[精品论文]Root-WaterNitr ogen-Uptake and Root Nitr ogen Mass of W i nter Wheat under Optimal Wa ter Condition 1.doc

精品论文root-water/nitr ogen-uptake and root nitr ogen mass of w i nter wheat under optimal wa ter condition 1jianchu shiqiang zuo*department of soil and water sciencesdepartment of soil and water sciences college of resources and environment college of resources and environment china agricultural university china agricultural universitybeijing, prc 100094beijing, prc 100044abstractroot water and nitrogen uptake are key processes for plant growth and soil water and nitrogen transport in the soil-plant system. enormous effort has been made to set up the rational root-water-uptake (rwu) and root-nitrogen-uptake (rnu) models on the basis of root length to simulate soil water and nitrogen dynamics. however, the diversities of root uptake activity among different roots or different parts of a root, and the effect of soil nitrogen level on rwu were ignored. in this study, two greenhouse experiments with winter wheat seedlings cultured in nutrient solution (exp. 1) and sand soil columns (exp. 2) were conducted to investigate the relationshipbetween root water/nitrogen uptake activity and root length, root nitrogen mass (rnm)of winter wheat under optimal water condition. results showed that the potential rwu and rnu coefficients (per unit root length) changed sensitively with root age and soil nitrogen level. nevertheless, when rnm was taken into consideration, rwuand rnu were found to be linearly correlated with rnm, and the potential rwu coefficient per unit rnm was almost a constant for both the whole root zone and different soil layers, independent of root age and soil nitrogen level but dependentlinearly on the free water surface evaporation rate. the interrelations between rwu, rnu and rnm of winter wheat under optimal water condition would be helpful to set up rational rwu and rnu models for simulating soil water and nitrogen movementin the soil-wheat system.keywords: root-water-uptake, root-nitrogen-uptake, root nitrogen mass, winter wheat.1introduction root uptake is time and space dependent, and affected by the conditions of climate (e.g., illumination, air temperature, humidity, and wind, etc.), soil (soil water, nutrient, osmotic potential, and texture, etc.), and plant (plant species, growth period, roots, etc.).the climate conditions are usually synthetically characterized with the potential transpiration rate in most root-water-uptake (rwu) and root-nitrogen-uptake1 support by the national natural science foundation of china (grant no.: 50639040, 50579072)and the specialized research fund for the doctoral program of higher education (grant no.:20040019001).-15-(rnu) models 1-6. enormous effort has been made to study the effect of soil factors on rwu and rnu, for example, the soil water and salinity reduction functions 7-9, michaelis-menten kinetic equation 10-12, incorporating the soil hydraulic parameters into the rwu model 13, and so on. as for a specific plant, roots are often taken into consideration as the main plant factor to influence root uptake function.plant root system plays a vital role in water and nutrient absorption, and the information about roots is very important for understanding mass flow in a soil-plant system. unfortunately, the uptake role played by plant roots is still incompletely understood because of its complexity 9, 14. supposing the uptake function of all roots in the root zone is identical and the maximal rwu rate is proportional to root length density (rld) under optimal water condition 3, 4, most models for rwu 5, 9, 15, 16 and rnu 10-12 were established on rld distribution. however, many other researchers reported that different roots in the root zone or various parts of a root undertook different uptake function, usually, younger roots (e.g., root tips, root hairs or fine roots) were considered to be much more active to extract water and nutrient than older roots 17-19. although the results in zuo et al. 20 partly supported the hypothesis that the maximal rwu rate is approximately proportional to rld during the seedling growth stage of winter wheat under the optimal water condition, their results were obtained over the whole root zone, not for different soil layers or roots. the effectiveness of roots and the