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IV. Stages of Seedling Development

IV. Stages of Seedling Development

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124



C. S. COOPER



Black (1956) reported emergence of subterranean clover seedlings from medium

and large seed 1 day earlier than seedlings from small seed when sown at a depth

of 3.2 cm. Jensen et al. (1972) reported that the emergence force of seedlings

from large-seeded alfalfa, red clover, alsike clover, and narrow leaf birdsfoot

trefoil was greater than that for small seeds. Williams (1956) found that differences in emergence force of seeds from several legume species was directly

related to seed size.

Depth of seeding is of major importance to emergence and establishment.

Stickler and Wassom (1963) obtained 53, 25, and 18% emergence of birdsfoot

trefoil from planting depths of 1.0, 2.5, and 3.8 cm, respectively. Moore (1943)

found best emergence of red, crimson, and alsike clover, white and yellow sweet

clover, alfalfa, and several Lespedeza spp. from a depth of 0.6 or 1.3 cm.

Seedlings from deeper plantings emerged more slowly and were less vigorous.

Erickson (1946) found that a 0.6-cm depth was most favorable for small alfalfa

seeds and that a 1.9-cm depth was best for large alfalfa seeds. He suggested a

1.3-cm depth as a desirable compromise. Peiffer et al. (1972) found that

‘Penngrift’ crown vetch had similar emergence from soil depths of 1.3, 1.9, and

2.5 cm but reduced emergence at 3.8 cm. Birdsfoot trefoil emergence decreased

at 2.5 cm and red clover and alfalfa at 3.8 cm.

For most forage legumes, seeding depth should be no greater than 1.3 cm.

Deeper planting may appear to be advantageous in order to place seeds in a

moist soil. However, deeper planting often results in weakened seedlings and

prolongs the period when seedlings are most susceptible to disease. Depth of

seeding may be important in terms of seedling competition. In mixtures, some

seeds will have an advantage over others at a given seed depth. Stapledon and

Wheeler (1948) concluded that optimum establishment of herbage seeds could

follow only from sowing the different fractions of a seed mixture at depths

suited to the individual seed size. Such a practice would be difficult with most

commercial seeding equipment.

During the heterotrophic stage both the amount and rate of germination may

be affected by osmotic concentration. Uhvits (1946) germinated alfalfa seeds in

substrates supplied with NaCl and mannitol at osmotic pressures ranging from 1

to 15 atmospheres. The rate and percentage of germination decreased as the

osmotic concentration increased. Germination was practically inhibited at 12 to

15 atmospheres of NaCl. Up to 9 atmospheres, however, alfalfa germination was

83% after 10 days of germination compared to 88% for tap water. Similar results

were obtained with a number of legumes germinated at a range of osmotic

concentrations by Young et al. (1970).

The legume seedling can assimilate and use externally supplied nutrients

at an early age. McWilliam e f al. (1970) reported an increase in weight of

legumes receiving nutrient solutions 5 days after imbibition. They reported

six- to tenfold increases in nitrogen content during the first 12 days of



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125



growth. Subterranean clover absorbed 32P as early as 4 days after imbibition

began.



B. TRANSITIONAL STAGE



When the cotyledon emerges, the seedling derives energy from photosynthesis

as well as from stored reserves. The duration of the transitional stage of forage

legume seedling growth may be very short and is dependent upon the amount of

reserves left at emergence. The quantity of reserves remaining at emergence is

primanly affected by depth of planting (Black, 1955). Temperature affects rate

of reserve utilization, which in turn determines rate of growth (QuaUs and

Cooper, 1968). The role of the cotyledon as a storage organ ends with the

complete utilization of reserves. At t h s time, the major role of the cotyledon is

photosynthesis (Cooper and Fransen, 1974). McWilliam et al. (1970) detected

photosynthesis in subterranean clover on the third day after imbibition and on

the fourth day the compensation point between respired C 0 2 and accumulated

COz was reached. Black (1955) found that the percentage of cotyledonary

reserves remaining at emergence was dependent upon depth of seeding and

temperature. When seeded at a 1.3-cm depth at a temperature of 2I0C, 61%of

the reserves were present at emergence, but when seeded at a 5-cm depth and a

temperature of 28OC, only 33% were present at emergence. He states that

cotyledon reserves at emergence are not likely to limit seedling growth. In

sainfoin, however, cotyledonary reserves decreased at the same rate when seeds

were germinated in the dark or the light even though seedlings grown in light had

