引用本文
陆姣云, 段兵红, 杨梅, 杨晗, 杨惠敏. 植物叶片氮磷养分重吸收规律及其调控机制研究进展. 草业学报, 2018,27(4): 178-188
LU Jiao-yun, DUAN Bing-hong, YANG Mei, YANG Han, YANG Hui-min. Research progress in nitrogen and phosphorus resorption from senesced leaves and the influence of ontogenetic and environmental factors. Acta Prataculturae Sinica,2018,27(4): 178-188  
Doi:10.11686/cyxb2017223
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植物叶片氮磷养分重吸收规律及其调控机制研究进展
陆姣云1, 段兵红1, 杨梅1, 杨晗2, 杨惠敏1,*
1.草地农业生态系统国家重点实验室,兰州大学草地农业科技学院,甘肃 兰州 730020
2.中国科学院遗传与发育生物学研究所,植物基因组学国家重点实验室与植物基因研究中心,北京 100101
*通信作者Corresponding author. E-mail: huimyang@lzu.edu.cn

作者简介:陆姣云(1989-),女,甘肃兰州人,在读博士。E-mail: lujy09@lzu.edu.cn

收稿日期: 2017-05-08
基金项目:国家自然科学基金(31572460),国家重点研发计划(2016YFC0400302)和兰州大学中央高校基本科研业务费专项资金(lzujbky-2016-178和lzujbky-2017-ot01)资助
摘要

植物养分重吸收是植物组织或器官在衰老脱落前将部分养分(主要如氮、磷)转移到其他成活组织的过程,能延长养分在植物体内的存留时间,提高养分利用效率,减少植物对土壤养分的依赖,是植物适应环境的重要策略之一。本研究综述了土壤水分和肥力、光照、温度等环境因子和寿命、个体发育等遗传特性对植物叶片氮磷养分重吸收的影响。不同生活型或功能型的叶片养分重吸收不同;施肥会改变土壤速效养分的含量和比例,从而调控叶片养分重吸收;土壤水分通过调节土壤养分有效性而影响叶片养分重吸收;植物在生长过程中,不同时期对养分的需求有所差异,对养分的敏感程度和吸收能力也各不相同,从而表现出不同叶片养分重吸收特征;除了叶片之外,植物的其他组织器官(如,细茎、树木的芯材和能够储存养分的根)也可以进行养分重吸收。因此,植物叶片养分重吸收易受多种因素的影响,不同物种的重吸收能力存在差异,同一物种在不同时、空及相关因素变化下的重吸收特性也不同。深入研究植物养分重吸收规律和调控机制有助于进一步揭示植物的环境适应性,能为提高水肥管理水平,权衡生产力和适应性,实现系统生产和生态可持续发展提供理论依据。

关键词: 养分重吸收; 遗传因素; 环境因子; 调控
Research progress in nitrogen and phosphorus resorption from senesced leaves and the influence of ontogenetic and environmental factors
LU Jiao-yun1, DUAN Bing-hong1, YANG Mei1, YANG Han2, YANG Hui-min1,*
1.State Key Laboratory of Grassland Agro-ecosystems, College of Pastoral Agriculture Science and Technology, Lanzhou University, Lanzhou 730020, China
2.State Key Laboratory of Plant Genomics and Center for Plant Gene Research, Institute of Genetics and Developmental Biology, Chinese Academy of Sciences, Beijing 100101, China
Abstract

