Response And Adaptation Mechanism Of Phoebe Bournei To Light Environment At Different Age

Phoebe bournei is an important afforestation tree species in China.It needs some shade at the seedling stage,but the morphological structure and physiological characteristics of P.bournei in different light environments at different stages are still unclear.The study of the morphological structure and photosynthetic physiology of P.bournei of different age classes in different light environment changes in the field natural forest can provide a basic basis for the protection of P.bournei population in natural forest,forest construction and further the selection of artificial light environment.Further study on the light response mechanism,photosynthetic nitrogen distribution strategy and molecular mechanism of seedlings adapting to weak light under artificially controlled light environment can provide an important reference basis for the research of P.bournei tending,plantation management and regeneration.The natural forest of P.bournei in Taijiang County is divided into four light intensity levels,with 10%of all light as weak light（WL）,30-40%as low light（LL）,50-60%as medium light（ML）and 80-100%as high light（HL）.Under low light（WL,LL and ML light intensity levels）,the distribution of older P.bournei is less,on the contrary,under high light（HL）,the distribution of younger seedlings and young trees is less,especially the distribution of age I P.bournei is not under HL light intensity level.The understory cultivation experiment of Pinus massoniana Plantation in Longli County was carried out.Taking 100%photosynthetic effective radiation in bare land as the reference,the daily mean values of FG1、FG2、FG3and FG4 were 80%,70%,40%and 10%of the light intensity in bare land,respectively.Under the control of shading net,set the light intensity of 100%,80%,60%,35%and 10%.（1）Among the four understory light environments,FG2（70%light intensity）light environment can significantly improve the growth of P.bournei sapling,and the growth of tree height and diameter and net growth of ground diameter are significantly greater than other light environment treatments.P.bournei saplings grow slowly in weak light（10%light intensity）.Under 70%light intensity and the phenomenon of"tender shoot burn"in summer,the seedling height and ground diameter are still the highest,followed by80%ground diameter.P.bournei saplings grow faster under medium high light intensity,Take a risky growth strategy.Among the five shading treatments,the seedling height,ground diameter and biomass are the highest at 60%and 35%light intensity.The saplings growth strategy are as follows:the P.bournei under 10%-60%light intensity is mainly aboveground growth,and the P.bournei under 80%-100%light intensity is mainly underground growth.（2）In the same natural forest environment,the leaf thickness increased with the increase of age class.The average thickness of leaf tissue increased from low light to high light intensity,and the average thickness of leaf tissue increased from low light to high light intensity level.The anatomical structure parameters（palisade tissue,sponge tissue and leaf thickness）of the leaves of P.bournei saplings gradually decreased with the weakening of light intensity.Except for full light,the law of anatomical structure parameters of P.bournei sapling was the same as that of P.bournei saplings.Except for the age class I,the leaf area of P.bournei increased with the weakening of light intensity in indoor and outdoor light environment.The leaf area of P.bournei increased first and then decreased with the increase of age class.The adjustment of anatomical structure and leaf area is conducive to the growth and survival of P.bournei under low light.The chloroplasts of P.bournei seedlings under 35%and 10%light intensity contained large and full-bodied starch granules,the thickness of grana became thicker,the grana lamellae were stacked neatly,and there were lipid spheres in the chloroplast matrix;Under weak light conditions,P.bournei seedlings can increase the thickness of grana,which is conducive to the formation of a long metabolic transport belt and promote the absorption,transmission and conversion of light energy.（3）With the decrease of light intensity,the photosynthetic pigment parameters（Chl a,Chl b,Car,Chl（a+b）,Chla/b,Car/Chl（a+b））of P.bournei seedlings and P.bournei saplings decreased with the decrease of light intensity P<0.05).Under weak light（10%light intensity）,P.bournei seedlings adapt to the weak light environment by increasing chlorophyll b synth Chla/b,Car/Chl（a+b）.The absorption spectral rate of seedlings decreased and the shade intensity increased（P<0.05）,and the absorption spectral rate of young trees was the highest at 70%.The photosynthetic parameters of P.bournei saplings under the four light environments reached the maximum in summer.FG2 saplings had the highest maximum net photosynthetic rate(A_max),maximum carboxylation rate(V_cmax),maximum electron transfer rate(J_max)and maximum light saturation point（LSP）.Among the five shading treatments,the LSP,V_cmax,J_maxand apparent quantum efficiency of P.bournei seedling leaves were the largest under 35%light intensity.Between 100%-35%light intensity,the diurnal variation of net photosynthetic rate（Pn）showed an obvious"bimodal"curve.Under extremely weak light environment（10%light intensity）,the diurnal variation of Pn showed a"single peak"curve,and the maximum value of Pn was(2.45μmol·m^-2·s^-1)appeared at 12:00 noon.The corresponding young trees of P.bournei showed an obvious"bimodal"curve in the vigorous growth period of Pn in summer.