Different Density-tolerant Maize Varieties Vascular Bundle Characters And Its Source-sink System

The vascular bundles are the significant system for mechanical framework and matter transportation of maize, and playing an important role in the flow that harmonizes the source-sink system. Four maize varieties that belong to two different density-tolerant types were selected in this experiment. The shape, development and structure of the vascular bundles and the difference of vascular bundles between varieties were observed through optic microscope. Meanwhile the responses of flow intension and its components, leaf-source and ear-sink to densities were studied to analyze the function of vascular bundles in source-sink system and the relationship between vascular bundles and lodging. In addition the effects of water stress on vascular bundle characteristics were also studied in this experiment. The understanding of the structure and function of vascular bundles and its relationship to source-sink system could perfect the source-sink theory and provide an insight for high-yield cultivation and breeding for maize. The main results showed as follows:1 The vascular bundle characteristic of different density-tolerant maizeThere was no significant difference of basal stem vascular bundle within the same type and between the different types at jointing stage, but the significant difference was found at tasselling stage. From the tasselling stage to the milk maturity, the significant difference always existed in vascular bundle of ear-located stem in all four varieties. The difference of vascular bundle in the ear first stalk and ear cob among varieties was clearer with the developmental process of maize. Namely, there was no difference in vascular bundle of four maize varieties until tasselling stage.From the aspect of different plant parts of different density-tolerant maize varieties, the vascular bundle development in basal stem of the thin density varieties was better than that of the density-tolerant varieties, especially in Dongdan60. The vascular bundle development in ear first stalk and ear cob of the density-tolerant varieties was better than that of thin-density varieties, especially in Zhengdan958. From the aspect of vascular bundle structure in different plant parts, the vascular bundle structure of the basal stem, ear first stalk and ear cob except ear-located ear of the density-tolerant varieties was up to the plant development.2 The responses of vascular bundle and source-sink system of different density-tolerant maize to densitiesThe vascular bundle areas of the thin-density varieties at all density treatments were bigger than that of the density-tolerant varieties. The basal stem vascular bundles of density-tolerant varieties and Dongdan60 at early growing stage were sensitive to the change of density, and became insensitive at later growing stage. However, Liaodan565 was opposite. The ear-located vascular bundles of the density-tolerant varieties were not sensitive at the change of the density at tasselling and milk maturity stage, but it appeared more sensitive in the thin-density varieties. Ear stalk vascular bundle in Liaodan565 was sensitive at density at tasselling. Till milk maturity effects of density on ear stalk vascular bundle of both types were great. Increasing of density had mainly negative impacts on the vascular bundle of maize plant.The sap of the basal stem was considered as a flow to study its activities in this research. The sap content of basal and ear-located stem in density-tolerant varieties were more than that of the thin-density varieties, thus, it could be deduced that the transportation capacity of density-tolerant were better than that of the thin density varieties. The sap content of four varieties basal stem at all density treatments declined from the early growing period to the later growing period, however it acted on opposition in the ear located. The response of the basal stem sap content in Zhengdan958 to densities showed the same way at early and later growing stage, and there was no apparent rule in the other three varieties. The ear-located stem sap content within the same types showed the same change rule at both early and later growing stage.The contents of soluble sugar, inorganic phosphates and nitrate nitrogen in the basal stem sap decreased with the development, and yet the soluble protein and total amino acid content increased. The main component in basal stem sap of density-tolerant maize changed from soluble sugar to amino acid and inorganic phosphates became the main component in basal stem sap in thin-density maize varieties from soluble sugar except Dongdan60 at the top and higher density. Consequently the different density-tolerant maize varieties had their own adjusted vascular bundle transportation system. At early stage, the components of sap in Zhengdan958 were balanced compared with the other three varieties; thus, it was considered that zhengdan958 had strong self-adjustment ability and its ability of absorbing and transporting in root was better. At later stage, the components of sap in basal stem of all varieties were more sensitive to the density.Though the relative single leaf area of density-tolerant maize varieties was small and its chlorophyll content was low, LAI was higher and the photosynthesis demands were met. The relationship of leaf area with densities in density-tolerant maize was second-degree curve which helped to release the density press. However, the relationship of leaf area with densities in density-tolerant maize was linear; the leaf areas of thin-density varieties were relatively higher and sensitive to densities. Photosynthetic rate, transpiration rate and stomatal conductance of density-tolerant varieties and Liaodan526 were at the highest level at the contrast density, but photosynthetic rate of Dongdan60 continuously declined with the density increased. Form the filling stage to wax ripeness stage, the decrease rate of photosynthetic rate of density-thin varieties was larger than that of density-tolerant varieties. The high rate of photosynthesis and water metabolism in density-tolerant varieties remained function of photosynthesis for a relative long time. The highest yield was gained at the proper density, and the yield of the density-tolerant varieties was higher than that of thin-density varieties. One-dimensional quadratic regression equation was build between the yield and the density in all varieties. There were no significant effects on grain nutrition by densities in all varieties.3 The relationship between the vascular bundle characteristic and lodgingThe response of stalk anti-press from the first stem to the seventh stem in different density-tolerant maize varieties to densities was sensitive. Correlation coefficient between diameter and anti-press of different stems was significant at 0.01 level at filling and milk maturity stage, in particular the third stem had the largest correlation coefficient. So the analysis of correlation suggested that the stem anti-press demanded on the stem diameter. The densities and stem anti-press were negative correlated which indicated higher density induced lower stem anti-press. Therefore the stem anti-press could be an index to determine the effect of density on stem intension to anticipate the lodging. The coefficient of variation of stem length/diameter of the third stem was the largest but the ear-located height/plant height was relatively stable. And the ear-located height/plant height was inversely proportional to the lodging rate. The relationship of lodging with stem length/diameter and plant/stem diameter in the third stem was closest through the path analysis.The responses of the basal 1-3 stem internodes vascular bundle to the densities showed the vascular areas change was similar with the phloem area and xylem area changed smoothly. The vascular bundle of the third stem was the most sensitive to the densities which explained the main reason for the lodging often caused by the third stem. The relationship of the lodging with third stem traits in the density-tolerant varieties was closer than that in the thin-density varieties. In addition the lodging was no necessary connected with the plant type but sensitive to the stem traits.4 The relationship between the vascular bundle characteristic and water stressWater stress occurred in early stage not only affected the development of vascular bundle in the nutrient period but also produced laggard effects on vascular bundle in generative period. The vascular bundle size at jointing stage was more sensitive to the water stress occurred at seedling stage with the evident decrease in vascular bundle size of all maize varieties, but the vascular bundle structure was stable. The effects of water stress on vascular bundle of ear-located stem at tasselling stage was negative which reduced the vascular bundle area and slow down the development, but the changes of vascular bundle structure were patent. At anthesis vascular bundle area in the ear stalk was increased and its development was facilitated under the water stress.The difference of vascular bundle induced by water stress in varieties was larger than that in types. The similar changes were not found in the same types. Zhengdan958, Liaodan565 and Dongdan60 were more sensitive to the water stress, and negative impacts by water stress were apparently observed at early growing stage and positive impacts at later growing stage. Liaodan526 was not sensitive to the water stress at all growing stage, maybe it was absent in self-adjustment for the change of the environment.