Seed anatomy and morphology of
Thlaspi arvense (pennycress) and
preliminary germination results
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
100
90
80
70
60
Mid
50
Late
Alberta
40
30
20
10
0
control
Ex emb
Ex emb+GA
Punct+GA
GA
KNO3
Chill
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
100
90
80
70
60
Mid
50
Late
Alberta
40
30
20
10
0
Ctrl 20/30
Ctrl 10/20
20/30+chill
10/20+chill
20/30+KNO3
10/20+KNO3
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
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 (nonendospermic 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
• 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 afterripening period, and these factors include the genotype of
plants, the seed maturation environment and the postharvest seed storage conditions.
Rukuni & Taylor, Cornell, Geneva
• 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
• 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
• 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
• 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. Adhoc 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
• 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. Afterripening 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 longterm efforts to domesticate T. arvense for biofuel production.
Rukuni & Taylor, Cornell, Geneva