Mechanisms of waterlogging tolerance in wheat – a review of root and shoot physiology

We review the detrimental effects of waterlogging on physiology, growth and yield of wheat. We highlight traits contributing to waterlogging tolerance and genetic diversity in wheat. Death of seminal roots and restriction of adventitious root length due to O₂ deficiency result in low root:shoot ratio. Genotypes differ in seminal root anoxia tolerance, but mechanisms remain to be established; ethanol production rates do not explain anoxia tolerance. Root tip survival is short‐term, and thereafter, seminal root re‐growth upon re‐aeration is limited. Genotypes differ in adventitious root numbers and in aerenchyma formation within these roots, resulting in varying waterlogging tolerances. Root extension is restricted by capacity for internal O₂ movement to the apex. Sub‐optimal O₂ restricts root N uptake and translocation to the shoots, with N deficiency causing reduced shoot growth and grain yield. Although photosynthesis declines, sugars typically accumulate in shoots of waterlogged plants. Mn or Fe toxicity might occur in shoots of wheat on strongly acidic soils, but probably not more widely. Future breeding for waterlogging tolerance should focus on root internal aeration and better N‐use efficiency; exploiting the genetic diversity in wheat for these and other traits should enable improvement of waterlogging tolerance.     

Root growth,aerenchyma development,and oxygen transport in rice genotypes subjected to drought and waterlogging

Soils under field conditions may experience fluctuating soil water regimes ranging from drought to waterlogging. The inability of roots to acclimate to such changes in soil water regimes may result in reduced growth and function thereby, dry matter production. This study compared the root and shoot growth, root aerenchyma development, and associated root oxygen transport of aerobic and irrigated lowland rice genotypes grown under well-watered (control), waterlogged, and droughted soil conditions for 30 days. The aerobic genotypes were as tolerant as the irrigated lowland genotypes under waterlogging because of their comparable abilities to enhance aerenchyma that effectively facilitated O₂ diffusion to the roots for maintaining root growth and dry matter production. Under drought, aerobic genotypes were more tolerant than the irrigated lowland genotypes due to their higher ability to maintain nodal root production, elongation, and branching, thus, less reduction in dry matter production. Aerenchyma was also formed in droughted roots regardless of genotypes, but was resistant to internal O₂ transport under O₂ deficiency. The ability of roots to resist temporal variations in drought and waterlogging stresses might have strong implications for the adaptation of rice growing in environments with fluctuating soil water regimes.     

Plant growth, nutrient acquisition and mycorrhizal symbioses of a waterlogging tolerant legume (Lotus glaber Mill.) in a saline-sodic soil

Seedlings of Lotus glaberMill., were grown in a native saline-sodic soil in a greenhouse for 50 days and then subjected to waterlogging for an additional period of 40 days. The effect of soil waterlogging was evaluated by measuring plant growth allocation, mineral nutrition and soil chemical properties. Rhizobiumnodules and mycorrhizal colonisation in L. glaberroots were measured before and after waterlogging. Compared to control plants, waterlogged plants had decreased root/shoot ratio, lower number of stems per plant, lower specific root length and less allocation of P and N to roots. Waterlogged plants showed increased N and P concentrations in plant tissues, larger root crown diameter and longer internodes. Available N and P and organic P, pH and amorphous iron increased in waterlogged soil, but total N, EC and exchangeable sodium were not changed. Soil waterlogging decreased root length colonised by arbuscular mycorrhizal (AM) fungi, arbuscular colonisation and number of entry points per unit of root length colonised. Waterlogging also increased vesicle colonisation and Rhizobium nodules on roots. AM fungal spore density was lower at the end of the experiment in non-waterlogged soil but was not reduced under waterlogging. The results indicate that L. glaber can grow, become nodulated by Rhizobium and colonised by mycorrhizas under waterlogged condition. The responses of L. glaber may be related its ability to form aerenchyma.     

