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Cycads and Ginkgo biloba are the only extant seed plants that produce flagellated male gametes. Superficially, the cells of both are similar in structure and function. In both the motile organelles arise from multicentriolar bodies, the blepharoplasts, and, in both forms, these give rise to a complex fibrous band, the multilayered structure (MLS), which bears numerous flagella. Generally speaking, these structures are much alike in cycads and Ginkgo. However, there are marked differences in details of their development, particularly in the presence of a “nucellar beak” in Ginkgo.
We describe the occurrence of arbuscular mycorrhizae in the roots of Zamia pumila and Dioon edule. Seedlings were grown on native, unsterilized soil taken from local pinelands of south Florida, where Z. pumila occurs naturally. Arbuscules, hyphae, hyphal coils, and vesicles occur in the parenchyma cells of the root cortex, especially the half of the cortex next to the stele. Hyphae of the arbuscular mycorrhizal fungi (AMF) occur mainly in longitudinal intercellular spaces and conform to the Acorus type. The finest, ultimate roots have AMF, but these roots are extremely brittle, detach with the slightest disturbance, and are usually lost when plants are uprooted from the ground. No AMF were found in the cortex of coralloid roots. Vovides (1991) previously reported that AMF occur on Dioon edule and Ceratozamia mexicana, and we reconfirm this in D. edule. In this species, AMF appear to be mostly associated with the outer and to a lesser extent the inner cortex. However, roots of a potted plant of C. hildae growing in native soil lacked AMF. When grown on low phosphorus soils, legumes are known to require AMF in order for their Rhizobium nodules to fix nitrogen. Without AMF, the legumes are deficient in phosphorus, which inhibits nodule production and nitrogen fixation. It is probable that cycads, with their nitrogen-fixing coralloid roots containing Nostoc, may also require AMF for successful nitrogen fixation when phosphorus is limiting.
The seed coat furnishes protection with a thick cuticle, tannin cells, mucilage, and a hard sclerotesta. The external layer of the seed coat is a sarcotesta; a thick cuticle covers the external walls of its epidermal cells. This epidermis bears stomates and, in the early stages, trichomes. The subepidermal cells have druses. Starch grains are abundant in the sarcotesta from June through August, but they disappear during dispersal in September. The parenchyma is interrupted by mucilage canals lined by epithelial cells. Tannin cells are found in the sarcotesta, sclerotesta, and pachychalaza. Ten sectors of an areole in the sclerotesta around the micropyle may correspond to the tips of the integumentary segments in some fossil plants, such as Genomosperma kidstonii.
It has been possible to regenerate a few cycad species in vitro by somatic embryogenesis, either from zygotic embryos (Ceratozamia hildae, C. mexicana, Encephalartos cycadifolius, E. dyerianus, E. natalensis, Zamia fischeri, Z. furfuracea, and Z. pumila) or from leaves of mature phase trees (C. euryphyllidia, Ceratozamia hildae, and C. mexicana). This strategy has great potential for the commercial vegetative propagation of certain highly endangered species (e.g., C. euryphyllidia) and should indirectly protect wild populations of these species by discouraging collection in situ. Embryogenic cultures of several cycad species have grown vigorously and are highly morphogenic more than 11 years after induction. The long-term conservation of cycad genetic resources can also be addressed for species that can be regenerated by somatic embryogenesis. Preliminary studies indicate that embryogenic cultures that have been pretreated on plant growth medium containing 0.75 M sucrose for two days, encapsulated in sodium alginate, and desiccated for six hours can survive immersion in liquid nitrogen (−196°C).
Individual somatic proembryos of Ceratozamia hildae were exposed to media that differed only in gelling agent utilized. Five different gelling agents were compared in the first experiment: Bacto agar, Agargel, Gel-Gro, Phytagar, and TC agar. In addition, the effect of agar was examined at two levels. Growth, proliferation, and development were assessed. The lower level of agar did not support good somatic proembryo growth. Agargel and the high agar concentration produced cultures with good proliferation. Proembryos exposed to Phytagar, Gel-Gro, and TC agar had the highest proliferation rates. Overall, Gel-Gro was considered the best gelling agent tested. The three concentrations of Gel-Gro used in the second experiment were 2, 4, and 6 g·l−1, with the lowest concentration representing the control, the recommended concentration. As gelling agent concentration increased, so did mortality; however, the highest Gel-Gro concentration also produced the highest numbers of good-quality, mature somatic embryos. Proliferation rate was greatest at the lowest concentration. These results suggest that Ceratozamia cultures should be exposed to different gelling agents or concentrations of gelling agents at different developmental stages in order to produce the greatest number and highest quality of somatic embryos.
