植物营养与肥料学报   2017, Vol. 23  Issue (4): 1054-1064 
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磷与水分互作的根土界面效应及其高效利用机制研究进展
王昕1, 李海港1, 程凌云1, 王宝兰2, 申建波1    
1. 中国农业大学资源与环境学院植物营养系,植物–土壤相互作用教育部重点实验室,北京 100193;
2. 中国科学院植物研究所,北京 100093
摘要: 【目的】 磷与水分利用率低是制约作物生产的重要因子。磷必须在水分的作用下通过根土界面才能被作物吸收利用,磷和水分在根土界面的互作效应是影响其高效利用的关键环节。本文以根际为核心,重点综述了磷与水分在根土界面的互作机制,并剖析了通过强化根土界面磷与水分的协同,提高农田水肥资源利用效率的根际调控途径。【主要进展】 根系的形态和生理变化深刻影响磷和水分的有效性,而根系生长和根际过程依赖于植物的营养和水分供应状况,作物根层适宜的水分和养分供应水平能最大化根系和根际过程的效率,从而促进作物对磷与水资源的高效利用。作物根系除了能对根层土壤中磷和水分的系统供应做出响应外,也对局部磷和水分的变化产生形态和生理上的反应。根系响应磷和水分的表型可塑性与植物激素的调控作用密切相关。ABA、乙烯、NO 均参与磷和水分互作的调控过程,质外体 pH 在调控植物抵抗水分胁迫过程中具有重要作用,并与植物的营养状况密切相关。【展望】 深入理解根土界面水与磷互作的协同过程及其调控机制是提高集约化作物体系水分和磷利用效率的关键。未来的研究方向与重点包括:进一步揭示磷和水分互作与激素信号途径之间的关系,探明农田生态系统中磷与水分互作的根土界面效应及其高效利用的协同机制,建立不同种植条件下水肥资源高效利用的根际调控途径,为通过根系、根际的定向调控,发挥其生物学潜力,提高集约化农田水肥资源的利用效率提供科学依据。
关键词:      水分     作物     根土界面     根际互作     利用效率    
Advances of root-soil interface effect of phosphorus and water interaction and mechanisms of their efficient use
WANG Xin1, LI Hai-gang1, CHENG Ling-yun1, WANG Bao-lan2, SHEN Jian-bo1    
1. College of Resource and Environmental Sciences, China Agricultural University, Beijing 100193, China;
2. Institute of Botany, Chinese Academy of Science, Beijing 100093, China
Abstract: 【Objectives】 Low use efficiencies of phosphorus and water are key limiting factors in intensive farming system. Only under the driving flow by soil water, phosphorus can be absorbed by plants mainly via the root-soil interface. Root-soil interface effects of phosphorus and water interactions are the key points in their efficient use mechanism by crop. This review summarizes recent advances on the mechanisms of interactions of phosphorus and water in root-soil interface and puts forward the rhizosphere management strategies of improving phosphorus and water use efficiency through enhancing their synergy in the plant-soil system. 【Advances】 The biological availability of phosphorus and water can be highly affected by root morphological and physiological plasticity, and in turn, root development and rhizosphere physiological processes are also dependent on phosphorus and water supply status. Under an appropriate intensity of water and phosphorus supply, root and rhizosphere process efficiency can be maximized to increase the resource use efficiency. Plant roots can highly respond to not only integral supply strength of phosphorus and water in root zone, but also heterogeneous spatio-temporal distribution of phosphorus and water with changing root morphological and physiological reactions. Phytohormones are closely related to the root plasticity under different phosphorus and water supply. ABA, ethylene, and NO are involved in the regulation processes of plant responses to phosphorus and water supply. Apoplast pH could play an important role in plant regulation processes to cope with water and nutrient deficiency. 【Prospects】 Better understanding of synergy effects and regulating mechanisms of phosphorus and water interactions in the root-soil interface is critical to increase use efficiency of phosphorus and water in intensive farming system. Hence, future research directions and emphases are proposed: exploring the mechanism of phosphorus and water interactions and the relationships with phytohormone in the plants, clarifying the interface effects of phosphorus and water interaction processes and their synergy mechanisms in the rhizosphere, and establishing effective rhizosphere management strategies and pathways for increasing phosphorus and water use efficiency in various cropping systems. These advances will provide scientific basis for improving use efficiency and coupling of water and nutrients in intensive crop production through engineering crop root and rhizosphere interaction processes.
Key words: phosphorus     water     crop     root-soil interface     rhizosphere interaction     use efficiency    

