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.