📚 Adaptations of Rice for Wet Environments | 水稻对湿润环境的适应
Rice (Oryza sativa) is unusual among cereals because it can grow in waterlogged paddy fields where oxygen availability in the soil is severely limited. This article explains the structural, physiological and biochemical adaptations that allow rice roots and shoots to survive, grow and reproduce in wet environments. Understanding these adaptations is a common A-level Biology topic linking plant anatomy, respiration and plant hormones.
水稻是一种不寻常的谷物，因为它能在土壤氧气严重不足的淹水稻田中生长。本文解释使水稻根和茎能在湿润环境中存活、生长和繁殖的结构、生理和生化适应。理解这些适应是 A-level 生物学的常见主题，涉及植物解剖、呼吸作用和植物激素。
1. The Problem of Waterlogged Soil | 淹水土壤带来的问题
When a field is flooded, water displaces the air in the spaces between soil particles. Because gases diffuse about 10 000 times more slowly in water than in air, oxygen cannot be replenished quickly from the atmosphere. Soil microorganisms and plant roots continue to respire, so the remaining oxygen is used up within hours or days, and the soil becomes hypoxic and then anoxic.
当田地被水淹没时，水取代了土壤颗粒之间的空气。由于气体在水中的扩散速度大约比在空气中慢 10 000 倍，氧气无法从大气中迅速补充。土壤微生物和植物根系持续呼吸，因此剩余的氧气在数小时或数天内被耗尽，土壤先变为低氧， سپس 进而变为无氧。
As oxygen disappears, the redox potential of the soil falls. Anaerobic bacteria then dominate decomposition, and reduced, potentially toxic substances accumulate around the roots. These include iron(II) ions, manganese(II) ions, hydrogen sulfide, ammonium, methane and organic acids, while ethylene also builds up because it diffuses away very slowly in water.
随着氧气消失，土壤的氧化还原电位下降。厌氧细菌随后主导分解过程，还原态的、可能有毒的物质在根周围积累。这些物质包括亚铁离子、锰(II) 离子、硫化氢、铵离子、甲烷和有机酸；同时乙烯也会积累，因为它在水中扩散得非常慢。
- Oxygen deficiency stops aerobic respiration in root cells. | 缺氧使根细胞的有氧呼吸停止。
- Fe²⁺, Mn²⁺ and H₂S can damage membranes and enzymes. | Fe²⁺、Mn²⁺ 和 H₂S 会损伤膜和酶。
- Nutrient availability changes: phosphate may increase, but zinc and copper availability often falls. | 养分有效性改变：磷酸盐可能增加，但锌和铜的有效性通常下降。
- Soil pH tends to move towards neutral, which affects enzyme activity and ion uptake. | 土壤 pH 趋于中性，影响酶活性和离子吸收。
2. Aerenchyma Tissue and Oxygen Transport | 通气组织与氧气运输
Waterlogged soil has very little oxygen because water fills the air spaces between soil particles and microorganisms rapidly consume the remaining oxygen. Roots still need oxygen for aerobic respiration, so rice plants form aerenchyma, a spongy tissue containing large interconnected air spaces.
淹水土壤中氧气非常少，因为水填满土壤颗粒之间的空隙，微生物也迅速消耗剩余的氧气。根系仍然需要氧气进行有氧呼吸，因此水稻形成通气组织，一种含有大型相互连通气腔的海绵状组织。
In rice, cortical cells in the root and stem undergo programmed cell death to create continuous air channels. These channels connect the aerial parts of the plant to submerged roots, allowing oxygen to diffuse down its concentration gradient from the atmosphere to root cells.
水稻根和茎的皮层细胞通过程序性细胞死亡形成连续的气体通道。这些通道将地上部分与水下根连接起来，使氧气能够沿浓度梯度从大气扩散到根细胞。
Aerenchyma also reduces the resistance to gas diffusion along the root and lowers the metabolic oxygen demand of the cortex, because fewer living cells remain in the pathway. The efficiency of this internal ventilation can be described qualitatively by Fick's law, which states that the rate of diffusion is proportional to the cross-sectional area and the concentration gradient, and inversely proportional to the length of the diffusion path.
