Total Dry Matter Yield of field crops results from accumulation of net CO2 assimilation throughout the growing season. Because CO2 assimilation results from solar energy (irradiance) absorption and because solar radiation, on a seasonal basis, is distributed uniformly over a land surface, the primary factors affecting total dry matter yield are the solar radiation absorbed and the efficiency of utilizing that energy for CO2 fixation.
Controlled experiments under laboratory conditions have provided detailed information about CO2 assimilation at these levels, but there is less information on CO2 assimilation at the crop community level of organization.
Crop communities compound the problem because:
(1) The environmental factors (external micro- and macro-environments) in crop communities are constantly changing (e.g., seasonal changes in irradiance, day length, temperature, water availability, CO2 concentration, nutrient availability, oxygen concentration, air turbulence).
(2) Plants respond to the complexity of cropping environments in many different ways. The concept of light interception by crop canopies and its relationship to crop productivity is emphasized here.
Leaf Area and Solar Radiation Interception of Field Crops:
For a crop to use solar radiation efficiently, most of the radiation must be absorbed by green, photosynthetic tissue. Leaves, the primary organs for light interception and photosynthesis in crop plants, develop either from embryos in seeds or from meristematic tissue in stems. Some perennial crops maintain a nearly complete ground cover (ground area shaded by leaves) in tropical or subtropical climates, but in temperate regions the low winter temperatures terminate it.
In the spring when temperatures favor growth, a new leaf canopy is regenerated from quiescent buds supported by stored foods. In perennials the regenerative organs are overwintering buds. In annuals the initial leaf area develops from seedlings and is small for much of the early growth; this results in absorption of most of the solar radiation by the soil surface, producing sensible energy (heat).
Efficient crop species tend to invest most of their early growth in expansion of leaf area, which results in efficient use of solar radiation. Many agronomic practices such as starter fertilizer, high plant, densities, and more uniform plant-spacing arrangements (e.g., narrow rows) are used to hasten ground cover and increase light interception.
Leaf area development in an annual, determinate crop (vegetative growth stops at flowering) is illustrated in Figure 2.1 A. As leaf area develops, radiation interception by leaves increases. Early leaf area develops at an exponential rate, but since the initial leaf area is small, significant radiation interception does not occur for several weeks.
Since flowering terminates leaf area development, cultural objectives are to maximize photosynthesis by the crop intercepting all or nearly all of the solar radiation. This is an efficient pattern for grain crops, in which the majority of seed weight comes from photosynthesis after flowering.
Early growth of individual plants, with little plant-to-plant competition, is exponential and is described by the relative growth rate, which is based on the rate of dry matter increase in relation to the dry matter weight of the whole plant or crop.
As leaf area develops and there is shading of lower leaves, descriptions of crop growth are based on leaf area or land area rather than on individual plants. Watson (1947) coined the term leaf area index (LAI), which is the ratio of the leaf area (one side only) of the crop to the ground area. Because solar radiation is evenly spread over the land surface, LAI is a rough measure of leaf area per unit of available solar radiation.
Leaf Area Index and Rate of Dry Matter Production:
i. Crop Growth Rate:
The concepts of growth analysis must be introduced here to facilitate discussion of yield in crop canopies. The most meaningful growth analysis term for crop canopies is dry matter accumulation per unit of land area per unit of time, or the crop growth rate (CGR). CGR is measured by harvesting (sampling) a crop community at frequent intervals and calculating the increase in dry weight from one sampling to the next.
It is usually expressed in units such as g . m-2 (land area) . day-1. Ideally, all living tissue of the crops growing in the sampled area should be measured, but the difficulty of sampling roots often excludes their use in some CGR studies. CGRs of a species are usually closely related to interception of solar radiation (Fig. 2.1).
ii. Net Assimilation Rate:
Since leaf surfaces are the primary photosynthetic organs of the plant, it is sometimes desirable to express growth on a leaf area basis. The dry matter accumulation rate per unit of leaf area per unit of time is called the net assimilation rate (NAR) and is usually expressed in g . m-2 (leaf area) . day-1. The NAR is a measure of the average photosynthetic efficiency of leaves in a crop community. It is highest when the plants are small and most of the leaves are exposed to direct sunlight.
As the crop grows and LAI increases, more and more leaves become shaded, causing a decrease in NAR as the growing season progresses (Fig. 2.1B). In canopies with a high LAI, the young leaves at the top absorb the most radiation; have a high CO2 assimilation rate, and translocate large amounts of assimilate to other plant parts.
