Results and Discussion
The average 100-seed weight was 134.1±6.8 g. Freshly collected seeds had
a moisture content of 36.2±1.9% and 92.5±0.4% germination (Fig. 1).
After 7-day drying, seed germination decreased significantly to 27.5%
where moisture content declined to 22.5%. The viability of seeds dried
to 20.7% moisture content (seeds dried for 14 days) was 5% and further
drying to lower moisture contents resulted in complete loss of viability
(Fig. 1). The sharp decline in germination when seeds moisture content
reduced to c. 20% f. wb. indicates that the seeds are ‘recalcitrant’
Compared with other desiccation-sensitive seeds of Fagaceae species,
e.g. Q. robur (Finchsavage, 1992), Q. coccifera L.,Q. pubescens Willd. and Quercus pedunculiflora K. Koch
(Ganatsas and Tsakaldimi, 2013), C. sclerophylla seeds had a
similar desiccation response, showing that the percent germination of
seeds decreased with the progress of drying. The seeds lost their
viability completely when the moisture content of the seeds dropped
lower than 20% in 2-3 weeks. The result is also consistent with drying
rates reported on Quercus seeds, where these studies reported
that the drying rate of Fagaceae seeds is relatively slow. Further, it
required 15-25 days to reach moisture levels that are lethal to seeds
when dried in silica gel. However, Tian and Tang (2010) reported thatC. fissa and Q. fabri seeds can be dried to approximately
20% moisture content within 72 hours, which is contradicting with our
results. This is not surprising, as we found in our earlier work (Li et
al., 2018) thatQ.
fabri seeds required a longer drying period of at least 20 days to
reach approximately 20% when dried above silica gel, which is in
agreement with Xia et al. (2012b). The contradicting result on the
drying rate could be attributed to the differences in the drying method.
Both whether or not the seeds were dried in a sealed environment and the
amount of silica gel used was not provided by Tian and Tang (2010).
In this study, we show that X-ray computed tomography (CT) can be a
useful tool to study the internal structures of seeds without altering
the intact structure of seeds. CT results show that the pericarp ofC. sclerophylla is similar to other Fagaceae seeds, such asQuercus (Xia et al., 2012b). Pericarp in Quercus is
divided into three areas: (1) the apex; (2) the scar; and (3) main
pericarp (the remaining area between the apex and the scar), and our
results indicate that these regions are clearly distinguishable inC. sclerophylla (Fig. 2.3a). Xia et al. (2012b) found that water
loss during seeds desiccation of Quercus species occurs only
through the scar instead of main pericarp or the apex in some species
including Q. nuttallii, Q. suber and Q. palustris . Xia et
al. (2012a) showed that the moisture content of Q. franchetiionly reached 23% after 164 days of drying in silica gel. For this
species, a comparatively small scar (ca. 6% of the pericarp area) might
be the mechanistic basis of limited water loss through the pericarp (Xia
et al., 2012b). For Q. suber , Eduardo and Belén (2000) studied
the morphological and micro-morphological differences of the scar and
main pericarp. The main pericarp provides a protective barrier in
slowing down water loss and it is mainly contained of a layer of
epidermal wax, a thick cuticle covered epidermal layer and a single
layer of palisade cells. However, the scar lacks this protective
structure, which is comprised by vascular strand presumably facilitating
moisture loss. Thus, this feature was reported to have ecological
advantage, because the embryos of Quercus seeds cannot reach
lower water content during brief exposure to drought conditions in
natural environment.
Whilst
investigating the route of water loss in C. schlerophylla might
be arguably an important area worth exploring, we believe the results of
photomicrographs and CT scan suggested otherwise. Clearly, in C.
sclerophylla , embryo is on the opposite side of the apex and sits just
below the scar region (Fig. 2). This is in marked contrast withQuercus embryo, which is located on the apex side and far away
from scar (Finch-Savage and Blake, 1994; Xia et al., 2012b). Thus,
during desiccation no matter water loss only occurs through scar or
intact pericarp, embryo of C. sclerophylla seed will be the first
part within the seeds to lose water due to embryo position and
continuous drying may kill the seeds, so there is no practical
significance to study the water loss pathway during seed drying.
However, our drying results indicate that C. sclerophylla seeds
lose viability only when the moisture content drops to c. 20% f.wb.
There appears to be strong difference in the germination pattern betweenQuercus and C. sclerophylla seeds. Notably, in C.
sclerophylla seeds, embryo elongates and grew thicker initially before
protrude through the scar, then elongated into root and radicle (see
Fig. 2.3). For Quercus seeds, the radicle protrudes through the
apex region during germination (Bonner and Vozzo, 1987; Löf et al.,
2019). It is possible to suggest that this difference could be due to
the position of embryo between these two genera. However, this
standpoint requires detailed investigation given that seeds of manyQuercus have epicotyl physiological dormancy (Allen and Farmer
Jr, 1977; Baskin and Baskin, 2014; Farmer Jr, 1977).