green cotyledons by the third day (Cooper and Fransen, 1974). In this species, it

appears that cotyledonary reserves are nearly completely utilized before photosynthesis begins. Stored energy in cotyledons of seedlings grown in darkness was

not sufficient to allow normal first leaf formation. In some crops, such as corn,

new leaf area formed must be able to supply energy at a rate equivalent to the

energy derived from endosperm if seedling growth is t o progress normally

(Cooper and MacDonald, 1970). Removal of part of the endosperm will decrease

growth.

During the transfer of cotyledonary reserves t o the seedling axis, total weight

of the seedling decreases due to weight losses from respiration until reserves are

utilized. Sainfoin lost 38% of its weight from imbibition until 9 days of age at a

temperature of 20°C (Cooper and Fransen, 1974). Greenhouse-germinated birdsfoot trefoil seedlings lost more weight when grown under low light intensity

than under high (Lin, 1963). The amount of dry matter lost varied from 25 to

45% among different varieties grown under 25% of greenhouse light intensity.

The efficiency of conversion of stored substrate to new growth, calculated by

dividing new growth by weight of dry matter lost from cotyledons of endo-



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C. S. COOPER



sperm, varies among crops. Conversion efficiencies of 56% (Cooper and Fransen,

1974), 65% (Cooper and MacDonald, 1970), and 57% (Cooper, unpublished

data) have been reported for sainfoin, corn, and barley, respectively, when

grown in the dark.

Cotyledons have not been extensively studied in forage legumes. In other

species, epigeal cotyledons have been shown to vary in size and shape and in the

length of time in which they become functional (Love11 and Moore, 1971). Some

plants such as cucumber (Cucumis sativus L.) develop leaflike structures with

high photosynthetic rate. Others such as Phaseolus spp. have thick cotyledons

which senesce rapidly following utilization of reserves. Forage legume cotyledons are between these two extremes, but they differ in size and thickness and

probably in rate of senescence among species. In alfalfa, birdsfoot trefoil, and

sainfoin, each milligram of seed weight produced a cotyledonary area of 14.0,

10.1, and 3.5 mm, respectively (Lin, 1963). In studies with birdsfoot trefoil

(Lin, 1963), subterranean clover (Black, 1957), and sainfoin (Fransen and

Cooper, 1976), however, the ratio of cotyledonary area to seed size within a

species appears nearly constant.

Within a species, seed size is the major factor affecting the size of the

cotyledon. The initial growth of legumes in a newly seeded field is dependent

upon the amount of cotyledonary area present per unit of land surface. The

quantity of cotyledonary area in turn is related to the number of seedlings per

unit of land area and to the size of seed. The contribution of the cotyledon in

total seedling photosynthesis decreases as total leaf area increases (Cooper and

Fransen, 1974).

Differences in cotyledonary senescence which might occur among different

forage species have not been studied. Opik and Simon (1963, 1966) state that

cotyledon senescence begins as food reserves are utilized. At this time, cotyledons have a maximum water content, but dry weight and respiration rate have

declined rapidly. Chloroplasts begin breaking down, leaving the nucleus with an

irregular membrane. Cotyledons turn different hues of yellow owing to breakdown of chlorophyll, allowing the carotenoids to become visible. Finally, an

abscission layer forms between cotyledons and the remaining plant, and cotyledons are shed.



C. AUTOTROPHIC STAGE



Upon exhaustion of food reserves within the cotyledon, the legume seedling

becomes a true autotroph. Its ability to establish itself and to compete with

weeds and other crop species is dependent upon inherent seedling vigor and the

effects of environment.