Nutrient resorption of a plant is a fundamental process in which the nutrients, i.e. nitrogen and phosphorus, in older tissues or organs are partly retranslocated into other vital parts before the tissues or organs senesce and fall. This process can retain nutrients within the plant that would otherwise be lost and enhance the nutrient use, thus reducing the dependence of a plant on soil nutrient availability. Resorption is an important strategy for adaption to adverse environments and for promotion of competitive ability. This review summarizes findings on the effects of environmental factors (i.e. soil fertility and water) and ontogenetic influences (i.e. growth stage and leaf lifespan) on nitrogen (N) and phosphorus (P) resorption from senesced leaves. Nutrient resorption varies among plant life forms or functional groups (e.g. N2-fixing and non-N2-fixing). Fertilizer application and irrigation can change nutrient availability in soils, influencing nutrient resorption. Plants sense and absorb nutrients differently at different growth stages, leading to ontogenetic changes in patterns of nutrient resorption. Resorption is not confined to leaves and may occur in other plant organs. In summary, nutrient resorption is influenced by multiple factors including ontogenetic and environmental signals. Improved knowledge about nutrient resorption may help understanding of how plants adapt to adverse environments and provide theoretical knowledge to support the development of more sustainable agricultural production practices.

Keyword: nutrient resorption; genetic feature; environmental factor; regulation

20世纪20年代, 植物学界就已经认识到植物养分利用效率在植物研究中的重要意义。1986年, Killingbeck[1]首次明确提出养分重吸收(nutrient resorption)的概念, 从另一个角度对植物养分利用进行了阐释。植物将衰老组织中的部分养分转移到其他组织的过程称为养分重吸收[1]。生长在贫瘠环境下的植物养分保存效率较高, 可能并不是因为它们对土壤养分的吸收能力更强, 而是因为养分重吸收更充分[2]。对植物而言, 重吸收可以提高养分(主要如氮N和磷P)利用效率, 降低植物生长对土壤养分条件的依赖程度[3, 4, 5, 6, 7, 8, 9]。重吸收能延长养分在植物体内的存留时间, 为生长组织所需的生物量生产充实了物质基础[10]。此外, 养分重吸收还有助于减少因凋落物分解、淋溶造成的养分损失[11]。因此, 养分重吸收增强了植物对养分贫瘠环境或因逆境(如干旱)导致的养分获取困难环境的适应能力[3, 12, 13, 14, 15], 体现了植物对环境多样性的适应, 是植物增强竞争力、提高生产力和适应逆境的重要机制之一。

养分重吸收易受多种因素影响, 不同植物的养分重吸收不同, 同一种植物对不同时、空环境变化的响应也有差异。因此, 深入研究养分重吸收变化规律, 阐明调控机制, 有助于充分了解系统养分运动规律和调控机制, 并可根据养分重吸收强度(重吸收效率和重吸收度)评估整个系统的生产力水平、土壤养分循环速度等, 从而为系统的科学管理提供参考。本文综述了环境因子和遗传特性对植物叶片氮磷养分重吸收的影响, 重点阐述了重吸收在不同因子影响下的响应规律, 并分析其调控机理。

1 养分重吸收强度的表征
1.1 重吸收效率

植物养分重吸收强度多以养分重吸收效率(nutrient resorption efficiency, NuRE, 从衰老组织转移到幼嫩组织中的养分比例)[6]来表征。高养分重吸收效率使植物更少地依赖于当前的养分吸收并增加了植物(尤其是养分贫瘠的生态系统)的适应性[16]。养分重吸收效率可以通过下式计算:

NuRE=(1-Nusen/Nugr)× 100% (1)

其中, Nusen和Nugr表示单位生物量中衰老器官(如衰老叶)和幼嫩器官(如新叶)的养分浓度。在各种各样关于养分重吸收的研究中(大部分数据来自美国和欧洲的森林生态系统)发现, 叶片氮重吸收效率(nitrogen resorption efficiency, NRE)和磷重吸收率(phosphorus resorption efficiency, PRE)分别为50%(n=287)和52%(n=226)[4], 而通过meta分析, 在覆盖了全球31个国家(大部分数据来自欧洲、北美、俄罗斯和非洲)关于陆生植物养分重吸收的86个研究中发现, NRE和PRE的平均值分别为62.1%和64.9%[9]。这主要是因为Vergutz等[9]考虑了叶片衰老时发生的质量损失, 而这部分损失会使养分重吸收效率低估约10%[17]。

考虑到叶片衰老过程中质量的损失, 使用校正后的公式:

NuRE=(1-Nusen/Nugr× MLCF)× 100% (2)

其中, MLCF为质量损失校正因子(mass loss correction factor), 一般为老叶与新叶的干重比, 或者当叶片质量损失率(leaf mass loss, LML)有效时, 由1-LML/100 估算。不同生长类型植物的MLCF值不同, 因为植物叶片的养分浓度通过质量和面积来表达, 质量浓度不能计算在衰老过程中因叶片生物量的减少而引起的叶片可溶性碳和非目标元素的变化, 从而引起重吸收的变化; 而随着衰老植物叶片面积缩小, 也会改变重吸收的大小[17, 18], 相比之下, 以叶片面积为基准得到的重吸收效率则较高[19]。根据大量数据分析表明, LML平均值为(24.2± 2.1), 则MLCF的平均值在0.737与0.779之间[9, 17]。然而现有研究中, 用到校正公式的情况很少[9, 17], 因为重吸收也会导致叶片物质减少和厚度减小, 而且在成年阶段表现更突出, 而叶片物质减少又会导致低估重吸收效率[20], 因而这是一个相互影响的过程。

1.2 重吸收度

植物养分重吸收能力大小也可以用养分重吸收度(nutrient resorption proficiency, NuRP)[5]来表征。NuRP是指植物衰老叶片中养分浓度能够减少到的最低水平, 可以用养分转移后衰老组织中的最低养分浓度来表示, 养分浓度越低表明重吸收度越高[5, 21]。根据多年生木本植物衰老叶片养分浓度大小, 养分重吸收度可分为完全重吸收(complete resorption)和不完全重吸收(incomplete resorption), 其中衰老叶中氮浓度低于7.0 mg· g-1, 落叶植物和常绿植物磷浓度分别低于0.5和0.4 mg· g-1被视为完全重吸收, 而二者分别大于10.0和0.8 mg· g-1被视为不完全重吸收[5]。养分重吸收度比养分重吸收效率更能直接表征养分重吸收的强度, 因为养分重吸收度不受绿叶取样时间和空间的影响[5]。NuRP直接影响了凋落物的质量, 凋落物的分解速率以及土壤的速效养分[5, 16, 22]。衰老叶片中的养分浓度越低, 表明从衰老叶片中转移到幼嫩组织的养分越多, 增加了养分在植物体内的留存时间, 同时使得凋落物分解时的养分淋溶量减少, 从而减缓养分从整个系统的损失[23]。

2 叶片养分重吸收的遗传差异

植物养分重吸收受内禀遗传特性控制, 不同物种间的重吸收差异很明显。这不仅与特定物种的养分需求、植物对养分组分的权衡(如N∶ P)[24, 25]、吸收能力等有关, 还可能受植物本身调节物质转移的内源因子影响, 如库容大小[26, 27]、韧皮部转运率[20, 28]、叶片的结构性防御力[29]、叶片脱落动态[30, 31]等的影响。

不同类型植物间叶片养分重吸收有明显差异[32, 33]。通过对大部分来自美国和欧洲的养分重吸收综合分析发现, 常绿树、落叶树、禾本科和非禾本科草本植物的NRE分别为33%~82%、55%~83%、28%~78%和40%~81%, PRE分别为25%~98%、45%~81%、34%~92%和62%~88%[4], 但常绿植物和落叶植物类型间重吸收可能无差异[3]。在养分完全重吸收后, 落叶树的N重吸收度(nitrogen resorption proficiency, NRP, 实际应该是衰老叶片N浓度)为< 7.0 mg· g-1、P重吸收度(phosphorus resorption proficiency, PRP, 实际应该是衰老叶片P浓度)为< 0.5 mg· g-1, 而常绿树的PRP为< 0.4 mg· g-15。对中国北方6种森林类型137种木本植物研究发现, 乔木NRE和PRE均显著高于灌木, 针叶树高于阔叶树[34]。有研究表明, NRE的大小可表现为禾本科> 非禾本科> 灌木[35]。在全球尺度上, NRE表现为禾草类> 非禾草类> 松柏类> 落叶植物> 蕨类植物> 常绿植物, PRE表现为禾草类> 松柏类> 非禾草类> 蕨类> 落叶植物> 常绿植物[9]。