磷是植物生长发育所必需的大量营养元素之一,是作物生长的重要物质基础。与氮相比,磷是不可再生资源,研究表明磷矿资源的短缺日益突出[1]。磷在土壤中移动性差,扩散速率低,且极易被固定,导致其生物有效性大大降低。我国的磷肥用量超过了世界总量的 37%,主要粮食作物磷肥的利用率平均只有 15%~20%。养分利用效率低不仅造成资源浪费和环境污染,也对粮食安全构成巨大的挑战。

水分是地球表面重要的生理生态因子。除直接影响植物生长外,还影响养分利用效率。适宜的水分条件不仅能促进土壤磷向根表的迁移,而且还能通过强大的“蒸腾流”提高养分的吸收和运输速率,从而提高磷的利用效率。我国局部地区的季节性缺水大大降低了作物对养分的利用效率,造成干旱减产。因此,探索调控植物营养的新途径,提高养分水分利用效率已经成为新形势下我国农业生产的重大需求。

根土界面 (根际) 是作物–土壤相互作用最剧烈的区域,是养分和水分从土壤进入作物系统的必经门户[2]。磷与水分在根–土界面存在强烈的共济效应:一方面,根际磷的活化吸收显著影响根系的形态和生理特性,从而强化对水分的吸收能力;另一方面,水分供应显著影响磷的有效性和吸收利用效率。因此,充分理解根土界面中磷与水分的互作效应,对于通过根际调控挖掘作物高效利用磷和水分的生物学潜力,具有重要的理论研究价值和生产实践意义。本文针对根土界面中的磷与水分互作效应进行系统综述,进一步剖析集约化农作系统养分水分高效利用的作物–土壤互作机制,为强化养分水分协同效应,提高土壤磷和水分利用效率提供科学依据。

1 磷和水分与根系形态和生理可塑性的相互作用 1.1 根系形态和生理变化影响养分和水分的生物有效性

磷在土壤中易被吸附和固定,难以移动,作物必须通过根系形态变化,扩展吸收范围以主动获取磷;或者磷必须借助于水分的扩散作用迁移到根土界面被根系吸收。根系的形态变化和生理过程深刻影响磷的生物有效性,如植物细根比例、侧根和根毛密度及长度的增加等形态学变化[35],扩大了根系与土壤颗粒的接触面积,使根系主导的根际范围增加,帮助植物获取更多的磷[6]。土壤中的磷和水分是异质性分布的,从垂直方向看,表层土壤中含有更多的磷,而水分主要集中在土壤的底土层[7]。研究表明,浅层土壤中侧根发达、细根广泛分布的根系构型,更有利于植物对土壤表层磷的吸收[7],也有利于植物充分利用较少的降水资源[11];同时,少量深层根系对于植物吸收水分至关重要,特别是在干旱条件下,植物可利用的水大部分存在于深层土壤中,根系深度的增加显著提高了植物对水分的吸收[811]。植物在进化过程中形成了“表层细根吸磷、深层主根吸水”的根构型来提高养分水分吸收效率。

另一方面,植物根系生理的适应性改变对磷和水分的吸收同样重要。植物在生长过程中可将 5%~25%的光合产物以根分泌物的形式通过根系分泌到根际环境中,目的是为根系生长创造一个良好的环境条件,以高效获取养分资源[6]。植物根系分泌的质子、有机酸、酸性磷酸酶可以显著提高根际土壤中磷的有效性[1214],此外,水溶性根分泌物的释放提高了根与土的附着程度,根系分泌的粘胶物质可以粘结土壤颗粒,在植物根际形成根鞘[15],显著改善根际土壤的物理结构和水分状况[1617],提高根系贮水能力,利于植物的水分吸收[18]。研究发现,根际土壤水分含量明显高于非根际土壤[1920],当根系分泌的粘胶物质浓度在 6 × 10–5 g/cm2 时,增强了水分流向根际的过程,提高了根际持水性[21],表明根系分泌物对于保持根际土壤水分有重要作用。在干旱胁迫下,植物释放的阴离子[22]、有机酸和酶类[23]增加,维持甚至提高了植物对根际磷的溶解和吸收,表明水分诱导的根系生理变化同时影响了根系对水分和磷的吸收。