relationship between root uptake and rld for winter wheat need further research 20. since then, it is still a challenging task to find other reasonable and easily acquired root parameter(s) to characterize root uptake activity (or the influence of roots on root uptake) and set up rational root uptake models.nitrogen, one of the most active elements in soil nutrients, takes about 17% in plant dry matter, and is an important component of some vital substances for plant growth such as protein, nucleic acid, enzyme, chlorophyll, etc., which are crucial for plant photosynthesis, respiration, metabolism, and root absorption 21, 22. on the other hand, nitride is also a perfect solute mostly existing in plant cell vacuole and cytoplasm, and plays an important role in root absorption as a particular penetrant 23. attention has been paid to the changing of root nitrogen concentration (rnc, root nitrogen mass per unit dry root weight) with growth of different plants 22, 24. similar to root uptake activity, rnc of plants also decreased with increasing root age 22. furthermore, when rnc was more than 14.4 and 1.3 mg g-1, the potential rnu coefficient per unit dry root weight (i.e., the mass of nitrogen absorbed by roots per unit dry root weight per unit time under the optimal water condition) for pisum sativum l. cv. marma and lemna gibba l. was proportional linearly to rnc, respectively 24. the results appeared to point to the possibility of a constant relation between the potential rnu coefficient (per unit dry root weight) and a fraction of the total nitrogen in the roots, and further showed that the root uptake capacity was probably dependent on the allocation of nitrogen to the root system 24.therefore, the objectives of this study were to: (1) discuss in further the relationship between root uptakeactivityandrootlengthdensity;and(2)exploretherelationshipbetween root-water/nitrogen-uptake and root nitrogen mass of winter wheat under the optimal water condition, through two greenhouse experiments of culturing winter wheat in nutrient solution and sand soil columns as well.2material and methods 2. 1root-w ater-uptake and root-ni t rogen-uptake root-water-uptake (rwu) rate s ( z, t ) (cm3 cm-3 d-1) is used to characterize the plant rwu function and often expressed as 3:s ( z, t ) = (h)s max ( z, t)(1)where z is the vertical coordinate originating from the soil surface and positive downwards (cm); t the time (d); h the soil matric potential (cm); (h) a dimensionless reduction function corresponding to water stress, and can be expressed with the measured distribution of soil matric potential h(z, t) 3, 7, 8, 9 or soil water content (z, t)25; smax(z, t) the maximal rwu rate under the optimal water condition, namely without water stress (cm3 cm-3 d-1). supposing all roots in the root zone have the same uptake function and the maximal rwu rate is proportional to root length density (rld), smax(z, t) is defined by 3, 4:s max ( z, t) = cr ld ( z, t)(2)where ld(z, t) is the rld (cm cm-3); the potential rwu coefficient cr (per unit root length) represents the精品论文maximal rwu rate per unit rld, i.e. the volume of water uptake per unit root length per unit time under the optimal water condition (cm3 cm-1 d-1).with known smax(z, t) and measured rld, the coefficients cr for different soil layers can be calculated easily using equation (2). since the rwu rate cannot be measured directly, the inverse method of zuo and zhang (2002) was applied to estimate the average rwu rate distributions during the experimental period in this study. on the other hand, by integrating equation (2) from soil surface to the rooting depth lr (cm), cr overthe root zone was calculated as follows 20:vctp(t )r =trl(t )(3)where vtp( t) is the daily potential transpiration water volume (cm3 d-1); trl( t) the total root length over the whole root zone (cm). on the assumption that the uptake function of all roots is identical, the potential rwu coefficient cr calculated using equation (2) and (3) should be a constant for the whole root zone and as well as for the different soil layers.plant roots only absorb inorganic nitrogen (i.e. nitrate and ammonium) in soils directly. root-nitrogen-uptake (rnu) is usually assumed to follow michaelis-menten kinetic equation 10, 11, 12:ni = i max (c n cmin )(4)k m + c n cminwhere