Studies On The Photosynthesis Antioxidation And Cultural Conditions Of Sargassum Thunbergii

In this paper, a liquid-phase oxygen electrode system, chlorophyll-fluorescencetechniques, an antioxidant enzymes kit were used to determine the effects ofilluminance, temperature, salinity, pH, nutrient level on photosynthetic characteristicsand antioxidant system at different physiological stage of Sargassum thunbergii. Inaddition, the suitable artificial cultural conditions were discussed.The results showed that:(1) The larger values of specific growth rate (SGR) andnet photosynthetic oxygen evolution rate (Pn) of receptacles appeared in8000lx,20℃and20salinity. As illuminance and temperature increased, pigment contentsdecreaced. Chlorophyll a and carotenoid contents were influenced by environmentalfactors significantly, while Chlorophyll c keeping stable. The suitable ratio wasbetween5:1and1:1, and it would be inhibited while the value was50:1. Changes ofconcentration of phosphorus affected the photosynthesis of primary receptacles moresignificantly than nitrogen. It showed that phosphorus element was vital important forS.thunbergii to build reproductive organs. The result indicated that suitable culturalconditions were8000lx,20℃, salinity20and N/P ratio5:1~1:1.(2) The effect of temperature, salinity on the SGR of germlings was significant(P<0.05). The maximum value of SGR was10.87%when temperature was25℃anddecreased gradually as the temperature dropping. The germlings grew faster at salinity30, whereas the SGR decreased as salinity reduced. Effect of illumination (2000lx~8000lx) on the SGR was not significant. The nitrogen concentration of the naturalsea water used in the experiments basically met the requirements of the germlings,however the phosphorus was relatively lacking. When nitrogen enrichment and N/Pratio over15:1, it was adverse to germlings growth and the suitable ratio was about10:1~2:1. In conclusion, temperature25℃, salinity30, illumination2000lx, nitrogenconcentration0.45~1mg/L, N/P ratio10:1~2:1were the suitable cultural conditionfor germlings.(3) The Pnand maximum relative electron transport rate (rETRmax) of bladeseedlings increased first and then decreased as illuminance or temperature rose. The higher Pnand rETRmaxvalues were respectively at11~26℃,10000lx~16000lx and11~21℃,4000~6000lx. Light saturation and compensation point increased astemperature rose. The rETR was low for dark adapted blade seedlings and increasedsignificantly when adapted in light. The SGR increased as temperature illuminancerose at the range of6~16℃,2000~8000lx and the SGR<0at the second weekexposed at21℃. It indicated that blade seedlings had higher photosynthetic capacitybut cann’t be cultured for long at21℃. the suitable cultural condition was11~16℃,over4000lx.(4) The Pnand dark respiration rate (Rd) in young adult tip was higher thanmiddle and base (P<0.05). For low light (LL)-adapted samples it took some time forphotosynthesis to begin, and in higher light conditions immediately PS Ⅱ wasinhibited. In medium light (ML) conditions, photochemical reaction and heatdissipation was in balance. In high light (HL) conditions ETR was near saturation,and NPQ was higher. ETRmaxin tip and middle was higher than base. For LL and MLadapted leaf NPQ recovery was fast, whereas in HL it was slow. In the tip, NPQrecovered rapidly and completely, indicating highly efficient restoration, but not in themiddle and base indicating photodamage. Pigment content was higher in the tip thanthe middle and base. Chlorophyll a contents were higher than carotenoid in the tip,which was the contrary in the middle and base. The SGR values relative to position onthe algae were as follows: tip>middle>base (P<0.05), indicating fast organic matteraccumulation in tip. Small gaps in the narrow leaf tip might form and change intovesicles, inferring that temperature was the main inducing factor.(5) Pnrapidly decreased at pH6.0and pH9.5for24h, which were significantdifference (P<0.05) compared with that of the control (pH8.5) after48h. BothSuperoxide dismutase (SOD) and Catalase (CAT) activity increased significantly(P<0.05) at pH9.5after48h. And MDA content firstly increased and then decreasedat pH9.5after48h. compared with that of control (salinity31), Pndecreasedsignificantly (P<0.05) at salinity15after24h and at salinity25or salinity20after48h. However, Pnhad some rise at salinity25,20and15after72h. MDA contentincreased significantly (P<0.05) at first24h and then rapidly decreased at salinity20.And both SOD and CAT activity increased significantly (P<0.05) at salinity20from24h to48h. MDA content increased significantly (P<0.05) with SOD, CAT activityrelatively low at salinity15after72h. Above results indicated that photosyntheticcapacity was inhibited at pH6.0and pH9.5. S. thunbergii was damaged by reactive oxygen species (ROS) under strong alkaline stress. Antioxidant enzymes eliminatedROS readily at salinity over15but had low activity at salinity15.The suitable photosynthetic and cultural conditions were discussed in this papar,it aimed to provide a reference for the research and production of S. thunbergii.