Some species such as cicer milk vetch, crown vetch, birdsfoot trefoil, and



LEGUME SEEDLING GROWTH



127



sericia lespedeza, are slow to become established and often have thin unproductive stands because of competition from companion crops or weeds (Hensen and

Tayman, 1961). Numerous studies have been reported relative to legume seedling response to light. Cooper (1966, 1967) using growth analysis techniques,

studied growth responses of alfalfa and birdsfoot trefoil when grown under low

light intensity in growth chambers and when grown under various levels of

shading in the field. Under light intensities of 21 and 86 Klux in growth

chambers, or 8 t o 100% of full sunlight in the field, birdsfoot trefoil had a higher

relative growth rate than alfalfa. Thus, the poorer seedling vigor of birdsfoot

trefoil was not due to differences in the effect of decreased light intensity on

relative growth rate. He states that “although shading tolerance of birdsfoot

trefoil and alfalfa may be similar, birdsfoot trefoil is more susceptible to being

shaded, both in the seedling stage and later. Seedlings and mature plants of

birdsfoot trefoil begin growth later in the spring and recover more slowly after

clipping. These factors increase the likelihood of birdsfoot trefoil becoming

shaded by associated species.” Gist and Mott (1958) reported that growth

response of birdsfoot trefoil to low light was similar to alfalfa and red clover, but

seedling growth was always less. Cooper (1966) found that the ability of alfalfa

to outyield birdsfoot trefoil in the seedling stage is due entirely to initial seed

size or, in terms of Watson’s (1952) definition of growth dependency, to “initial

capital.”

Shading or low light intensity affects distribution of dry matter into tops and

roots. Cooper (1966) reported that less dry matter is partitioned into seedling

roots with decreasing light intensity. As a result, shaded seedlings may become

more susceptible to drought because of restricted root development (Cooper,

1966; Shirley, 1945). Cooper and Ferguson (1964) reported that rooting depth

of birdsfoot trefoil was only 20 cm and alfalfa only 40 cm when the barley

companion crop with which they were grown was harvested. At the same time,

roots of both species had penetrated to a depth greater than 61 cm when grown

without a companion crop. McKee (1962) found that birdsfoot trefoil shaded

for 5 weeks required 6 to 11 weeks of growth in full daylight t o restore its

original top/root ratio. He also found that the leaf area per plant of “Pennscott”

red clover often increased with shading.

Pritchett and Nelson (1951) reported that one of the most striking effects of

reduced light intensity on alfalfa was the proportional decrease in nodulation.

They found that nodulation ceases at less than 2.6 Klux and felt that this may

contribute to loss of seedlings in the field. McKee (1962) reported that “Vernal”

alfalfa and “Empire” and “Viking” birdsfoot trefoil required at least 25% of

daylight to be functionally nodulated and 50% to be adequately nodulated. In

contrast, functional nodules of Pennscott red clover were observed at 12.5% of

fill daylight.

Temperature has a marked effect on metabolic processes and consequently



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C. S. COOPER



affects the growth of the legume seedling. Richards et al. (1952) state "Such

physiological phenomena as cytoplasmic streaming, bio-electrical potential, synthesis of organic materials, translocation and respiration are all influenced by

temperature."

Temperature affects nutrient absorption. Potassium (K), nitrate, and bromide

ion uptake increases with temperature increases from 6" to 30°C (Hoagland and

Broyer, 1936). Temperature coefficients are higher for anion absorption than for

cation absorption (Wanner, 1948). In alfalfa, K increased in tops and decreased

in roots with increasing temperature, but Mg and Ca content of both tops and

roots decreased with increasing temperature. Potassium content of soybeans

increased with increasing temperatures of 12', 22", and 32"C, while divalent

cation content decreased (Wallace, 1957).

Each legume species has an optimum growth temperature (Stapledon and

Wheeler, 1948). When temperatures exceed this optimum, roothop ratios are

reduced, and when temperatures are less than optimum, root/top ratios are

increased (Sprague, 1943).

Smoliak et al. (1972) germinated and grew alfalfa, cicer milk vetch, and

sainfoin with soil temperatures controlled at 7", 13", and 27°C for a 28-day

period. Cicer milk vetch failed to emerge at 7°C and emerged and grew slowly at

18°C. Both alfalfa and sainfoin emerged and grew at 7°C. Alfalfa and cicer milk

vetch developed best at 27°C but sainfoin grew equally well at 18" or 27°C. Late

seeding at warmer temperatures seems to favor cicer milk vetch establishment,

and sainfoin can evidently be established early in the spring with cool temperatures.