The effect of two light intensities (25 µmol m−2s−1 and 50 µmol m−2s−1) on four developmental stages of Ceratozamia mexicana somatic embryos growing on semisolid plant growth medium at 25oC was measured. Growth parameters included fresh weight, morphology, and invertase and peroxidase activity. Under low light conditions, fresh weight was greater in stages 1 and 2 than in stages 3 and 4. In addition, there was a high frequency of hyperhydricity and polyembryogenesis in stages 1 and 2, whereas stages 3 and 4 were nonhyperhydric and unbranched. Stages 2–4 were green. Under high light conditions, embryos had lower fresh weights and less hyperhydricity, and stages 2–4 were green. Under low light conditions, peroxidase activity was less, although stage 1 embryos under both light conditions showed the highest activity. Stage 1 embryos required three to four months to develop to stage 2 under high light conditions and two to three months under low light conditions. Invertase activity under low light conditions was minimal in stage 2. All embryos had low invertase activity under high light intensity, and stages 2–4 had high levels of glucose. Embryo development from stage 2 to the next and for each subsequent stage under high light conditions required three to four months, and under low light conditions required four to five months. Higher light intensity therefore promotes the speedy recovery of plants.
The photosynthetic characteristics of Cycas micronesica K.D. Hill were studied from August 1998 until February 1999 using chlorophyll fluorescence and gas-exchange techniques to determine the responses to long-term shade of 35% ambient light transmission, followed by the transfer of shade-grown leaves into full-sun conditions. The shade-grown leaves exhibited increased photosynthetic light use efficiency and effective quantum efficiency of photosystem II (PS II) and decreased photosynthetic light saturation point and dark respiration when compared with leaves grown in full sun. Shade was removed from shade-grown C. micronesica leaves during midday on December 14, 1998, when effective quantum efficiency of shaded leaves was 45% greater than that of sun leaves. Following one hour in full sun, effective quantum efficiency of the shade-grown leaves declined to below that of the sun-grown leaves. After receiving full sunlight for the rest of the photoperiod, maximum quantum efficiency of PS II photochemistry for shade-grown leaves was below that of sun-grown leaves throughout the night. The damage caused by excessive light to shade-grown leaves progressed for the first three days after shade removal. On day 3, effective quantum efficiency during midday was 30%, net photosynthesis was 47%, apparent quantum yield was 65%, and light compensation point was 136% of that for sun-grown leaves. After day 3, the relationship between full-sun leaves and the previously shaded leaves for these response variables was relatively stable. Two months following removal of shade, the previously shaded leaves continued to exhibit damage from high light. These results have application to transplanting cycad plants from a shaded nursery to a field site or, after tropical cyclones, where protective forest canopy cover has been destroyed and cycad plants in the forest subcanopy are abruptly exposed to full-sun conditions.
This study examines the effects of doubled CO2 concentration on the ultrastructure and function of chloroplasts from cycads and, for control from two other herbaceous angiosperms. Under a doubled CO2 concentration condition, the chloroplast ultrastructure of the two cycads (Cycas multipinnata with a shade-type chloroplast and C. panzhihuaensis with a sun-type chloroplast) changed little: The conformation of the thylakoid membrane system kept well, and almost no starch grains accumulated. In contrast, under the same conditions the chloroplast ultrastructure of soybean and foxtail millet changed considerably, with starch grains accumulating in their chloroplasts and some of thylakoids (especially stroma thylakoid) membranes being destroyed to some degree by the more numerous and larger starch grains that accumulated in the chloroplasts. Interestingly, the changes in the ultrastructure of the chloroplasts from the two cycads was correlated with the 77K fluorescence emission spectra of their chlorophyll; i.e., the F685/F734 (PS II / PS I) ratio within the chloroplasts, which were minimal. The absorption spectrum showed decreases in the red and blue peaks. These changes in the absorption spectrum may be related to changes in the structural arrangement of the thylakoid membranes. Preliminarily, this experimental result shows that the cycads may adapt themselves to environmental changes under doubled CO2 concentration in the coming centuries. However, more studies on this aspect are necessary.
