The stem consists of internodes spaced between the nodes, with attached leaves. The number of nodes and internodes is equal to the leaf number, all three having a common origin in the phytomer. Shoots of temperate grasses have compacted or untelescoped nodes (without internode elongation), which, until elongation after floral initiation are positioned below the soil surface.
At flowering, four to five of the upper internodes elongate and vertically space the upper leaves. A similar number of internodes remain short and confined at or below the soil surface (referred to as the crown). Many dicot species are stem less until flowering. On the other hand, tropical species may produce vegetative stems, that is, internode growth without flowering.
Westmore and Steeves (1971) classified plants based on internode length as follows:
(1) Short stem (those without conspicuous internodes, such as plantains and the first-year growth of biennials)
(2) Long stem (those with conspicuous internodes, such as maize and the second-year growth of biennials).
Internode Elongation of Stem:
Growth in height of stem occurs in the intercalary meristems of the internodes. Internodes lengthen both by increases in cell number and (primarily) by cell expansion, the latter resulting in an increase of up to 25 cm or more. Growth by cell division is at the internode base (i.e., intercalary) rather than in apical meristems. However, intercalary meristematic activity is distributed throughout the length of the leaf lamina, sheath, and internode at the primordial stage. With maturation, the meristem activity moves to the basal regions and eventually terminates.
The peduncle (internode supporting the inflorescence in grasses) and the flowering stalk (of dicots) grow from intercalary meristems. Generally internode growth is determinate for reasons not fully understood but apparently due to a limitation of the potential number of active cells.
An exception is found in the mesocotyl (first internode in grasses) which, within food reserve limits, continued, to elongate indefinitely in darkness or in infrared light. Growth of the mesocotyl is inhibited immediately by exposure to red light; that is, growth is phytochrome (pigment) controlled but modifiable by organic nutrition.
In addition to growth limitation because of the number of active cells, the amount of growth hormones in the intercalary meristems may be limited since they are not self-generating as in apical meristems. Consequently plant growth regulators (PGRs) must be supplied from plant parts outside the meristem. Dwarf plants can respond to an exogenous (external) source, generally to gibberellin (GA) applications.
Maize remains stemless until reaching a height of about 40 cm and developing eight fully expanded leaves, which arise from the pseudo stem (a vegetative shoot). At this stage there is no perceptible internode growth. Due to compacted nodes and internodes, biennials produce a stemless rosette during the first year. Until flower initiation, temperate grasses produce pseudostems. At floral initiation the internodes of grass and biennial shoots that bear an inflorescence elongate. In early season a grass plant usually has both vegetative and reproductive tillers (culms).
In dicots with no stems (e.g., Plantago) the last internode below the inflorescence greatly elongates to produce a flowering stalk. Long flowering stalks are evident in certain other species, such as white clover. The gynophore (peg) of peanut can be considered a nodeless fruiting stalk although its morphogenesis by originating from the flower differs somewhat from the typical flowering stalk.
In monocots and dicots with long stems internode length generally increases acropetally, but patterns alternating long and short internodes are characteristic of some species. Basal internodes of many species may be short enough to escape observation, whereas the uppermost internodes, especially the peduncle of a grass culm, may exceed 25 cm.
Normally two or more internodes elongate simultaneously, but in sunflower a new internode does not initiate elongation until the preceding internode has completed it. Meristematic activity to cause elongation of internodes, except in the gynophore of peanut, is concentrated at the basal end, as indicated by the presence of mitotic activity in stained cells.
Crown Development in Stem:
The lower, closely spaced nodes of a plant form the crown, which is located at or just below the soil surface. In grasses these densely spaced nodes give rise to the successive whorls of adventitious roots called the nodal, crown, or coronal root system. The lower nodes of perennial legume plants such as alfalfa form a crown but without adventitious root development.
The location of growing points in the grass crown below the soil surface and resultant exposure of new leaves from sheaths of older leaves (pseudo- stem) have crop management implications. Since maize maintains this condition for 4 or more wk (until approximately eight leaves are fully exposed), an early frost or clipping usually injures only the above-ground foliage, the oldest and smallest leaves.