研究表明, 固氮物种NRE大大低于非固氮物种[5], 而PRE则较高[34]。段兵红等[36]结果显示, 紫花苜蓿(Medicago sativa)叶片NRE和PRE的均值低于已经报道的50%和52%[4]或者62%和65%[9]。也有特例发现, 山龙眼科植物斑克木(Banksia integrifolia)PRE远高于豆科植物金合欢(Acacia farnesiana)[37]。每个物种只是反映了在当前环境条件下的实际重吸收效率值, 并不能反映该植物在任意条件下的最大重吸收值[8, 37]。同一物种在不同地区的重吸收值不同是因为受到了水分有效性、衰老时长、叶片养分情况和遮阴等多种因素的影响[5, 9]。

3 叶片养分重吸收对主要环境因子变化的响应
3.1 土壤养分供应及有效性

施肥实验[6, 22, 38]和沿自然养分梯度的观测[39, 40, 41]均表明, 低养分地区的植物NuRE和NuRP较高[5, 42, 43], 而高养分供应下则较低[6, 10, 41, 44]。虽然生长较慢, 但是养分物质损失率低的物种在养分贫乏生境中将占优势, 而生长较快、养分物质损失速率高的物种则在养分充足生境中占优势[45, 46]。对生长于迥异生境下的多年生草本植物而言, 在恒定养分供给水平下, 处于最适生境条件下物种的N素利用效率先随养分有效性提高而增大, 而当养分有效性增加到一定水平后, N素利用效率又开始下降[47], 这可能意味着吸收到体内的N素再次利用减少了[48]。此外, 贫瘠生境, 低生产力的物种叶片中所观察到的高浓度酚醛树脂可能会造成蛋白质合成之前发生蛋白质沉降现象, 而这种现象会通过降低养分重吸收作用而使养分利用率降低[2, 20, 45]。因此, 养分贫瘠样地中的物种未必是由于具有高养分利用效率才能适应贫瘠环境, 更多地可能是由于具有低养分损失率而适应贫瘠环境。

养分重吸收随土壤养分有效性加强而降低。一般地, NuRE随土壤养分供应的增加而降低[6, 35, 38, 49, 50, 51, 52], NuRP也随土壤养分供应的增加而减小[42, 50, 53, 54, 55], 意味着衰老叶养分浓度随土壤速效养分的增加而相应增加了。如Enoki等[56]发现, 日本滋贺县黑松(Pinus thunbergii)叶片NRE随着土壤N有效性的降低从43%提高到77%。又如Vourlitis等[57]发现, 巴西萨王那地区, NRP随土壤全N的减少而升高, PRP和PRE随土壤可提取P的减少而升高。此外, 与NuRE相比, 叶片NuRP与土壤养分有效性的关系更为密切[6, 13, 42, 53, 54, 57]。

在农业系统中, 施肥能改善土壤养分供应, 提高土壤养分有效性, 极大地提高作物的生产性能。但是, 施肥如何影响养分重吸收尚无一致的结论。如Aerts[4]对60个物种的施肥试验数据进行总结, 发现其中63%的物种NuRE对施肥没有响应, 而Son等[58]的研究表明, 施肥(N和P)可使日本落叶松(Japanese larch)NRE提高。土壤高N无P的控制条件下, P重吸收加强; 仅N添加可能导致P成为次生限制性养分[59]。淡水湿地中, N添加降低了小叶章(Deyeuxia angustifolia)叶片的NRE、PRE、NRP和PRP, 导致凋落物中的N和P浓度较高[16]。因而土壤养分供应与NuRE间尚没有完全一致的关系[6, 22, 28, 30, 49, 51, 60, 61, 62]。施肥可能会改变土壤中其他养分的含量, 间接影响土壤水分、热量以及微生物活性[52], 从而最终多途径调控养分重吸收。因此, 在实践中, 通过施肥管理措施调控养分重吸收的难度很大, 还需要深入研究, 以更好地指导生产。

