氮有效性增加对细根解剖、形态特征和菌根侵染的影响

氮(N)有效性增加对森林生态系统结构和功能有重要影响.细根作为树木地下最为活跃的组分,其对N有效性增加的响应已成为森林生态学研究的热点.本文对N有效性增加条件下细根解剖构造、形态和菌根侵染的响应趋势及潜在机制进行了综述.N有效性增加导致细根皮层厚度、皮层层数和皮层细胞直径下降,外皮层上通道细胞数量减少,而中柱和木质部横截面积,以及导管(或管胞)直径、数量和壁厚度均增加,根解剖构造的响应与植物激素浓度变化有关;N有效性增加后菌根侵染率下降,比根长(单位根干质量的长度)在外生菌根树种中普遍下降,而在内生菌根树种中增加;根直径和组织密度的变化趋势则表现出较大的种间差异.这些个体根和树种水平上的结果对于理解森林生态系统水平上的碳和养分循环具有重要意义.最后,根据目前研究中存在的不足提出了今后的研究方向与问题.     

Phosphate Availability Alters Lateral Root Anatomy and Root Architecture of Fraxinus mandshurica Rupr. Seedlings

Plants have evolved some mechanisms to maximize the efficiency of phosphorus acquisition. Changes in root architecture are one such mechanism. When Fraxinus mandshurica Rupr. seedlings were grown under conditions of low phosphorus availability, the length of cells in the meristem zone of the lateral roots was longer, but the length of cells in the elongation and mature zones of the lateral roots was shorter,compared with seedlings grown under conditions of high phosphorus availability. The elongation rates of primary roots increased as phosphorus availability increased, but the elongation rates of the branched zones of the primary roots decreased. The number of lateral root primordia and the length of the lateral roots decreased as phosphorus availability increased. The topological index (altitude slope) decreased as phosphorus availability increased, suggesting that root architecture tended to be herringbone-like when seedlings were grown under conditions of low phosphate availability. Herringbone-like root systems exploit nutrients more efficiently, but they have higher construction costs than root systems with a branching pattem.     

Phosphate Availability Alters Lateral Root Anatomy and Root Architecture of Fraxinus mandshurica Rupr. Seedlings

Plants have evolved some mechanisms to maximize the efficiency of phosphorus acquisition.Changes in root architecture are one such mechanism. When Fraxinus mandshurica Rupr. seedlings were grown under conditions of low phosphorus availability, the length of cells in the meristem zone of the lateral roots was longer, but the length of cells in the elongation and mature zones of the lateral roots was shorter,compared with seedlings grown under conditions of high phosphorus availability. The elongation rates of primary roots increased as phosphorus availability increased, but the elongation rates of the branched zones of the primary roots decreased. The number of lateral root primordia and the length of the lateral roots decreased as phosphorus availability increased. The topological index (altitude slope) decreased as phosphorus availability increased, suggesting that root architecture tended to be herringbone-like when seedlings were grown under conditions of low phosphate availability. Herringbone-like root systems exploit nutrients more efficiently, but they have higher construction costs than root systems with a branching pattern.     

关于氮有效性影响细根生产量和周转率的四个假说

在全球变化如氮沉降及温度升高等可能导致土壤氮有效性增加的背景下,细根动态如何变化一直是陆地生态学研究中的一个重要内容.本文综述了前人提出的细根动态响应土壤N有效性提高的4个代表性假说:1)细根生产量和周转率都提高;2)细根生产量和周转率都下降;3)细根生产量下降,周转率提高;4)细根生产量提高,周转率下降.根据2000年以来以微根管方法为主测得的根系动态数据,笔者认为假说1)和假说2)得到的支持最为充分.此外,还探讨了树种、细根异质性、菌根真菌、细根采样方法和施肥方法等对上述各假说检验的影响.     