小麦湿害营养及其营养调控效应研究

通过盆钵试验,研究了湿害条件下几种营养调控措施对营养吸收及其产量的效应。结果表明,渍水严重抑制了小麦对Ｎ、Ｐ、Ｋ、Ｚｎ、Ｃｕ的吸收。可再利用营养元素（Ｎ、Ｐ、Ｋ、Ｚｎ）以叶部下降最为明显,蕨是茎部,穗部下降极少;不可财利用元素Ｃｕ在受渍小麦各部位均衡下降;湿害使单株产量大幅度下降。采用调控措施对湿害较轻的皖麦１９,追施氮肥或喷施Ｎ、Ｐ、Ｋ均可有效地提高其对营养元素的吸收并能有效提高产量;对湿害较严     

Effects of waterlogging and drought on winter wheat and winter barley grown on a clay and a sandy loam soil

Summary The effects of winter waterlogging and a subsequent drought on the growth of winter barley and winter wheat have been examined. We used lysimeters containing soil monoliths with facilities to control the water table and a mobile shelter to control rainfall. Winter wheat was grown on a clay and on a sandy loam, but winter barley only on the clay soil. Lysimeters were either freely-drained during the winter or waterlogged with the water table 10 cm below the soil surface from 2 December until 31 March (that could occur by rainfall with a return period of 2 to 3 years). The lysimeters then were either irrigated so that the soil moisture deficit did not exceed 84 mm, or subjected to drought by limiting rainfall (equivalent to a 1 in 10 dry year in the driest area of England) so that the deficits reached maximum values of 150 mm in the clay and 159 mm in the sandy loam by harvest.Winter waterlogging restricted tillering and restricted the number of ears for all crops; grain yield of the winter barley was decreased by 219 g/m² (30%), and that of winter wheat by 170 g/m² (24%) and 153 g/m² (21% on the clay and sandy loam respectively.The drought treatment reduced the straw weight of winter barley by 75 g/m² (12%) but did not significantly depress the grain yield. For winter wheat on the clay, where the soil was freely-drained during the winter, drought depressed total shoot weight by 344 g/m² (17%) and grain weight by 137 g/m² (17%), but after winter waterlogging, drought did not further depress total or grain weight. In contrast, the winter wheat on the sandy loam was not significantly affected by drought.From these results, which are discussed in relation to other experiments in the United Kingdom, it seems that winter waterlogging is likely to cause more variation in the yield of winter barley and winter wheat than drought.     

渍害胁迫下小麦生长的响应机理及调控措施研究进展

全球气候变化导致近年来渍害频发,而旱地作物小麦对渍害敏感。受气候、土壤、轮作制度等因素的影响,我国长江中下游小麦主产区的渍害灾情严重。渍害引起的土壤溶氧量降低可以导致小麦根系生长受到抑制,进而限制植株生长,最终降低小麦产量和品质。本文基于国内外相关研究,从根系呼吸代谢、水分传导、矿质养分吸收、光合作用、氧化还原代谢等方面概述了渍害胁迫抑制小麦生长的生理机理;讨论了小麦通过无氧呼吸维持能量供应和改变根系形态维持氧气供应等渍害适应机制;总结了肥料调控、生长调节剂调控和胁迫记忆等栽培措施在小麦抗渍上的应用及其机理,并对未来小麦抗渍研究进行了展望,以期为小麦的耐渍栽培和稳产高产管理提供理论支撑。     

Root Respiration and Carbohydrate Status of Two Wheat Genotypes in Response to Hypoxia

