Introduction
Based on their ability to withstand drying, two broad groups of seeds
have been recognised by Roberts (1973). One group includes nearly 90%
of the known plant species that produces seeds which can survive drying
to the low moisture content (MC), c. 3-10% on a fresh weight basis (f.
wb), thus desiccation-tolerant, i.e. orthodox. Another group of species
produces seeds that lose viability if dried to the moisture range of
20-30% f.wb, therefore referred to as desiccation-sensitive or
recalcitrant (Roberts, 1973). The seeds of desiccation-sensitive species
are not amendable to low temperature storage and storing them at higher
temperatures in the range of 0-10 °C does not preserve viability for
more than 1 year (Berjak and Pammenter, 2008). Ellis et al. (1990)
described a third group of seed storage, which they referred to as
intermediate, wherein seeds can tolerate desiccation to c. 9-12%
moisture content, but further drying reduces seeds viability, and/or
seeds lose viability during low temperature storage.
The family Fagaceae is widely distributed in the temperate rainforests
and includes nine genera with approximately c. 1000 species (Manos and
Zhou, 2001). Information on seed storage behavior and germination
ecology of Fagaceae seeds are restricted to the largest genusQuercus , which includes c. 400 species (Bonner, 1996; Pritchard,
1991; Xia et al., 2012a). These studies suggest that most of theQuercus seeds are large and they do not withstand drying to
moisture content less than 15-20% f. wb, thus recalcitrant. However,
little is known about the seed storage behavior of species in other
genera within Fagaceae. Castonopsis is the second largest genus
in Fagaceae which includes approximately 120 species (Huang et al.,
1999; Wu et al., 2014). Current knowledge about seed storage behavior ofCastonopsis species are unclear. For example, Kew’s seed
information database (SID, 2020) reports seed storage behavior of 10Castonopsis species, yet 6 of them are labelled as uncertain and
4 species (1 orthodox and 3 recalcitrant), are doubtfully assigned.
Such confusion is further complicated by previous studies lacking
desiccation experiments or failing to report the moisture content.
Shopmeyer (1974) reported that seeds of Castanopsis chrysophyllacollected from North America reduced viability to 5% following hermetic
air-dry storage at 5°C for 5 years, but the moisture content of the
seeds was not mentioned. Similarly, Campbell (1980) showed that seeds
viability of C. indica , C. purpurella and C.
tribuloides from Nepal can only maintain viability for 2 months in
moist storage at room temperature, but did not provide any information
about viability following desiccation. Tian and Tang (2010) found that
fresh seeds of C. fissa with 42.7% moisture content had 100 %
germination. However, drying seeds to 25.3% moisture content reduced
the germination to 17%. Further desiccation to 17.3% moisture content
within 72 hours resulted in complete seed death. However the quick
drying rate of C. fissa seeds is contrasting with other
Fagaceae species reported by various authors (Li et al., 2018;
Pritchard, 1991; Xia et al., 2012b). In general, Quercus seeds
dried in silica gel require about 20 days to reach approximately 20%
MC. Recently, Basrudin et al. (2019) reported that the seeds C.
buruana Miq. dried in the sunlight after washing reduced viability,
neither the moisture content measurements nor any desiccation
experiments were performed. Apparently, there are no other studies on
seed desiccation sensitivity of Castanopsis.
Xia et al. (2012b) reported that seed anatomy plays an important role in
controlling water loss of Fagaceae species. Using nine Quercusspecies, those authors reported that the water loss occurs (only)
through the scar during desiccation in many -but not all- species, and
the main pericarp being impermeable to water, protects the embryo by not
allowing the water to leave seeds. In most Quercus , embryo sits
on the opposite side of the scar (Baskin and Baskin, 2007; Martin, 1946;
Nixon, 1985; Nixon, 1993), thus water loss occurring only through scar
will dry the cotyledons and the embryo is kept in a hydrated stated for
long-time, (Finch-Savage and Blake, 1994). Consequently, this mechanism
could protect the seeds under ecological drying due to drought. However,
whether a similar mechanism operates in other genus of Fagaceae is
unknown.
Castanopsis
sclerophylla distributed in southern and eastern subtropical forests of
China is one of the dominant species of evergreen broad-leaved forest
(Huang et al., 1999). It is an important economic tree providing
valuable timber for construction and water-soil maintenance. Further,
the seeds of C. sclerophyllais rich in starch, and consumed as food (Xin et al., 2007). However, the
population is shrinking with poor establishment of new trees and
expansion of agricultural land area adds further pressure for
regeneration. Clearly, we know little about the physiological and
morphological changes during desiccation and nothing is known about the
desiccation tolerance ability of seeds of this species. Thus, the first
objective of this study was to define the germination characteristics
and drying pattern of C. sclerophylla seeds. In particular, we
wanted to identify the seed storage behavior.
X-ray computed tomography (CT) is a non-invasive approach which have 3-D
visualization and can quantify biological structure (Burg et al., 1994;
Dell’Aquila, 2007; Foucat et al., 1993; ISTA, 2020). In Syagrus
flexuosa fruits, Stuppy et al. (2003) used high-resolution X-ray
computed tomography (HRCT) to clearly distinguish all morphologically
relevant parts and organs such as the endocarp with fibrovascular
bundles and endosperm with embryo. Kunishima et al. (2020) used a
laboratory-based X-ray microscope to study 3D-structural on the
development of pansy seeds without any pretreatment. Therefore, the
second objective of the present study was to use CT to scan the whole
seed without any treatment, so as to show an intuitive view of the
internal structure of seeds and better understand the effect of the
morphological structure of seeds during drying.