in is the potential rnu coefficient per unit root length (mg cm-1 d-1), representing the mass of nitrogen absorbed by roots per unit root length per unit time under the optimal water condition; imax the maximal rnu coefficient per unit root length at high nitrogen concentrations (mg cm-1 d-1); cn the nitrogen concentration in solution (mg cm-3); km the nitrogen concentration in solution when in is one-half of imax (mg cm-3); cmin the minimal nitrogen concentration in solution when rnu ceases (mg cm-3). however, the kinetic parameters such as imax, km and cmin cannot be measured directly, either in the laboratory or in the field. they were usually optimized through the laboratory experiment of culturing the plant in the nutrient solution when the potential rnu coefficient in was estimated usingthe measured daily nitrogen uptake mass msu (mg d-1) and the total root length trl under the optimalwater condition, viz.:m su (t)i n =trl(t)(5)2.2culturing winter wheat in nutrient solution an experiment (exp. 1) with winter wheat (triticum aestivum l. cv. jingdong 8) cultured in nutrient solution was conducted in the greenhouse to investigate the relationship between root length, root nitrogen mass (rnm) and root uptake activity of winter wheat under the optimal water condition.winter wheat seeds were surface-sterilized with a 0.83 mol l-1 hydrogen peroxide solution for 0.5 h, then washed with deionized water for three changes, and soaked for 4 h in saturated caso4 solution at25 c. seeds on moist filter paper were initially germinated in darkness for 2 d under a constant temperature of 25 c. on 1 mar. 2006, seedlings were transferred into quartz sands. before 20:00 on12 mar. 2006 (11 days after planting, 11 dap), all seedlings were irrigated sufficiently with standardnutrient solution, viz. half-strength hoagland solution, which contained (mg cm-3): no-n, 0.105; so222+342+26-s, 0.032; h2po 4 -p, 0.016; mg, 0.024; k , 0.117; ca, 0.100. microelements in the solutionwere (10-6 mg cm-3): cu, 63.6; zn, 156.9; mn, 109.8; b, 10.8; fe, 1675.5; mo, 20.2 27. during this period, the conditions for winter wheat growth in the greenhouse were kept as: photosynthetic photon flux density of 500 mol m-2 s-1 over the canopies for 12 h per day (from 8:00 to 20:00); day/night airtemperature: 20/12 2 c; relative humidity of 40 5%.exp. 1 was performed using 16 basins (54 cm in length, 36 cm in width, and 15 cm in height), each covered with a foamed plastic board in which 40 holes (2 cm in diameter) were bored symmetrically. at 20:00 on 12 mar. 2006 (11 dap), all basins for 16 treatments were filled with 29 l specific nutrient solution, with nitrate concentration of 0, 0.007, 0.014, 0.028, 0.042, 0.056, 0.07, 0.084, 0.105, 0.14,0.21, 0.42, 0.105, 0.105, 0.105, 0.105 mg cm , labeled as treatment 1, 2, , and 16, respectively. the concentration for other nutrient elements (e.g., p, k, and mg) and microelements in the nutrient solution was kept the same as that in the standard nutrient solution. the ionic charge in solution was2-32+balanced with so 4 and cl(when nitrate concentration was less than 0.105 mg cm) or ca(whennitrate concentration was more than 0.105 mg cm-3). one winter wheat seedling was planted and fixed with sponge in each hole on the covered foam boards. 640 seedlings in total were used in exp. 1.treatment 9, 13, 14, 15 and 16, with the standard nutrient solution, were set up to consider the effect ofmeteorological condition on root uptake activity. the winter wheat growth condition was still kept thesame as that before 11 dap for treatment 1-12, but was adjusted a little for treatment 13-16: the photosynthetic photon flux density over the plants for treatments 13 and 14 was designed as 200 and800 mol m-2 s-1 (for 12 h per day); the daily illumination time (photosynthetic photon flux density: 500mol m-2 s-1) for treatment 15 and 16 was 8 h (6:00-14:00) and 16 h (6:00-22:00), respectively. the nutrient solution in each basin was replaced every 3 d, and fresh air was injected continuously with an air exhauster.exp. 1 lasted 24 d from 12 mar. 2006 (11 dap) to 5 apr. 2006 (35 dap). sampling work was conducted once every 6 d (totally 5 times during the experimental period), and the first sampling work was started at 20:00 on 11 dap. at each sampling time, 6 duplicate seedlings for each