| With the increase of food requirements worldwide, intercropping facilitation has been playing an important role in improving utilization efficiency of land, in increasing land areas for food and grain production. Maize-soybean relay strip intercropping system has been popular in South China recently. The rapid promotion and development of this system, with the development of soybean grain production, has effectively relieved supply and demand balance of soybean in China. Due to unwanted excessive precipitation and excessive application of nitrogen, soybean usually grows prosperously at flowering and pod stages, at expense of reproductive growth, which ultimately affects the soybean yield. Four experiments, different nitrogen application levels at different application time, different nitrogen application levels at different application ratios, different nitrogen application levels at different water application levels under relay strip intercropping system and different nitrogen application under relay strip intercropping system and sole cropping system were conducted at field and shed experimental site from 2009 to 2012 to study morphological effects, yield effect and related physiological and biochemical mechanism of excessive growth of soybean. The results showed as follow:1. Morphological traits of excessive growth soybeanExcessive N application level significantly increased plant height, stem length, first internode length, low pod height, branch numbers, LAI, the ratio of stem length and stem diameter, the ratio of stem length and node number, and the ratio of first internode length and stem diameter compared with no N application or suitable N application level. Excessive water application improved plant height, stem length, first internode length, low pod height, branch numbers, LAI, the ratio of stem length and stem diameter, the ratio of stem length and node number, and the ratio of first internode length and stem diameter. Under suitable N application level, relay intercrops increased plant height, first internode length, low pod height, the ratio of stem length and stem diameter, and the ratio of first internode length and stem diameter, but stem length, stem diameter, branch numbers and LAI were reduced under excessive N application level.2. Yield traits of excessive growth soybeanTotal pod number, pod number of main stem, pod number of branch, seed number of main stem, seed number of branch, HI, seed yield of main stem, seed yield of branch, total seed yield were increased at first then reduced sharply with increasing N application levels, the values of above were highest at suitable N application level (60 kg·hm-2), but were lowest at excessive N application level (180 kg·hm-2), significantly lower than other treatments. Excessive water application promoted pod number of main stem, pod number of branch, seed yield of main stem and seed yield of branch and total seed yield under suitable N application level, while opposite trends were found at excessive N application level. Relay intercrops decreased pod number of main stem, total pod number, seed yield of main stem and total seed yield under suitable application levels, but increased pod number of branch and seed yield of branch were at excessive N application level compared with sole crops.3. Evaluation index of excessive growth soybeanThe relations between morphological triaits and yield of soybean at different N application levels under relay intercrops or sole crops were quadratic equations. Significant quadratic correlations between plant height, stem length, branch number and yield were found in field experiment 2009, the plant height, stem length, branch number were 98.2 cm, 78.9 cm, and 4.9 plant-1 when achieving highest yield at different N application levels. Similarly found existed in field experiment 2010, stem length, low pod height, branch number, LAI were 76 cm,22.6 cm,4.5 plant-1, and 6.8 respectively when yield was of the highest. Significant quadratic correlations were also found in field and shed experiments from 2011-2012, significant quadratic correlations between stem length, first internode length, branch number, LAI and yield was found in sole cropping system, stem length, first internode length, branch number, LAI were 83.8cm,5.2cm,5.7 plant-1 and 7.5 respectively when yield was of the highest. Stem length, low pod height, branch number, LAI were found significant quadratic correlation with yield, and the extreme values of stem length, low pod height, branch number, LAI were 86.5cm,27.3cm,5.3,7.1. Different correlations existed in different experiments during 2009-2012, stem length, branch number, and LAI were the common indexes for evaluating excessive growth, the quadratic equation in sole cropping system was below:Y= 58.861+1.690X1-1.459X2-6.704X3-0.015X1*X1+0.015X2*X2+1.431X3*X3-0.010X1*X2-0.122X2*X3 (R2= 0.9938, P=0.008, F=29.91), while that in relay intercropping sytem was Y=54.248+0.633X1-1.449X2+0.023X1*X1+0.017X2*X2+0.809X3*X3-0.029X1*X2+0.153X1*X3-0.129X2 *X3(R2= 0.9917, P=0.013, F=22.20). Suitable morphology with suitable stem length, branch number and LAI was benefit to the formation of seed yield, excessive morphological growth were unfavorable for yield formation under both cropping sytems.4. Physiological and biochemical reason of excessive growth4.1 Imbalance of biomass accumulation and distributionN application significantly increased shoot biomass accumulation from V5-R7 growth stage, relative growth rate from V5-R1 stage and R1-R3 stage, biomass of main stem, biomass of branch, and total biomass. With increasing N amount, the above indexes increased, but the percentage of vegetative organ in total biomass declined firstly then increased, the percentage was highest when application excessive amount of N (180 kg·hm-2), reproductive organs showed the opposite trend. Excessive water application increased shoot biomass accumulation at R3 and R7 stages, the percentage of vegetative organ in total biomass from R3-R7 stage, and relative growth rate from R1-R3 stage. Total biomass of main stem and total shoot biomass were reduced by relay intercrops, while total biomass of branch was improved. At excessive N application level, relay intercrops caused a reduction in total shoot biomass and total biomass of main stem, but an increase in total biomass of branch.4.2 Imbalance of carbon metabolism and nitrogen metabolismTotal carbon content in leaf, soluble sugar content in vegetative organs (leaf, stem petiole) and pod, total non-structural carbohydrates (TNC) in leaf, ratio of total carbon content and total nitrogen content (C:N), ratio of TNC and amino acid (AA), and amount of xylem sap were increased firstly then dropped with increasing N application amount, and the values were lowest at excessive N application levels. With increasing N application amount, total nitrogen amount of leaf, soluble protein content, NO3--N content of vegetative organs and amino acid (AA) and NO3-N in xylem sap were increased, NH4+-N content of vegetative organs and NH4+-N in xylem sap were decreased. Relay intercrops promoted amino acid (AA), amount of xylem sap, NH4+-N in xylem sap, but reduced NO3-N in xylem sap. Under suitable N application levels, relay intercrops reduced total carbon content in leaf, soluble sugar content in vegetative organs, total non-structural carbohydrates (TNC) in leaf, ratio of C:N, ratio of TNC and AA, NH4+-N content in vegetative organs, increased total nitrogen amount of leaf, soluble protein content, NO3-N content of vegetative organs and NO3--N in xylem sap, opposite trends were found at excessive N application level in relay intercrops. Positive liner correlation between C:N ratio and yield, TNC:AA ratio and yield in soybean leaves were found in sole and relay strip intercropping systems at different nitrogen application levels. Yield increased with C: N ratio and TNC:AA ratio. The quadratic relations between morphological traits (stem length, first internode length, low pod height, branch number, LAI) and ratios of C:N ratio and TNC:AA were significant.4.3 Plant hormone metabolism was influenced by excessive N applicationWith increasing N application amount, GA3 content, IAA content and ZR content in main stem leaves and branch leaves were increased, ABA content was reduced. Relay intercrops decreased ZR content and ABA content in main stem leaves and branch leaves. GA3 content, IAA content and ZR content in main stem leaves and branch leaves were increased by relay intercrops under suitable N application levels, but were declined by relay intercrops at excessive N application level. Positive liner correlations between morphological traits (stem length, branch number, LAI) and ZR content of main stem and branch were found in sole and relay strip intercropping systems at different nitrogen application levels. Similar correlations between seed yield of main stem and ABA content of main stem at R3 stage, seed yield of branch and ABA yield of branch at R3 stage were found. |
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