Study On Leaf Structure And Physiological Characteristics Of Liquidambar Formosana During The Change Of Leaf Color

Liquidambar formosana is a deciduous tree of Liquidambar in Hamamelidaceae.After autumn,its leaves turn red or orange yellow,which is of great ornamental value.L formosana is an excellent landscape ecological tree species.At present,there are no systematic studies on the discoloration mechanism of L formosana under natural conditions.Therefore,this study takes the plant of leaf turning red as the research object,through continuous monitoring of the changes of leaf color,structural characteristics,pigment,content and photosynthetic characteristics in the process of leaf turning red,in order to systematically explore the mechanism of maple leaf turning red from the perspective of morphological physiology.The main results are as follows:（1）There were significant differences in leaf color parameters in different color changing stages.With the leaves becoming red（S1-S5）,lightness L^*and yellow blue attribute b^*increased first and then decreased;red green attribute a^*and saturation c^*increased gradually;hue angle h°decreased gradually.In S1 stage,leaves are green,a^*is negative,L^*,b^*,c^*are smaller,h°is larger.With the increasing of the red color of the leaves,the values of L^*and b^*reached the maximum in S2,then decreased,a^*and c^*continued to increase,and h°continued to decrease.When the leaves of S5turned red completely,the values of L^*,b^*and h°decreased to the minimum,and a^*and c^*reached the maximum.（2）The thickness of lower epidermis,upper epidermis,palisade tissue,spongy tissue,mesophyll,xylem,main vein,and the ratio of palisade tissue,spongy tissue,mesophyll and main vein gradually decreased during the process of maple leaf turning red,among which the thickness of palisade tissue,spongy tissue,mesophyll and main vein decreased by 28.3%-36.5%.The number and length of chloroplasts in a single cell decreased with the color of leaves becoming red gradually.Before the color change,the chloroplasts were irregular ellipse,the inner capsule layer was clear and the starch granules were large.At the end of color change,the chloroplasts became oblate,the inner capsule disintegrated and the starch granules disappeared.（3）There were significant differences in pigment content and ratio in leaves of different discoloration stages.With the leaves turning red gradually,the total chlorophyll content and carotenoid content decreased continuously,and decreased by 64.0%and 26.7%respectively in the period of complete color change（S5）;the anthocyanin content,anthocyanin/chlorophyll and anthocyanin/carotenoid increased continuously,and increased by 142.3%,594.3%and 232.8%respectively in the period of S5.（4）There were significant differences in soluble sugar content,CHI enzyme activity and PAL enzyme activity at different discoloration stages.The content of soluble sugar increased by 96.7%,the activity of Ch I decreased,the activity of PAL decreased.（5）During the discoloration process of L formosana,the diurnal variation trends of P_n,G_s and T_r were the same,which were double peaks curves in S1 and S2,single peak curves in S3,S4 and S5.In S1,S2 and S3,P_nand G_s reached their maximum peaks at 11:00,while in S4 and S5,P_n and G_sreached their peak at 13:00,and T_r reached their maximum peaks at 11:00.The diurnal variation curve of intercellular CO₂ concentration（C_i）showed a"V"shape.In S1,S2,S4 and S5,it was at a low at 13:00,and in S3,it was at a low at 11:00.（6）Correlation analysis showed that during the reddening process of sweetgum leaves,the leaf color attributes were significantly or extremely significantly related to the leaf structure,pigments,inclusions and photosynthetic characteristics.Among them,the red-green attribute a^*was related to the thickness of palisade tissue,chlorophyll content,PAL activity,Pn,etc.showed significant or extremely significant negative correlation,and were significantly or very significantly positively correlated with anthocyanin/chlorophyll,soluble sugar content,C_i,etc.Factor analysis showed that the changes in pigment content,inter-pigment ratio,and soluble sugar content had the highest scores,which were the factors that had the greatest impact on leaf color changes,followed by photosynthetic index scores,and leaf structure and enzyme activity index scores the lowest.In summary,the internal cause of the redness of the leaves of the L formosana is not caused by a single factor,but the result of the coordinated regulation of multiple factors.Among them,the change in the pigment content and the ratio between the pigments are the main reasons that affect the redness of the leaves.Affected by the low temperature and low light environment,the thickness of the tissue in the leaf is reduced,the number of chloroplasts is reduced,the chlorophyll content is reduced,and the photosynthetic capacity is reduced,and it cannot provide enough energy to maintain physiological metabolism.In this case,the leaves of L formosana can synthesize a large amount of soluble sugars and anthocyanins to reduce low-temperature damage,protect their own physiological metabolism,delay leaf senescence,and cause leaf color changes.