Ching-Te Chien

Physical Dormancy in Seeds of the Holoparasitic Angiosperm Cuscuta australis (Convolvulaceae, Cuscuteae): Dormancy-breaking Requirements, Anatomy of the Water Gap and Sensitivity Cycling

BACKGROUND AND AIMS Dormancy in seeds of Cuscuta (Convolvulaceae, tribe Cuscuteae) is due to a water-impermeable seed coat (physical dormancy). In nondormant seeds of several species of this family, bulges adjacent to the micropyle have been identified as the initial route of water entry into seeds (water gap). However, there are claims that water enters seeds of Cuscuta spp. via the entire seed coat. Although several studies have been done on seed coat anatomy of Cuscuta, none has identified and/or characterized the morphology/anatomy of a water gap. Thus, the primary aim of this research was to identify and describe the morphology and anatomy of the water gap in seeds of Cuscuta australis. It was also determined if sensitivity cycling to dormancy-breaking treatments occurs in seeds of this species. METHODS Light microscopy, scanning electron microscopy, tissue-sectioning and dye-tracking and blocking experiments were used to investigate the morphology and anatomy of the water gap. Treatments simulating natural conditions were used to break seed dormancy. Storage of seeds at different temperatures was tested for their effect on sensitivity to dormancy-breaking treatment. KEY RESULTS Dormancy-breaking treatments caused the tightly closed hilar fissure to open. Staining was observed in cells below the hilum area but not in those below the seed coat away from the hilum. Sensitivity to dormancy-breaking treatment was induced by storing seeds dry and reduced by storing them wet. CONCLUSIONS Whereas bulges adjacent to the micropyle act as the water gap in other species of Convolvulaceae with physical dormancy, the hilar fissure serves this function in Cuscuta. Cuscuta australis can cycle between insensitivity <--> sensitivity to dormancy-breaking treatments.

Dormancy-break and germination in seeds of Prunus campanulata (Rosaceae): role of covering layers and changes in concentration of abscisic acid and gibberellins

Intact seeds (seed+endocarp) from freshly harvested fruits of Prunus campanulata were dormant, and required 4–6 weeks of warm followed by 8 weeks of cold stratification for maximum germination percentage. Removing both endocarp and seed coat, however, promoted germination in a high percentage of non-stratified seeds. Treatment of intact, non-stratified seeds with gibberellic acid (GA3) was only partially effective in breaking dormancy. However, GA3 promoted germination of non-stratified seeds in which the endocarp (but not the seed coat) had been removed. The order of abscisic acid (ABA) concentration in fresh seeds was endocarp > seed coat > embryo, and its concentration in endocarp plus seed coat was about 6.2-fold higher than that in the embryo. Total ABA contents of seeds subjected to warm and/or cold moist stratification were reduced 6- to 12-fold. A higher concentration of GA4 was detected in embryos of non-dormant than in those of dormant seeds. Fluridone, a carotenoid biosynthesis inhibitor, was efficient in breaking dormancy of Prunus seeds. Paclobutrazol, a GA biosynthesis inhibitor, completely inhibited seed germination, and the inhibitory effect could be partially reversed by GA4, but not by GA3. Thus, dormancy in P. campanulata seeds is imposed by the covering layers. Dormancy break is accompanied by a decrease in ABA content of the covering layers and germination by an increase of embryonic GA4 content.

Germination of Viburnum odoratissimum seeds: a new level of morphophysiological dormancy

Previous studies indicated that seeds of Viburnum odoratissimum had only physiological dormancy (PD), but no measurements of embryos were made during the dormancy-break treatments. Thus, we investigated embryo growth and radicle and cotyledon emergence over a range of temperatures. Seeds have underdeveloped embryos, and their length increased about 300% before radicle emergence. Embryos also had PD, as evidenced by delays in beginning of embryo growth (2–3 weeks) and of germination after embryos were elongated (4 weeks). After radicle emergence, epicotyl emergence was delayed 1–8 weeks, depending on incubation temperature, but cold stratification was not required to break PD of the epicotyl. Unlike seeds of many previously studied Viburnum spp., epicotyls of V. odoratissimum have non-deep, rather than deep, PD. Hence, a new level of MPD called non-deep, simple, epicotyl MPD has been identified.

Storage behavior and changes in concentrations of abscisic acid and gibberellins during dormancy break and germination in seeds of Phellodendron amurense var. wilsonii (Rutaceae).