To investigate root respiration and carbohydrate status in relationto waterlogging or hypoxia tolerance, root respiration rateand concentrations of soluble sugars in leaves and roots weredetermined for two wheat (Triticum aestivum L.) genotypes differingin waterlogging-tolerance under hypoxia (5% O₂) and subsequentresumption of full aeration. Root and shoot growth were reducedby hypoxia to a larger extent for waterlogging-sensitive Coker9835. Root respiration or oxygen consumption rate declined withhypoxia, but recovered after 7 d of resumption of aeration.Respiration rate was greater for sensitive Coker 9835 than fortolerant Jackson within 8 d after hypoxia. The concentrationsof sucrose, glucose and fructose decreased in leaves for bothgenotypes under hypoxia. The concentration of these sugars inroots, however, increased under hypoxia, to a greater degreefor Jackson. An increase in the ratio of root sugar concentrationto shoot sugar concentration was found for Jackson under hypoxicconditions, suggesting that a large amount of carbohydrate waspartitioned to roots under hypoxia. The results indicated thatroot carbohydrate supply was not a limiting factor for rootgrowth and respiration under hypoxia. Plant tolerance to waterloggingof hypoxia appeared to be associated with low root respirationor oxygen consumption rate and high sugar accumulation underhypoxic conditions.Copyright 1995, 1999 Academic Press Oxygen consumption rate, sugar accumulation, Triticum aestivum L., waterlogging tolerance     

Mechanisms for the increase in phosphorus uptake of waterlogged plants: soil phosphorus availability, root morphology and uptake kinetics

Waterlogging frequently reduces plant biomass allocation to roots. This response may result in a variety of alterations in mineral nutrition, which range from a proportional lowering of whole-plant nutrient concentration as a result of unchanged uptake per unit of root biomass, to a maintenance of nutrient concentration by means of an increase in uptake per unit of root biomass. The first objective of this paper was to test these two alternative hypothetical responses. In a pot experiment, we evaluated how plant P concentration of Paspalum dilatatum, (a waterlogging-tolerant grass from the Flooding Pampa, Argentina) was affected by waterlogging and P supply and how this related to changes in root-shoot ratio. Under both soil P levels waterlogging reduced root-shoot ratios, but did not reduce P concentration. Thus, uptake of P per unit of root biomass increased under waterlogging. Our second objective was to test three non-exclusive hypotheses about potential mechanisms for this increase in P uptake. We hypothesized that the greater P uptake per unit of root biomass was a consequence of: (1) an increase in soil P availability induced by waterlogging; (2) a change in root morphology, and/or (3) an increase in the intrinsic uptake capacity of each unit of root biomass. To test these hypotheses we evaluated (1) changes in P availability induced by waterlogging; (2) specific root length of waterlogged and control plants, and (3) P uptake kinetics in excised roots from waterlogged and control plants. The results supported the three hypotheses. Soil P avail-ability was higher during waterlogging periods, roots of waterlogged plants showed a morphology more favorable to nutrient uptake (finer roots) and these roots showed a higher physiological capacity to absorb P. The results suggest that both soil and plant mechanisms contributed to compensate, in terms of P nutrition, for the reduction in allocation to root growth. The rapid transformation of the P uptake system is likely an advantage for plants inhabiting frequently flooded environments with low P fertility, like the Flooding Pampa. This advantage would be one of the reasons for the increased relative abundance of P. dilatatum in the community after waterlogging periods. Received: 15 February 1997 / Accepted: 20 May 1997     

氮肥后移对抽穗后水分胁迫下冬小麦光合特性及产量的影响

采用子母桶栽土培法模拟冬小麦抽穗后不同的水分胁迫状态,研究了氮肥后移对冬小麦光合特性及产量的影响.设置3个氮肥处理,分别为N₁(基肥∶拔节肥∶开花肥=10∶0∶0)、N₂(6∶4∶0)和N₃(4∶3∶3),模拟冬小麦抽穗后2种水分胁迫(渍水胁迫、干旱胁迫),设正常供水为对照.结果表明：相同供水条件下,N₂和N₃处理较N₁处理显著提高冬小麦灌浆期旗叶的SPAD和光合速率,确保了收获时较高的穗数、穗粒数和地上部分生物量;氮肥后移处理显著提高了冬小麦的耗水量,但其籽粒产量和水分利用效率也显著提高.相同氮肥条件下,干旱胁迫和渍水胁迫处理较正常供水显著降低了冬小麦开花期和灌浆期旗叶的光合速率、千粒重、穗粒数和产量.与正常供水相比,各氮肥条件下干旱胁迫和渍水胁迫处理花后旗叶光合速率及籽粒产量的减小幅度均表现为N₁＞N₂＞N₃.表明氮肥后移通过提高旗叶SPAD、减缓花后旗叶光合速率的下降幅度、增加地上部分干物质积累量,调控产量及其构成要素,以减轻逆境灾害(干旱和渍水胁迫)对产量的影响.     

