Seed anatomy and morphology of Thlaspi arvense (pennycress) and preliminary germination results

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Seed anatomy and morphology of Thlaspi arvense
(pennycress) and preliminary germination
results
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Terms used on slides

• Testa seed coat and can act as a barrier for
  germination
• Embryo living portion of seed that grows into
  the seedling
• Endosperm In the case of Pennycress this is an
  envelope or sack that surrounds the embryo
  and acts as a barrier for germination.
• Cotyledons embryonic leaves that emerge from
  soil after germination
• Radicle embryonic root
• Micropylar end point on seed where radicle
  emerges
• GA3 gibberellic acid (a germination promoter)
• KNO3 - potassium nitrate (promotes germination
  in some species)

Rukuni Taylor, Cornell, Geneva
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Figure 1. Germination of pennycress at 20/30oC
under various treatments. GA3 was used at 100
µM, and KNO3 was at 0.2. Excised embryos (Ex
emb) germinated 100 after 4 days when GA was
added, but in the Mid and Late it took 18 days
without GA. Punct punctured testa and
endosperm.
Rukuni Taylor, Cornell, Geneva
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Figure 2. Germination of pennycress at 20/30oC
or 10/20oC under various treatments. Chilling
was done at 5oC for 7 days prior to the
germination test, and KNO3 was at 0.2.
Rukuni Taylor, Cornell, Geneva
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• SUMMARY
• The anatomy and morphology of T. arvense seeds
  resembles that of the model species Lepidium
  sativum and Arabidopsis thaliana, which are
  frequently used to study germination and dormancy
  physiology. These three species belong to the
  Brassicaceae family, also known as the Cruciferae
  or Mustard family. The seeds consist of a seed
  coat, a single cell-layer of endosperm
  (endospermic seeds), and a dicotyledonous embryo,
  but other brassica species may not have an
  endosperm (non-endospermic seeds). The embryo
  consists of the cotyledons (embryonic leaves),
  the radicle (miniature root) and an embryonic
  shoot between the cotyledons (not visible in
  pictures).

Rukuni Taylor, Cornell, Geneva
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• In these endospermic brassica seeds, dormancy is
  normally classified as combinational, in the
  sense that the endosperm and seed coat act as
  physical barriers to germination and the embryo
  itself has physiological dormancy. Physiological
  dormancy is known to decline under suitable
  after-ripening conditions, normally at ambient
  conditions (temperature and relative humidity).
  After-ripening is a little understood phenomenon
  and many factors affect the length of the
  after-ripening period, and these factors include
  the genotype of plants, the seed maturation
  environment and the post-harvest seed storage
  conditions.

Rukuni Taylor, Cornell, Geneva
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• In order to germinate, seeds have to first
  overcome physiological dormancy of the embryo,
  and when the embryo has acquired the ability to
  grow, it also has to gain the growth strength or
  vigor to overcome the restrictive physical forces
  exerted on it by the seed coat and endosperm. A
  good example of physiological dormancy is
  illustrated in Figure 1, where excised embryos
  take about 4 days to germinate when gibberellic
  acid (GA) is added but takes 18 days without GA
  for the Mid and Late seed lots. In many cases,
  seed pre-treatments like GA, cold stratification
  (chilling) or potassium nitrate do not overcome
  physiological dormancy (Figure 1), but
  after-ripening will overcome dormancy in time.

Rukuni Taylor, Cornell, Geneva
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• Of the two outer layers, the seed coat normally
  ruptures first and then the endosperm follows.
  Puncturing the seed coat and endosperm, and
  adding of GA (Figure 1) promoted germination,
  further proof that these two seed tissues are a
  physical barrier to germination, though the more
  dormant Mid seed lot had limited germination due
  to the deeper physiological dormancy. The
  endosperm has the ability to inhibit germination
  even when the testa has ruptured. In some
  species, enzymes that digest the endosperm are
  known to exist, and endosperm weakening through
  digestion has to occur before germination
  proceeds. In endospermic seeds, the endosperm is
  the major physical germination barrier.

Rukuni Taylor, Cornell, Geneva
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• After-ripening relieves dormancy, and non-dormant
  seeds germinate in a wider range of environmental
  conditions, especially various soil temperatures.
  The behavior of all pennycress seed lots
  demonstrates that the seed lots have varying
  degrees of dormancy. Figure 2 shows the
  unpredictable germination behavior of dormant or
  partially dormant pennycress seed lots under
  different temperature and pre-treatment (chilling
  or potassium nitrate) regimes. The most dormant
  is the Mid then the Late, and the least dormant
  is the Alberta.

Rukuni Taylor, Cornell, Geneva
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• It is possible to enhance germination in such
  seed lots, but the degree of dormancy determines
  the success of these treatments, and more chances
  of success lie with the least dormant. A
  treatment that might work with one seed lot might
  not necessarily be the best for another seed lot,
  this being influenced by the physiological status
  of the seeds. Ad-hoc seed enhancements could be
  used, but more reliable techniques need more time
  to develop, and this begins with appropriate seed
  production and handling methods, seed
  conditioning and sanitation and seed storage
  under suitable conditions (temperature and
  relative humidity) to maintain longevity.

Rukuni Taylor, Cornell, Geneva
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• Cardinal conditions that promote after-ripening
  need to be established and these will determine
  how long the seeds need to be after-ripened
  before long-term storage. After-ripening
  durations may vary with the degree of dormancy
  even for seed lots of the same variety or
  landrace harvested in the same or different years
  or at various locations. Therefore, a periodic
  monitoring system needs to be employed to
  ascertain when seeds have after-ripened and also
  to avoid seed aging after seeds have fully
  after-ripened. With this in mind, it is apparent
  that a more in-depth seed physiology study needs
  to be commissioned to support long-term efforts
  to domesticate T. arvense for biofuel production.

Rukuni Taylor, Cornell, Geneva