The medicinal Asian plant genus Phellodendron is known to contain several very important compounds that have biological action. The main purpose of this study was to determine whether seeds of Phellodendron amurense var. wilsonii can be stored and to characterize their dormancy. Seeds of this taxon stored at -20 and -80 degrees C and in liquid nitrogen retained their high germinability, indicating that they have orthodox storage behavior. Intact seeds from freshly collected fruits were dormant and required 12 weeks of cold stratification at 4 degrees C for complete germination. Scarifying the seed coat was partially effective in breaking seed dormancy. Exogenous gibberellins (GA(3), GA(4) and GA(4+7)) promoted germination of scarified seeds, GA(4) and GA(4+7) being more effective than GA(3). Fluridone, an abscisic acid (ABA) biosynthesis inhibitor, was efficient in breaking dormancy, but it was less effective than GA(4) or GA(4+7) alone. Paclobutrazol, a GA biosynthesis inhibitor, inhibited seed germination, and the inhibitory effect was reversed completely by GA(4) and by GA(4+7). ABA content of seeds subjected to cold stratification or to incubation at 35/10 degrees C, which enhanced seed germination, was reduced about four- to sixfold compared to that of fresh seeds. Higher concentrations of GA(3), GA(4) and GA(7) were detected in nondormant seeds and in seeds with an emerged radicle than in fresh seeds. Present results seem to indicate that dormancy in P. amurense var. wilsonii seeds is imposed partially by the seed coat and partially by high ABA content. ABA content decreased and GA(3), GA(4) and GA(7) content increased during germination.

Deep simple epicotyl morphophysiological dormancy in seeds of two Viburnum species, with special reference to shoot growth and development inside the seed

BACKGROUND AND AIMS In seeds with deep simple epicotyl morphophysiological dormancy, warm and cold stratification are required to break dormancy of the radicle and shoot, respectively. Although the shoot remains inside the seed all winter, little is known about its growth and morphological development prior to emergence in spring. The aims of the present study were to determine the temperature requirements for radicle and shoot emergence in seeds of Viburnum betulifolium and V. parvifolium and to monitor growth of the epicotyl, plumule and cotyledons in root-emerged seeds. METHODS Fresh and pre-treated seeds of V. betulifolium and V. parvifolium were incubated under various temperature regimes and monitored for radicle and shoot emergence. Growth of the epicotyl and cotyledons at different stages was observed with dissecting and scanning electron microscopes. KEY RESULTS The optimum temperature for radicle emergence of seeds of both species, either kept continuously at a single regime or exposed to a sequence of regimes, was 20/10 °C. GA(3) had no effect on radicle emergence. Cold stratification (5 °C) was required for shoot emergence. The shoot apical meristem in fresh seeds did not form a bulge until the embryo had grown to the critical length for radicle emergence. After radicle emergence, the epicotyl--plumule and cotyledons grew slowly at 5 and 20/10 °C, and the first pair of true leaves was initiated. However, the shoot emerged only from seeds that received cold stratification. CONCLUSIONS Seeds of V. betulifolium and V. parvifolium have deep simple epicotyl morphophysiological dormancy, C(1b)B (root)-C(3) (epicotyl). Warm stratification was required to break the first part of physiological dormancy (PD), thereby allowing embryo growth and subsequently radicle emergence. Although cold stratification was not required for differentiation of the epicotyl--plumule, it was required to break the second part of PD, thereby allowing the shoot to emerge in spring.

Epicotyl Morphophysiological Dormancy in Seeds of Daphniphyllum glaucescens, a Woody Member of the Saxifragales

Available information on seed dormancy for various members of the Saxifragales and phylogenetic relationships within this order allowed us to accurately predict that Daphniphyllum glaucescens seeds have morphophysiological dormancy (MPD). However, our hypothesis that seeds had deep simple epicotyl MPD, i.e., nondeep physiological dormancy (PD) in root and deep PD in shoot, was not supported. Both the root and the shoot (cotyledons) of the underdeveloped embryo of D. glaucescens have nondeep PD. Exposure to moderate (15°/6°, 20°/10°C), rather than high (25°/15°C), temperatures for 10–12 wk broke the PD of the hypocotyl/root. After hypocotyl emergence, seeds with an attached developing root system did not require cold stratification to break the PD of the shoot. The PD of the shoot was broken by an additional 10–12 wk at moderate temperatures, during which time cotyledons slowly grew inside the seed. As cotyledons grew, all of the hypocotyl was pushed out of the seed, and the final cotyledon length was almost twice that of the seed; at this point, the folded cotyledons emerged. Since the level of PD in root and shoot may or may not be the same, we advocate stating the level of PD in both root and shoot when epicotyl MPD is described in a species. Thus, seeds of D. glaucescens have nondeep simple (root)–nondeep simple (epicotyl) MPD, which is written as C1bB(root)‐C1bB(shoot) in the formula system of Nikolaeva. This is the first report of this level of epicotyl MPD in the Saxifragales.

