📚 Structural and Physiological Adaptations of Rice to Aquatic Environments | 水稻适应湿生环境的结构与生理机制
Rice (Oryza sativa) is one of the few major crop species capable of thriving in waterlogged or flooded conditions. Its ability to survive and produce grain in anaerobic, reduced soils is the result of a suite of structural and physiological adaptations that allow oxygen supply to roots, detoxification of harmful by-products, and maintenance of metabolic activity under low-oxygen stress.
水稻（Oryza sativa）是少数能够在淹水或涝渍条件下正常生长并结实的 major 作物之一。它在缺氧、还原性土壤中存活并产生籽粒的能力，源于一系列结构与生理适应机制。这些机制使其能够向根系供氧、解除有害代谢产物的毒性，并在低氧胁迫下维持正常的代谢活动。
1. Oxygen Demand and the Challenge of Flooded Soils | 需氧量与淹水土壤的挑战
Flooded soils are rapidly depleted of oxygen because water has a low diffusion coefficient for O₂ — approximately 10,000 times slower than in air. Furthermore, microbial respiration in waterlogged soils consumes the remaining oxygen, creating strongly reducing conditions. For a root system embedded in such an environment, the inability to obtain oxygen for aerobic respiration would normally be fatal within hours.
淹水土壤中的氧气会被迅速消耗殆尽，因为氧气在水中的扩散系数约为空气中的一万分之一，扩散速度极为缓慢。同时，涝渍土壤中微生物的呼吸作用会耗尽仅存的氧气，形成强烈的还原性环境。对于根系深埋于这种环境的植物而言，若无法获得氧气进行有氧呼吸，通常数小时内便会死亡。
Rice overcomes this challenge through a combination of internal oxygen transport pathways, metabolic flexibility, and aerenchyma formation — adaptations that together constitute its “flood tolerance syndrome.”
水稻通过内部氧气运输通道、代谢可塑性以及通气组织形成——这些共同构成了其“耐涝综合征”——成功克服了这一挑战。
2. Aerenchyma Formation in Roots | 根系通气组织的形成
The most conspicuous structural adaptation in rice roots is the formation of aerenchyma — large, gas-filled spaces in the root cortex. Rice roots typically develop constitutive aerenchyma, meaning it forms even under well-oxygenated conditions, though its development is greatly enhanced under flooding.
水稻根系最显著的结构适应是通气组织的形成——即根皮层中大型的气体填充腔隙。水稻根系通常形成组成型通气组织，即使在氧气充足的条件下也会发育，但在淹水条件下其发育会显著增强。
Aerenchyma arises through two distinct mechanisms: (a) lysigenous aerenchyma, produced by programmed cell death (PCD) of cortical cells, and (b) schizogenous aerenchyma, formed by cell separation without cell death. In rice, lysigenous aerenchyma is the dominant type, with ethylene acting as the primary signal that triggers cortical cell death and subsequent gas-space formation.
通气组织通过两种不同的机制产生：（a）溶生性通气组织，由皮层细胞的程序性细胞死亡（PCD）形成；（b）裂生性通气组织，通过细胞分离而不发生细胞死亡形成。在水稻中，溶生性通气组织是主要类型，乙烯作为触发皮层细胞死亡和随后气体空间形成的主要信号。
Lysigenous aerenchyma pathway: ethylene → PCD of cortical cells → gas-filled spaces
溶生性通气组织形成路径：乙烯 → 皮层细胞程序性死亡 → 气体填充腔隙
The aerenchyma network is continuous from the leaves, through the stem, and into the roots, providing a low-resistance pathway for O₂ diffusion from the aerial parts to submerged tissues.
通气组织网络从叶片经茎延伸至根系，形成连续的低阻力通路，使 O₂ 能够从地上部分扩散至水下组织。
3. Radial Oxygen Loss (ROL) and the Rhizosphere Barrier | 径向氧损失（ROL）与根际屏障
While aerenchyma delivers oxygen to roots, some oxygen inevitably leaks radially out of the root into the surrounding soil — a process termed radial oxygen loss (ROL). In rice, ROL is largely confined to the basal regions of the root, while the apical (younger) zones exhibit minimal oxygen leakage.