Waterlogging Tolerance Among a Diverse Range of Trifolium Accessions is Related to Root Porosity, Lateral Root Formation and 'Aerotropic Rooting'

Waterlogging tolerance, root porosity and root anatomy wereevaluated for 20 Trifolium accessions (species and sub-species,all annuals) selected from the eight Sections of the genus.Nine accessions were sensitive [relative growth rate (RGR) reducedby up to 80%] to waterlogging, nine accessions were tolerant(RGR not reduced), and in two accessions RGR increased (up to1.9-fold), when compared to drained controls. Growth of themain (i.e. tap) root axis was severely reduced in all accessionswhen waterlogged. Lateral roots formed the bulk of the rootsystem of tolerant accessions when grown in waterlogged soil.Lengths of the longest lateral roots were up to three-timeslonger than the main root axis. Root porosity varied from 0.7–12%among accessions when grown in aerated solution and from 1.1–15.5%in plants grown in hypoxic (0.031–0.045 mol O₂m^-3) solution.In some accessions aerenchyma formed by cell lysigeny; in othersit formed by schizogenous cell separation, or a combinationof both processes. O₂consumption rates of expanded lateral roottissues varied by up to 1.7-fold (on a mass basis) among thesix accessions tested and was reduced by an average of 24% forroots of plants grown in hypoxic solution prior to measurements.Accessions with the highest root porosity tended to have longerroots when grown in waterlogged soil. Three accessions formed‘aerotropic roots’ and the lateral root lengthsof these plants exceeded those of all other accessions, suggestingenhanced O₂movement to the submerged lateral root axis via theaerotropic roots. Waterlogging-tolerant accessions were identifiedin seven of the eight Sections in Trifolium, and the tolerantaccessions tended to be those with extensive lateral root systemsof relatively high porosity. Copyright 2001 Annals of BotanyCompany Waterlogging, Trifolium, aerenchyma, hypoxia, flooding, root respiration, clover, root anatomy, root porosity, pasture, aerotropic roots     

The Use of Agar Nutrient Solution to Simulate Lack of Convection in Waterlogged Soils

Agar at 0.1% in nutrient solution (‘stagnant solution’)was used to prevent turbulence (convection), thus simulatingthe slow gas movements which occur in waterlogged soils. Wheat,aged between 6 and 16 d at the start of the treatment, was usedto test plant growth and development in this stagnant solutionfor 8–15 d. K-MES buffer at 5 mol m^-3was used to retainthe pH of the rhizosphere in the stagnant solution at pH 6.5. The prevention of convection reduced dissolved oxygen concentrationsin the bulk solution from 0.275 to below 0.05 mol m^-3after 1d, while ethylene accumulated over 10 d to 6.5x10^-6m³m^-3(ppm). Aerenchyma of nodal roots grown in stagnant solution comprised22% of the cross sectional area of the root 50 mm behind theroot tip; this was similar to values recorded earlier for nodalroots of wheat in waterlogged soil and contrasts with 7.6% forroots in non-flushed solution without agar (referred to in thispaper as ‘semi-stagnant solution’) and 2.4% in N₂-flushedsolution. Increases in dry weight and numbers of nodal roots with timewere larger for stagnant and N₂-flushed, than for semi-stagnantor aerated solution. In contrast, seminal roots did not growin stagnant solution, while seminal roots in N₂-flushed solutiongrew much less than in semi-stagnant or aerated solution. In the stagnant solution, relatively high concentrations ofN, K and P were required to avoid limitations in mineral uptakeinto the roots, due to the long diffusion pathway from the bulksolution imposed by the lack of convection. Nevertheless, ourdata show that the slow growth imposed by the lack of convectionwas due to factors other than low mineral nutrition. The mostlikely cause was the change in the dissolved gas compositionof the root media, particularly of the rhizosphere. In conclusion, in terms of anatomy and morphology the rootsgrown in the stagnant solution more closely resembled thosefrom waterlogged soil than did those grown in either semi-stagnantor N₂-flushed solution. Triticum aestivum; wheat; waterlogging; lack of convection; aerenchyma; root development; nutrient uptake     

