In this article we will discuss about:- 1. Initiation and Emergence of Leaves 2. Number of Leaves Produced on a Shoot 3. Senescence 4. Factors Affecting Growth.
Initiation and Emergence of Leaves:
Leaf initials (primordia) begin with certain cells in the apical dome, which divide (become meristematic) and produce swellings or protuberances on the stem apex. The protuberances spread and encircle the apex, particularly the sheath primordium of a grass leaf.
After the leaf collar is formed, cells in the sub-hypodermis become meristematic and produce an axillary bud. Subsequent growth of leaf blades (lamina) and sheaths or petioles and stem internodes is from intercalary meristems (those between differentiated tissues).
In a grass leaf the intercalary meristem is divided into, two parts by formation of the ligule. The upper part was found to contribute to lamina growth, the tower to the sheath. Growth of a grass leaf occurs while the leaf is enclosed in the pseudostem (the rolls of older leaves). In dicots leaves emerge from short bud scales; therefore growth by expansion is principally in the open.
In a constant environment leaf initials appear on the stem apex at a constant rate for a given genotype. The interval of time between the appearances of successive leaf primorida is termed the plastochron. The interval of time between the appearances of the tip of successive leaves is termed the phyllochron and may differ from the plastochron. Time intervals in which phyllochrons are longer than plastochrons result in longer shoot apexes.
In wheat the emergence of a given leaf tip is at plastochron 5; that is, the fifth leaf is initiated as the first leaf emerges. Since emergence in dicots is from bud scales, the distinction between phyllochron and plastochron is not as useful as in grasses in which leaf growth occurs in the pseudostem.
Research on leaf initiation rate and appearance in crop plants is limited. Temperature, light, and other factors have been shown to influence plastochron development. With ryegrass it was observed that high temperatures (18-25°C) and light intensity increased plastochron and phyllochron rates. This is not surprising, since rate of plant development is temperature driven.
Raising the temperature from 15 to 20°C increased leaf appearance rate in wheat by over 50% and shortened plastochron rate by 50%, from 5 or 6 days to 2 or 3 days. Rate of leaf appearance in barley was linear as light increased from 7.8 to 32.5 W. m-2, but these effects could have been influenced by temperature changes.
Number of Leaves Produced on a Shoot:
The number of leaves produced on a shoot or tiller is determined by inflorescence initiation. Formation of leaf initials on the apex gives way to floral initial formation, which fixes leaf number. Secondary and higher order tillers or branches generally have one to two fewer leaves than the primary shoot, since they emerge later and receive the same environmental cues to flower. Thus floral initiation is at a lower leaf number.
Characteristic leaf numbers are 7 to 9 for wheat, oats, and barley; 7 to 14 for sorghum; 14 to 21 for most U.S. maize hybrids; and 10 to 16 for upper- latitude U.S. soybean cultivars. Maize cultivars adapted from 50° latitude to the equator vary from 7 to 48 leaves. Height and maturity of maize are highly correlated to leaf number.
The number of primordial leaves present in a mature seed embryo is characteristic of the species. Most cereal grains, such as wheat, have three leaves in the mature seed, while five leaves can be recognized in the embryos of maize seeds. Plastochron 6 is during emergence or early seedling growth.
Leaf Senescence:
The leaf number and leaf area index (LAI) peak and then remain quite constant until general senescence begins. This equilibrium in LAI results from loss of lower leaves at a rate equaling the production of new (upper) leaves. Therefore LAI tends to plateau at a maximum of about 4 to 7 for crop canopies, regardless of plant population above a moderate level. Forage grasses with narrow, upright leaves usually have LAIs higher than 7.
Langer (1972) reported the mean numbers of living leaves as 4.5 to 5.8 per shoot for several different forage grasses growing in a heated greenhouse in England, compared with means of 3.1 to 3.7 in an unheated greenhouse. Additional N raised the number slightly at the higher temperature. Grass plants low in N, tend to fire (senesce) the lower leaves.
Senescence of an individual grass leaf begins at the oldest part of the leaf (the tip) and progresses downward. Senescence of an individual plant begins at the basal (older) leaves and progresses upward. By the time maize has produced 10 to 12 leaves, 4 to 5 leaves have been lost to senescence (the loss in LAI is relatively small since the lost leaves are small). Usually leaf 5 is the first healthy leaf on a maize plant at tasseling.
The cause of senescence is generally thought to be mobilization and redistribution of mineral and organic nutrients to more competitive sinks, such as young leaves, fruits, tillers, and roots. Contribution from leaves to these organs declines progressively with senescence. There is no evidence of reverse flow (parasitism), by aging leaves, as was once commonly thought to occur.