In contrast, older leaves at the bottom of the canopy under shaded conditions have low CO2 assimilation rates and contribute less assimilate to other plant parts. The net assimilation rate does not take into account non-laminar photosynthesis, which is the photosynthesis of plant parts other than leaves (i.e., petioles, stems, sheaths, and inflorescences), which can significantly contribute to crop yield.
The NAR is a measure of the average net CO2 exchange rate per unit of leaf area in the plant canopy. Thus, when it is multiplied by the LAI, the product is the CGR.
CGR = (NAR)(LAI)
Critical and Optimum Leaf Area Indexes:
i. Critical Leaf Area Index:
Two kinds of relationships have been found between the CGR and LAI. Brougham (1956) in New Zealand hypothesized that if enough leaf area was kept in a pasture to intercept most of the solar radiation, the maximum growth rate should be maintained. To test this hypothesis, Brougham clipped plots of ryegrass-clover mixture to heights of 12.7, 7.6, and 2.5 cm.
He measured dry matter, LAI, and light interception of the canopies every 4 days for a total of 32 days after clipping. He found that immediately after clipping, the plots clipped to 12.7 cm intercepted 95% of solar radiation whereas plots clipped to 2.5 cm intercepted less than 20%. Brougham showed that the CGR increased as the LAI increased up to 5, where the canopy intercepted 95% of solar radiation.
Leaf area indices above 5 did not change the CGR significantly so Brougham termed the LAI at which the canopy first reached maximum CGR (which occurred at 95% light interception) as the critical LAI.
The LAI with 95% solar radiation interception has been adopted as the critical LAI by most crop physiologists for two reasons. First, radiant energy interception approaches a maximum asymptotically, which means the LAI at 100% interception would be impossible to measure. Second, 95% interception approaches under maximum solar radiation of 2300 µmol photons. m-2 . sec-1 means that the radiation level at the bottom of the canopy is 115 µmol photons. m-2 . sec-1; this is the light compensation point for many species.
Increasing the LAI above 95% radiation, interception would result in insignificant CGR increases. Growth analysis of soybeans by Shibles and Weber (1965) showed the critical LAI response and classical relationships among the interception of solar radiation, LAI, and CGR.
ii. Optimum Leaf Area Index:
In England, Watson (1958) ran an experiment similar to that of Brougham’s. Watson grew kale and sugar beets in rows and varied the number of plants per row to change the LAI. Measuring the LAI and dry weight at 10-day intervals, Watson showed results similar to Brougham’s except that with kale the CGR peaked at a LAI of about 3.5 and then declined at greater LAI values.
This finding was similar to theoretical calculations by Kasanaga and Monsi (1954) in Japan, who labeled the LAI at maximum CGR as optimum LAI because the CGR decreased as the LAI increased beyond the optimum. In Watson’s experiments sugar beets were more efficient than kale and did not seem to reach a maximum CGR even at a LAI of 5.
Critical and Optimum LAI Concepts:
Canopies exhibiting either optimum or critical LAI concepts show an increase in CGR with an increased LAI up to the LAI at which most of the solar radiation is intercepted. After the maximum CGR is attained, however, the two concepts differ because of respiration. Photosynthesis increases until all the radiation is intercepted by photo- synthetic surfaces.
Any further increase in leaf area would only shade lower leaves, which would then be unable to produce enough photosynthesis to meet respiration requirements and might use photosynthetic products from other leaves (become parasitic), thus resulting in a reduced CGR. In most species new leaves are produced at the top of the plant and older, bottom leaves are shaded. Fully expanded leaves have not been shown to import photosynthetic products from other leaves.
In addition, as leaves become shaded their respiration is reduced along with reduced photosynthesis. In such species we would expect a critical LAI response. Research by King and Evans (1967) in Australia confirmed that increases in respiration are much reduced once the critical LAI is reached in wheat and alfalfa. Therefore, these species showed a critical LAI response.
The optimum LAI response may occur when young leaf tissue is shaded. Pearce et al. (1965) showed an optimum LAI response in orchard grass with growth that occurred after flowering. Since orchard grass leaves elongate from intercalary meristems near the non-elongated stem, at a high LAI older leaf parts at the top of the canopy shade younger leaf parts at the bottom.
