Relationship of functional traits and site conditions with NO−3 uptake capacity of tree root
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摘要:目的
养分是干旱瘠薄立地中树木生长的重要限制因素，树木对干旱瘠薄立地中养分的吸收利用方式决定其生态适应对策。本研究通过野外原位测定，探究根系氮吸收动力学及其与根系形态性状间的耦合关系，为揭示林木根系在干旱瘠薄环境中的生理功能奠定基础。
方法以北京市百望山森林公园内的山桃、栓皮栎和元宝枫为对象，采用以硝态氮（NO−3）为唯一氮源的改良型梯度霍格兰营养液，在一般程度的干旱瘠薄立地和极端程度的干旱瘠薄立地中分别开展野外原位测定根系NO−3 吸收动力学研究，并通过Pearson相关性分析和通径分析研究根系NO−3 吸收速率与根系功能性状间的关系。
结果树种、立地条件和两者的交互效应对NO−3 吸收速率和动力学参数均有显著或极显著的影响。3个树种对氮的亲和性均较高，元宝枫根系对NO−3 的吸收速率偏低，在2种立地条件中均显著低于山桃和栓皮栎。生长在更加干旱瘠薄立地中的速生树种对NO−3的吸收具有补偿性。根系功能性状与NO−3的吸收速率有很好的耦合关系，比根长和比根表面积对根系NO−3吸收速率有显著正效应，根直径和根组织密度则相反。分支结构性状中，分支强度和链接数对根系NO−3吸收速率的作用较弱。
结论生长速度较快的山桃和栓皮栎根系的NO−3 吸收速率在极端干旱瘠薄立地胁迫下显著降低，元宝枫则相反。采取提高最大吸收速率和降低氮亲和力的“速度策略”可保障速生树种根系对NO−3的补偿性吸收。高比根长、高比根表面积、低根直径和低根组织密度的形态性状组合，可有效提高根系在干旱瘠薄立地中对NO−3 的吸收速率。
Abstract:ObjectiveNutrient is an important limiting factor for tree growth in drought and barren sites, and the way trees absorbing and using nutrients in drought and barren sites determines their ecological adaptation strategies. In this paper, the kinetics of root nitrogen uptake and the coupling relationship between root morphological traits were measured in situ in the field, which laid a foundation for revealing the physiological function of trees in drought and barren environments.
MethodWe took Prunus davidiana, Acer truncatum and Quercus variabilis in Baiwangshan Forest Park of Beijing as the research objectives. We used modified Hogland nutrient solution with NO−3 concentration gradients to carry out in-situ measurement of root NO−3 uptake kinetics in generally and extremely drought and barren site conditions, respectively. The relationship between root NO−3 uptake rate and root functional traits was analyzed by Pearson correlation and path analysis.
ResultTree species, site conditions and the interaction of above two factors all had an significant or extremely significant effect on root NO−3 uptake rate and kinetic parameters. Three tree species all had high nitrogen affinity. The uptake rate of NO−3 in the root of A. truncatum was lower, and it was significantly lower than that of P. davidiana and Q. variabilis under the above two site conditions. Under the site conditions of more drought and barren, fast growing tree species had compensatory absorption of NO−3. Root functional traits and the uptake rate of NO−3 had a good coupling relationship. The result showed that the morphological traits of specific root length (SRL) and specific root surface area (SRA) had significantly positive effects on NO−3 uptake rate of roots, while root diameter (RD) and root tissue density (RTD) had negative effects. In terms of branching structure traits, branching intensity and number of links had weak effects on NO−3 uptake rate.
ConclusionThe NO−3 uptake rate of P. davidiana and Q. variabilis with a faster growth rate decreases significantly under the extremely drought and barren site stress, while A. truncatum is the opposite. The “speed strategy” of increasing the maximum absorption rate and reducing nitrogen affinity ensures the compensatory absorption of NO−3 by the roots of fast-growing tree species. The combination of morphological traits with higher SRL, higher SRA, lower RD and lower RTD can effectively improve the uptake rate of NO−3 by roots in drought and barren sites.
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Keywords:
- in situ test /
- site conditions /
- interspecific difference /
- NO− 3 uptake /
- functional trait /
- uptake kinetics
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图 1 不同立地条件中山桃、元宝枫、栓皮栎根系的NO−3 吸收差异
Site A. 一般程度的干旱瘠薄立地；Site B. 极端程度的干旱瘠薄立地；Pd. 山桃；At. 元宝枫；Qv. 栓皮栎。大写字母表示同一树种不同NO−3浓度处理之间的差异显著性，小写字母表示同一NO−3浓度处理不同树种之间的差异显著性。Site A, a generally drought and barren site; Site B, an extremely drought and barren site; Pd, Prunus davidiana; At, Acer truncatum; Qv, Quercus variabilis. Capital letters indicate significant difference among different NO−3 concentration treatments of the same tree species, and small letters indicate significant difference among varied tree species under the same NO−3 concentration treatment.
Figure 1. Difference of NO−3 uptake of Prunus davidiana, Acer truncatum and Quercus variabilis under different site conditions
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图 2 根系功能性状与NO−3 吸收速率的相关性分析
V. NO−3 吸收速率 ；RD. 平均根系直径；SRL. 比根长 ；SRA. 比根表面积；RTD. 根组织密度； RDMC. 根干物质含量；BI. 根分支强度；Tips.根尖数；NL. 链接数 ；红色代表负相关关系，蓝色代表正相关关系。***代表p < 0.001，**代表p < 0.01，*代表p < 0.05。下同。V, NO−3 uptake rate; RD, average root diameter; SRL, specific root length; SRA, specific root area; RTD, root tissue density; RDMC, root dry matter content; BI, branching intensity; NL, number of links. Red represents negative correlation, and blue represents positive correlation. *** means p < 0.001; ** means p < 0.01; * means p < 0.05. The same below.