Restoration of Catchment Water Balance: Responses of Clonal River Red Gum (Eucalyptus camaldulensis) to Waterlogging

Effects of waterlogging were studied in the field and under glasshouse conditions on two clonal lines of Eucalyptus camaldulensis Dehnh (river red gum), which are used in the rehabilitation of damaged agricultural catchments in Western Australia. The plantation of 9-year-old trees was in a position that covered a range of waterlogging and salinity conditions. Up-slope the water table was deeper (0.65–1.5 m), whereas the water table was closer to the ground surface down-slope (0.45 m in winter; 1.25 m in summer). Salinity was greater downslope and increased at the end of the dry summer, remaining high until diluted by the winter rains. Trees of both clonal lines were smaller downslope and used less water over the year. Clone M80 used more water in winter; clone M66 more in summer. In the field, the roots of clone M80 were evenly distributed through the soil profile, while roots of clone M66 decreased with increasing depth. Production of new root terminals varied with season. Greatest production was in spring and early summer, with much lower production over late autumn and winter. Only clone M66 produced new root terminals at depth (60–75 cm) during the drier months of late summer and early autumn. At this time, saline ground-water was the main source for water uptake. To explore clonal differences more closely, the effects of prolonged waterlogging were studied under glasshouse conditions. Clone M80 grew similarly under freely drained and continuously waterlogged conditions for the experimental period (21 weeks). The response under continuously waterlogged conditions was achieved through adventitious root production. By contrast, growth of clone M66 was suppressed under continuous waterlogging, a response associated with the lack of adventitious root production. The results from field and glasshouse studies suggest that clone M80 is more adapted to waterlogging by relatively fresh water than clone M66, but that clone M66 may use water of higher salinity than clone M80. Clone M80 would be better suited to higher positions in partially cleared catchments, where rainfall provides relatively fresh soil water. Clone M66 is better suited to lower catchment positions due to its ability to utilize more saline groundwater. Restoration of the water balance of damaged agricultural catchments can be best managed by matching specialized genotypes with particular catchment positions.     

断根对冬小麦竞争能力与产量的影响

 采用简化的deWit替代系列法研究了盆栽冬小麦‘长武135’（Triticum aestivum cv. Changwu 135）在拔节初期断根对冬小麦根冠关系、竞争能力及其与产量性状的关系。无论干旱或湿润，单栽条件下断根降低了小麦的根冠比，在混栽条件下，断根小麦的相对穗重和相对地上部生物量均显著低于未断根小麦，说明断根降低了小麦的竞争能力。根冠比越大竞争能力越强，即作物的竞争能力与根冠比存在正相关关系。单栽湿润条件,断根降低了小麦的穗重和籽粒产量，而在中等干旱条件下，断根小麦穗重和籽粒产量高于未断根小麦 。说明在水资源充分的条件下，较高的作物个体竞争能力则具有较高的生产能力，而在水资源有限的情况下，降低作物个体竞争能力反而提高了群体籽粒产量。     

Photosynthesis and growth of winter wheat in response to waterlogging at different growth stages

