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CHAPTER 4. GROWTH OF THE LEGUME SEEDLING

CHAPTER 4. GROWTH OF THE LEGUME SEEDLING

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120



C. S. COOPER



The growth of a legume seedling depends on its inherent vigor and the

environmental conditions present during seed germination, maturation, and

growth. During this century there has been much research concerning growth of

the legume seedling. With the exception of review papers dealing with the effect

of seed size upon seedling growth, however, few attempts have been made to

summarize these data. This paper is concerned with factors affecting the development and growth of the forage legume seedling.

I I. Physiological Predetermination



Fifty-six years ago Kidd and West (1919) pointed out that environmental

conditions during seed formation could influence seed size and subsequent

progeny performance. They designated growth responses which could be traced

to environmental conditions at some stage of previous development as “physiological predetermination” in order t o distinguish them from those which are

due to hereditary causes. They listed three conditions which could affect the

“potentiality” of the seed or the capacity of the resulting plant for growth and

yield. These were: (1) parental conditions; (2) conditions immediately preceding

germination, during germination, or in the early stages of the seedling’s growth;

and (3) harvesting conditions.

The most noticeable manifestation of physiological predetermination is seed

size. Seed size may be affected by the position of seed on the plant, the amount

of substrate and nutrients available during seed formation, and the environment

during seed development. Seed size varied from 2 to 34 mg in one strain of

subterranean clover (Black, 1957) and from 2.7 to 24.6 mg in one strain of

sainfoin (Fransen and Cooper, 1976). Similar ranges of seed size are observed in

most forage legume species. Legumes such as birdsfoot trefoil or crown vetch

with indeterminate flowering habit experience a greater range of environmental

effects during seed development than those with a determinate flowering habit.

Anderson (1955) reported that umbels of birdsfoot trefoil set in early season

produced more pods and usually more seeds per pod than umbels set in late

season.

Time of harvest may also affect seed size and embryo maturity. With determinate species, harvest may be timed to occur when most seeds are mature,

but indeterminate species display seeds of various stages of maturity at any given

harvest date. Immature seeds are usually smaller and of lower viability than

mature seeds (Anderson, 1955; Carleton et al., 1967). Birdsfoot trefoil seed

reaches maximum dry weight at 35 to 40% moisture (Anderson, 1955) and

sainfoin seed at 40% moisture (Carleton et al., 1967). Maximum dry weight for

most forage legume seeds probably occurs at 35-40% moisture.

Early seedling growth is proportional to seed size in alfalfa (Beveridge and

Wilsie, 1959; Carleton and Cooper, 1972), birdsfoot trefoil (Carleton and



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Cooper, 1972; Hensen and Tayman, 1961; Stitt, 1944; Twamley, 1967), sainfoin

(Carleton and Cooper, 1972; Cooper, 1966), and subterranean clover (Black,

1956, 1957, 1959). Fransen and Cooper (1976) showed that seedlings from large

sanfoin seeds emerged and developed first and second leaves faster than seedlings

from small seeds. Embryo axis and leaf primordia length were directly proportional to seed size. The more rapid seedling growth from larger seeds during the

heterotrophic phase is probably due to a more advanced stage of embryology

rather than to more stored reserves.

Environmental conditions during maturation may affect the performance of

seed. Dotzenko et al. (1967) found that alfalfa seeds produced under a wide

range of temperatures in the field displayed significant variation in the percentage of hard seed, quick germination, and total germination. In general, hard

seed content is greatest when seed is produced in cooler climates (Gunn, 1972)

and is greater in small seeds within a species than in larger seeds (Black, 1959).

Although climate during maturation may affect hard seed content and size of

seed, it does not necessarily affect the subsequent growth of the seedling.

Cooper (unpublished data) found no difference in relative growth rate, net

assimilation rate, or leaf area ratio of seedlings grown from alfalfa seed produced

from an identical 2-clone cross in Arizona, Montana, and Nevada. In his work,

seeds were screened to remove seed size effects which may have occurred as a

result of environment.



I I I . Germination



A. HARDSEED



Legumes imbibe water much more rapidly than grasses and nearly all of the

water needed for germination is imbibed during the first 4-8 hours (McWilliam

e t aZ., 1970). The presence of a seed pericarp slows the rate of imbibition in

crimson clover (Stitt, 1944) and in sainfoin (Carleton et al., 1968).

Hard seeds are impermeable to water and thus incapable of imbibition until

the seed coat is scarified. Many legumes contain a large percentage of hard seeds

if hand-harvested, but are scarified to some degree in the process of threshing.

Rincker (1954) reported an average of 64% hard seed for 66 samples of

Wyoming machine-threshed certified alfalfa seed, and Cooper (1957) reported a

hard seed content of 80 t o 90% for an annual native clover in Oregon. Hard-seed

content of crown vetch (Brant et ab, 1971) and of cicer milk vetch (Carleton et

al., 1971) may be more than 75%.

