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CHAPTER 4. GROWTH OF THE LEGUME SEEDLING
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
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
LEGUME SEEDLING GROWTH
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
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)
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
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.
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
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.
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