McKell et al. (1962) studied growth response and phosphorus (P) utilization of

four native and four introduced legumes at 10.Oo, 15.5", and 21.1"C. Root and

top growth and P content increased with increasing temperature. Trifolium

subterraneum and T. incamaturn grew better at 15.6"C than did the other six

species. Four species, T. tridentatum, T. subterraneum, T. incamaturn, and

Medicago hispida responded to P at 15.6" and 21.1"C better than T. variegatum,

T. hirtum, T. microcephalum, and Lotus purshianus. Stimulation of root growth

with P fertilization during cool winter months was suggested as a major factor in

the subsequent production of these legumes. Many winter annuals of the genus

Wfolium germinate rapidly at temperatures of 20°C or below but have limited

germination above 30" to 35°C.

The temperature environment of seedling legumes is largely dependent upon

the climate of the area in which they are seeded. Temperature during the early

stages of growth may be controlled to some degree by time of seeding, although

extremes in temperature variation from year to year are likely to be the rule

rather than the exception. The presence or absence of competition will also

affect temperature of the microenvironment. Cooper and Ferguson (1964)



LEGUME SEEDLING GROWTH



129



found soil temperatures 1.6” to 4.4”C lower under a barley companion crop at

soil depths of 7.6 to 61 cm. The presence of an overstory species such as a

companion crop or weeds reduces the insolation intensity and influences the

wavelength of radiation reaching the soil surface. Geiger (1950) reported that

the insolation intensity of the soil surface under plants 1 meter high was only

one-fifth that at the surface of bare ground. Reflectivity was 8-20% for visible

light but rose to 45% for infrared (Geiger, 1950). Thus, both intensity and

quality of light reaching the soil surface differ under plant cover and may

influence soil and air temperatures near the soil surface. Geiger (1950) points

out that the intensity of incoming radiation upon a growing crop and bare soil is

the same during the day. Likewise, the intensity of outgoing radiation at night is

equal. The effect of the plant cover is on the distribution of heat gained or lost,

Dense stands of plants result in cooler air temperatures near the ground during

the day. At night, however, outgoing radiation is from the top surface of the

vegetation and the air is consistently warmer near the ground. In contrast, in the

absence of an overstory species, radiation is from the soil surface, resulting in a

cooler layer of air near the ground. As a result, legumes seeded with a companion crop have lower growth temperatures during the day but warmer growth

temperatures during the night than legumes seeded in pure stands.

Competition for the young legume seedling is generally from weeds or companion crops. Legume species differ in their abilities to compete, primarily as a

result of differences in growth rate. These differences may affect the vegetative

composition of those mixtures which contain more than one legume, since those

legumes which compete best may have a greater survival rate under the stress of

competition. Blaser er al. (1956) ranked aggressiveness of some common forage

legumes as follows:

Very aggressive

Alfalfa (“Kansas common”)

Red clover (“Kenland” or “Northern Neck”)

Sweet clover

Crimson clover

Aggressive

Alsike clover

White clover (Ladino)

Birdsfoot trefoil (“Granger” or “Italian”)

Nonaggressive

White clover (S-100, Virginia, and Lousiana)

Birdsfoot trefoil (“Empire”)

Big trefoil

From that ranking, it is evident that the degree of aggressivenessvaries within a

species as well as among species.



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C. S. COOPER

V.



Improvement of Legume Seedling Vigor



Seedling vigor of legumes is proportional to seed size. However, cultivars of a

species with the same seed size may differ in vigor. Thus, in breeding for seedling

vigor, both seed size and inherent seedling vigor must be considered. Birdsfoot

trefoil has received much attention in breeding programs for seedling vigor,

mostly because of difficulties encountered in establishing this species. Draper

and Wilsie (1965) in three cycles of recurrent selection increased seed size of

“Viking” birdsfoot trefoil 30% per cycle and of “Empire” 6% per cycle.