通气组织还降低了气体沿根扩散的阻力，并降低了皮层的代谢需氧量，因为通道中剩余的活细胞更少。这种内部通气效率可以用菲克定律定性描述：扩散速率与横截面积和浓度梯度成正比，与扩散路径长度成反比。
扩散速率 ∝ (A × ΔC) ÷ L
In this relationship, A is the total cross-sectional area of the air channels, ΔC is the oxygen concentration difference between the shoot and the root, and L is the distance over which oxygen must diffuse. Rice maximises A by forming many wide, continuous channels and minimises L by keeping roots relatively short and well branched.
在这个关系中，A 是气体通道的总横截面积，ΔC 是地上部与根之间的氧气浓度差，L 是氧气必须扩散的距离。水稻通过形成许多宽大而连续的气腔来增大 A，并通过使根系相对较短且分支良好来减小 L。
3. How Aerenchyma Forms: Programmed Cell Death | 通气组织的形成：程序性细胞死亡
Rice aerenchyma is described as lysigenous, meaning it is created by the controlled death and dissolution of cells rather than by cells simply pulling apart. The process is a neat example of programmed cell death, in which specific cells are dismantled for the benefit of the whole organism.
水稻的通气组织属于溶生型，即由细胞受控死亡和溶解形成，而不是细胞简单地相互分离。该过程是程序性细胞死亡的典型例子，特定细胞为了整个生物体的利益而被拆解。
Signalling begins when roots sense low oxygen. Ethylene accumulates in the root cortex because it cannot diffuse out of waterlogged tissue. Ethylene then triggers the expression of genes for cell wall degrading enzymes such as cellulase, xylanase and pectin methylesterase, together with enzymes that control reactive oxygen species.
当根系感知到低氧时，信号传导开始。乙烯在根皮层中积累，因为它无法从淹水的组织中扩散出去。乙烯随后触发细胞壁降解酶（如纤维素酶、木聚糖酶和果胶甲酯酶）以及控制活性氧的酶类的基因表达。
The affected cortical cells show vacuolation, chromatin condensation, loss of plasma membrane integrity and finally breakdown of the cell walls. Neighbouring cells remain alive, so the tissue keeps its structural framework while the spaces between cells become connected into long tubes.
受影响的皮层细胞出现液泡化、染色质凝聚、质膜完整性丧失，最终细胞壁崩解。相邻细胞仍然存活，因此组织保留其结构骨架，而细胞之间的空隙则连成贯通的长管。
- Lysigenous aerenchyma: formed by cell death, typical of rice roots. | 溶生通气组织：由细胞死亡形成，是水稻根的典型特征。
- Schizogenous aerenchyma: formed by separation of cells along the middle lamella, common in some wetland species. | 裂生通气组织：细胞沿中层分离形成，见于某些湿地植物。
- Formation can be detected within 24 to 48 hours of waterlogging in the basal and middle zones of the root. | 淹水后 24 至 48 小时内在根的基部和中部区域即可检测到通气组织的形成。
4. The Radial Oxygen Loss Barrier | 径向氧损失屏障
Oxygen that travels down the aerenchyma would be wasted if it leaked sideways into the surrounding reduced soil. Rice therefore deposits suberin and lignin in the outer cell layers of older root zones, especially the exodermis, forming a barrier to radial oxygen loss, often abbreviated as the ROL barrier.
沿通气组织向下运输的氧气如果向周围还原性土壤侧向泄漏，就会被浪费。因此，水稻在较老根区的外层细胞（尤其是外皮层）中沉积木栓质和木质素，形成径向氧损失屏障，常缩写为 ROL 屏障。
The barrier is strongest in the basal and middle regions of the root and weaker or absent near the root tip and in young lateral roots. This arrangement is functionally elegant: the root apex and the elongation zone, which contain the most metabolically active cells, receive the oxygen, while a small amount of leakage near the tip is allowed for detoxification.
该屏障在根的基部和中部最强，在根尖附近和幼嫩侧根中较弱或缺失。这种安排在功能上很巧妙：代谢最活跃的根尖和伸长区获得氧气，而根尖附近允许少量泄漏以进行解毒。
The oxygen that does leak out oxidises toxic reduced ions in the rhizosphere. Iron(II) is oxidised to iron(III), which precipitates as iron plaque on the root surface, and sulfide is oxidised to less harmful sulfate. This detoxification protects the root from Fe²⁺ and H₂S poisoning, although a very thick plaque can also restrict the uptake of zinc and other micronutrients.
