| Sargassum wightii is an economically and ecologically important macroalge. It has long been used as high quality feed for abalone and sea cucumber, and it has considerable commercial potential for artificial culture. Abundant S. wigitii were found in xuwen sea area, Guangdong, China. Recently, there was a sharp degradation of S.wightii due to anthropogenic perturbations. So, we hope to develop a commercial-scale aquaculture of this species, by artificial seed production. But, the sexual reproduction and the production of artificial seedlings of S. wightii have not been studied.In order to study the sexual reproduction and the growth of seedlings of S. wightii, the growth pattern in wild, ovulation and the development of zygotes were systematically recorded from June,2013 to May,2015. And also the effect of temperature, illumination and salinity on the ovulation, development of zygotes and the growth of seedlings at different stage were studied. In addition to this, the effect of different concentration of IAA, GA3,6-BA and brassinolide on the growth of seedlings were researched aimed at finding a way to promote the growth of young S. wightii. The main experimental results were as follows:1. The holdfast of S. wightii were perennial, but its sporophytes were annual. The peak reproductive period of S. wightii in the field was from the late April to the early May. In June the sporophytes began to rot except the holdfast, and the biomassof S. wightii in the field was the least from July to October. During this time the growth rate of the seedlings that regenerated from holdfast was very low, which is also called summer-dormancy period. From the middle of November to May of the next year was the rapid growth stage of S. wightii. Up to April of the next year the thalli began to mature again and began the next life cycle.2. The female and the male reproductive part were from the same receptacle, and the sperm and eggs developed from the same conceptacle. The ovulation was a continual progress and the optimum temperature for it was 21℃. The eggs belong to 8-nucleui type, who develop into a 8-nucleui eggs before they were released. After being fertilized the eggs began to divide immediately. The first and the second division were transverse division, and the third and forth division were longitudinal division. About 6-8 hours after fertilization, the zygotes developed into landmine-like multicellular spore. After about 24 hours of development, the zygote grow into seedling that with rhizoids. Before the rhizoids were formed the size of the seedling was almost stay the same. But, once the rhizoids were formed the seedlings began to grow faster than before, about 48 hours after fertilization, the seedlings began to shoot out a bud form the up side of the seedling. After about 6 days of air-charging incubation the seedling began to form its first leaf. After about 30 days air-charging incubation, the seedlings developed into a young thalli that with 4 leaves.3. The optimum temperature for zygote develop was 21℃, and during the first 10 days, the optimum temperature for the growth of the seedling was still 21℃, but after that the optimum temperature was 24℃. When the temperature was over 27℃, it will do harm to both the development of zygotes and the growth of seedlings. The optimum illumination for the development of zygote was 20 μmol photons m-2 s-1, and during the first 15 days, the optimum temperature for the growth of the seedling was still 20 μmol photons m-2 s-1, but after that the optimum illumination was 60 μmol photons m-2 s-1. When the illumination was over 60 μmol photons m-2 s-1, it will do harm to both the development of zygotes and the growth of seedlings. S.wightii was euryhaline. The optimum salinity for zygotes development and growth of seedling were both 15-30, during this range the survival rate of zygotes and the growth rate of seedlings showed no significant difference. However, When the salinity was over 35 or below 10, it will both do harm to the development of zygotes and the growth of seedling.4. The effect of temperature, illumination intensity and salinity on physiology of the seedling that after being cultured for 30 days was significant. When the temperature was at 21～24℃, the content of photosynthetic pigments were higher than other treatmens, but the activity of SOD and CAT was lower. However, when the temperature was over 27℃, the content of photosynthetic pigments was lower than than other treatments, but the activity of SOD and CAT was higher, thus indicated that most suitable temperature for the growth of seedlings at this stage is at 21～24℃ and the growth of seedlingswas constrained when the temperature was over 27℃. When the illumination intensity was at 20-80 μmol photons m-2 s-1, the content of photosynthetic pigments decreased with the rise of illumination intensity, but when the illumination intensity was at 80 μmol photons m-2 s-1, the activity of SOD and CAT of the seedling at 80 μmol photons m-2 s-1 was higher than other treatment, this indicated that the growth of seedlings was constrained. When the salinity was at 15～30 the content of photosynthetic pigments was hither than that at 35, meanwhile the activity of SOD and CAT of seedlings at the salinity was higher than other treatments, it can be determined the suitable salinity was 15～30, when the salinity was over 35 the growth of seedlings was constrained.5. The effect of different concentration of IAA,6-BA, GA3 and Brassinolide on the growth of seedlings that after being cultured for 60 days was significant. When the concentration of IAA was at 0.1～0.7 mg/L, it could accelerate the growth rate of the seedling, and the seedling got the highest growth rate which was 5.23 mg/d at 0.5mg/L. However, when the concentration of IAA was over 0.8mg/L, the growth of seedlings was constrained.When the concentration of 6-BA was at 0.01-0.09 mg/L it could accelerate the growth rate of the seedling, and the seedling got the highest growth rate which was 2.78 mg/d at 0.06mg/L. However, when the concentration of 6-BA was over 0.09 mg/L, the growth of seedlings was constrained. When the concentration of GA3 was at 0.5～3.0 mg/L, it could accelerate the growth rate of the seedling, and the seedling got the highest growth rate which was 4.91 mg/d at 2.5 mg/L. However, when the concentration of GA3 was over 3.5 mg/L the growth of seedlings was constrained.When the concentration of Brassinolide was at 0.1-0.6 mg/L, it could accelerate the growth rate of the seedling, and the seedling got the highest growth rate which was 3.00 mg/d at 0.4 mg/L.However, when the concentration of IAA was over 0.8 mg/L the growth of seedlings was constrained. |
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