理想的根系构型能显著提高水分和养分的利用效率[2426]。表层土壤含有大量的磷,高量施磷进一步加剧了磷在土壤剖面的异质性分布;而土壤水分则主要分布在底层土壤。理想的根系构型具有三个特点:1) 表层土壤大量分布根系,以高效获取土壤磷资源;2) 具有发达的深层根系,以获取深层的水分资源;3) 根系构型能够高效协同空间异质性的水分和养分资源。因此,根系的构型与土壤剖面水分养分的空间匹配对于提高资源的利用效率具有重要的作用[2425]

1.2 磷和水分对根系形态和生理变化的影响

植物根系的形态和生理变化深刻影响养分和水分的有效性,而根系生长与生理活性主导的根际过程高度依赖于植物的营养状况[24]。低磷条件下,植物的主根生长受到抑制,而侧根发育受到诱导,侧根密度和长度显著升高[2729],这与主根分生区细胞分裂速率降低及伸长区的细胞生长受到抑制有关[3032]。供磷强度与植物根毛的发育也密切相关。低磷条件下,拟南芥的根毛长度可达到正常供磷条件下的 3 倍,而过量供磷会完全抑制其根毛的生长[4]。低磷可以诱导白羽扇豆形成大量密集的排根,排根不仅极大地增加了根系吸收养分水分的面积,同时分泌柠檬酸活化被土壤固定的难溶性磷,提高磷的吸收利用效率[3334],在增加外界环境磷的供应时,排根的数量显著减少[35]。进一步的研究表明,只有当植株地上部磷浓度低于 2~3 mg/g 这一临界水平时,才能诱导排根的形成[3637]。最近的研究表明,当土壤有效磷 (Olsen-P) 供应水平达到 20 mg/kg 时,小麦可达到高产,同时根系保持了较高的生物学效率 (磷的吸收能力)[38]。当降低磷的供应强度时,在一定程度上可以增强根系对磷的吸收效率,但小麦产量显著降低。相反,继续增加磷肥投入,产量不再增加,根分泌物的释放速率显著被抑制,根际过程的强度降低,根系对磷的利用效率也大大下降[38]。类似地,玉米根层养分浓度控制在临界水平时,玉米主根与侧根发达,显著提高了土壤养分水分的空间有效性;过量施肥反而抑制侧根发育,养分和水分的空间有效性和利用效率显著下降[24]。这表明改变根际磷的供应水平能显著调控根系吸收能力及根际强度,从而提高作物对土壤养分资源的捕获和利用能力[6, 39]

土壤水分的供应状况显著影响根系的生长发育以及根分泌物的释放,从而影响磷和水分的吸收效率。例如,即使在集约化高投入条件下,由于北方早春的气温较低,土壤磷的有效性显著下降,导致石灰性土壤上春玉米季节性缺磷相当普遍,严重影响春玉米根系发育和幼苗生长,春季干旱缺水进一步加剧了土壤磷的缺乏;当作物遭受季节性干旱时,大量光合作用合成的碳水化合物被分配到地下,根冠比增加,主根的下扎能力增强,根分泌物的释放量增加,提高作物对深层土壤养分水分的获取效率。水分供应状况还显著影响植物侧根的生长发育,侧根增生是植物根系扩展对水分养分吸收范围、响应土壤环境变化的重要方面。在遭遇短暂水分胁迫时,大麦和玉米的侧根早期发育受到显著抑制[40]。这表明土壤水分是根系生长发育的重要限制因子,进而影响植物对根际水分和养分的吸收效率[24]

以上分析表明,土壤养分水分的供应强度低时,根系生长和根际效应得不到充分发挥;而土壤养分供应过高时根际效应又会受到抑制,只有在供应强度处于适宜水平时,根系和根际过程的效率才能达到最大[6, 2425, 39],水分和养分协同强化根系形态和生理作用的叠加效应促进了养分和水分资源的高效利用。这为通过外部养分和水分调控强化根际效应,最大化作物根系对土壤养分和水分的利用潜力,减少外部水肥资源的投入提供了重要依据,是当前我国集约化作物体系从最大化生物学潜力的角度实现“减肥增效”的关键所在。