A study on photosynthetic and yield effects of waterlogging of winter wheat at four stages of growth was conducted in specially designed experimental tanks during the 2007–2008 and 2008–2009 seasons. Compared with the control, waterlogging treatments at tillering and jointing-booting stages reduced photosynthetic rate (P _N) and transpiration (E) significantly, it also decreased average leaf water-use efficiency (WUE, defined as the ratio of P _N to E) by 3.3% and 3.4% in both years. All parameters returned quickly to the control level after soil was drained. Damage to the photosynthetic apparatus during waterlogging resulted in a lower F_v/F_m ratio, especially at the first two stages. A strong reduction in root length, root mass, root/shoot ratio, total dry mass, and leaf area index were observed. The responses from vegetative plants at tillering and jointing-booting stages were greater than in generative plants at onset of flowering and at milky stages. The number of panicles per hectare at tillering stage and the spikelet per panicle at the stages of jointing-booting and at onset of flowering were also significantly reduced by waterlogging, giving 8.2–11.3% decrease of the grain yield relative to the control in both years. No significant difference in yield components and a grain yield was observed between the control and treatments applied at milky stages. These responses, modulated by the environmental conditions prevailing during and after waterlogging, included negative effects on the growth, photosynthetic apparatus, and the grain yield in winter wheat, but the effect was strongly stage-dependent.     

渍水对小麦扬麦5号旗叶和根系衰老的影响

1　引　　言生育中后期渍水是长江中下游麦区小麦高产稳产的主要限制因子[13 ,16] .该区由于普遍实行稻麦多熟种植制度 ,前茬水稻使土壤浸水时间过长 ,土壤粘重 ,排水困难 ,透气性差而造成湿害 ;另外 ,该区常年麦季降雨量 5 0 0～ 80 0ｍｍ (浙江省可达 10 0 0ｍｍ)多集中于小麦生长的中后期 ,大大超过了小麦正常需水量 (35 0～ 4 5 0ｍｍ) ,从而加剧渍害[5] .　　研究表明 ,渍水小麦株高、地上部干重、分蘖数、主茎绿叶片数、绿叶面积等都受到影响[1,12 ,16] ,叶片光合速率、气孔导度、细胞间隙ＣＯ2 浓度下降[8] ,ＲｕＢＰＣＯ活性降低[14…     

Process of aerenchyma formation and reactive oxygen species induced by waterlogging in wheat seminal roots

The development and regulation of aerenchyma in waterlogged conditions were studied in the seminal roots of wheat. Evans blue staining and the first cell death position indicated that the cortical cell death began at the root mid-cortex cells in flooding conditions. Continuous waterlogging treatment caused the spread of cell death from the mid-cortex to the neighboring cells and well-developed aerenchyma was formed after 72 h. Meanwhile, the formation of radial oxygen loss barrier was observed in the exodermis owing to the induction of Casparian bands and lignin deposition. Analysis of aerenchyma along the wheat root revealed that aerenchyma formed at 10 mm from the root tip, significantly increased toward the center of the roots, and decreased toward the basal region of the root. In situ detection of radial oxygen species (ROS) showed that ROS accumulation started in the mid-cortex cells, where cell death began indicating that cell death was probably accompanied by ROS production. Further waterlogging treatments resulted in the accumulation of ROS in the cortical cells, which were the zone for aerenchyma development. Accumulation and distribution of H₂O₂ at the subcellular level were revealed by ultracytochemical localization, which further verified the involvement of ROS in the cortical cell death process (i.e., aerenchyma formation). Furthermore, gene expression analysis indicated that ROS production might be the result of up-regulation of genes encoding for ROS-producing enzymes and the down-regulation of genes encoding for ROS-detoxifying enzymes. These results suggest that aerenchyma development in wheat roots starts in the mid-cortex cells and its formation is regulated by ROS.     

高温下干旱和渍水对冬小麦花后旗叶光合特性和物质转运的影响

以蛋白质含量不同的两个冬小麦品种扬麦9号和豫麦34为材料,研究了不同温度和水分条件下小麦花后旗叶光合特性的变化、营养器官花前贮藏干物质和氮素转运特征及其与籽粒产量和品质形成的关系.结果表明,高温及干旱和渍水均明显降低了旗叶光合速率和叶绿素含量（SPAD值）,但高温下干旱和渍水对光合作用的影响加重.小麦营养器官花前贮藏干物质、氮素转运量和转运率在适温下表现为干旱>对照>渍水,高温下则表现为对照>干旱>渍水.适温下花后同化物积累量表现为对照>渍水>干旱,高温下则表现为对照>干旱>渍水.花后氮素积累量在适温和高温下均表现为对照>渍水>干旱.籽粒淀粉含量以适温适宜水分处理最高,高温渍水下最低;蛋白质含量以高温干旱下最高,适温渍水下最低.温度和水分逆境下小麦粒质量和淀粉含量的降低与花后较低的光合能力及干物质积累有关,而蛋白质含量则与花前贮藏氮素的转运量和转运率有关.     