虽然通气组织将氧气输送至根系，但部分氧气不可避免地会沿径向从根部泄漏到周围土壤中——这一过程称为径向氧损失（ROL）。在水稻中，ROL 主要局限于根的基部区域，而根尖（较幼嫩）区域的氧气泄漏极小。
This ROL restriction is achieved by the deposition of suberin and lignin in the cell walls of the exodermis and sclerenchyma layer in the mature regions of the root. These hydrophobic barriers prevent oxygen from diffusing outward, ensuring that the oxygen supply is conserved for the growing root tip.
这种 ROL 限制是通过根成熟区外皮层和厚壁组织细胞壁中木栓质和木质素的沉积实现的。这些疏水性屏障阻止氧气向外扩散，确保氧气供给得以保留给正在生长的根尖。
Moreover, the ROL around basal roots oxygenates the rhizosphere, promoting the growth of aerobic microorganisms and the oxidation of phytotoxic reduced ions such as Fe²⁺ and Mn²⁺, thereby preventing their excessive uptake.
此外，基部根周围的 ROL 可氧化根际环境，促进好氧微生物的生长，并氧化植物毒性还原态离子如 Fe²⁺ 和 Mn²⁺，从而防止其过度吸收。
4. Stem and Leaf Structural Adaptations | 茎和叶片的结构适应
Rice stems possess distinct structural features that facilitate gas transport. The culm contains aerenchymatous spaces in the pith cavity, which are continuous with the root aerenchyma. These spaces allow efficient longitudinal O₂ transport and also serve as channels for the removal of volatile compounds such as methane and ethylene produced in the rhizosphere.
水稻茎秆具有促进气体运输的独特结构特征。秆的髓腔中含有通气组织间隙，与根系通气组织相连通。这些间隙不仅允许 O₂ 的高效纵向运输，还充当甲烷和乙烯等根际挥发性化合物排出的通道。
In leaves, rice possesses large air spaces in the mesophyll and prominent bulliform cells on the adaxial surface. The bulliform cells enable leaf rolling under water stress, reducing transpirational water loss. Additionally, the stomatal distribution in rice leaves — with stomata present on both surfaces (amphistomatous) — facilitates gas exchange when the lower surface is in contact with floodwater.
在叶片中，水稻叶肉组织含有大型气腔，且在近轴面具有显著的泡状细胞。泡状细胞使叶片在水胁迫下能够卷曲，减少蒸腾失水。此外，水稻叶片的气孔分布为两面生型（amphistomatous），上下表皮均有气孔，这有利于当叶片下表面接触淹水时的气体交换。
These aerial and stem structures work in concert with root aerenchyma to maintain a continuous gas-phase pathway from the atmosphere to the root tip.
这些地上部分和茎的结构与根系通气组织协同作用，维持从大气到根尖的连续气相通路。
5. Root Pressure and Guttation | 根压与泌水
Root pressure in rice is notably high, typically ranging from 50 to 150 kPa. This positive pressure is generated by the active accumulation of ions in the xylem, creating an osmotic gradient that drives water entry. In flooded conditions, root pressure supplements transpiration-driven water flow and is especially important during the night or when stomata are closed.
水稻的根压显著较高，通常在 50 至 150 kPa 之间。这种正压源于离子在木质部中的主动积累，形成驱动水分进入的渗透梯度。在淹水条件下，根压补充了蒸腾驱动的水流，尤其在夜间或气孔关闭时尤为重要。
Guttation — the release of water droplets from the margins of leaves through hydathodes — is commonly observed in rice seedlings growing in flooded fields. This process is a visible manifestation of active root pressure and also serves to excrete solutes that might accumulate to toxic levels in the root zone.
泌水——即通过排水器从叶片边缘释放水珠——在水稻田中生长的幼苗中常见。这一过程是活跃根压的可见表现，同时也用于排出可能积累到毒性水平的溶质。
Root pressure may also facilitate gas exchange by helping to push gas bubbles through the aerenchyma. However, its primary importance in wet environments lies in maintaining water balance despite reduced root hydraulic conductivity caused by low oxygen.