The young leaf tissues utilize photosynthate from other leaf tissues as a result of their growth function. This increases their respiration, which they cannot match in photosynthesis because of shading, and the optimum LAI response occurs.
Although critical and optimum LAIs differ in definition, the same quantity is being characterized, that is, the minimum LAI to attain the maximum CGR. Both assume a LAI that intercepts most of the light.
In this article we will discuss about radiation attenuation through canopies for photosynthesis in plants.
Radiation Attenuation through Crop Canopies:
Crop communities intercept both direct sunlight and indirect or diffuse skylight. The upper leaves receive both direct and diffuse radiation, while leaves lower in the canopy receive a smaller portion of direct radiation (e.g., sun flecks). Indirect radiation becomes more prominent due to radiation transmitted through leaves and reflected from plant and soil surfaces. Both quantity and quality of radiation change with depth in the canopy because light transmitted by leaves is predominantly infrared.
Since plants preferentially absorb energy in the 400- to 700-nm wavelength range, the longer wavelengths become more prominent at lower levels. For that reason, in photosynthetic studies most instruments used to measure radiation in crop communities only measure quantum or energy levels between wavelengths of 400 and 700 nm. The radiation thus measured is called photosynthetic photon flux density (PPFD) as a quantum measurement or photosynthetically active radiation (PAR) as either a quantum or energy measurement. The term radiation hereon will mean PAR.
The attenuation of radiation down through the plant canopy has been shown to approximate that of colored solutions or algae cell suspensions. This pattern of extinction conforms to the Lambert- Beer law of absorption, which states that each layer of equal thickness absorbs an equal fraction of radiation that traverses it. For plant canopies the layer of equal thickness is based on units of LAI.
The mathematical expression is:
Ii/Io = e-kL
Where Io. = photosynthetically active radiation above the canopy, Ii = photosynthetically active radiation below the ith layer of leaves, L = leaf area index of the ith layer of leaves, k = extinction coefficient characteristic of the canopy, and e = base of natural logarithms (2.71828).
Thus the amount of sunlight penetrating through the canopy is affected by the LAI and the pattern of leaf display. The extinction coefficient (k) gives a numerical indicator of light attenuation in the canopy. The k is a characteristic of canopy leaf display, which primarily consists of leaf inclination and the way leaves are grouped within the canopy.
Leaf Inclination within Canopies:
Types of leaf inclination have been defined and illustrated by de Wit (1965). These idealized patterns range from planophile, with most leaves nearly horizontal (<35° from horizontal), to erectophile, with most leaves nearly vertical (>60° from horizontal). Trenbath and Angus (1975), who tabulated studies showing how different crop species fit different patterns of inclination, cited studies in which plant species have shown all the leaf inclination patterns except extremophile.
The inclination of leaves affects radiation interception and distribution in the canopy. The planophile clover canopy needs less leaf area to intercept most of the radiation than the erectophile grass canopy (Fig. 2.8). Approximate k values for clover and grass stands are 0.6 for clover and 0.25 for grass.
Warren Wilson (1959), using the frequency of foliage contact with vertical and horizontal needles passed through various strata to calculate mean foliage angle (Fig. 2.8), illustrated the planophile and erectophile inclination patterns of clover and ryegrass canopies, respectively. According to Brougham’s (1956) theory of critical LAI, the clover canopy in Figure 2.8 intercepts 95% of the radiation at a LAI of 5, so the critical LAI for clover is 5 whereas the CGR of grass continues to increase up to a critical LAI of 9.
A high proportion of the crops studied for leaf inclination are the planophile type. This could be due to competition to weed development in crop stands; most weeds are severely hampered in growth by shading, so crop plants reduce competition for water, nutrients, and radiation by maximum shading of weeds during vegetative development.
Leaf Inclination and Photosynthetic Efficiency:
Leaf photosynthesis is most efficient (i.e., CO2 fixed per unit of light) at low radiation levels. Most individual leaves are radiation saturated in direct sunlight (Fig. 2.9). In a planophile canopy, upper leaves are radiation saturated and lower leaves have reduced photosynthesis due to shading.
Theoretically, a planophile canopy would be more efficient if radiation were distributed more evenly over leaf surfaces. Such an equitable distribution could be accomplished by having leaves, at least the upper leaves, at a vertical leaf inclination when the sun is at high elevations.