3.2 土壤水分状况

土壤水分不仅是影响土壤养分有效性的关键因素, 也是影响陆地生态系统养分循环进程的关键因素[63]。它影响了土壤的氮矿化速率、矿质氮的移动和微生物活性[51, 63, 64], 从而改变了植物养分的吸收和积累模式, 最终改变了养分重吸收[4, 5]。一般的, 植物NuRE与植物的水分利用效率呈显著的负相关关系[65], 但不同的养分元素对土壤水分的响应不一致, 如土壤水分增多可导致NRE降低, 而对P重吸收没有显著影响[51]。干旱会导致叶片出现健康存活、干燥死亡、干旱诱导衰老(表现为失绿)、正常衰老(常在秋季)等几种状态, 叶片衰老方式不同的树种, 养分重吸收强度也存在差异。干旱诱导落叶型植物NRP和PRP比其他植物的更高(即重吸收更完全)[66]。土壤水分状况通过调节土壤养分有效性而影响植物NuRE, 在维持养分循环中扮演非常重要的角色[63, 64]。因此, 合理控水, 对于减少成本, 提高养分重吸收, 从而提高生产力具有重要意义。

3.3 光照

光照能影响植物叶片的光合作用, 从而影响气孔的开闭、蒸腾等, 最终对根系吸收养分的能力产生影响。有效光的改变可能会引起植物幼苗的养分循环组分和NRE的变化[67]。实施突然遮阴会减小桦树(Alaskan)的NRE[11, 28], 这种影响对长期生长在阴影下的植物不明显[68]。耐阴植物的NuRE低于需光植物, 因此, 单位面积的枯叶中氮浓度更高[69], 对应的NuRP更低。遮阴下植物衰老叶片中的N 和P的含量较高, 从而导致对应元素的NuRE比对照低[11]。但是, 对葡萄牙栎(Quercus faginea)而言, 适度的遮阴下NRE最大, 而全光照下NRE最小[67], 强光处理下的挪威槭(Acer platanoides)和糖槭(Acer saccharum)幼苗比适宜光照处理下的NRE分别高42%和36%[70]。也有研究表明, 当植物没有光合作用时, 仍具有重吸收作用, 但是植物不可能重吸收枯叶中所有的氮, 如, 细胞壁上嵌入的蛋白质不会从枯叶中转移[68]。因此, 植物在不同光照下养分重吸收的差异, 不仅受到光强的影响, 也可能与植物本身对光的响应有关。

3.4 温度

温度是影响植物生长的重要因素之一, 每种植物都有其自身适宜生长的温度。环境温度的变化可调节根系酶活力, 影响植物养分吸收, 因而导致养分重吸收发生变化。研究表明, 夏季温度对NRE具有显著的影响, 并表现出显著负相关性[71]。高温胁迫导致活性氧清除能力降低, 使叶片中活性氧大量产生, 从而加剧叶片细胞的膜脂过氧化程度, 加速叶片的衰老[72]; 同时, 高温也会使小麦(Triticum aestivum)的剑叶丙二醛含量升高, 超氧化物歧化酶活性和可溶性蛋白质含量降低, 加速植株衰老[73], 从而使得养分来不及转移而留存在衰老叶片中, 降低重吸收能力。

