Effects Of Planting Density On The Anatomical Structure And Synthesis Of Structural Carbohydrate In Maize Stalk

【Objective】Increasing plant density is one of the most effective means to improve maize（Zea mays L.）yield in China and around the world in recent years.However,stalk lodging limits yield seriously under close density.Under high-density planting conditions,the external morphology and internal anatomical structure of the stalk will change,and the content of structural carbohydrates such as cellulose and lignin will change accordingly.How does dense planting affect the morphogenesis,structural carbohydrates accumulation and distribution of maize stalks,and then affect the formation of stalk strength?Both of these are worth exploring.Therefore,it is of great significance to explore the planting density on the anatomical structure of maize stalk and the synthesis of structural compounds by enzymatic activity.【Methods】External morphology,internal anatomical structure,cellulose and lignin synthesis and accumulation of maize internodes were determined under field condition in 2018-2020.Split plot design,3planting as the main plot（low densi:45,000 plants/hm²;D2:90,000 plants/hm²;D3:135,000 plants/hm²）,3varieties with different density tolerance（Xianyu 335 and Zhengdan958,2 varieties with better density resistance;Xinyu 41,a variety with poor density resistance）as the split plot.To study the effect of stalk anatomy on stalk strength,and clarify the accumulation and distribution of structural compounds during stalk development,reveal the relationship between the dynamic changes of cellulose and lignin accumulation and their related enzyme activities under dense planting conditions.The main results are as below.【Results】1)Stalk length increased and stalk diameter decreased with the increase of planting density,and the increase of the stalk length of Xinyu 41 was higher than that of Xianyu 335 and Zhengdan 958.analyse the response of the internal anatomical structure of the stalk showed that the number and thickness of the mechanical tissue layer of the stalk epidermis,the number and area of the large vascular bundle,and the number and area of the small vascular bundle were significantly reduced.The big vascular bundle area decreased by 51.8%-54.6%,and the small vascular bundle area of the stalk decreased by 44.9%-77.8%.The number of layers of epidermal mechanical tissue,the thickness of epidermal mechanical tissue,and the total area of small vascular bundles of stalk of the varieties with poor density tolerance were lower than those of the density resistant varieties.Comparing the response of different density tolerance varieties,it was found that the small vascular bundle area of the stalk of the density tolerance varieties Xianyu 335 and Zhengdan 958 decreased by 48.8%and 44.9%,respectively,while the poor density tolerance varieties of Xinyu 41 decreased by 77.8%,small vascular bundles area of varieties with poor density tolerance decreased more greatly with the increase of density,and decreases more with increasing density.2)With the increase of planting density,the cellulose and lignin content per unit node length of stalks decreased significantly.There were some differences in the cellulose and lignin content and distribution characteristics in the stalks of different dense-tolerant varieties of maize.The dry matter accumulation in the stalk pith tissue did not change,but the morphogenesis and material synthesis of the rind tissue were still active after the 13-leaf stage in the third internode of the maize.Comparing the distribution of chemical components in the tasseling stage of the stalk,it was found dry weight of the rind tissue accounts for 66.9%-70.8%of the stalk,cellulose content of the rind tissue accounts for 75.8%-82.3%of the total cellulose content of the stalk,and the lignin content of the rind tissue accounts for 84.6%-91.2%of the total stalk lignin content.With the increase of planting density,the content of cellulose and lignin per unit length of stalk decreased significantly.3)The rapid accumulation period of stalk cellulose was from the 9-leaf stage to the tasseling stage.Analyse the direct substrate sucrose and its related synthase activities during the rapid accumulation period of cellulose showed that with the increase of planting density,the activities of sucrose synthase（SS）and sucrose phosphate synthase（SPS）in stalk decreased significantly.The change of planting density showed that there was no significant difference in the SPS activity of different varieties at low planting density at the 15-leaf stage,while the varieties Xianyu 335 and Zhengdan 958 with better density tolerance at medium and high densities were significantly higher than Xinyu 41,a variety with poor density tolerance.The rapid accumulation rate of stalk lignin decreased with the increase of planting density.The enzyme activities related to lignin were measured,and it was found that different enzyme activities had different responses with the increase of planting density.Activity of phenylalanine ammonia lyase（PAL）decreased with the increase of planting density from the 9-leaf stage to the tasseling stage.The activities of cinnamyl alcohol dehydrogenase（CAD）and peroxidase（POD）decreased with the increase of planting density from the12-leaf stage to the tasseling stage.It can be seen that the regulation period of dense planting on PAL enzyme activity was longer than that of tyrosine ammonia lyase（TAL）,CAD and POD.TAL is less affected by planting density.Rapid accumulation rates of lignin was different among varieties with different density tolerance.Comparing the enzyme activities,it is found that during the period of rapid lignin accumulation,the stalk PAL activity,CAD activity and POD activity of the dense-tolerant variety Xianyu335 was higher than Xinyu 41,a variety with poor density tolerance.4)With the increasing of planting density,the bending strength（BS）and rind puncturing strength（RPS）of the stalks decreased significantly,the poor dense variety Xinyu 41 showing a greater decrease in strength with increasing planting density,and the puncturing and bending strength of the stalks of Xinyu 41were significantly lower than those of the more dense varieties Xinyuyu 335 and Zhengdan 958 at medium and high planting densities.Correlation analysis was carried out on the stalk RPS,BS and stalk anatomical5)Structure under different planting densities.The results showed that the stalk RPS was significantly positively correlated with the mechanical organization of the stalk epidermis,while there was no correlation between the number of large vascular bundles and RPS.Stalk bending strength was significantly positively correlated with the mechanical organization of stem epidermis,the number and area of small vascular bundles;the total area of large vascular bundles had no correlation with stalk RPS and BS.6)Comparing the grain yield of different varieties,it can be found that the grain yield of each variety was no significantly different under the density of 45,000 plants/hm^-2,and the grain yields of Xianyu 335and Zhengdan 958,which have better density tolerance under the density of 135,000 plants/hm²,were significantly higher than Xinyu 41,Xianyu 335 and Zhengdan 958 were 31.3%and 29.9%higher than those of Xinyu 41,respectively.The decreased of grain yield for Xinyu 41 under 135,000 plants/hm²was related to its higher lodging percentage.【Conclusions】With the increase of planting density,the stalk length increases,and the stalk diameter decreases,the number and area of stalk epidermis mechanical tissue and vascular bundles decrease significantly,and the content and proportion of structural carbohydrate in stalk epidermis decrease significantly.resulting in a decrease of stalk strength.Further analysis of the synthesis of stalk structural carbohydrate and related enzyme activities showed that the sucrose content of the stalk from the 12-leaf stage to the tasseling stage decreased,and the activities of SS and SPS also decreased,so that the cellulose accumulation rate of the stalk slowed down and the cellulose content was reduced.The rapid accumulation period of lignin was from the 9-leaf stage to the tasseling stage.The PAL activity decreased with the increase of the planting density from the 9-leaf stage to the tasseling stage.Stalk CAD and POD activities decreased with the increase of planting density from 12-leaf stage to tasseling stage,and stem lignin content decreased.Compared with the cultivar Xinyu 41 with poor density tolerance,the cultivars Xianyu335 and Zhengdan 958 with better density tolerance under high density had thicker stalk epidermis mechanical tissue thickness,larger area of total small vascular bundles,higher cellulose and lignin content,and higher stalk strength.Varieties with better density tolerance had better anatomical characteristics and structural carbohydrate content under high density,which ensured the lodging resistance of stalk,and finally obtained higher grain yield.