1.3 植物根系吸收磷和水分过程的协同作用

在田间土壤条件下,磷和水分在土壤中存在明显的互作关系,这种互作效应与根土界面效应交织在一起,对于磷和水分转化及有效性产生重要影响。由于其复杂的化学属性,磷在土壤中有多种形态 (如土壤溶液中的可溶性磷、土壤颗粒上的吸附磷、被固定的磷、有机磷等),并随着土壤环境的改变,不断发生着形态间的转化[6]。植物直接吸收的磷主要是无机形态的正磷酸盐 (Pi)[41],而由于其易被吸附、固定,在土壤中移动性差,很难通过质流和截获的方式被根系大量吸收。有研究表明,根系通过质流和截获吸收的磷很少,仅占植物总磷需求的不到 5%[4243]。所以,扩散是植物从土壤中获取磷的重要途径,而土壤含水量是影响磷扩散过程的重要因素。可溶性磷通常吸附在土壤颗粒表面,土壤微细孔隙中的水膜是其扩散的介质,一旦水膜中断,可溶性磷便无法在土壤颗粒间进行扩散。研究表明,根土界面水分下降 0.1 个单位 (体积含水量从 0.3 降低到 0.2 cm3/cm3),磷的扩散速率下降 98%[44]。用32P 的标记试验表明,缺水胁迫处理降低了番茄对磷的吸收速率[45]。根土界面的水分含量不仅影响了土壤磷的扩散速率,还在一定程度上决定了根系吸收土壤磷的范围,即根际的范围。对大豆的研究结果表明,干旱条件下根系对磷的吸收受到抑制,而复水后根系的磷吸收速率显著上升,这很大程度上与复水后土壤磷的扩散速率和范围增加有关[46]。综上所述,根土界面上磷的生物有效性受到根际水分含量的深刻影响。

田间的研究表明,供磷可以提高干旱条件下作物根系的水分利用效率 (WUE)[4749],特别是在保水性较差的砂质土壤上[50]。磷对水分利用效率的促进作用主要与其对光合作用、蒸腾[49]及根系水导[51]的影响有关。根系水导是表征根系吸收运输水分能力的重要指标,水导主要由定位于细胞膜上的水通道蛋白的活性决定。矿质养分的缺乏显著抑制根系水导。研究表明,缺磷导致小麦根系水导显著降低,气孔导度下降,而对缺磷的小麦恢复供磷后,被抑制的根系水导可以恢复正常[51]。干旱胁迫下的高粱在恢复供水后根系水导可恢复正常,且与缺磷植株相比,供磷植株根系水导恢复的速率更快[52]。植物缺磷显著降低根系水导,其原因可能是缺磷抑制了水通道蛋白的活性。有研究表明,外源添加水通道蛋白抑制剂 Hg 显著降低正常供磷小麦的根系水导,将 Hg 清除后,根系水导可以恢复;而缺磷小麦的根系水导与被 Hg 抑制的供磷小麦的根系水导接近,且添加或清除 Hg 对缺磷小麦的根系水导没有影响,暗示缺磷可能抑制了 Hg 敏感型水通道蛋白活性[53]。在缺水条件下提高叶片磷浓度能够提高植物的水分利用率和植物的耐旱性[54]。这表明根系对水分的吸收和运输受到植株磷营养状况的调节。

由此可见,根系对磷和水分的吸收过程存在重要的相互作用:水分可以扩展根际范围,增加根土界面磷的生物有效性;植物的磷营养状况可以调节根系吸收运输水分的能力。通过理解根土界面磷与水吸收过程的协同机制,可为实现农业生产中“以水促磷,以磷节水”的目标提供重要的理论依据。

2 植物根系对异质性水分与养分的响应 2.1 根系对异质性水分或养分供应的响应

土壤中水分和养分通常是异质性分布的,作物根系除了对整体养分和水分的供应做出形态和生理反应外,也可对局部养分水分变化做出响应。土壤水分有效性在剖面的水平和垂直方向上以厘米级的尺度变化[55],植物根系对水分的吸收也会进一步造成土壤水分的异质性分布[5657]。通常植物根系被认为具有向水性反应。对拟南芥的研究表明根系向水性的感受中心是根冠,但其机制与向重力性相互独立[58]。最近的研究表明,根系具有明显的向水性,当根系同时接触干燥和湿润表面时,侧根和根毛会优先朝湿润的方向生长[59]。这个过程有别于传统的水分胁迫响应,其与植物体内的 ABA 信号过程相对独立:局部较高的水分含量调控了生长素的合成和分布,通过色氨酸转氨酶及生长素流出蛋白 (PIN) 介导的生长素途径,促进了局部湿润区域的侧根发育[59]。尽管如此,根系向水性的分子生理与生态学意义仍需进一步探索[60]