4 叶片养分重吸收的时间变异性
4.1 生长阶段转变

植物体内的氮和磷不仅受到非生物因素的影响, 而且随着植物的生长发育发生变化[52]。在植物的整个生长阶段中, 不同时期对养分的需求有所差异, 因而对养分的敏感程度和吸收能力也各不相同, 从而表现出不同养分重吸收特征。养分重吸收在生殖结构形成时增加, 移除时降低[28, 39]。在温带地区, 进行生殖生长的植物个体叶片养分含量低于进行营养生长的个体叶片养分含量, 表明生殖生长状态会极大地影响养分分配, 包括重吸收这一养分循环的重要环节[41, 74, 75]。可能是成年阶段新叶片的出现很快(一波), 常伴随成花, 对养分需求大且快, 土壤供应不及, 则重吸收发挥作用[19]。短期或长期生殖需求较大的树木比生殖需求较小的树木具有更低的枯叶氮磷含量, 表明NuRP增大[41]。此外, 相比幼苗而言, 成年植株的叶片NRE较高(绿叶N浓度也较高), 但是NRP无差异。相比成年阶段, 早期阶段的绿色叶片N浓度和光合养分利用率较低[76]。同时, 早期阶段绿色叶片单位面积N含量与NRE正相关; 成年阶段则不存在[19]。可能是在早期阶段快速生长导致的组织(如叶片)N和P浓度的减小, 但在后期因为生长暂缓而得到补充或因养分重吸收的启动而补充[77]。

4.2 叶片和个体寿命差异

植物叶片的寿命长短对叶片的养分重吸收也具有影响。一般地, 长寿命叶片NuRE比短寿命叶片更高[3, 78]。因此, 植物叶片的寿命越长, 保持的叶片生物量越大, 植物从衰老叶中转移的养分就越多, 从而使其在气温回升、土温仍然较低的时期能维持正常生长[79]。同时, 叶片滞留于植物体上的时间更长, 养分利用效率也就越大。然而, 也有研究表明, 叶片衰老持续时间较长的物种(如云杉Picea asperata和冷杉Abies fabri)NuRE较低, 而叶片脱落持续时间较短的物种(如落叶松Larix decidua)NuRE较高[30, 79]。也有研究发现生长早期植物往往追求较高的光合同化, 中午水势较低[80], 从而提早了叶片的死亡, 导致养分重吸收降低。此外, Chapin等[28]研究发现, 延长叶片在树上的生长时间对NRE、PRE没有影响。

除了叶片寿命以外, 植物自身的寿命对其养分利用也有重要影响。一般地, 长寿命植物的叶片通常具有较小养分浓度, 其较高NuRE可减少养分损失[3], 从而有助于植物的适应和生存。然而, 对于寿命较长物种而言, 不同年龄时的养分供应、吸收能力等也有差异, 因而, 养分重吸收不同。一般地, 老龄或成龄植株NRE、PRE高于幼龄[80, 81]。但是, 也有研究表明, 植物NRE和PRE随年龄增大先升高后降低[25, 82, 83]。此外, 植物NuRE也可能随年龄增加而降低[23, 84]。这种养分保存能力的降低, 表明随着年龄的增加, 植物对生境的适应性逐渐降低, 在对养分的保存上出现了衰退, 从而直接导致重吸收功能减弱, 使得养分重吸收与年龄的关系更为复杂。

5 叶片衰老对矿质养分重吸收的影响

叶片的衰老过程与源叶同化物的供应及源的大小(叶面积)有关[85]。影响植物从衰老叶片内转移养分的主要原因并不是土壤肥力而是植物养分转移中的“ 源-库” 关系, 即养分从衰老叶片(“ 源” )中转移到活跃组织(“ 库” )的过程, 同时, 植物体内(尤其叶片)养分浓度的变化可能是养分重吸收变化的直接原因, 转移养分数量的多少比植物本身的养分状态及土壤养分有效性更为重要[86], 但是目前对于叶片养分浓度的变化与重吸收变化的关系并没有一致的结论, 既表现出正相关[32], 又表现出负相关[9, 18, 87, 88]和无关[33]。但是, 也有研究表明, 加强“ 库” (对麦穗进行遮阴)或减弱“ 源” (摘去第一片叶)都能提高养分的内迁移效率, 因而源-库关系理论在重吸收调控中并不完全发挥作用[89]。