Radiation Attenuation and Crop Growth Rate:
How vertical should leave be? Figure 2.9 illustrates the theoretical concept of maximizing canopy photosynthesis through more vertical leaf orientation. For example, when leaves are angled 75° from the horizontal with a vertical radiation source they intercept 26% as much radiation as horizontal leaves and the effective radiation level on the leaf is 26% of that on a horizontal leaf. Because the photosynthetic response to radiation is curvilinear and radiation efficiency is greatest at low radiation levels, the vertical leaf is more efficient per unit of radiation intercepted.
In the red clover example (Fig. 2.9), when the leaf is angled 75° from horizontal and is intercepting only 26% of the’ light, leaf photosynthesis is reduced only 21% from that of horizontal leaves.
A small reduction in upper leaf photosynthesis because of vertical leaf inclination allows much more radiation to penetrate to lower leaves. Canopy photosynthesis and CGR can theoretically be increased dramatically by vertically orienting leaves at a high LAI.
The C4 species usually do not reach radiation saturation in direct sunlight, which means they use high radiation levels more efficiently than do C3 species. But they still use radiation more efficiently at low levels than under full sunlight.
Loomis and Williams (1969), using a computer modeling program, estimated the influence of leaf inclination and leaf amount on the CGR of maize and clover. The critical LAI (95°7o radiation interception) is lowest for canopies with horizontal leaves and highest for canopies with vertical leaves. Canopies with horizontal leave provide the highest CGR at LAIs below 3.
A canopy of vertical leaves needed an LAI of 4 or greater to have a distinctly higher CGR than canopies with horizontal leaves. At low LAIs there is little shading among leaves, so canopies with horizontal leaves have a slight advantage over canopies with vertical leaves because of higher irradiance at the leaf surface. At high LAIs canopies with vertical leaves have the advantage because light is more evenly distributed over the canopy leaf area; less radiation interception by upper leaves allows more light to be intercepted by lower leaves.
Solar Angle, Radiation Attenuation and Crop Growth Rate:
The sun is not always vertically overhead; the angle of the sun’s rays shining into crop canopies changes seasonally and diurnally. Duncan et al. (1967) calculated the effect of leaf angle and leaf amount on the CGR during the diurnal period.
During the early morning and late evening, when the sun’s rays come in at nearly horizontal angles, the leaf angle or LAI had little effect on the CGR. At solar noon horizontal leaves had the advantage at an LAI of 2 and leaves 80° from the horizontal at an LAI of 8. Duncan (1971) estimated that for maize in the U.S. Corn Belt a leaf angle of 80° would be the most productive.
Leaf Inclination Variation within Canopies:
Leaf inclination may vary in different strata within the canopy (Fig. 2.8). Canopies with leaves vertically inclined at the top and gradually becoming more horizontal closer to the ground have been termed the ideal foliage display by Trenbath and Angus (1975).
Pendleton et al. (1968) showed that maize canopies with the leaves above the ear tied- in a vertical inclination yielded more than those with the leaves in their normal planophile position or those with all plant leaves tied into vertical inclinations.
The pattern of vertical upper leaves and more horizontal lower leaves allows vertical leaves in the radiation-rich environment to intercept less radiation, putting them at a more photosynthetically efficient radiation level and allowing more radiation to pass to lower leaves. With radiation more evenly distributed over the total leaf area, the canopy may not require the extremely high LAIs that are necessary for a high CGR in canopies with all leaves vertically inclined.
Advantages and Disadvantages of Erectophile Canopies:
Trenbath and Angus (1975) cite four studies on sugar beet, barley, rice, and tea in which the relationship between CGR and leaf inclination was measured. Erectophile type CGRs were 19 to 108% greater than planophile type CGRs. They also cited fourteen studies on wheat, barley, rice, and maize in which grain yield was measured in relation to leaf inclination.
Three studies showed a yield advantage for planophile types (5-18%), and eleven showed a yield advantage for erectophile types (4-68%). In all cases, erectophile canopies that performed to advantage were planted at plant densities achieving or exceeding the critical LAI.
Most of the studies concerning leaf inclination have been conducted on grasses. Broadleaf (dicotyledonous) plants often change their inclination in response to the sun (heliotropic movement). Many crops, including legumes, cotton, and sunflower, have heliotropic responses. Some orient their leaves perpendicular to direct solar radiation; others may actually angle the leaf surface in relation to direct solar radiation.