部分根区干旱 (PRD) 是 Dry 等首先提出的节水灌溉理论[62],对部分根区进行适度干旱胁迫,处于干旱区的根系通过 ABA 信号系统将干旱信息传递到地上部使部分气孔关闭,减少奢侈蒸腾,有利于作物保存水分[6163],同时维持作物产量与品质不降低[61, 6465],有效提高了水分利用效率。提水作用是由于夜间植物气孔关闭,蒸腾拉力降低,水分从根中向周围干燥土壤排出的现象[62],通常根系从含水量较高的深层土壤吸收水分,而在较为干旱的浅层土壤排出水分,这种水分的垂直运动被称为提水作用。提水作用具有重要的生态价值,包括通过增强活化过程来提高表层土壤养分的有效性[62],延长根系寿命[66],保护菌根真菌等根际微生物[67],影响相邻植物的水分养分状况[68]等。相对干旱条件下,根系的提水作用对提高土壤表层的养分水分利用率有重要的影响。

由于磷的扩散速率低、易被固定及土壤生化过程等原因,土壤中的磷通常呈现异质性斑块状 (patches) 分布。为了充分获取土壤中异质性分布的磷,植物根系会优先分布到能大量获得养分资源的区域 (磷富集区)。Drew 对大麦的经典研究表明,根系在局部供应磷的区域大量增生,根长、根干重显著增加,一级、二级侧根密度显著增加[69]。对白羽扇豆的研究表明,局部供应富含磷的有机物质能显著影响根系的分布,有机质矿化释放的磷是刺激局部根系增生的主要原因[70]。局部供磷不仅诱导了大量根系的增生,磷富集区中的根系的生理过程也显著增强,一方面启动快速的养分吸收系统,提高根系对磷的吸收速率[71];另一方面局部供磷刺激了根系有机酸、酸性磷酸酶的分泌,强化了根系生理过程,利于根系对富集区中磷的活化。我们前期的研究表明,在田间条件下局部根系供应磷和铵态氮不仅增强了玉米侧根的发育,根长密度和细根的比例显著增加,而且还诱导根际的强烈酸化,提高根际酸性磷酸酶的活性和养分的活化吸收效率[7273]。这种促进效应对于低温条件下提高玉米早期的抗逆性,促进养分、水分的协同吸收具有重要作用。

2.2 根系对异质性养分与水分的综合协同响应

自然条件下,养分和水分的异质性分布通常同时存在,而根系如何协调多种环境信号并做出响应仍然没有统一认识。有研究认为植物根系对水分和养分的觅食有交互作用,从而影响植物根系的可塑性变化;而另外一些研究认为根系对异质性水分和养分的响应是相互独立的。揭示根系对局部供磷和水分响应的协同机制,是深入理解植物如何感应并快速捕获异质性土壤磷和水分资源,提高环境适应性及养分水分吸收的关键。一个典型的例子是短命植物对干旱沙漠环境的适应性反应,研究表明,生长在干旱沙漠环境条件下的植物在遇到短暂的降雨时,形成了典型的沙漠背景下的土壤水分局部富集区,短命植物能通过根系生长和提高吸收速率快速捕获土壤水分局部富集区中的水分资源和由此释放出来的养分[7475],这反映了植物高效协同利用局部水分、养分资源,从而适应极端环境的重要机制。在农田生态系统中,作物也会遇到养分、水分异质性分布的环境,基于作物根系对异质性养分水分的响应规律,采用局部供应养分水分的根际调控方式能显著增强作物的生长,从而提高养分利用效率。研究表明,局部同时供应磷和水显著促进根系生长,提高了根系吸收磷和水分的效率[76]。在干旱条件下,给部分根系供应磷和水分,即可满足植株的生长需求,形成的地上部生物量不低于均匀供应磷水处理,节约资源投入,提高磷和水分的利用效率[76]。我们的研究表明,在集约化农田条件下,当局部养分和水分在空间上耦合时,显著提高了玉米的生长和养分吸收,而当局部养分和水分的供应在空间上相分离时,则制约了玉米的生长,降低了玉米根系对土壤空间异质性水肥资源的利用效率[77]。然而,这种反应取决于植物本身的性状和具体的养分环境条件。我们的研究表明,玉米能够整合利用空间分离的氮磷资源,通过协调同化物碳在不同资源富集区的投入比例,针对不同资源采取相应的觅食策略,极大地提高玉米对空间分离氮磷养分的吸收效率[78]。说明了植物根系具有高效整合利用处于不同位置资源的潜力。因此,在农田条件下,构建合理的磷水资源空间配置及其协同效应,进而强化这种协调关系与作物根系生长和分布的匹配关系,对于提高作物对养分、水分资源的利用效率,实现可持续生产具有重要的指导作用[2425]