Figure 2. Correlation analysis of root functional traits and NO−3 uptake rate
表 1 树种、立地条件及NO−3 梯度浓度处理对根系NO−3 吸收影响的方差分析
Table 1 ANOVA results for compound effects of tree species, site conditions and NO−3 concentration gradient on root NO−3 uptake rate
变异来源
Source of variationdf NO−3 吸收速率 NO−3 uptake
rate （V）/（mmol·g−1·h−1）F p 树种 Tree species (Sp) 2 137.441 < 0.001 立地 Site (Si) 1 27.182 < 0.001 NO−3 浓度
NO−3 concentration (Con)4 15.086 < 0.001 Sp × Si 2 10.440 < 0.001 Sp × Con 8 3.562 0.002 Si × Con 4 0.293 0.881 Sp × Si × Con 8 1.343 0.241 
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表 2 树种、立地条件及其交互效应对NO−3 吸收动力学参数影响的方差分析
Table 2 ANOVA on the tree species, site conditions and their interaction on kinetic parameters of NO−3 uptake
动力学参数
Kinetic parameter树种 Tree species 立地 Site 树种 × 立地 Tree species × site F p F p F p Vmax 542.3 < 0.001 5.5 0.038 189.9 < 0.001 Km 994.2 < 0.001 1120.6 < 0.001 1701.6 < 0.001 Cmin 1328.0 < 0.001 1697.0 < 0.001 2556.0 < 0.001 α 785.7 < 0.001 303.7 < 0.001 119.8 < 0.001 注：Vmax.最大吸收速率；Km.米氏常数；Cmin.根系养分离子内流与外溢浓度相等时（V = 0）的介质浓度，α为离子流入根系的速度。Notes: Vmax, maximum uptake rate; Km, Michaelis constant; Cmin, medium concentration at which nutrient ions inflo and outflow from roots are equal (V = 0); α, velocity of ions flowing into roots.
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表 3 不同立地条件中山桃、元宝枫、栓皮栎根系NO−3 吸收的动力学参数及方程
Table 3 Kinetic parameters and equations of NO−3 uptake of P. davidiana, A. truncatum and Q. variabilis roots under different site conditions
动力学参数
Kinetic parameterPd Qv At Site A Site B Site A Site B Site A Site B Vmax/（mmol·g−1·h−1） 0.134 ± 0.002Aa 0.077 ± 0.004Ba 0.070 ± 0.005Ba 0.144 ± 0.005Aa 0.010 ± 0.000Ab 0.012 ± 0.002Ab Km/（mmol·L−1） 0.346 ± 0.009Ab 0.291 ± 0.005Ba 0.214 ± 0.010Bc 1.239 ± 0.012Aa 0.340 ± 0.017Ab 0.259 ± 0.009Bb Cmin/（mmol·L−1） 1.236 ± 0.017Ab 1.007 ± 0.005Ba 0.708 ± 0.003Bb 4.216 ± 0.066Aa 1.227 ± 0.008Ab 0.991 ± 0.026Ba α/（L·g−1·h−1） 0.386 ± 0.011Aa 0.266 ± 0.004Bb 0.326 ± 0.010Aa 0.116 ± 0.009Bb 0.030 ± 0.004Bc 0.046 ± 0.003Ac 方程 Equation 1/V = 3.057/C + 5.020 1/V = 5.892/C + 6.056 1/V = 4.312/C + 9.663 1/V = 16.925/C − 1.187 1/V = 35.082/C + 74.124 1/V = 16.387/C + 73.634 R2 0.987 0.956 0.860 0.996 0.980 0.935 注：C代表营养液中NO−3 浓度，V代表NO−3 吸收速率。大写字母表示同一树种内不同立地条件之间的差异显著性，小写字母表示同一立地条件中不同树种之间的差异显著性。Notes: C represents NO−3 concentration. V represents NO−3 uptake rate. Capital letters indicate significant difference among varied site conditions of the same tree species, and small letters indicate significant difference among varied tree species under the same site conditions.
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表 4 根系功能性状与根系NO−3 吸收速率的通径分析
Table 4 Path analysis of root functional traits and NO−3 uptake rate
性状
Trait简单相关系数
Simple correlation
coefficient直接通径系数
Direct path
coefficient显著性
Significance间接通径系数 Indirect path coefficient 决策系数
Decision
coefficientRD SRL SRA RTD RDMC BI Tips NL 合计
TotalRD −0.352 0.018 0.873 −0.230 −0.154 −0.022 0.011 0.011 0.015 0.000 −0.369 −0.013 SRL 0.654 0.412 0.016 −0.010 0.322 −0.091 0.023 −0.002 0.001 0.000 0.243 0.369 SRA 0.610 0.387 0.032 −0.007 0.342 −0.162 0.048 0.001 0.000 0.000 0.222 0.322 RTD −0.129 0.298 0.017 −0.001 −0.126 −0.210 −0.067 −0.001 −0.023 0.001 −0.427 −0.166 RDMC −0.182 −0.117 0.295 −0.002 −0.080 −0.158 0.171 −0.016 0.022 −0.002 −0.065 0.029 BI 0.027 0.057 0.606 0.003 −0.015 0.007 −0.007 0.032 −0.055 0.005 −0.030 0.000 Tips 0.012 −0.106 0.597 −0.003 −0.002 −0.001 0.064 0.024 0.030 0.007 0.118 −0.014 NL −0.001 0.008 0.969 −0.001 −0.014 0.005 0.031 0.029 0.036 −0.095 −0.009 0.000
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