Preliminary Studies On The Sexual Reproduction And Photosynthetic Physiology Of Sargassum Thunbergii

Sargassum thunbergii, belonging to Sargassaceae （Phaeophyta）, is one kind ofwarm temperature marine brown alga in the rocky intertidal zone along the west ofNorth Pacific Ocean, mainly inhabiting the coast of China, Russia, Japan and Korea. S.thunbergii is of great economic and ecological value, and has potential of exploitationin the domains of medicine and mariculture as well as chemical and food industries. Itis also used as an important source of feed for Stichopus japonicus aquaculture, whichis rapidly developing in China recently. The increasing demand for S. thunbergii hasresulted in the depletion of natural populations along the northern coast of China.Hence, there is an urgent need for large-scale mariculture and artificial breeding of S.thunbergii. A large number of studies have been carried out on the ecology andcultivation technique of S. thunbergii in China recently. However, the basic biologicacquaintance of S. thunbergii, such as photosynthetic physiology, is poorly understood,which will limit the development of the large-scale mariculture and artificial breeding.The need for a large quantity of seedlings is still a bottleneck for successful largescale farming. The preliminary studies on the sexual reproduction and photosyntheticphysiology of S. thunbergii were researched in this paper, including as follows:（1）The mature receptacles of S. thunbergii were used for materials. The effects oftemperatures, light intensities and photoperiods and Cu²⁺on egg releasing or youngsporophyte growth of S. thunbergii were investigated.（2） The chlorophyllfluorescence instrument and liquid-phase oxygen electrode system were used forobtaining detailed data of chlorophyll fluorescence parameters and photosyntheticcharacteristics of growing seedlings under different light intensities, temperatures,salinities and nutrient concentrations. The present work aims to know about theecophysiological adaptability of S. thunbergii in-depthly for successful control andmanagement in artificial breeding. The main conclusions are as follows:（1） The effects of different temperatures （13²3°C）, different light intensities（60²20μmol photons/（m2s）） and different photoperiods （15L:9D,12L:12D,9L:15D）on egg releasing and young sporophyte growth of S. thunbergii were investigated in this paper. The main results are as follows: Under60⁷0μmol photons/（m²s）, it wasconducive to releasing eggs at20²3°C. Within60²20μmol photons/（m²s）, thehigher light intensity, the more favorable for egg releasing at suitable temperature.The photoperiod was not the main factor to affect the speed of releasing eggs. Thelight intensity was an important but not key factor for the growth of young sporophyteof S. thunbergii. Temperature was key factor for the growth of S. thunbergii youngsporophyte. It was not conducive to the fast growth of young sporophyte of S.thunbergii below16°C.（2） Effect of Cu²⁺on the growth of S. thunbergii young sporophyte at differentgrowth stage was investigated. We found that both the early and late stages of S.thunbergii young sporophyte were very sensitive to Cu²⁺treatment. Growth of S.thunbergii young sporophyte at the early stage （170³00μm） was seriously inhibitedby Cu²⁺at concentration of5μg/L and above, and the growth was almost terminatedunder250μg/L Cu²⁺. For S. thunbergii young sporophyte at the late stage （750¹300μm）, the growth was seriously inhibited by Cu²⁺at10μg/L and above, the growthwas accelerated by1μg/L Cu²⁺, and the growth of young sporophytes was slower oreven ceased under more than50μg/L Cu²⁺. Due to the special characteristics ofstructure and function, rhizoid of S. thunbergii young sporophyte was found to beinsensitive to Cu²⁺. At the beginning of Cu²⁺treatment, rhizoid still presented fastgrowth.（3） Effects of different temperatures （5³4°C）, different salinities （10⁵0）,different nutrient concentrations and different nitrogen forms on chlorophyllfluorescence parameters of young seedlings of S. thunbergii were researched in thispaper. The main parameters determined in the study including the optimal chlorophyllfluorescence quantum yield （F_v/F_m） and the maximum relative electron transport rate（rETRmax） of photosystem II. The results were shown as follows:1) If nitrogenconcentration or phosphorus concentration was greater than8mg/L and1.2mg/Lrespectively, or temperature was greater than28°C, there were significant effects onthe photosynthesis of young seedlings of S. thunbergii.2)24h recovery for normalcultivation after1h treatment in5°C or6h treatment in10⁵0salinity, the Fv/Fmofyoung seedlings of S. thunbergii could both be back to normal.3) If nitrogenconcentration and phosphorus concentration was2⁴mg/L and0.2⁰.8mg/Lrespectively, and moreover, NH₄⁺-N:NO₃--N was equal to1³, it was beneficial forthe photosynthesis of young seedlings of S. thunbergii. （4） Effects of different temperatures （10³1°C）, different salinities （10⁵0）,different light intensity on photosynthesis and respiration of young seedlings of S.thunbergii were researched in this paper. The results were shown as follows:1) thelight saturation point and light compensation point of unfertilized egg or youngsporophytes which was2or3days post-fertilization were in the range of90¹20μmol photons/（m²s） and14²2μmol photons/（m²s）, respectively.2) Thephotosynthetic rate or respiratory rate of seedlings had significant reduction whentemperature was smaller than10°C or greater than28°C for short period.21°C isoptimum temperature for growth under the experimental condition in this paper.3)Short-term high or low salinity tests had an significant influence on photosyntheticrate and respiratory rate of young seedlings, especially in salinity10and salinity50.However, the rates recovered to the normal level after an24h recovery period,demonstrating a powerful ion transport system of the seedlings. The present work willbe useful for successful control and management in artificial breeding of S.thunbergii.（5） The newborn branches of S. thunbergii were used for materials. The effectsof different light intensity （30²20μmol photons/（m2s））, temperatures （5³4°C） anddifferent salinities （0⁶0） on chlorophyll fluorescence parameters net photosyntheticrate （Pn） and dark respiration rate （Rd） of newborn leaves of S. thunbergii were studiesin this paper. The preliminary analysis of the newborn branch about resistancephysiology could provide reference for the artificial cultivation of S. thunbergii basedon vegetative reproduction.The optimal chlorophyll fluorescence quantum yield ofphotosystem II （F_v/F_m）, maximum relative electron transport rate (rETR_max) andinitial slope of rapid light curve were determined. The results show:1)15²4°C isoptimum temperature for growth of newborn branch under the experimental conditionin this paper.2) The newborn leaves of S. thunbergii suffered from high intensity lightand high temperature stress easily and the Fv/Fmdecreased significantly under highintensity light. The photosystem showed irreversible injury when the temperature washigher than30°C.2) One hour treatment at5°C or nine-hour treatment at0⁶0salinity affected the photosystem II obviously. However, the chlorophyll fluorescenceparameters and Pncould nearly recover to normal level after24hours understandard culture condition except those in60salinity group.4) During the wholestress process, Rdof salinity40and salinity50treatment both showed high value andthe stress of salinity0and salinity60both had an significant influence on Rd. However, Rdcould recover to the normal level after an24h recovery period whenexposure to different salinities for9h.（6） The differences of maximal net photosynthetic rate (P_nmax), saturationirradiance (I_sat), light compensation point （I_c）, initial quantum yield α, dark respirationrate （R_d）, pigment content, specific leaf area and microstructure of heteromorphismleaf （broad leaf, narrow leaf, pneumathode） were analyzed to reveal the probablereason about the change of leaf shape of S. thunbergii.