Evolutionary considerations of the presence of both morphophysiological and physiological seed dormancy in the highly advanced euasterids II order Dipsacales

Although the underdeveloped embryo, and thus morphological (MD) or morphophysiological (MPD) seed dormancy, is basal in angiosperms, it also occurs in advanced groups. A synthesis of the literature, combining phylogeny and the kind of seed dormancy in the highly evolutionarily advanced order Dipsacales , shows that MPD (or MD) occurs throughout all clades except the most advanced one, Valerina . Seeds of taxa in the Valerina clade have fully developed embryos and physiological dormancy (PD) or are non-dormant (ND); thus, PD and ND are derived conditions in Dipsacales . Assuming that types of seed dormancy have not changed since the Early Tertiary, the fossil record suggests that MPD (or MD) was present in extant genera of Dipsacales by the Palaeocene, but PD (or ND) not until the Miocene. Molecular dating indicates that the ages of dipsacalean lineages with MPD and PD are older than those indicated by the fossil evidence.

Deep simple morphophysiological dormancy in seeds of the basal taxad Cephalotaxus

Although mature seeds of the monogeneric conifer family Cephalotaxaceae sensu stricto have underdeveloped embryos, no definitive studies have been done to classify dormancy in this family. Our primary purpose was to determine the kind of dormancy in seeds of Cephalotaxus wilsoniana and to put the results into a broad phylogenetic context for gymnosperms. The species is of horticultural and medicinal value, and information is needed on how to propagate it efficiently from seeds. Embryo growth and germination were monitored for seeds at warm, cold and warm plus cold temperatures, and germination was monitored for seeds subjected to: (1) cold rn warm rn cold rn warm; and (2) warm rn cold rn warm rn cold rn warm temperature sequences. The effects of gibberellic acids GA3 and GA4 were tested on radicle emergence in ungerminated seeds and on shoot emergence in root-emerged seeds. Germination was promoted byngn36 weeks of warm stratification followed byngn8 weeks of cold stratification, but only if seeds were returned to high temperatures. The underdeveloped embryo must increase in length by g120% before the radicle emerges. Neither GA3 nor GA4 was effective in promoting radicle emergence; however, both plant growth regulators increased rate (but not percentage) of shoot emergence in root-emerged seeds. We conclude that seeds of C. wilsoniana have the deep simple level of morphophysiological dormancy (MPD), C1b-C3-B1b; thus, warm stratification followed by cold stratification and then warm-temperature incubation are required for germination. In gymnosperms, MPD is known in cycads, Ginkgo and now in three families of conifers.

Biomechanical features of eccentric cambial growth and reaction wood formation in broadleaf tree branches

Tree branches and stems have different physiological functions that work collaboratively to maximize light interception. Light penetration in tree crowns is controlled by the orientation of the branches. However, mechanisms of branch bending have not received the attention they deserve. This study approached the problem by investigating the growth strain distribution in the upper and lower sides of branches of broadleaf trees, estimating the bending tendency of branches, and observing the branch eccentricity and the distribution of gelatinous fibers. The strain distribution was compared between the branches of 11 species (including 8 examined species and 3 referenced species) and tilted stems of 37 species from both our data and previous reports. Compressive strain was generally observed on the lower side of branches, but little was measured in tilted stems. The pith eccentricity of branches was in a reverse pattern to the corresponding strain distribution of stems. The radial growth of branches was hypotropic in contrast to the epitropic eccentric growth in inclined trunks. Furthermore, on the upper side of branches, G-fibers within the fiber arcs formed in an intermittent manner rather than in the continual manner found in artificially inclined stems. The resultant upward bending moment might not suffice to counteract the branch’s own weight; therefore, most of the measured branches, differing from tilted stems, tended to bend downward. In conclusion, by comparing the biological and mechanical aspects of the strain distribution, bending tendency, and eccentricity, our experiments could discriminate the bending dynamics and role of G-fibers in tree branches from that of main stems.

Seed dormancy and germination in Neolitsea acuminatissima (Lauraceae)

【Summary】 Fresh seeds of Neolitsea acuminatissima germinated slowly at 30/20℃ in light (with a 12-h daily photoperiod) and required > 20 wk to complete germination. Seeds cold-stratified at 4℃ for 9 mo or for 1 yr not only retained their original viability, but the germination rate significantly increased. Fresh seeds have a fully-developed embryo and a water-permeable seed coat and endocarp, and they require > 4 wk to germinate in a warm temperature regime. Thus, we concluded that