（4）The leaf nitrogen content per unit area(N_area)of FG2（about 70%light intensity）was significantly higher than that of FG1.The P_R（proportion of photosynthetic nitrogen allocated to Rubisco）,P_B（proportion of photosynthetic nitrogen allocated to bioenergetic components）and photosynthetic nitrogen utilization efficiency（PNUE）of FG2 were the largest,indicating that FG2 light environment enabled the young leaves of Phoebe to allocate more leaf nitrogen content to P_Rand P_Bphotosynthetic mechanisms,significantly improving the photosynthetic utilization efficiency of nitrogen,followed by FG1.Narea of young Phoebe trees in low light environment fg4 is lower than that in FG1,FG2 and FG3 light environment treatment.More photosynthetic nitrogen energy is allocated to nitrogen and allocated to light capture components（P_L）.The photosynthetic nitrogen utilization efficiency（PNUE）is the lowest.The photosynthetic action of P.bournei saplings in low light environment decreases and the corresponding carbon assimilation capacity decreases.N_areadecreased with the weakening of light intensity.Under five light environments,the photosynthetic nitrogen distribution ratio of P.bournei seedlings was the highest in P_L,the second in P_Rand the lowest in P_B.Proper shading is helpful to improve P_Rand P_B.Under weak light（10%）,P.bournei seedlings allocate more nitrogen to P_Lto adapt to the weak light environment.The PNUE of P.bournei young increased first and then decreased with the change of light intensity,with the lowest light intensity of 100%and the highest light intensity of 35%.（5）Under light stress（strong light and weak light）,the protective enzyme activity of P.bournei seedlings decreased,the contents of malondialdehyde（MDA）MDA and relative conductivity（REC%）increased,while the protective enzyme activities（POD,SOD and cat）were higher under 35%and 60%light environment,so as to reduce peroxidation damage.With the decrease of light environment,q_p、NPQ、Fv’/Fm’and Fv/Fo of photosystem II（PS II）increased first and then decreased.The parameter indexes were the highest at 35%light environment,followed by 60%and 80%light intensity,and the lowest at100%and 10%.Y（NPQ）is a very important light protection mechanism of plant leaves.Y（II）and Y（NPQ）were significantly higher than those of each treatment group under 35%light environment.Y（NO）is an important index of light damage,which is easily inhibited by light at 100%light environment.（6）The physiological level of Phoebe seedlings reflected that it grew better under 35%light environment,and the PNUE of P.bournei seedlings under 10%light environment was second only to 35%light intensity.In order to study the molecular mechanism of low light tolerance,the transcriptome samples of CK control（100%light environment）T1（35%light environment）and T2（10%light environment）were collected for shading experiment.Analysis showed that compared with CK control,724 and 3248 genes were differentially expressed due to the low light intensity of T1 and T2,respectively.Through functional enrichment analysis,differentially expressed genes（DEGs）are related to photosynthesis,nitrogen metabolism,plant hormone signal transduction,secondary metabolite biosynthesis and endoplasmic reticulum protein processing.In addition,after T2 and T1 low light,HSP（heat shock protein）,CAB（chlorophyllA-Bbinding）,hema1（l-glutamate-1-semialdehyde）,GSA（glutamate-1-semialdehyde-2,1-amino mutase）,DVR（3,8-divinylprotochlorophyll-8-vinyl reductase）,MYB（v-MYB avian myeloblastosis virus oncogene homologue）,b HLH（basic helix）,PORA（protochlorophyll oxidoreductase a）The expression of CAO（chlorophyll a oxygenase）,GLK（biosynthesis related factor）and photosystem I and II complex related genes increased significantly.The research shows that the rapid growth of P.bournei seedlings in low light（35%and 10%light intensity）may be the result of the accelerated expression of genes related to photosynthesis and chlorophyll biosynthesis,which enables the plant to maintain a high photosynthetic rate even under weak light conditions.（7）The selection of appropriate light environment in seedling and sapling stage is particularly important for the growth of P.bournei into a big tree.P.bournei seedlings are more suitable for the growth of medium and low light（35%light intensity）environment,and saplings are more suitable for the growth of medium and high light environment（70%）.According to the shading net control experiment,it is recommended that P.bournei below grade III（plant height 100cm）grow in the light environment of about35%-60%of full light;The third age class（sapling stage）and above of Pinus massoniana plantation are about 70%of the total light intensity,which is suitable for growth.Therefore,according to different age classes,artificial intervention should be carried out to prevent the emergence of"small and old"trees.Its light regulation is an important measure for the phased growth of P.bournei.