3 植物激素在磷与水分互作中的调控作用 3.1 ABA 和质外体 pH介导磷和水分的协同响应

植物在适应缺磷和水分胁迫过程中伴随着一系列的形态结构和生理过程的变化,以提高磷和水分的利用效率,这种根系形态和生理过程的响应与激素调控作用密切相关。一些植物激素或信号物质同时参与植物对磷和水分的响应,它们在磷与水分互作中起到了关键作用[79]。如赤霉素 (GA) 通过 GA-DELLA 蛋白途径参与调控拟南芥根形态、生理和分子水平对低磷的响应[80],而水分胁迫也能改变 GA 的代谢,稳定 DELLA 蛋白,改变根系的发育[81]。许多研究表明,水分胁迫下根系合成大量脱落酸 (ABA),并作为长距离化学信号通过韧皮部运输到地上部[82],影响保卫细胞质膜上的离子通道,调控胞内钾离子浓度,调节气孔开闭和叶片蒸腾,从而在水分胁迫条件下维持植物的存活[83]。研究表明,ABA 也参与了缺磷诱导的植物根系的形态变化,ABA 能够刺激根毛增生,提高根冠比等[79]

木质部汁液/质外体 pH 参与 ABA 介导的植物在水分胁迫下的气孔运动。已有的研究表明,木质部汁液的 pH 呈酸性状态时,有利于气孔的开启,促进蒸腾作用和叶片的生长[84]。用低 pH 的缓冲液处理叶片,显著降低了质外体 pH,使气孔的导度和开启程度增加,从而增强了植物的生长和光合作用[84]。水分胁迫会显著提高质外体 pH,其机制为水分胁迫影响植物对氮、磷等矿质养分的吸收,引起阴阳离子吸收的不平衡,改变离子的电荷平衡和植物体内的有机酸代谢,从而导致木质部汁液/质外体 pH 升高[8586]。ABA 是一种弱酸,其分子形态是亲脂性的,容易穿过细胞质膜。水分胁迫下的木质部汁液/质外体的高 pH 改变了木质部质外体中 ABA 分子的形态,使其主要解离为疏脂性的酸根离子形态,不能转移进入叶肉细胞的胞质,而大量积累到保卫细胞附近的质外体中,加速气孔关闭;当木质部中 pH 较低时,ABA 通常保持分子状态,并快速分配到叶肉细胞中,从而减少了 ABA 在保卫细胞附近的积累,引起气孔开启。这些调控气孔的过程,显著影响了叶片对水分的利用和其他生理功能[83, 87]

植物质外体的 pH 在很大程度上受矿质营养状况的调控,植物缺磷上调 H+-ATPase 的活性,抑制硝酸盐的吸收和同化,阴阳离子吸收的不平衡导致根系分泌大量质子,使根际 pH 和质外体 pH 显著下降,铵态氮的供应会加剧根际的酸化作用[73]。此外,质外体 pH 和矿质营养还可以通过对水通道蛋白基因表达的影响来调控根部水分的传导[5153, 8889]。以上研究结果表明,质外体和根际 pH 在调控植物抵抗水分胁迫过程中具有重要作用,矿质养分的吸收通过改变根际 pH 影响植物对水分的吸收。质外体和根际 pH 可能是根土界面上磷和水分调控机制的重要连接点。有关研究亟待加强。