Under cloudy conditions soybeans have been found to orient their leaves perpendicular to the brightest part of the sky while maintaining an oblique angle to direct solar radiation. Few studies have been made to determine if this characteristic can be stabilized and used for better radiation interception.
Vertical Separation of Leaves:
Vertical leaf density influences the skylight pattern within the canopy. A leaf just beneath a top leaf can receive both direct shade and direct sunlight according to its position; the farther away the leaf is from the top leaf the less prominent the sun-flecks and shade because of diffusion of shadow edges.
Large but widely separated leaves like those of sunflower may actually create a diffuse light pattern in the plant canopy similar to that of shorter plants such as alfalfa. If leaves are widely spaced vertically or are very narrow (e.g., asparagus and conifers), shadow edges will be diffuse and the distinction between direct radiation and diffuse radiation may be lost.
Most plants seem to have evolved with leaves maintaining a vertical distance twice that of their width. In breeding for dwarf varieties of cultivated species (e.g., sorghum and maize), this distance factor has changed so that wide leaves are closer together in relation to leaf width. Loomis and Williams suggest that the leaf arrangements in dwarfs might be improved by reducing leaf with, by having fewer leaves, and/or by arranging leaves in a whorled pattern.
Efficient interception of radiant energy incident to the crop surface requires adequate leaf area, uniformly distributed to give complete ground cover. This is achievable by manipulating stand density and its distribution over the land surface.
Measurements of the LAI and CGR in crop communities reveal much about how crops achieve high yields. However, they are difficult to measure and so, for practical reasons, crop managers use plant densities (plants per unit of ground area) and final yield.
Plant and Environmental Factors Affecting Optimum Plant Density:
Selections of the most suitable stand density must be based on the following plant and environmental factors:
1. Plant size (which primarily reflects leaf area per plant):
Four species — oats, soybeans, sorghum, and maize—are compared in Figure 2.18 at commonly used plant densities that result in LAIs of 4.2, 6.7, 3.5, and 4.2, respectively. The leaf area per plant determines the number of plants needed to develop a critical LAI. Maize hybrids adapted to northern latitudes have one to three fewer leaves than those in the southern United States and require higher stand densities for maximum yield. Leaf inclination would modify the critical LAI, and the stand density would have to be adjusted accordingly.
2. Tillering and/or Branching:
Branching, an effective way of increasing the leaf area per plant, decreases the sensitivity of yield to plant density. In sorghum plants, which tiller (branch from nodes close to or below the soil surface), the number of heads per acre increased only slightly when plant density was changed from 13,000 to 52,000 plants per acre (32,000 to 128,000 per ha) (Fig. 2.19), indicating over three tillers per plant at 13,000 plants per acre.
When plant density was doubled from 52,000 to 104,000 plants per acre the heads per acre also doubled, indicating that little tillering occurred at 52,000 plants per acre. Increased plant density did not increase grain yield because as heads per acre increased, the seeds per head decreased proportionately. Modern maize varieties do not tiller much, even at low plant densities, and usually produce only one ear per plant.
Therefore maize grain yield is much more sensitive to plant density than is sorghum because both the LAI and the number of ears per acre increase or decrease with density. Maize does not have the flexibility of most crop species, which can increase leaf area and number of reproductive units by branching at low crop densities.
Increased density causes plants and stems to become smaller, weaker, and often taller. Thus, strong-stemmed cultivars are required or plant density has to be decreased to reduce lodging (the leaning or falling over of plants). Lodging decreases harvestable yield by putting the seeds too close to the ground for equipment to harvest them and decreases absolute yield by ruining leaf display.
4. Reduction in fruit set:
As density increases, potential flowers and fruits do not set or are aborted. This reduces the possible seed yield by decreasing the total possible amount of assimilate that the seed could retain.
The environment also influences the optimum plant density for yield.
The primary environmental factors include:
(3) Soil fertility
Limitation of these environmental factors lowers the optimum plant density for maximum production. Weeds compete with crop plants for these environmental factors, which decrease the optimum plant density.
Plant Density and Yield:
Holliday (1960 a, b) summarized a large volume of literature and emphasized the two density-yield interactions that occur when crop plant density is increased. These interactions depend on whether the yield is a product of the plant’s growth in the reproductive phase (seed yield) or a product of growth in the vegetative phase. The key consideration is whether the economic yield is a plant component (e.g., seed weight) or the entire plant (biological yield).