Effect of phosphorus availability on basal root shallowness in common bean

Root gravitropism may be an important element of plant response to phosphorus availability because it determines root foraging in fertile topsoil horizons, and thereby phosphorus acquisition. In this study we seek to test this hypothesis in both two dimensional paper growth pouch and three-dimensional solid media of sand and soil cultures. Five common bean (Phaseolus vulgaris L.) genotypes with contrasting adaptation to low phosphorus availability were evaluated in growth pouches over 6 days of growth, and in sand culture and soil culture over 4 weeks of growth. In all three media, phosphorus availability regulated the gravitropic response of basal roots in a genotype-dependent manner. In pouches, sand, and soil, the phosphorus-inefficient genotype DOR 364 had deeper roots with phosphorus stress, whereas the phosphorus-efficient genotype G19833 responded to phosphorus stress by producing shallower roots. Genotypes were most responsive to phosphorus stress in sand culture, where relative root allocation to the 0–3- and 3–6-cm horizons increased 50% with phosphorus stress, and varied 300% (3–6 cm) to 500% (0–3 cm) among genotypes. Our results indicate that (1) phosphorus availability regulates root gravitropic growth in both paper and solid media, (2) responses observed in young seedlings continue throughout vegetative growth, (3) the response of root gravitropism to phosphorus availability varies among genotypes, and (4) genotypic adaptation to low phosphorus availability is correlated with the ability to allocate roots to shallow soil horizons under phosphorus stress.     

Upscaling from Rhizosphere to Whole Root System: Modelling the Effects of Phospholipid Surfactants on Water and Nutrient Uptake

While the rhizosphere presents a different chemical, physical and biological environment to bulk soil, most experimental and modelling investigations of plant growth and productivity are based on bulk soil parameters. In this study, water and nutrient acquisition by wheat (Triticum aestivum L.) roots was investigated using rhizosphere- and root-system-scale modelling. The physical and chemical properties of rhizosphere soil could be influenced by phospholipid surfactants in the root mucilage. Two models were compared: a 2-dimensional (2D) Finite Element Method rhizosphere model, and a 3-dimensional (3D) root architecture model, ROOTMAP. ROOTMAP was parameterised to reproduce the results of the detailed 2D model, and was modified to include a rhizosphere soil volume. Lecithin (a phospholipid surfactant) could be exuded into the rhizosphere soil volume, decreasing soil water content and hydraulic conductivity at any given soil water potential, and decreasing phosphate adsorption to soil particles. The rhizosphere-scale modelling (5 × 5 mm² soil area, 10 mm root length, uptake over 12 h) predicted a reduction in water uptake (up to 16% at 30 kPa) and an increase in phosphate uptake (up to 4%) with lecithin exudation into the rhizosphere, but little effect on nitrate uptake, with only a small reduction in dry soil (1.6% at 200 kPa). The 3D root model reproduced the water (y = 1.013x, R² = 0.996), nitrate (y = 1x, R² = 1) and phosphate (y = 0.978x, R² = 0.998) uptake predictions of the rhizosphere model, providing confidence that a whole root system model could reproduce the dynamics simulated by a Finite Element Method rhizosphere model. The 3D root architecture model was then used to scale-up the rhizosphere dynamics, simulating the effect of lecithin exudation on water, nitrate and phosphate acquisition by a wheat root system, growing over 41 d. When applied to growing and responsive roots, lecithin exudation increased P acquisition by up to 13% in nutrient-rich, and 49% in relatively nutrient-poor soil. A comparison of wheat (Triticum aestivum L.) and lupin (Lupinus angustifolius L.) root architectures, suggested an interaction between the P acquisition benefit of rhizosphere lecithin and root architecture, with the more highly-branched wheat root structure acquiring relatively more P in the presence of lecithin than the sparsely-branched lupin root system.     

Soil fertility and fine root dynamics in response to 4 years of nutrient (N,P, K) fertilization in a lowland tropical moist forest,Panama

The question of how tropical trees cope with infertile soils has been challenging to address, in part, because fine root dynamics must be studied in situ. We used annual fertilization with nitrogen (N as urea, 12.5 g N m^?2 year^?1), phosphorus (P as superphosphate, 5 g P m^?2 year^?1) and potassium (K as KCl, 5 g K m^?2 year^?1) within 38 ha of old‐growth lowland tropical moist forest in Panama and examined fine root dynamics with minirhizotron images. We expected that added P, above all, would (i) decrease fine root biomass but, (ii) have no impact on fine root turnover. Soil in the study area was moderately acidic (pH = 5.28), had moderate concentrations of exchangeable base cations (13.4 cmol kg^?1), low concentrations of Bray‐extractable phosphate (PO₄ = 2.2 mg kg^?1), and modest concentrations of KCl‐extractable nitrate (NO₃ = 5.0 mg kg^?1) and KCl‐extractable ammonium (NH₄ = 15.5 mg kg^?1). Added N increased concentrations of KCl‐extractable NO₃ and acidified the soil by one pH unit. Added P increased concentrations of Bray‐extractable PO₄ and P in the labile fraction. Concentrations of exchangeable K were elevated in K addition plots but reduced by N additions. Fine root dynamics responded to added K rather than added P. After 2 years, added K decreased fine root biomass from 330 to 275 g m^?2. The turnover coefficient of fine roots <1 mm diameter ranged from 2.6 to 4.4 per year, and the largest values occurred in plots with added K. This study supported the view that biomass and dynamics of fine roots respond to soil nutrient availability in species‐rich, lowland tropical moist forest. However, K rather than P elicited root responses. Fine roots smaller than 1 mm have a short lifetime (<140 days), and control of fine root production by nutrient availability in tropical forests deserves more study.     

