashion as leaves in an aerial shoot. As the leaves form, they also aid in pushing the rhizome through the soil. With the elongation of stem internodes, the leaves are separated and occur alternately along the rhizome. When the tip is exposed to sunlight, internode elongation ceases and new cataphylls transform to conventional, chlorophyll- bearing leaves. The determinate rhizomes of Kentucky bluegrass vary in length depending on environmental conditions. During the summer months, the rhizomes tend to be relatively long because of extensive horizontal growth; these are called "extensor" rhizomes. During the spring and fall months, Kentucky bluegrass rhizomes may turn up almost immediately; these are called "sprout" rhizomes. Let's review the various types growth occurring in the aerial shoot. Notice how new leaves emerge from the growing point atop the crown while tillers and rhizomes begin forming from axillary buds located along the sides of the crown. A new leaf eventually becomes evident as its tip emerges from the older, enclosing leaf sheaths. At the same time, new tillers also become evident as the leaves of the parent shoot are either pushed away or senesce. And new rhizomes appear as they grow outward from the parent shoot. With further development, the newer leaves supplant the older, senescing leaves in a continuing cycle of replacement growth. The new tillers and their parent shoot become almost indistinguishable. And the new rhizome tips eventually reach the surface and begin forming rhizome daughter plants. The "perenniality" of a turfgrass population is thus due not so much to the longevity of any particular component of the plant, as the life of these components is relatively short, but to the capacity of the plant to continually replace all of its components during the course of the growing season. Turfgrass management involves ensuring that replacement growth occurs at the rate needed by losses due to natural senescence, injury and disease.