根压还可能通过推动气泡穿过通气组织来促进气体交换。然而，在湿生环境中其主要重要性在于，尽管低氧导致根系水力导度降低，根压仍能维持水分平衡。
6. Metabolic Adaptation: Anaerobic Respiration and Ethanol Fermentation | 代谢适应：无氧呼吸与乙醇发酵
Despite the efficiency of aerenchyma-mediated O₂ transport, the root apex and other tissues may still experience oxygen deficiency. Under such conditions, rice roots switch partially to anaerobic respiration via the glycolytic pathway followed by ethanolic fermentation.
尽管通气组织介导的 O₂ 运输效率很高，根尖和其他组织仍可能经历缺氧。在这种条件下，水稻根系部分转换为通过糖酵解途径和乙醇发酵进行的无氧呼吸。
Glucose → 2 Pyruvate → 2 Ethanol + 2 CO₂ + 2 ATP (per glucose)
葡萄糖 → 2 丙酮酸 → 2 乙醇 + 2 CO₂ + 2 ATP（每分子葡萄糖）
Compared with flood-intolerant species, rice exhibits higher activities of pyruvate decarboxylase (PDC) and alcohol dehydrogenase (ADH), the key enzymes of ethanolic fermentation. This enhanced fermentative capacity allows rice to produce sufficient ATP for essential cellular processes and to regenerate NAD⁺ for continued glycolysis.
与不耐涝物种相比，水稻的丙酮酸脱羧酶（PDC）和乙醇脱氢酶（ADH）活性更高，这两种酶是乙醇发酵的关键酶。这种增强的发酵能力使水稻能够在缺氧条件下产生足够的 ATP 以维持基本细胞活动，并再生 NAD⁺ 以继续糖酵解。
Importantly, rice can also oxidize ethanol to acetaldehyde and then to acetyl-CoA via the ethanol oxidation pathway, enabling the detoxification of ethanol accumulated during anaerobiosis. This metabolic flexibility provides a significant advantage over species that accumulate ethanol to toxic levels.
重要的是，水稻还能通过乙醇氧化途径将乙醇氧化为乙醛，再转化为乙酰辅酶A，从而解除无氧呼吸期间积累的乙醇毒性。这种代谢可塑性比那些乙醇积累到毒性水平的物种具有显著优势。
7. Photosynthetic Adaptations and the C₃ Pathway | 光合适应与C₃途径
Rice is a C₃ plant, meaning it fixes CO₂ via the Calvin cycle using ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). While C₃ photosynthesis is generally less water-use-efficient than C₄, this is not a disadvantage in flooded environments where water is abundant.
水稻是 C₃ 植物，即通过核酮糖-1,5-二磷酸羧化酶/加氧酶（Rubisco）利用卡尔文循环固定 CO₂。虽然 C₃ 光合作用的水分利用效率通常低于 C₄，但在水分充足的淹水环境中这并非不利条件。
In partially submerged rice, leaves that remain above the water surface carry out normal C₃ photosynthesis. However, submerged leaves exhibit a unique adaptation: they can take up dissolved inorganic carbon (H₂CO₃/HCO₃⁻) from the floodwater directly through the cuticle. This “aquatic photosynthesis” supplements aerial carbon fixation and helps maintain carbohydrate reserves.
在部分淹没的水稻中，保持在水面以上的叶片进行正常的 C₃ 光合作用。然而，水下叶片显示出独特的适应：它们能直接通过角质层从淹水中吸收溶解的无机碳（H₂CO₃/HCO₃⁻）。这种“水生光合作用”补充了地上部分的碳固定，并有助于维持碳水化合物储备。
Additionally, rice leaves possess relatively high chlorophyll content and a high photosynthetic rate per unit leaf area in flooded environments, reflecting a compensation for the relatively low CO₂ diffusion rates in water.