Impermeability in legume seed is ascribed to the cuticle or the macrosclerid

layer, also known as the pahade layer or the Malpighian layer (Brant et aZ.,

1971). Rincker (1954) made hard seeds of alfalfa permeable by exposing them

to a temperature of 105°C for 90 seconds or to 42°C for 1 hour. Barton (1947)



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C. S. COOPER



made seeds of whlte sweet clover (Melilotus alba Desr.) absorb water by plunging

them into liquid N (-1953°C). Brant'et al. (1971) successfully scarified seed of

crown vetch by: (1) immersion in 18 N sulfuric acid for 15 minutes, (2) immersion in boiling hot water for 15 seconds, followed by a dip in cold water,

(3) mechanical scarification, and (4) two 2-minute immersions in liquid N. They

also reduced hard-seed content slightly by treatment with hemicellulase and

pectinase.

The degree of scarification required differs with species and among seed lots

within a species. Carleton et al. (1971) reported that cicer milk vetch required

much more intense mechanical scarification than alfalfa. They developed a

"quick swell test" for determining the effectiveness of scarification. Following

scarification treatment, seeds are germinated on wet blotter paper. After 24

hours, the percentage of seeds swollen is determined. They reported that 30 to

50% swollen seeds is commensurate with good scarification. Higher percentages

were often associated with a high degree of seed injury which resulted in poor

emergence when seeds were planted.



B. TEMPERATURE



Most forage legume seeds germinate over a wide range of temperatures but the

optimum germination temperature varies among species. Those species which are

easiest to establish in the field also have the ability t o germinate completely and

rapidly over a range of temperature (Townsend and McGinnies, 1972). Townsend and McGinnies (1972) grew a number of legume species at three alternating

temperatures (5"-2OoC, 1So-25"C, and 2Oo-25"C) and at a constant 2OoC.

Duration of alternating temperatures was 12 hours. They considered alfalfa and

sainfoin to be temperature insensitive for total germination over the range of

temperatures tested. Germination rate and total germination of cicer milk vetch

increased through 15"-25"C but decreased at higher temperatures. McElgunn

(1973) germinated sweet clover, alfalfa, birdsfoot trefoil, and sainfoin at constant temperatures of 7", lo", 13", or 21°C and 12-hour alternating temperatures of 2"-13OC, 4"-15"C, 7"-18"C, or 16"-27OC. Rate of germination averaged across all temperatures was in the order of sweet clover > alfalfa >

birdsfoot trefoil > sainfoin. He concluded that cold alternating temperatures

reduced both germination rate and total germination. Sainfoin had the slowest

germination rate when averaged across all temperatures. For the first 5 days,

germination rate at constant temperatures was faster than at alternating temperatures for all species. Qualls and Cooper (1968) found that respiration and

germination rate of birdsfoot trefoil increased with increasing temperatures from

15.6" t o 26.7"C. They found differences in germination rate of seeds of the

same size from different varities. Carleton et al. (1968) showed that sainfoin



LEGUME SEEDLING GROWTH



123



seed germinated best at a temperature range from 15" to 23°C but germination

decreased at 30" or 35°C. During the first 8 days of germination, seedling length

increased most at 20" to 30°C and least at 35°C.

Young et al. (1970) found that subterranean clover, woolypod vetch (Vicia

dasycarpa Ten. var. Lana), cup clover (Trifolium cherileri L.), and sainfoin had a

remarkable amount of germination at 5" and 0.5"C. They state that these

legumes appear to be hghly adaptable t o germination in late fall or early spring

in cold seedbeds of arid rangelands. McWilliam et al. (1970) found that germination rate of whte clover and alfalfa increased from 5" to 30°C but germination

of subclover declined markedly at temperatures above 30°C.



IV. Stages of Seedling Development



Once a seed germinates, the resultant seedling goes through three stages of

development which have been defined by Derwyn et al. (1966) as (1) heterotrophic, (2) transitional, and (3) autotrophic. For a legume seedling, the heterotrophic phase is from imbibition of water until emergence and commencement

of photosynthesis in cotyledons. The transition stage occurs when cotyledons

begin to photosynthesize but before the exhaustion of reserves. The autotrophic

stage follows the exhaustion of the cotyledonary reserves. At this time the

seedling is entirely dependent upon photosynthesis and is a true autotroph.



A. HETEROTROPHIC STAGE



During the heterotrophic stage of seedling development, the rate of transfer of

stored reserves to the embryo axis is highly dependent upon temperature

(Derwyn et al., 1966). Rapid extrusion of the seedling root is important to

establishment, particularly where surface soils dry rapidly, or where conditions

favorable to germination may be of short duration such as on arid rangelands.

Rate of root extrusion is more rapid for annuals than for perennials and is a

factor in the capacity for regeneration, which permits the use of annuals in

pastures (McWilliam et al., 1970). Rate of seedling elongation varies within a

species and is directly correlated with seedling vigor (Qualls and Cooper, 1968).

The rate of hypocotyl elongation often determines the duration of the heterotrophic stage of development because cotyledons begin photosynthesis upon

emergence and the seedling enters the transition stage. Size of seed and depth of

seeding are two major determinants of emergence rate. Fransen and Cooper

(1976) studied growth and development of sainfoin seedlings from four seed

sizes of 10 sainfoin accessions. In all accessions, seedlings from large seed

emerged earlier and developed more rapidly than seedlings from small seed.



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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CHAPTER 4. GROWTH OF THE LEGUME SEEDLING

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