To illustrate yield from the reproductive phase, wheat grain yields in England were cited. These data described a parabolic response curve (Fig. 2.20), typically a flat-topped one with decrease in yield on both sides of an optimum. When seed yield is the desired product, there is an optimum plant density beyond which plant density can become too high because available photosynthate is partitioned more to the vegetative growth or to maintenance respiration than to seed growth.
The curve for seed yield in Figure 2.19 can be fitted to the following quadratic equation:
Y = a + bx – cx2
Where Y = yield per unit area, x = plant density (plants/area), and a, b, and c are regression constants.
When yield is the product of growth of vegetative material, the yield response to increasing plant density is asymptotic, similar to the critical LAI. In this case a dense stand for maximum radiation interception must be achieved as rapidly as possible; but if the stand is too dense, the only loss is from greater seeding expense.
This partially explains why recommended seeding rates for forages are often so high. Although there is no loss from being over the critical plant density, there is also no gain because only 100% of solar radiation can be intercepted. Uniform stands of forages are often difficult to establish, which motivates super-high seeding rates.
The curve for biological yield in Figure 2.20 can be defined by the following expression for a rectangular hyperbole:
Y = Ax 1/(1 + Abx)
Where Y = yield of dry matter per unit, A = apparent maximum yield per plant, x = number of plants per unit area, and b – linear regression coefficient.
In this expression the term 1/(1 + Abx) represents the manner in which the maximum individual plant yield (A) is reduced by the increasing competition resulting from greater plant density. It may be termed the competition factor.
While plant density and yield have been determined in many studies, the three parameters, plant density, the yield of dry matter, and the yield of grain, have not often been measured together. Six such studies, however, are cited by Donald (1963); in each instance the peak of the grain curve occurred approximately at the density at which the biological yield (dry matter yield) leveled off (Fig. 2.20).
Therefore grain yield had an optimum LAI at the critical LAI for biological yield. At this plant density any gain in total yield per acre due to the addition of extra plants is offset by the decrease in the weight per plant. No doubt these relationships represent conditions of either limiting radiation or limiting nutrients and would not hold, for example, under conditions in which water supply becomes exhausted before grain is formed.
Duncan (1958) presented an interesting discussion of the relationship between plant density and yield of maize, with special emphasis on the interaction of number of plants and yield per plant. He formed the hypothesis, confirming it with results from many field experiments, that the logarithm of average yield for individual plants had a negative linear relationship to plant density. He concluded that one could grow a maize variety at two widely divergent plant densities (6,000 and 25,000 plants per acre, or 15,000 to 62,000 per ha) and calculate the density at which the maximum yield could be expected from that variety.
The yield per plant decreases as the number of plants increase; this relationship can be determined by plotting the yield per plant on semilog paper. Since it is a straight line plot, one only needs to know the yield per plant at two plant densities.
The yield per acre is the yield per plant multiplied by the plant density, so the yield at any plant density can be calculated and the results plotted into the usual graph form (Fig. 2.22). When the logarithm plot is compared with the arithmetic plot in Figure 2.22, it can be seen that the flatter the logarithm slope, the higher the plant density required for maximum yield.
Willey and Heath (1969) commented that while Duncan’s approach is sound, it would seem safer in practice to include a third intermediate density, so that the point calculated for maximum yield is not too far from an experimental treatment.
Plant Responses to Stand Density Changes:
Donald (1963) presented an explanation of plant responses to stand density changes. Both he and Duncan (1969) relied extensively on the work of Hozumi et al. (1955), who studied the yield of adjacent plants.
Donald suggested that the greater seed weight and number of seeds per inflorescence at intermediate densities are due to the timing of interplant (between plants) and intraplant (within a plant) competition. At the widest spacing (lowest plant density), both types of competition are absent during early stages of growth. Flower primordia are formed in large numbers.
As growth proceeds, there is little interplant competition and even less intraplant competition until after flowering and seed setting. The large loads of inflorescences leads to competition for assimilate among inflorescences and seeds on the same plant, that is, intraplant competition.
This loss of efficiency at the widest spacing reflects greater intraplant competition, resulting in fewer seeds per inflorescence and reduced seed size compared with denser stands. Thus intraplant competition may be intense at low densities.
In moderately dense stands interplant competition apparently becomes operative at the time of flower initiation or formation. The number of floral primordia laid down by each plant is considerably reduced; this reduced load lies more closely within the capacity of the plant as interplant competition intensifies.