此外，水稻叶片在淹水环境下具有较高的叶绿素含量和单位叶面积的光合速率，这反映了对水中 CO₂ 扩散速率较低的一种补偿。
8. Hormonal Regulation of Flooding Adaptation | 淹水适应的激素调控
Ethylene plays a central role in orchestrating rice adaptations to flooding. Under submerged conditions, ethylene accumulates in plant tissues for two reasons: (a) its synthesis is enhanced because ACC (1-aminocyclopropane-1-carboxylic acid), the precursor, is trapped in anoxic tissues and converted to ethylene; and (b) the low diffusivity of ethylene in water prevents its escape.
乙烯在水稻淹水适应中起核心调控作用。在淹水条件下，乙烯在植物组织中积累，原因有二：（a）其合成增强，因为其前体 ACC（1-氨基环丙烷-1-羧酸）被困在缺氧组织中并被转化为乙烯；（b）乙烯在水中扩散性低，难以逸出。
Ethylene triggers multiple downstream responses:
乙烯触发多种下游反应：
- Promotion of aerenchyma formation via induction of cell death in cortical cells.
- 促进通气组织形成，通过诱导皮层细胞死亡实现。
- Induction of adventitious root formation at stem nodes, providing new roots that emerge into the aerated water column rather than the anoxic soil.
- 诱导茎节处长出不定根，使新根伸出至含有氧气的水层中而非缺氧的土壤中。
- Stimulation of shoot elongation in deepwater rice varieties through modulation of gibberellin (GA) signaling.
- 通过调控赤霉素（GA）信号传导，促进深水稻品种的茎伸长生长的加速。
Gibberellins and abscisic acid (ABA) also participate. In deepwater rice, ethylene promotes GA biosynthesis and enhances GA responsiveness, leading to rapid internode elongation that keeps leaves above the rising floodwater. Conversely, ABA inhibits these responses and its levels decrease under flooding.
赤霉素和脱落酸（ABA）也参与其中。在深水稻中，乙烯促进赤霉素的合成并增强赤霉素的响应，导致节间迅速伸长，使叶片保持在水面以上。相反，ABA 抑制这些反应，其在淹水条件下水平下降。
9. Detoxification of Reduced Toxic Ions | 还原性毒害离子的解毒机制
Waterlogged soils accumulate reduced ions that are phytotoxic at high concentrations, notably Fe²⁺, Mn²⁺, and H₂S. Rice has evolved multiple strategies to cope with these:
涝渍土壤中会积累对植物有毒害作用的还原态离子，尤其是 Fe²⁺、Mn²⁺ 和 H₂S。水稻演化出多种策略来应对这些毒害离子：
First, ROL around roots creates an oxidized rhizosphere, converting Fe²⁺ to Fe³⁺. The precipitated iron oxides form a plaque on the root surface, reducing Fe²⁺ entry. This iron plaque can also adsorb other toxic metals, providing additional protection.
首先，根系周围的 ROL 形成氧化性根际，将 Fe²⁺ 氧化为 Fe³⁺。沉淀的铁氧化物在根表形成铁斑，减少 Fe²⁺ 的进入。这种铁斑还能吸附其他有毒金属，提供额外保护。
Second, rice roots can exclude Fe²⁺ at the root plasma membrane via specific transport regulation. When iron does enter, it is sequestered in vacuoles or bound to ferritin, minimizing cytosolic toxicity.
其次，水稻根系能通过特定的转运调控在根质膜水平排斥 Fe²⁺。当铁进入细胞后，会被隔离在液泡中或与铁蛋白结合，从而将细胞质毒性降至最低。
Third, rice has a high tolerance for H₂S because its cytochrome oxidase system is less sensitive to sulfide inhibition than that of non-tolerant species. Additionally, the oxidation of sulfide to sulfate in the rhizosphere by ROL-supported microorganisms reduces H₂S toxicity.