衰老是植物生长发育、形态建成和对环境应答反应中一个重要的生理现象, 伴随一系列生理变化和分子事件, 是一个受内外因子直接或间接影响的、高度有序的细胞程序化死亡过程[90, 91, 92]。在衰老的叶片细胞中, 经过高度有序地去组装和降解过程, 代谢产物(如营养物质)又会被重新运输到更年轻或再生器官中[93, 94, 95]。因此, 养分重吸收与衰老密切联系。Nooden等[96]提出三段式理论, 将叶片衰老分为3个阶段, 即起始(initiation)、衰退(degeneration)和终末(terminal)。外界环境信号和内源发育信号共同作用诱发叶片衰老, 首先会使得叶片的光合作用下降, 源-库关系转变, 幼叶作为“ 库” 器官直到成熟, 而老叶作为“ 源” 器官提供糖类; 当幼叶发育成熟, 光合作用达到最大, 其对糖的需求量开始降低, 有限的需求量将导致老叶中糖的积累, 并诱导老叶的衰老[97, 98]。其次细胞组分去组装、大分子物质降解, 从而使得降解的产物作为营养物质被重新运送到“ 库” 器官; 在大分子降解过程中, 各种蛋白降解系统和脂类降解系统也被激活, 营养物质会被重新利用, 从而促进衰老[94, 99]。最后细胞死亡, 叶片脱落[95]。

随着叶片的逐渐衰老, 有机氮和磷被水解, 在叶片脱落前无机磷和氨基酸态氮被转移出衰老叶片[20]。随着衰老, 叶片中被重吸收的磷占核酸和磷脂水解化合物全磷含量的40%~47%, 被重吸收的氮占蛋白质水解及后续的氨基酸再转移氮的82%~91%[20]。

6 非叶片器官的养分重吸收特征

养分重吸收是植物“ 获取” 养分的重要途径。除了叶片之外, 植物的其他组织器官也可以进行养分重吸收, 包括细茎、树木的芯材和能够储存养分的根等[100, 101]。叶片养分重吸收在过去的40多年间已经被广泛研究, 但是关于茎和根养分重吸收的研究却很少。研究表明, 在植物非叶片器官中, 茎秆的重吸收较其他器官高, 且在植物养分经济和生态系统养分循环中扮演重要角色[13, 102]。茎与植物其他组织相比, 最大不同点在于衰老速度特别慢[100], 可能导致其重吸收较大。也有研究表明, 中国杉的叶片NRE大于细枝[103]。另外, 植物细根和老根间的养分含量几乎没有差异[26, 104, 105], 表明根的养分重吸收与叶片或其他组织相比是可以忽略的[106, 107]。在养分重吸收过程中, 根不仅是库也是源; 同样地, 茎既是养分从叶片重吸收后的库, 又是营养生长和生殖生长的源[20, 26, 107, 108]。因此, 与叶片相比, 植物其他组织(茎、根、叶鞘等)养分重吸收更为复杂, 相关研究较少[101]。

目前, 在全球尺度上, 植物叶片的养分重吸收强度, 已经有一些明确的阈值范围, 而其他器官(如茎秆和根)的养分重吸收强度, 则尚不确定。

7 展望

植物一旦开始衰老, 大量养分会从植物的衰老部分通过韧皮部转移到正在生长的库中, 如种子[46]。在营养生长阶段, 衰老器官将养分转运到植物幼嫩部分[109]; 在生殖生长阶段, 随着整个衰老过程开始, 母本植物的所有组织和器官死亡, 养分从衰老组织重新转移到种子中[110]。但是, 植物的养分重吸收过程与衰老的关系尚不明确, 需作进一步的研究。

植物养分重吸收过程受多种因子的调节, 在不同的物种、不同组织中表现出不同的模式和反应。应加强对控制条件下重吸收规律的研究, 深入了解其调控机制, 在实践中结合水肥管理等措施来调节植物养分重吸收, 从而提高植物(作物)的适应性和生产性能。

此外, 还可根据需求, 选择特定重吸收强度的作物(如牧草), 充分利用其重吸收特性, 保证产量和品质。

The authors have declared that no competing interests exist.

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