New insights to lateral rooting: Differential responses to heterogeneous nitrogen availability among maize root types

Historical domestication and the "Green revolution" have both contributed to the evolution of modern, high-performance crops. Together with increased irrigation and application of chemical fertilizers, these efforts have generated sufficient food for the growing global population. Root architecture, and in particular root branching, plays an important role in the acquisition of water and nutrients, plant performance, and crop yield. Better understanding of root growth and responses to the belowground environment could contribute to overcoming the challenges faced by agriculture today. Manipulating the abilities of crop root systems to explore and exploit the soil environment could enable plants to make the most of soil resources, increase stress tolerance and improve grain yields, while simultaneously reducing environmental degradation. In this article it is noted that the control of root branching, and the responses of root architecture to nitrate availability, differ between root types and between plant species. Since the control of root branching depends upon both plant species and root type, further work is urgently required to determine the appropriate genes to manipulate to improve resource acquisition by specific crops.     

磷空间有效性对拟南芥根形态构型的影响

磷空间有效性显著影响拟南芥主、侧根生长。在均一的磷处理下,极度磷胁迫或过量供磷均会导致拟南芥主根变短和侧根密度降低。在分层的磷处理下,上层高磷下层低磷能明显促进主根伸长生长,提高侧根在高磷区域的密度,说明植物根系在下层低磷区感受到磷胁迫信号后,可促进上层高磷区侧根的形成和发育。     

柠条细根的分布和动态及其与土壤资源有效性的关系

受土壤资源有效性时空异质性的影响,植物细根会表现出明显的垂直分布和季节变化特征。揭示这些特征对认识细根的养分和水分吸收规律,预测C在地下的分配特点具有重要意义。本研究运用Minirhizotron技术对晋西黄土丘陵区30年生柠条（Caragana korshinskii Kom.）人工林0-100cm土层深度范围内细根的密度( FRD, N cm-2)、根长密度( FRLD, mm cm-2)、平均直径(FRDi, mm)和根表面积(FRSA, mm2 cm-2)的垂直分布特征和季节动态进行了一个生长季的观测,并分析了这些参数与土壤温度、水分和有效氮之间的关系。结果表明：(1)FRD、FRLD和FRSA均表现出随土层深度增加而先增大后减小的趋势,以40-60cm土层之值最大(分别占总数的34.3%、35.5%和37.3%);而FRDi随土层深度增加而减小,其最大值为0.31970.0231mm,最小值为0.28840.0109 mm;(2)受土壤资源有效性季节变化的影响,FRD、FRLD和FRSA在不同土层(除0-20cm外)表现出相似的季节动态,即随季节变化而先增大后减小,春季小(分别为0.2204 N cm-2,1.8482 mm cm-2,2.2647 mm2 cm-2)而秋季大(分别为0.5316 N cm-2,4.4046mm cm-2,4.3007mm2 cm-2);FRDi则表现由粗逐渐变细的过程,春季最粗(0.3659mm)而秋季最细(0.2712mm);(3) 各细根参数与土壤温度、水分和有效氮在各土层存在不同程度的相关性。从简单相关分析来看,细根的季节性变化主要受土壤温度和水分的影响,有效氮的影响不明显。FRD、FRLD和FRSA在0-20cm土层主要受土壤水分影响(r=-0.729--0.914, p<0.05),而在20-100cm土层则主要受土壤温度的影响,且显著性随土层加深而增加(r=-0.028-0.832, p<0.05)。各土层细根与土壤有效资源间的相关性反映了细根功能的季节性差异。综合分析表明,各细根参数季节变化的54.0%-98.6%是由土壤温度和水分的交互作用而引起。