Seeds per inflorescence and seeds per unit area achieve maximum values. Still higher plant densities should reduce seed number, causing reduced seed yield, because the interplant competition is already intense at the time of flower primordia formation.
Plant Distribution-Row Spacing:
It can be assumed that plants were uniformly placed in the field, producing a uniform canopy of leaves that uniformly intercepted solar radiation. In managed crops, however, this is not the case. Seeds are placed in the soil with a mechanical planter, usually in discrete rows. The wider the row, the more seeds must be planted per length of row to achieve a particular plant density.
The objective for higher yields is to intercept as much solar radiation as possible and equidistant planting would give the earliest and the maximum light interception. As rows are widened and spacing becomes less uniform, interplant competition occurs earlier. Plants in rows that are farther apart must be closer together within rows to achieve a particular plant density.
The major factor determining the distance between plants is plant density; the same factors that affect optimum plant density influence optimum row spacing. Crop plants with high leaf area per plant grown at lower plant densities (e.g., maize) respond less to reductions in row spacing than do smaller crop plants grown at higher plant densities (e.g., soybean).
Shibles and Weber (1966) conducted a classic comparison of a large soybean variety (‘Hawkeye’) at different plant densities and row spacings. Their results showed that a particular combination of earlier leaf area development, maximum interception of solar energy, conditions favoring reduced lodging, and efficient translocation of dry matter to the seed was needed to get the highest yields. They attained a 33% increase in yield by reducing row spacing from 40 in. to 10 in. (102 to 25 cm) at 100,000 plants per acre (247,000 per ha).
Plant stature affects optimum plant density. Soybean seeded late in the season is usually short statured due to early flowering induced by photoperiod. Plant densities have to be increased and row widths narrowed to obtain maximum yield potential, compared with taller statured soybean.
Under unfavorable environments, narrowing the rows of most crop plants will not increase yield. Taylor (1980) tested the hypothesis that during years of low water supply soybean grown in wide rows would yield as much as or more than soybean grown in narrow rows.
In a season of high water supply, seed yield in narrow (25-cm) rows yielded 17% more than in 100-cm rows. During 2yrs of lower seasonal water supply, there was no difference in seed yield among 25-. 50-, 75-, and 100-cm row spacings. In dry years, severe water deficits occurred in the narrow rows first, resulting in plants smaller in both height and LAI.
Weeds compete with crop plants for environmental factors, so good weed control is important for high yields. Weed control is difficult in rows too narrow to cultivate. Weber (1962) observed that in soybean the yields were reduced in 6- to 7-in. rows compared with 21- to 28-in. (53- to 71-cm) rows because of poorer weed control. Narrow row culture calls for higher plant densities that ensure faster canopy development to compete successfully against weeds.
Using narrow rows appears to be one of a series of steps that has led to higher crop yields for producers. However, to obtain a high yield response from narrow row widths, the producer must have already adopted other managerial tools leading to high yields (e.g., using adapted varieties, fertilization, weed control, insect control, timely cultural practices, uniform plant distribution within the row, and optimum plant densities).
When shifting to narrow rows the producer must decide on the variety to plant; the seeding rate (plant density); how to deal with potential lodging problems, the expense of buying narrow row equipment (planting and harvesting), and the higher investment needed for seed and fertilizer; and how to get good early-season weed control.
Plant breeders and crop physiologists are attempting to identify genotypes adapted to high plant densities and narrow rows.
Total dry matter yield is a result of crop canopy efficiency in intercepting and utilizing the solar radiation available during the growing season. The primary plant organs intercepting solar radiation are the leaves. For maximum crop growth rates, enough leaves must be present in the canopy to intercept most of the solar radiation incident on the crop canopy.
When this occur the level of crop photosynthetic efficiency (or the CGR) is determined by the photosynthetic efficiency of leaves (or the NAR). The efficiency of the NAR can be influenced by the amount of solar radiation, the ability of leaves to photosynthesize, the LAI, how evenly the solar radiation level is divided among leaf surfaces, and the amount of plant respiration.
Crop plants do not maintain a critical LAI over the total growing season. Annuals start leaf area accumulation from seedlings, in which radiation interception by the crop canopy is almost zero. But the LAI increases and eventually intercepts most of the solar radiation. After total ground cover is achieved, total dry matter production is a factor of how long the crop can maintain an active, green leaf canopy.