确实泄漏出来的氧气会氧化根际中的有毒还原态离子。铁(II) 被氧化为铁(III)，以铁膜形式沉淀在根表面；硫化物被氧化为危害较小的硫酸盐。这种解毒作用保护根系免受 Fe²⁺ 和 H₂S 毒害，不过过厚的铁膜也可能限制锌等微量元素的吸收。
4 Fe²⁺ + O₂ + 4 H⁺ → 4 Fe³⁺ + 2 H₂O
5. Root Anatomy and Architecture | 根系解剖结构与构型
Rice roots are finely branched and relatively shallow, which suits a soil in which oxygen is confined to a thin surface layer. The root cap protects the meristem as it pushes through soft, waterlogged mud, and the apical meristem itself is small and compact, which reduces its oxygen requirement.
水稻根系分支细密且相对较浅，这适应了氧气仅局限于薄薄的表层土壤的环境。根冠在根系穿过柔软淹水的泥土时保护分生组织，而根尖分生组织本身小而紧凑，从而降低了需氧量。
Compared with cereals such as wheat or barley, rice has a thinner cortex and a much smaller proportion of living cortical cells in mature root zones. The stele, containing the xylem and phloem, is well developed, so water and mineral ions can still be transported efficiently even though the surrounding cortex is largely hollow.
与小麦或大麦等谷物相比，水稻皮层更薄，在成熟根区中活皮层细胞的比例小得多。含木质部和韧皮部的中柱发育良好，因此即使周围皮层大部分中空，水和矿质离子仍能高效运输。
Rice also produces adventitious roots from nodes above the water line and, in some varieties, floating roots that grow into the water. These additional roots shorten the diffusion path from the atmosphere to the respiring tissue and can absorb nutrients directly from the floodwater.
水稻还从水面以上的节产生不定根，某些品种甚至产生伸入水中的浮根。这些额外的根缩短了从大气到呼吸组织的扩散路径，并能直接从洪水中吸收养分。
6. Anaerobic Respiration and Fermentation | 无氧呼吸与发酵
Even with aerenchyma, the deepest root cells and the cells of germinating grains under water may experience periods of anoxia. Rice cells can therefore switch to anaerobic respiration, which yields far less ATP per glucose but keeps essential metabolism running.
即使有通气组织，最深的根细胞和水下萌发谷粒的细胞也可能经历一段缺氧期。因此，水稻细胞可以切换到无氧呼吸，每分子葡萄糖产生的 ATP 少得多，但能维持基本代谢继续进行。
Aerobic respiration in the mitochondrion produces roughly 30 to 32 ATP per glucose molecule, whereas fermentation produces only 2 ATP per glucose. This is why flooded rice plants grow more slowly, have lower yields in deep water and depend heavily on carbohydrate reserves stored in the stem and grain.
线粒体中的有氧呼吸每分子葡萄糖约产生 30 至 32 个 ATP，而发酵每分子葡萄糖仅产生 2 个 ATP。这就是为什么淹水的水稻生长较慢、深水下产量较低，并严重依赖茎和谷粒中储存的碳水化合物。
C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O（约 30–32 ATP）
C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂（净得 2 ATP）
C₆H₁₂O₆ → 2 CH₃CH(OH)COOH（乳酸，净得 2 ATP）
The critical biochemical point is that fermentation regenerates NAD⁺ from NADH, allowing glycolysis to continue when the electron transport chain cannot operate because oxygen is absent as the final electron acceptor. Without this recycling, glycolysis would stop once all the NAD⁺ had been reduced.
关键的生化要点是：发酵使 NADH 重新生成 NAD⁺，从而在氧作为最终电子受体缺失、电子传递链无法运行时让糖酵解继续进行。若没有这种再生，一旦 NAD⁺ 全部被还原，糖酵解就会停止。
In rice, alcoholic fermentation is carried out by pyruvate decarboxylase and alcohol dehydrogenase, while lactate dehydrogenase and alanine aminotransferase provide shorter-term alternatives. Ethanol diffuses out of the tissue, which reduces the risk of self-poisoning, whereas lactate tends to acidify the cytoplasm and must be exported or converted.
在水稻中，酒精发酵由丙酮酸脱羧酶和乙醇脱氢酶催化，而乳酸脱氢酶和丙氨酸氨基转移酶提供短期替代途径。乙醇扩散出组织，从而降低自我毒害的风险；而乳酸会使细胞质酸化，必须被排出或转化。
7. Hormonal Control: Ethylene, Gibberellin and Auxin | 激素调控：乙烯、赤霉素与生长素
Ethylene is the central hormone in rice flooding responses. It is a gas, so in waterlogged tissue it cannot escape and its concentration rises. The precursor ACC is made in oxygen-poor roots and can be transported in the xylem to aerated shoots, where ACC oxidase uses oxygen to convert it to ethylene. This ACC transport explains why shoots respond hormonally even when roots alone are submerged.
