Seasonal Changes Of Physiological And Biochemical Characteristics,morphological Structure Of Leaves Of Magnolia Sinostellata Under Different Light Intensities

Exploring the seasonal changes of photosynthetic,physiological and biochemical characteristics and morphological structure of Magnolia sinostellata leaves under different light intensities can provide theoretical basis for population reproduction,rejuvenation and ex-situ protection.In this study,two-year-old M.sinostellata seedlings were used as the research objects.Shading treatments was carried out by constructing sunshade nets with different aperture,and the light intensity was 40%(I₄₀)and 10%(I₁₀)of the total light,respectively.Full light(I₁₀₀)was set as the control.In spring,summer and autumn,photosynthetic characteristics were measured by Li-6400XT portable photosynthetic apparatus and MINI-PAM portable chlorophyll fluorescence analyzer,physiological and biochemical indices were measured by UV spectrophotometer,leaf morphological indices were measured by leaf area scanner,and leaf structure indices were measured by Scanning Electron Microscope(SEM)and Transmission Electron Microscope(TEM).The results are as follows:1.The seasonal variations of photosynthetic characteristics of M.sinostellata leaves under different light intensities are as follows:(1)The diurnal variations of the net photosynthetic rate(P_n)of M.sinostellata under I₁₀₀in spring and I₁₀₀and I₄₀in summer were shown as“double peaks”curves,while the others were shown as“single peak”curves.(2)The maximum net photosynthetic rate(P_nmax),light saturation point(LSP),light compensation point(LCP)under I₁₀₀ and I₄₀in summer were significantly higher than those under I₁₀,but P_nmax and LSP under I₁₀₀ in autumn were significantly lower than those under I₄₀and I₁₀.(3)The maximum carboxylation rate(V_cmax),maximum electron transfer rate(J_max)and triose phosphate utilization rate(TPU)under I₁₀₀ in summer were significantly lower than those under I₄₀ and I₁₀,and J_max and TPU under I₁₀₀ in autumn were also significantly lower than I₄₀treatment.(4)Under I₁₀₀,the F_v/F_m value was 0.68 in summer and 0.72 in autumn,which were significantly lower than 0.8,indicating the photoinhibition happened.The photochemical quenching(q _P)and the effective quantum yield of photochemical energy conversion Y(II)under I₄₀ in summer were significantly higher than in spring and autumn.Under I₁₀,Y(II),q _P in spring and NPQ in summer were significantly lower than those in the other two seasons.2.The seasonal variations of photosynthetic pigment of M.sinostellata leaves under different light intensities are as follows:The chlorophyll a(Chl a),Chl b and Chl a+b contents in three seasons increased with the decreasing of light intensity.Those under I₁₀ treatment was significantly higher than the other two treatments,while the carotenoid/chlorophyll(Car/Chl)values decreased with the decreasing of light intensity.The contents of Chl a,Chl b,Chl a+b and Car in the three light environments were the highest in spring,and the values of Chl a/b and Car/Chl were the highest in summer.3.The seasonal variations of antioxidation system of M.sinostellata leaves under different light intensities are as follows:(1)The activity of superoxide dismutase(SOD)decreased with the decreasing of light intensity in spring and autumn.The peroxidase(POD)activity increased with the decreasing of light intensity in summer and autumn.The catalase(CAT)activity was relatively lower in full light intensity treatment.SOD activity was the highest in autumn and CAT activity was the highest in spring under the three shading treatments.POD activity was the highest in spring under I₁₀₀ and I₄₀.(2)The content of ascorbic acid(As A)in three seasons was the lowest under I₁₀ treatment.The content of GSH was the highest in summer and autumn,respectively,under I₁₀₀and I₄₀ treatments.(3)The electrical conductivity under I₁₀₀ in summer was significantly higher than that in I₁₀treatment,and was significantly higher than those in shade treatments in autumn.The electrical conductivity under three shading treatments was the highest in autumn and the lowest in spring.(4)The soluble protein content under three light intensities in autumn was significantly lower than those in spring and summer,and the soluble protein content under I₁₀₀ treatment was significantly lower than those in shading treatments in autumn.The soluble sugar content in spring was the lowest in I₁₀ treatment,while that in summer was the lowest in I₁₀₀ treatment,and in autumn were the lowest in I₄₀treatment.The soluble sugar content in summer was significantly higher than those in spring and autumn under three light intensities treatments.The proline content under I₁₀₀ treatment in autumn was significantly higher than those in spring and summer,and also significantly higher than that under shading treatments.4.The seasonal variations of stoichiometric characteristics of M.sinostellata leaves under different light intensities are as follows:(1)Leaf C content under three light intensities was significantly higher than that under I₁₀ treatment in autumn under I₄₀ treatment.Leaf N content in three seasons and P content in spring increased with the decreasing of light intensity,while P content in summer and autumn was significantly lower than that in I₁₀₀ treatment.