第三，水稻对 H₂S 具有较高耐受性，因为其细胞色素氧化酶系统对硫化物抑制的敏感性低于非耐受物种。此外，ROL 支持的好氧微生物将硫化物氧化为硫酸盐，降低了 H₂S 的毒性。
10. Comparison with Non-Tolerant Species | 与不耐受物种的比较
The adaptations described above become clearer when rice is compared with a flood-intolerant crop such as maize (Zea mays). The following table summarizes key differences:
将水稻与不耐涝作物如玉米（Zea mays）进行比较，上述适应机制就更加清晰。下表总结了主要差异：
| Feature | 特征 | Rice 水稻 | Maize 玉米 |
| Aerenchyma in roots | 根系通气组织 | Constitutive and extensive | 组成型且发达 | Inducible only, limited | 仅诱导型，有限 |
| ADH activity under hypoxia | 低氧下ADH活性 | High and sustained | 高且持续 | Low and transient | 低且短暂 |
| Root pressure | 根压 | High (50–150 kPa) | 高（50–150 kPa） | Low (5–20 kPa) | 低（5–20 kPa） |
| Submergence survival | 淹水存活能力 | Days to weeks | 数天至数周 | Hours to 1–2 days | 数小时至1–2天 |
This comparison underscores that flood tolerance is not a single trait but a coordinated system of anatomical, biochemical, and physiological adaptations.
这一对比表明，耐涝性并非单一性状，而是解剖学、生物化学和生理学适应的协调系统。
11. Ecological and Agricultural Significance | 生态与农业意义
The adaptations of rice to aquatic environments have profound ecological and agricultural implications. Rice paddies support vast wetland ecosystems, providing habitat for diverse organisms. The ability to grow in flooded fields also allows rice cultivation in regions where waterlogged soils would preclude most other crops, underpinning food security for billions of people.
水稻对水生环境的适应具有深远的生态和农业意义。稻田支撑着广阔的湿地生态系统，为多种生物提供栖息地。在淹水田地中生长的能力也使水稻得以在大多数作物无法生长的涝渍土壤地区种植，为数十亿人的粮食安全提供了保障。
However, flooded rice paddies are a major source of methane (CH₄), a potent greenhouse gas. Studies of rice aerenchyma have revealed that these same gas spaces that deliver oxygen to roots also serve as conduits for methane release from the anaerobic soil to the atmosphere. Understanding the structural and physiological basis of this trade-off is essential for developing strategies to reduce methane emissions while maintaining rice productivity.
然而，淹水稻田是甲烷（CH₄）的主要排放源，甲烷是一种强效温室气体。对水稻通气组织的研究揭示，这些向根系输送氧气的同一气体通道也充当着甲烷从厌氧土壤释放到大气中的通路。理解这种权衡的结构与生理基础，对于制定在保持水稻产量的同时减少甲烷排放的策略至关重要。
Current research aims to develop rice varieties that combine enhanced aerenchyma for improved oxygen transport with reduced methane emission, through breeding or genetic modification of the suberin barrier or the microbial community in the rhizosphere.
当前研究的重点是开发既具有增强通气组织以改善氧气运输，又能减少甲烷排放的水稻品种，途径包括通过育种或遗传改良木栓质屏障或调控根际微生物群落。
12. Conclusion: An Integrated Adaptation System | 结论：一个整合的适应系统
Rice exemplifies how structural and physiological traits are integrated to permit survival in anaerobic wetland environments. Aerenchyma provides the oxygen conduit; radial oxygen loss barriers conserve oxygen and oxidize the rhizosphere; root pressure maintains water balance; ethanolic fermentation sustains energy production; and hormonal signaling coordinates these responses. Together, these adaptations form a coherent strategy that has made rice the most successful crop for flooded agriculture.
水稻是结构与生理性状如何整合以在厌氧湿地环境中存活的典范。通气组织提供氧气运输通道；径向氧损失屏障节约氧气并氧化根际；根压维持水分平衡；乙醇发酵维持能量产生；激素信号协调这些反应。这些适应机制共同形成了统一的策略，使水稻成为淹水农业中最成功的作物。
For A-Level biology students, the study of rice adaptations offers an excellent example of how exam questions might ask you to relate structure to function, compare species, or explain physiological responses using knowledge of transport, respiration, and plant hormones.
对于A-Level生物学学生而言，研究水稻的适应机制提供了极好的范例，考题可能要求你将结构与功能相联系、比较不同物种，或运用运输、呼吸和植物激素的知识解释生理反应。
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