Little permanent injury is caused by such early defoliation, since a new canopy of leaves soon emerges from the unspent intercalary meristems protected by the leaf roil and from newly formed leaves. The common practice of pasturing wheat during winter and early spring in the southern United States does not seriously damage gram production as long as the growing points of shoots remain vegetative, that is, below the soil surface.
After floral initiation and concomitant stem elongation with the onset of longer spring days, grazing can remove the inflorescence and destroy the grain production potential. Unlike temperate grasses, dicots and many tropical grasses grow from exposed buds of aerial stems.
Hence, freezing or destruction of above-ground shoots can destroy axillary buds and regrowth potential. If a killing frost should occur on soybean, for example, growth potential is destroyed because no buds are present below the cotyledon axils, which are above ground; reseeding of the crop is necessary.
Factors Affecting Stem Growth:
1. Growth Regulators:
The effect of plant growth regulators, especially GAs, on stem growth is well documented. They can overcome dwarfism in genetic dwarfs, such as dwarf maize and pea, promoting increased internode growth and normal height presumably by correcting an endogenous GA deficiency.
However, the dwarf habit in dwarf ‘RS 610’ sorghum was not corrected by GA sprays; only the below-ground nodes (mesocotyl and second internode) and the coleoptile responded. This comparative lack of response in sorghum was probably due to the fact that sorghum dwarfing is controlled by several genes, and maize and pea dwarfing by a single gene. Evidently GA is more effective in correcting dwarfing that is inherited simply.
Leopold (1949) showed that auxin has a pronounced effect on tillering (growth of shoots from crown buds) in barley (Table 11.2). When the shoot apex and source of auxin was destroyed, ‘Wintex’ barley tillered profusely unless given an application of the auxin naphthalene acetic acid (NAA). The NAA-treated plants with destroyed apexes tillered about the same as normal plants, that is, those with undisturbed apexes.
2. Light:
Light has a pronounced effect on stem growth. In the dark, etiolation (elongation of internodes) is extreme and similar to that of the mesocotyl internode. The internodes of shaded plants, such as in dense stands, are more etiolated. The shade effect is believed to be due to auxin enhancement, probably acting synergistically with GA. Theoretically photo destruction of auxin is less in shaded stands, since high irradiance decreases auxin and plant height.
Day length affects stem growth usually less conspicuously than it affects flowering. Consequently photoperiodic responses of stem development are not often reported. Long days cause increases in internode length and plant height, especially on short-day plants. Soybean cultivars adapted to northern latitudes had fewer and shorter internodes and flowered earlier when grown at lower latitudes.
Planting the same cultivars at higher latitudes than those to which they are adapted has the reverse effect and would also probably result in immature seeds at harvest. Much earlier seeding dates in an area of adaptation tend to result in the same response as to shorter days characteristic of lower latitudes. For example, early planting produces shorter internodes in maize, which results in sturdier plants.
As with leaf growth, internode growth of grasses is influenced by light quality. Both grow from intercalary meristems of common origin and are shielded from light in the roles of older leaf sheaths, producing a dark or far- red effect. Far-red light (maximum effectiveness at 730 nm) promotes, and red light (maximum effectiveness at 660 nm) inhibits, mesocotyl elongation, the mechanism controlling emergence from variable planting depths.
In wheat, emergence is primarily from elongation of the second rather than the first internode or mesocotyl; a distinct effect of monochromatic light on upper internodes has not been demonstrated. However, in grasses these internodes and young leaves are trapped in the darkness of the sheaths of older leaves for the greatest part of their total growth. Consequently the phytochrome-far-red response appears to be operative. Growth is inhibited when exposed to light.
Dicot internodes are not so enclosed in leaf rolls, probably indicating little or no phytochrome-monochromatic light response, but this relationship has not been well established.
However, a bush-type garden bean under long night-far-red radiation in effect assumed a climbing habit (internode elongation) due to development of long internodes, a single-gene response. Flowering, which was independent of the climbing habit, was promoted by short nights (long photoperiods). It was concluded that flowering response and climbing habit are both phytochrome controlled but are discrete, independently inherited responses.
Mineral nutrient and water availability affect internode growth, especially by cell enlargement, as in any vegetative or fruiting organ. Nitrogen and water, particularly, increase plant height, but the effect is complex since a larger leaf size results in more shading. Shading tends to increase auxin levels, which could affect internode length.
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