Review of wheat improvement for waterlogging tolerance in Australia and India: the importance of anaerobiosis and element toxicities associated with different soils

Background and Aims

The lack of knowledge about key traits in field environments is a major constraint to germplasm improvement and crop management because waterlogging-prone environments are highly diverse and complex, and the mechanisms of tolerance to waterlogging include a large range of traits. A model is proposed that waterlogging tolerance is a product of tolerance to anaerobiosis and high microelement concentrations. This is further evaluated with the aim of prioritizing traits required for waterlogging tolerance of wheat in the field.

Methods

Waterlogging tolerance mechanisms of wheat are evaluated in a range of diverse environments through a review of past research in Australia and India; this includes selected soils and plant data, including plant growth under waterlogged and drained conditions in different environments. Measurements focus on changes in redox potential and concentrations of diverse elements in soils and plants during waterlogging.

Key Results

(a) Waterlogging tolerance of wheat in one location often does not relate to another, and (b) element toxicities are often a major constraint in waterlogged environments. Important element toxicities in different soils during waterlogging include Mn, Fe, Na, Al and B. This is the first time that Al and B toxicities have been indicated for wheat in waterlogged soils in India. These results support and extend the well-known interactions of salinity/Na and waterlogging/hypoxia tolerance.

Conclusions

Diverse element toxicities (or deficiencies) that are exacerbated during waterlogging are proposed as a major reason why waterlogging tolerance at one site is often not replicated at another. Recommendations for germplasm improvement for waterlogging tolerance include use of inductively coupled plasma analyses of soils and plants.Key words: Waterlogging, microelements, toxicity, redox potential, wheat, anaerobiosis

‘No grain is ever produced without water, but too much water tends to spoil the grain and inundation is as injurious to growth as dearth of water.’ Narada Smriti XI, 19; circa 3000 bc.‘Waterlogging’ is defined as a condition of the soil where excess water limits gas diffusion; while ‘waterlogging tolerance’ is defined as survival or the maintenance of high growth rates, biomass accumulation or grain yield under waterlogging relative to non waterlogged (usually drained soil) conditions (Setter and Waters, 2003).

     

Aerenchyma and an Inducible Barrier to Radial Oxygen Loss Facilitate Root Aeration in Upland, Paddy and Deep-water Rice (Oryza sativa L.)

The present study evaluated waterlogging tolerance, root porosityand radial O₂ loss (ROL) from the adventitious roots, of sevenupland, three paddy, and two deep-water genotypes of rice (Oryzasativa L.). Upland types, with the exception of one genotype,were as tolerant of 30 d soil waterlogging as the paddyand deep-water types. In all but one of the 12 genotypes, thenumber of adventitious roots per stem increased for plants grownin waterlogged, compared with drained, soil. When grown in stagnantdeoxygenated nutrient solution, genotypic variation was evidentfor root porosity and rates of ROL, but there was no overalldifference between plants from the three cultural types. Adventitiousroot porosity increased from 20–26 % for plants grownin aerated solution to 29–41 % for plants grown instagnant solution. Growth in stagnant solution also induceda ‘tight’ barrier to ROL in the basal regions ofadventitious roots of five of the seven upland types, all threepaddy types, and the two deep-water types. The enhanced porosityprovided a low resistance pathway for O₂ movement to the roottip, and the barrier to ROL in basal zones would have furtherenhanced longitudinal O₂ diffusion towards the apex, by diminishinglosses to the rhizosphere. The plasticity in root physiology,as described above, presumably contributes to the ability ofrice to grow in diverse environments that differ markedly insoil waterlogging, such as drained upland soils as well as waterloggedpaddy fields.