乙烯是水稻淹水响应的核心激素。它是一种气体，因此在淹水组织中无法逸出，浓度升高。前体 ACC 在缺氧的根中合成，可经木质部运输到有氧的地上部分，在那里 ACC 氧化酶利用氧气将其转化为乙烯。这种 ACC 运输解释了为什么即使只有根被淹没，地上部分也会产生激素响应。
Ethylene promotes aerenchyma formation in the root cortex, restricts root elongation, and in deepwater rice it stimulates internode elongation by interacting with gibberellins. In deepwater varieties the SNORKEL genes promote rapid stem extension of several centimetres per day, lifting the leaves above the rising water so that photosynthesis can continue.
乙烯促进根皮层通气组织的形成，抑制根伸长，并在深水稻中通过与赤霉素相互作用刺激节间伸长。在深水品种中，SNORKEL 基因促进茎每天数厘米的快速延伸，把叶片托到上涨的水面之上，使光合作用得以继续。
By contrast, submergence-tolerant varieties carry the SUB1A gene, which suppresses ethylene-driven elongation and saves carbohydrate. This strategy, called quiescence, allows the plant to survive complete submergence for one to two weeks and then resume growth when the water recedes.
相反，耐淹水品种携带 SUB1A 基因，它抑制乙烯驱动的伸长并节省碳水化合物。这种被称为静止策略的机制，使植株能在完全淹没下存活一到两周，并在水退后恢复生长。
- Ethylene: triggers aerenchyma and elongation responses. | 乙烯：触发通气组织形成和伸长响应。
- Gibberellin: promotes cell elongation in internodes and coleoptiles. | 赤霉素：促进节间和胚芽鞘的细胞伸长。
- Auxin: regulates lateral root initiation and vascular patterning. | 生长素：调控侧根发生和维管束构型。
- Abscisic acid: generally antagonises growth and influences stomatal closure. | 脱落酸：通常拮抗生长并影响气孔关闭。
8. Shoot Adaptations above the Water | 地上部分的适应
The shoot is the plant's interface with the atmosphere, so rice invests heavily in shoot adaptations that maintain gas exchange. The coleoptile of a germinating grain grows rapidly towards the surface even under water, using anaerobic respiration and stored starch broken down by gibberellin-stimulated α-amylase.
地上部分是植物与大气的界面，因此水稻在维持气体交换的地上部分适应上投入很多。萌发谷粒的胚芽鞘即使在水下也迅速朝水面生长，利用无氧呼吸以及由赤霉素诱导的 α-淀粉酶分解储存的淀粉。
Rice stems are hollow, with aerenchyma running through the internodes, so they act as ventilation pipes. Leaves are amphistomatous, meaning stomata occur on both surfaces, and the mesophyll contains large intercellular spaces that connect to the stem aerenchyma.
水稻茎中空，节间贯穿通气组织，因此起到通气管道的作用。叶片为双面气孔型，即两面都有气孔，叶肉含大型细胞间隙，与茎的通气组织相连。
Submerged leaves of rice retain a thin gas film on the water-repellent surface, sometimes called a plastron. This film allows a limited amount of underwater photosynthesis and enables uptake of carbon dioxide from the water, which is a real advantage when the canopy is flooded for several days.
水稻被淹没的叶片在疏水表面保留一层薄气膜，有时称为气盾。这层气膜允许有限的水下光合作用，并能从水中吸收二氧化碳，在冠层被淹数天时是真正的优势。
Rice also shows plasticity in tillering and leaf angle. In dense, wet canopies plants reduce tiller number and orient leaves to reduce self-shading, which maximises light capture for the surviving shoots.
水稻在分蘖和叶角方面也表现出可塑性。在密集湿润的冠层中，植株减少分蘖数并调整叶片角度以减少自遮阴，从而最大化存活茎的光捕获。
9. Nutrient Uptake and Rhizosphere Chemistry | 养分吸收与根际化学
In flooded paddy soil, nitrogen is present mainly as ammonium rather than nitrate, because nitrification is limited by low oxygen. Rice is
更多咨询请联系16621398022（同微信）
或截图后用「扫一扫 → 相册」选择本图