(2)Leaf C:N and C:P in spring decreased with the decreasing of light intensity in three seasons,while C:P in summer in I₁₀₀ treatment was significantly lower than that in I₄₀ and I₁₀ treatments.Leaf N:P under the three light intensities was about 15 in spring,and increased gradually with the decreasing of light intensity in summer,all of which were higher than 16.In autumn,leaf N:P under I₁₀ treatment was significantly higher than those under I₁₀₀ and I₄₀ treatments.5.The seasonal variations of non-structural carbohydrate(NSC)of M.sinostellata leaves under different light intensities are as follows:The soluble sugar and NSC under three light intensities were the maximum in summer,and the soluble sugar and NSC in spring decreased with the decreasing of light intensity.The soluble sugar and NSC in summer under I₁₀₀ treatment were significantly lower than those in the shading treatments,while the soluble sugar and NSC in autumn under I₁₀₀treatment were significantly higher than those in I₄₀ treatment.NSC in full light treatment were higher than those in shading treatments.Under the three light intensities,starch content increased gradually with the change of seasons with significant differences.That was the lowest in spring and the highest in autumn.The starch content in I₁₀ treatment was significantly lower than those in I₁₀₀ and I₄₀treatments in spring and autumn.The ratio of soluble sugar starch in autumn was significantly lower than those in spring and summer.6.The seasonal variations of morphological and structural characteristics of M.sinostellata leaves under different light intensities are as follows:(1)The leaf width and area under I₄₀ treatment in spring and that in I₄₀ treatment in summer were significantly higher than those under I₁₀₀treatment,and the leaf width and area under I₁₀ treatment in autumn were significantly higher than those under full light intensity.I₁₀₀ had no significant change in the three seasons,I₄₀ had significantly higher leaf area in summer than in spring and autumn,and I₁₀ had significantly higher leaf area in autumn than in spring and summer.The specific leaf area and leaf water content increased with the decreasing of light intensity in three seasons.(2)Stomatal area was the smallest in spring and autumn in I₁₀₀treatment,while stomatal density and stomatal density percentage in I₁₀₀ treatment were significantly higher than those in I₄₀and I₁₀ treatments in three seasons.(3)The thickness of upper epidermis,palisade tissue and sponge tissue under I₁₀₀ treatment in spring was significantly higher than those under I₄₀ and I₁₀ treatments,the thickness of sponge tissue under I₁₀₀ treatment in summer was significantly higher than those under the shading treatments,and the thickness of upper epidermis,lower epidermis,palisade tissue and sponge tissue under I₄₀ treatment in autumn was significantly higher than those under I₁₀₀ and I₁₀treatments.Leaf thickness under I₁₀₀ treatment in spring was the maximum,that under I₁₀ treatment in summer was the minimum,and significantly higher than those under I₁₀₀,I₁₀ and I₄₀ treatments in autumn.The thickness ratio of palisade tissue to sponge tissue in I₁₀ treatment significantly lower than those in I₁₀₀ and I₄₀ treatments in all three seasons.S_c and S_mes in I₁₀₀ treatment in summer and autumn was the minimum,which is not conducive to CO₂transfer.(4)The chloroplast ultrastructure of M.sinostellata leaves in spring under the three light intensities showed a small amount of heavy particles of osmium,which become apparent in summer and autumn.Under full light treatment,osmium particles increased gradually in summer,while occupy the vast majority of the chloroplast in autumn and grana thylakoid become few and scattered.The condition of which were addicted to autumn,osmiophilic particles under I₄₀ and I₁₀ treatments were significantly smaller than that under I₁₀₀ treatment,and the stacking of grana thylakoid was better in three seasons.In conclusion,M.sinostellata is susceptible to high light,which results in inhibition of photosynthetic capacity of leaves,irreversible damage of leaf structure,and decrease of antioxidant enzyme activity.Finally,the photosynthetic capacity of M.sinostellata is significantly decreased in autumn.Under moderate shade condition,the enzyme activity and leaf structure were better,and the photosynthetic rate was higher in all three seasons.Under the I₁₀ treatment,the weak photosynthetic capacity might be due to the insufficient light intensity in the environment.Therefore,the photoinhibition is easy to happen under full light intensity and shading will benefit the growth of this species.But the shading intensity should not be too high and the effective radiation intensity should be kept above 40%of the natural light intensity.