3.2 乙烯和 NO 调控植物对磷和水分的综合响应

乙烯和 NO 是介导植物响应干旱和低磷胁迫的重要因子。水分亏缺条件下,根系合成乙烯量增加,通过调控气孔运动、改变水通道基因的表达和根系水导,介导一系列植物对干旱胁迫的响应过程[90]。根系水导受乙烯的调节,在淹水环境下生长的植物,将其根系短暂暴露在外源乙烯中,根的水通道蛋白活性受到调控,根系的水导增加,叶片的气孔开启[91]。根系中的乙烯同时受到植物磷营养状况的调节:低磷胁迫下,根系合成的乙烯量显著增加[92],根系对乙烯的敏感性增强[93],从而激活一系列低磷胁迫 (PSI) 诱导基因的表达[94]。另外,乙烯促进植物根尖 IAA 的合成[95],并对低磷诱导根毛和侧根发育具有重要的调控作用[96],磷高效基因型植物的根系产生的乙烯量较高[97],表明乙烯与植物对磷的获取密切相关。乙烯在植物水分和磷获取过程中都起到重要的调控作用,很大程度上影响了作物根系对根土界面上磷和水分的协同吸收和利用。一氧化氮 (NO) 是调节植物气孔运动、介导低磷胁迫下根系形态生理响应的重要信号分子[9899]。研究表明 NO 在保卫细胞中的积累是 ABA 调控气孔关闭的必要条件[100]。外源 NO 处理能减小植物叶片的气孔导度、蒸腾速率和离子渗透,从而增加植物对水分胁迫的抗性[101]。低浓度的磷能够增加植物根尖 NO 的含量,NO 介导了低磷诱导的排根的形成与柠檬酸的分泌[98, 102]。除此之外,乙烯、NO 与 ABA 之间的相互作用也与气孔运动及水分状况密切相关,乙烯一方面可以拮抗 ABA 对气孔的关闭作用[103],另一方面通过影响活性氧物质 (过氧化氢、NO) 的合成调控气孔运动[104]。ABA 的存在与否对乙烯调控气孔运动的过程有决定性作用:ABA 存在时,气孔导度随乙烯的增加而增加;ABA 不存在时,气孔导度随乙烯的增加而降低[83]。值得关注的是,乙烯的合成过程可能受到 NO 的抑制[105]。乙烯、NO 及其相互作用在植物整体水分平衡中发挥着重要调节作用,与此同时,磷的供应强度显著改变植物体内乙烯和 NO 的调控过程,表明乙烯和 NO 可能参与了植物对磷和水分协同响应的调控,有关分子生理机制的研究有待加强。

4 结论与展望

综上所述,根土界面上的磷和水分存在着复杂的相互作用:磷和水分调控植物的根系形态和根际过程,而植物的形态和生理变化又深刻影响了根际磷和水分的生物有效性,根系对磷和水分的吸收过程也存在重要的相互作用及协调机制;土壤环境中,磷和水分的分布不均匀,根系对异质性磷和水分存在协同响应;植物对磷和水分的响应通过一系列植物激素的调控来实现,其中质外体 pH、ABA、乙烯和 NO 作为调控磷和水分响应过程的信号物质,是根土界面上磷和水分高效利用机制的重要节点(图1)。

长期以来,有关磷和水分高效利用的研究处于相对分离状态。以往根际营养的研究主要集中在模拟缺磷胁迫条件下根际养分的活化与吸收方面,对田间自然条件下磷与水分的互作过程,特别是对磷和水分高效利用的根土界面效应及其协同机制并不清楚。未来研究重点包括以下几个方面:1) 探讨植物协同响应磷和水分的激素信号途径;2) 揭示磷和水分互作的根土界面效应及其调控机制;3) 研究不同种植方式下作物根系如何协同响应土壤异质性的磷和水分?如何通过根际调控强化作物对磷与水分的高效利用?探明这些问题对于进一步揭示集约化农田生态系统作物–土壤相互作用机制,强化养分和水分的协同效应,进而提高土壤磷和水分利用效率具有重要的理论和实践意义,这些问题可能是未来集约化条件下有关水肥耦合机制与调控途径研究的重要方向。

图1 磷与水分互作的根土界面效应及机制综合模式图 Fig. 1 The conceptual model of root-soil interface effect and mechanisms of phosphorus and water interactions
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