Effects of shading on photosynthetic carbon and nitrogen physiology,plant hormones,and andrographolide biosynthesis and accumulation in Andrographis paniculata（ Burm. f.） Nees

【Objective】 The effects of shading on photosynthesis and physiological characteristics of Andrographis paniculata （Burm.f.） Nees were studied to clarify the mechanism by which light intensity affects the accumulation of an drographolide,and to provide theoretical basis for optimizing the planting mode and growth regulation of A.paniculata【Method】 Using the medicinal plant A.paniculata as the experimental material,shading treatments were carried out by covering with 1-layer shading net （light shading,transmittance of 70%） and 2-layer shading net （severe shading,transmit tance of 30%） respectively.The plants grown under natural light conditions was used as the control.Then,photosynthetic light response characteristics,leaf protein nitrogen allocation,carbon metabolism,plant hormones,as well as androgra‐pholide accumulation and the expression of related genes in A.paniculata were studied. 【Result】 The aboveground bio‐mass and specific leaf weight of A.paniculata decreased gradually with the increase of shading rate,but the chlorophyll content did not change significantly（P＞0.05,the same below）.Severe shading significantly reduced the content of hydro‐gen peroxide （H₂O₂） and malondialdehyde （MDA） in the leaves of A.paniculata （P＜0.05,the same below）,but had no significant effect on the activity of superoxide dismutase （SOD）.The maximum net photosynthetic rate(P_max)and apparent quantum yield was significantly higher in the severe shading than that in control and light shading.Light compensation point was decreased with the increase of shading degree.Severe shading decreased the water soluble protein nitrogen con‐tent of leaves.Light shading significantly reduced the nitrogen content of cell wall proteins,while shading had no signifi‐cant effects on the nitrogen content of membrane-binding proteins.However,the proportion of membrane-binding protein nitrogen in total protein nitrogen was increased significantly under severe shading.The contents of soluble sugar and starch in leaves were decreased with the increase of shading degree.The content of salicylic acid was increased but auxin content was decreased under shading conditions.The content of andrographolide slightly decreased under severe shading,with no significant difference.The expression of key enzyme genes of andrographolide biosynthesis pathway,including HMGR,DXS,GGPS and ApCPS,was significantly up-regulated under severe shading.【Conclusion】 The photosynthesis of medicinal plant A.paniculata has a strong adaptability to low light environment.It alters leaf protein nitrogen allocation to increase the proportion of membrane-binding protein nitrogen.However,the decrease in carbohydrate accumulation and changes in plant hormones caused by low light environments can be adverse to the biosynthesis and accumulation of an‐drographolide.