It has repeatedly been noted that ready availability of well-grown cycad specimens would substantially reduce collection of plants from their natural habitats. Although some cycads are extensively produced in several countries, there is uncertainty as to their optimal growing conditions and fertilizer requirements. Using Zamia floridana (sensu lato) as a model, one-year-old seedlings were grown in 30% and 50% light-exclusion shadehouses for one growing season. They were fertilized with roughly equivalent nutrient proportions of 20–20–20 (N–P–K) Peters solution at 300 ppm applied biweekly, nine-month 18–6–12 controlled-release Osmocote granules, and 16–8–12 Controlled Release Sierra Tablets plus minors at two and three tablets per container, and in combinations. There was interaction between shade and fertilizer types in all parameters measured. Overall, plants grown in 30% shade had a larger caudex, more leaves, and higher leaf and stem-plus-root fresh and dry weights. Peters fertilizer at 300 ppm was least effective in all growth parameters, as compared with other fertilizer treatments. Alone, however, it was more effective in caudex enlargement in 50% shade. No differences were observed between any treatments involving granules, irrespective of supplemental Peters. There was no significant difference between three tablets plus Peters and two tablets only, in 30% and 50% shade. Two tablets plus Peters and three tablets only, however, had a significant effect on caudex enlargement in 30% shade. In 50% shade, three tablets plus Peters or granules alone were more effective. These results and personal experience show that nearly all cycads grow best in ± 30% shade and benefit from fertilizers that contain micronutrients and a higher ammoniacal nitrogen source.
Cycas taitungensis Shen, Hill, Tsou & Chen is an endemic species remaining in two remnant populations in southeastern Taiwan. Ecological studies showed that the sex ratio between female and male of the main population is approximately 1.7:1. Leaf production was found to be correlated with tree height before reaching 1 m in length (r = 0.95). The annual reproduction rate of female plants is highly variable, with seed numbering between 80 and 400 in each tree. The site study revealed a significant difference in vegetative growth and age structure between the subpopulations collected in two opposite microhabitats. Genetic studies using isozyme analysis showed low genetic variability (HE= 0.039) and little genetic differentiation between the populations (FST = 0.051). The genetic data are well correlated with the ecological observation that the differences reflect various microhabitat effects within a very local environment and that the impact influenced the extent of the degree of genetic differentiation within local populations. This work presents extensive genetic information for C. taitungensis that give rise to more ecological and genetic insights into the plant for better establishment of in situ and ex situ conservation programs.
From 1997 to 1999 Cycas debaoensis Y. C. Zhong & C. J. Chen and C. changjiangensis N. Liu were described from South China. The wild populations of Cycas szechuanensis were discovered in Fujian. Cycas guizhouensis K. M. Lan & R. F. Zou at the higher elevations and C. segmentifida D. Y. Wang & C. Y. Deng at the lower elevations along the Nanpanjiang River should be good species, which were treated by Chen and Wang (1995) and Chen and Stevenson (1999) as synonyms for C. szechuanensis. However, 11 other new species have been reduced by Chen and Stevenson (1999) and the present authors. They are C. longlinensis Huang T. Chang & Y. C. Zhong, C. xilinensis Huang T. Chang & Y. C. Zhong, C. multifida Huang T. Chang & Y. C. Zhong, C. longiconifera Huang T. Chang & Y. C. Zhong, and C. acuminatissima Huang T. Chang & Y. C. Zhong, all treated as synonyms of C. segmentifida. Cycas spiniformis J. Y. Liang, C. longisporophylla F. N. Wei, C. septermsperma Huang T. Chang & H. X. Zhang, C. brevipinnata Huang T. Chang et al. should be synonyms for C. exseminifera F. N. Wei. Cycas miquelii Warb. and Epicycas miquelii (Warb.) de Laub. should be the synonyms for C revoluta because their “type” specimens are somewhat like C. revoluta. The Honghe Nature Cycad Reserve for Cycas multipinnata and C. hongheensis was recently established in Yunnan. The Debao Cycad Reserve will be established soon. However, most existing cycad reserves in China have not been so successful because of shortages of funding and poor management. In ex situ conservation the Qingxiushan Cycad Garden in Nanning, Guangxi, was established. Cycad nurseries have begun to appear in some villages in South China.