Twamley (1974) increased seedling vigor of “Leo” birdsfoot trefoil 35 to 40% in

three cycles of selection. In his program, selection was made for large-seeded

plants and then for the most vigorous seedlings from seed of those plants.

VI. Seedbed Preparation



The ideal seedbed for forage legumes should be weed-free and firm enough

that a man walking across it does not leave a footprint deeper than 0.3 cm, and

should have enough loose surface soil to cover seed to a depth of 0.6 to 1.2 cm

deep (Cooper et al., 1973). Any tillage equipment that will provide this type of

seedbed will be satisfactory.

The moldboard plow turns under crop residue and makes the soil easier to

prepare for seeding. It prepares a loose seedbed 10-25 cm deep, which requires

additional treatment before it is ready for seeding. The major advantage of the

moldboard is that it destroys existing vegetation and buries seeds of grassy weeds

such as cheatgrass (Bromus tectorum L.) and foxtail (Hordeumjubacum L.) deep

enough to prevent their germination and reestablishment. Disadvantages are the

loose seedbed and a relatively high cost per hectare.

Disk plows are not as effective as the moldboard in eliminating existing

vegetation but are more effective under drier conditions in handling residue and

shrubby growth. The one-way disk plow effectively covers heavy residue and

controls weeds but leaves the soil loose. An offset disk is more effective than the

one-way disk in breaking down sod and large clods and in killing small weeds.

Following disking, harrowing will smooth the seedbed and firm the soil, although not to the degree needed for seeding. Spike harrows smooth the soil but

leave it subject to wind erosion. Spring tooth harrows leave the seedbed less

subject to blowing because they leave more clods on the surface.

Most seedbeds require rolling or cultipacking to acquire the degree of firmness

needed for the seeding operation. In the arid regions of the West, cultipacking is

essential for obtaining a good firm seedbed. In the more humid areas of the

Midwest and East, cultipacking during the spring is less necessary and may lead



LEGUME SEEDLING GROWTH



13 1



to excessive soil compaction (Tesar and Jackobs, 1972). Firming is best accomplished by rolling with a heavy roller or by cultipacking. If the needed equipment is not available for this, a weighted spike harrow or an irrigation float or

leveler will compact the soil before seeding.

VII. Seeding Forage Legumes



A. SEED TREATMENT



Before seeding, legume seed should be inoculated with symbiotic bacteria

(Rhizobium spp .).

The symbiotic bacteria are specific for many legumes such as birdsfoot trefoil

and sainfoin but in some cases bacteria will cross-inoculate with several species.

Inoculation is essential when a legume is seeded in an area for the first time, For

successful legume inoculation the following procedures should be adhered to:

1. Select the proper inoculant for the legume t o be grown.

2. Store the commercial culture in a cool, dark place until it will be used.

3. Plant seed within 48 hours after inoculation, or reinoculate.

4. Inoculate in all cases of doubt, and always inoculate on new land.

Small amounts of seed and inoculant may be mixed in a tub or bucket. Larger

amounts may be mixed in a small concrete mixer or by hand on a cement floor

or on the bottom of a truck bed. Addition of sticking agents such as milk or

diluted syrup will help inoculant adhere to seed, Most companies that sell

inoculant also sell sticking agents.



B. CALCULATION OF SEEDING RATES



Seeding rates are recommended to provide a given number of viable seeds per

linear meter of drill row. The percentage of viable seed in a seed lot is calculated

by multiplying germination percentage by purity percentage and dividing by

100. The value obtained is called pure live seed index (PLS). Thus if a seed lot

has a percentage germination and purity of 90 and 85, respectively, the PLS is

90 X SS/lOO = 76.5%. For legumes, the percentage of hard seed is added to the

germination percentage before multiplying it by the purity percentage.

The number of seeds planted per meter of row is dependent upon the number

of seeds per kilogram, the kilograms of seed planted per hectare (ha) and row

spacing. It may be computed as follows:

seed per meter of



= Number of PLS/kg X planting rate in Kg PLS/ha



row m in ha at width to be planted



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IV. Stages of Seedling Development

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