treatment were taken out from the basins randomly. the roots collected from each seedling were scanned with a scanner (snapscan 1236, agfa, germany), and analyzed with the winrhizo pro software package (regent instruments inc., canada) for root length. the roots and shoots were dried 48 h to a constant weight at 70 c and weighed for dry weights. nitrogen concentration of the roots and shoots (nitrogen mass per unit dry weight, mg g-1) for each seedling was measured with an element analyzer (chnso ea 1108, carlo erba, italy), respectively, which was used to estimate the daily nitrogen uptake mass msu in equation (5).standard evaporation pans, each with 20 cm in diameter, were used to measure the free water surface evaporation rate at the top of winter wheat under different illumination conditions for treatment 9, 13,14, 15, and16, respectively. water in each pan was replaced every day, and the free water surface evaporation rates for different treatments were obtained from the water loss by weighing the pans daily at 20:00.another parallel experiment (exp. 1p), also with 16 treatments corresponding to that in exp. 1, was designed to observe the transpiration rate of winter wheat under different treatment conditions. three barrels (20 cm in diameter and 15 cm in height) were used for each treatment, two for planting winter wheat seedlings (exp. 1p-et) and the other one without plant (exp. 1p-e). each barrel was also covered with a foamed plastic board, with 3 holes (2 cm in diameter) bored symmetrically, in which 3 seedlings were planted and fixed with sponge in exp. 1p-et, but only sponge was filled in exp. 1p-e. the other experimental conditions were respectively kept the same as that in exp. 1.the evapotranspiration rates for different treatments were obtained from the water loss by weighing the barrels in exp. 1p-et at 20:00 daily respectively. although the holes on the covered foam boards were filled with sponge, water loss resulted from evaporation still happened in this experiment. the evaporation rates were estimated from the water loss by weighing the barrels in exp. 1p-e at 20:00 daily. thereupon, the actual transpiration rate for each treatment was calculated by subtracting the estimated evaporation rate from the corresponding measured evapotranspiration rate.2 . 3culturing winter whea t in sa nd so il co lumns another experiment (exp. 2) with winter wheat (triticum aestivum l. cv. jingdong 8) cultured in sand soil columns was carried out to testify the research results in exp. 1. columns made of polyvinyl chloride (pvc) pipe were 53 cm high and 15 cm in diameter, and 64 columns were used in exp. 2. at the beginning of the experiment, each column was cleaved vertically into two halves, then the cleaved columns were stuck together and all the columns were sealed with pvc back covers at the bottom. the columns were packed with sand soil up to the height of 50 cm with a dry bulk density of 1.65 g cm-3. the particle size distribution of the soil included 92.33% of sand, 7.43% of silt, and 0.24% of clay. the organic matter and total nitrogen in the soil were 0.11 g kg-1 and 0.07 g kg-1, respectively. the soil water retention was described using the closed form of van genuchten 28 with the parameters as follows: saturated hydraulic conductivity ks = 60.8 cm d-1; saturated soil water content s = 0.383 cm3 cm-3; residual water content r = 0.010 cm3 cm-3; the fitted coefficients = 0.08 cm-1 and n = 1.615.the field water capacity f was chosen as 0.112 cm3 cm-3, corresponding to the soil water content at100 cm of soil matric potential 29.winter wheat seeds were germinated as the same way in exp. 1. on 16 dec. 2004, winter wheat was planted in the columns with a seed density of 7 plants per column, similar to that in the field (400-600 plants per m2). all columns were covered with 3 cm height fine quartz sand above the soil surface on19 dec. 2004 (3 dap) to reduce evaporation. until 26 dec. 2004 (10 dap), all the seedlings in the columns were irrigated with the half-strength hoagland solution to keep sufficient water and nutrient supply. two treatments, named as treatment hn and ln, were started at 20:00 on 10 dap. winterwheat for both treatments was irrigated every 6 d with different solutions: half-strength hoagland solution (high-n solution) for treatment hn,