Photosynthetic Characteristics And Physiological Responses To Temperature Stress Of Magnolia Wufengensis Seedlings

Magnolia wufengensis is a new Magnoliaceae species found and nominated by Lu Yi Ma and Luo Rong Wang in Wufeng county,Hubei province,China.M.wufengensis has very high scientific value and ornamental value.Because of the special particularities of its distribution region,the narrowness of its dispersal area,the scarcity of its individuals and the dangerousness of its population extinction,the research on M.wufengensis should be taken in salvage way immediately.The photosynthetic characteristics,the effect of different light intensities on growth photosynthetic characteristics and the effect of temperature stress on physiological characteristics were of M.wufengensis seedlings carried out in this study.The objective of this study is to provide a theoretical base for the research of germplasm conservation and utilization of M.wufengensis.The results were as follows:1,On the June sunny day,diurnal variations of photosynthetic characteristics of M.wufengensis seedling were studied under natural condition by Li-6400 photosynthesis analyzer.The results indicated that the diurnal variations of photosynthetic active radiation and air temperature were single-peaked curves;the diurnal variation of atmospheric relative humidity was inverted parabolic curve;the diurnal variation of atmospheric CO₂ concentration was like "L" basically.The diurnal variations of net photosynthetic rate and stomatal conductance were double-peaked curves,and photosynthetic midday depression was mainly caused by stomatal factors.The diurnal variation of transpiration rate was double-peaked curves,and the main factor affected transpiration were atmospheric relative humidity, vapor pressure deficit,photosynthetic active radiation and air temperature.M.wufengensis was sensitive to high temperature and light,so the water use efficiency and light use efficiency were lowest in midday.2,Under the Shading treatment(25%and 10%light transmission rate),the chlorophyll a, chlorophyll b and total chlorophyll of M.wufengensis seedlings increased;and along with the degree increasing of shading treatment,the chlorophyll a/b values decreased,the results indicated that shading treatment could make the plant use more low light quantum.Shading treatment reduced the maximum net photosynthetic rate,apparent quantum efficiency,light compensation point and dark respiration rate. The reduction of photosynthetic capacity inhibited the growth and the accumulation of biomass. Therefore M.wufengensis was not suitable to be planted in too shading environment.3,Under high temperature stress,the net photosynthetic rate ofM.wufengensis seedlings decreased, and that the reduction was mainly due to stomatal factors under 30℃,which was mainly caused by the non-stomatal factors under 45℃.Under 30℃,35℃,40℃and 45℃,there was a very significant correlation between the relative conductivity and processing time.under 30℃condition,MDA content changed small and increased undulantly.Under 35℃,40℃and 45℃,MDA content increased significantly compared with the control.After high temperature treatment,M.wufengensis enhanced the capacity of osmosis-regulating substances and heat tolerance of leaves by means of increasing soluble sugar and proline content.Under different temperatures,the changes of soluble protein content were distinguished with the treatment time:under 30℃and 35℃,it showed the trend of increasing-reducing-increasing;under 40℃,it showed the trend of reducing-increasing -reducing; under 40℃,it increased first and reduced later.Under 30℃and 35℃,the SOD activity,POD activity and CAT activity showed the trend of increasing-reducing-increasing,and they were coordinated to get rid of reactive oxygen species effectively.Under 40℃and 45℃,SOD and CAT responded to high temperature more rapidly than POD,the significant-increased time of SOD activity and POD activity was earlier POD activity.Correlation analysis showed that there was a very significant correlation between random two indexes of the relative conductivity,MDA content,soluble sugar content,proline content and net photosynthetic rate.4,In the cold stress of different temperatures and 5℃continually cold stress,the relative conductivity and soluble sugar content of M.wufengensis seedlings were significantly increased;MDA content increased undulantly;and soluble protein content decreased first and increased later.In the cold stress of different temperatures,proline content increased undulantly;and in the 5℃continually cold stress,proline content increased undulantly in the former 3d and decreased obviously at the 4d.The activities of protective enzymes(SOD,POD,CAT) in the cold stress of different temperatures and 5℃continually cold stress were different:in the cold stress of different temperatures,SOD activity continued to rise,POD activity and CAT activity decreased first and then increased;in the 5℃ continually cold stress,SOD activity,POD activity and CAT activity increased first and then decreased. Correlation analysis showed that there was a very significant correlation between random two indexes of the relative conductivity,MDA content and soluble sugar content:and CAT activity was very significantly correlated with MDA content.The relative conductivity,and MDA content of M. wufengensis seedlings were higher under low temperature with light than with dark.Under low temperature with light,M.wufengensis seedlings resisted the harmful environment by the means of accumulating more osmosis-regulating substances and maintaining higher activities of protective enzymes.