| Pinus massoniana Lamb is an important afforestation and timber tree species in the subtropical region of China.It is widely distributed in 17 provinces,from south of Qinling Mountains and Huaihe River and east of Yunnan-Guizhou Plateau.It has the characteristics of rapid growth,high yield,wide application and high comprehensive utilization.At present,studies on drought resistance of mycorrhizal masson’s pine are mainly focused on nutrient absorption,antioxidant enzyme activity and photosynthetic characteristics of seedlings,but there are few studies on the screening of drought resistance strains.Moreover,studies on drought resistance physiology of mycorrhizal masson’s pine are not comprehensive,lacking studies on cell biology.In this research,we selected three ectotrophic mycorrhiza fungi strains,Suillus Bovinus(Sb),Sinoboletus Placidus(Sp)and Sclerderma Citrinum Pers(Sc),as primary test strains,and simulated drought stress with PEG – 6000 to study growth,SOD activity,MDA content and the hyphae form of ectotrophic mycorrhiza fungi under different drought stress levels.Basing on the above physiological changes and mycelia morphologic changes,we selected two fungi with the strong drought resistance for further study of drought resistance mechanism of ectomycorrhizal fungi.At last,Sb and Sc were selected to inoculate on pinus massoniana seedling to carry on the study of different responses of mycorrhizal and non-mycorrhizal seedlings to drought stress.This study set 4 moisture levels(normal water,mild drought,moderate drought,severe droughts)by weighing control in pots,and reveal physiological and biochemical response mechanism of pinus massoniana seedling to drought stress through growth properties,the activity of antioxidant enzymes,osmoregulation substance content,photosynthetic characteristics and chlorophyll fluorescence and other physiological characteristics.By studying the anatomical structure and ultrastructure of Sb masson’s pine seedlings and non-mycorrhizal masson’s pine seedlings under drought stress,we revealed the mechanism of ectomycorrhizal fungi seedlings improving drought resistance in cell biology level.The research results are as follows:1.Different PEG-6000 concentrations had different effects on the growth,physiology and hypha morphology of ectomycorrhizal fungi(1)The growth of Sb mycelia was slightly affected by PEG-6000 concentrations,and the growth rate was similar at different time points.The growth rate of Sp mycelia was higher between 0 and 21 d,while that of Sc presented a rapid growth period between 14 and 28 d.Mild drought stress(PEG-6000 concentration 10%-20%)was beneficial to the accumulation of Sb and Sp biomass,promoted the secretion of H+ of Sb,increased the SOD activity of Sb,and reduced the MDA content of Sb,while the growth of Sc was inhibited by PEG-6000.(2)Under PEG-6000 treatment,the hypha morphology of the three ectomycorrhizal fungi was damaged variously,and the hypha morphological damage was further aggravated with the increase of PEG-6000 concentration,but the morphological structure integrity of Sb hypha was higher than that of Sp and Sc.Therefore,mild drought stress was beneficial to Sb growth,and the drought resistance of 3 ectomycorrhizal fungi from strong to weak was Sb>Sp>Sc.2.Effects of drought stress on seedling growth and physiological characteristics(1)With the deepening of drought stress,the biomass,seedling height growth,ground diameter growth and leaf water content of mycorrhizal and non-mycorrhizal seedlings all decreased.The biomass,seedling height growth,ground diameter growth and leaf water content of mycorrhizal seedlings under the same water condition were higher than those of non-mycorrhizal seedlings,with significant differences(P > 0.05).mild drought stress had no significant effect on the biomass of mycorrhizal and non-mycorrhizal seedlings.(2)Under drought stress,inoculation of Sb and Sc could increase the activity of antioxidant enzymes such as SOD,POD and CAT.Under moderate and severe stress,SOD,POD and CAT activities of mycorrhizal seedlings were significantly different from those of non-mycorrhizal seedlings(P > 0.05).With the deepening of drought stress,the soluble sugar content and soluble protein content in mycorrhizal and non-mycorrhizal seedlings increased at first and then decreased,and the content of proline increased.Under drought stress of the same degree,the content of soluble sugar,soluble protein and proline in Sb and Sc seedlings was higher than that in non-mycorrhizal seedlings.Based on the above indexes,the drought resistance of Sb,Sc and CK masson pine seedlings was as follows: Sb > Sc> CK.(3)Drought stress reduced Pn,Gs and Tr of mycorrhizal and non-mycorrhizal seedlings,while Ci decreased first and then increased with the deepening of drought stress.Under the same water condition,the photosynthetic parameters of mycorrhizal seedlings were better than that of non-mycorrhizal seedlings,with significant difference under moderate and severe drought stress(P > 0.05).The chlorophyll content decreased with the deepening of drought stress,and the difference between mycorrhizal seedlings and non-mycorrhizal seedlings was significant under moderate drought stress(P>0.05).The maximum photosynthetic efficiency(Fv/Fm)and photochemical quenching coefficient(q P)of mycorrhizal and non-mycorrhizal seedlings decreased with the increase of drought stress.Under the same water condition,Sb and Sc inoculated could increase Fv/Fm and q P of seedlings,and the difference was significant under moderate and severe drought stress(P>0.05).The quantum yield parameter Y(NPQ)of mycorrhizal seedlings all increased first and then decreased with the deepening of drought stress.The change trend of quantum yield parameter(Y(NO))was opposite to that of Y(NPQ),but non-mycorrhizal seedling Y(NPQ)decreased with the deepening of drought stress,which was significantly different from mycorrhizal seedlings(P>0.05);under moderate and severe drought stress,the actual quantum yield(Y(Ⅱ))was significant difference(P > 0.05)between mycorrhizal and non-mycorrhizal seedlings.3.Effects of drought stress on the anatomical structure and ultrastructure of coniferous cells(1)Mesophyll cells,skin cells,endothelial cells,such as needles group of pinus massoniana seedling were damaged aggravating gradually with the deepening of drought stress,inoculating Sb could alleviate drought stress on seedling of mesophyll cells,skin cells,endothelial cell damage,and protect the structure of needles.(2)With the deepening of drought stress,the damage of chloroplast in the mesophyll cells of seedlings was gradually damaged to disappear,the granule lamella and stroma lamella were fractured,the number of starch grains and the cavitation area increased.Sb inoculation reduced the damage of drought stress on chloroplast membrane structure of seedlings,kept the chloroplast structure intact,and reduced the number of starch grains.(3)Inoculation of Sb reduced the damage of endoplasmic reticulum and vacuoles in root tip cells of masson pine seedlings under drought stress,increased the number of secretory vesicles,reduced the separation degree of cytoplasmic wall,and maintained the integrity of cells. |
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