Rapid production and expansion of leaves are highly important in crop production in order to maximize light interception and assimilation. A full canopy also reduces weed competition and sheet erosion. Seeding rates of peanut are inordinately high, partly to reduce intra-row weed competition. Interestingly, assimilation rates are usually maximized at a LAI of 3 to 5 for most cultivated crop plants.
Factors Affecting Leaf Growth:
Leaf number and size are affected by genotype and environment. The position of the leaf on the plant (plastochron number), which is principally controlled by genotype, also has a pronounced effect on leaf growth rate, final dimensions, and capacity to respond to improved environmental conditions, such as available water.
Leaf length, width, and area generally increase progressively with ontogeny up to a point; then in certain species these parameters decrease progressively with ontogeny so that the largest leaves are near the center of the plant, such as on a maize plant (Fig. 11.2). The flag (uppermost) leaf of maize is shorter and narrower and has less area than the ear leaf. This type of profile is characteristic of many species.
In other grasses, such as barley, the length of lamina decreased with flower initiation but the width increased, resulting in a broad flag leaf. The cause of the diminution of upper leaves is not known but appears to be competition with the inflorescence for nutrients. The relative growth rate of leaves decreases with leaf number.
At stage 5 of soybean, 70% of the total plant dry weight was leaves. Leaf growth peaked at stage 6 and remained constant until stage 10, while total plant dry weight increased rapidly due to stem and fruit growth.
Size and weight of new leaves decreased after stage 6; after stage 10 total leaf weight decreased due to senescence of bottom leaves. The maximum weight and area of the leaves of a plant is reached early in the life cycle, after which the gain in leaves just equals the loss, a status referred to as the critical leaf area.
Even though the lower leaves on plants are smaller and frequently lost due to environmental stresses and senescence, they are important to the vegetative growth. For example, 14C fed to leaf 3 of a grass plant was active in leaf 4, 5, and 6.
The lower pods of soybean were primarily supplied by the subtending leaves. In barley the sheath plus stem contributed 50 to 70% as much of the apparent photosynthesis for grain production as did the blade. In dicots leaves with a long petiole and large petiole base make a significant photosynthetic contribution.
Nitrogen (N) fertilization had a pronounced effect on leaf expansion, especially on leaf width and area. With low N, leaf 4 of wheat was the largest in size; with high N, leaf 5 was largest. The shift in maximum size to an upper leaf was thought to have resulted from a reduction in the competition for N between upper leaves and emerging stem and inflorescence. A N deficiency also causes a reduction in leaf area due to senescence of older leaves.
Other minerals appear to have less effect than N on leaf growth and senescence, although the competition for most nutrients between new and old leaves and between fruits and leaves is evident.
For reasons, not completely understood, elongation of wheat leaves was significantly less during the night than during the day and dropped almost to zero if the period of darkness was extended. This decline was also related to lower irradiance during the previous light period. A night (far-red) effect is indicated, which probably interacted with organic nutrition.
Irrigation in a humid climate (Columbia, Mo.) promoted rapid leaf elongation of tall fescue (Festuca arundinaceae) during summer months. However, leaf growth in non-irrigated plots was greater than that in irrigated plots in the fall and the following spring, when moisture was available naturally.
Ralph (1982) showed that late-maturing cultivars of sunflower, unlike early cultivars, were benefited in leaf expansion by water stress during the vegetative phase. Leaves of late cultivars were less determinate with less competition from inflorescences, and expanded more once irrigation was renewed. The stressed plants of late cultivars, such as ‘Stenchurian,’ produced 60% more leaf area than plants under full irrigation. The leaf area of non-irrigated early cultivars was less than that of irrigated plants.
With ‘Marquis’ wheat, high temperatures (25°C), long days, and low irradiance (about 14-42 W. m-2) resulted in long, slender, thin leaves (Fig. 11.3). On the other hand, low temperatures (15°C), high irradiance, and short days resulted in wider, shorter, thicker leaves. The greatest leaf area was obtained at intermediate levels of these factors.
Studies with timothy supported these conclusions; moderately warm (warmer than outdoors) greenhouse temperatures increased leaf length 2.5- fold. Phyllochrons were 9.3 days and 13.5 days for indoors and outdoors, respectively. Increasing day length increased leaf growth rate. Vernalized (exposed to a cold period) wheat plants produced leaves with shorter lamina than non-vernalized.
The effect of photoperiod on leaf appearance rate is harder to evaluate, since longer photoperiods are often associated with greater heat input, which is the major driving force in plant development. Thus evidence on photoperiod effect is often conflicting and inconclusive.
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