Green manure technology: Potential,usage, and limitations. A case study for lowland rice

The growing concern about the sustainability of tropical agricultural systems stands in striking contrast to a world-wide decline in the use of soil-improving legumes. It is timely to assess the future role that soil-improving legumes may play in agricultural systems. This paper reviews recent progress, potential, and limitations of green manure technology, using lowland rice cropping systems as the example.Only a few legume species are currently used as green manures in lowland rice. Sesbania cannabina is the most widely used pre-rice green manure for rice in the humid tropics of Africa and Asia. Astragalus sinicus is the prototype post-rice green manure species for the cool tropics. Stem-nodulating S. rostrata has been most prominent in recent research. Many green manure legumes show a high N accumulation (80–100 kg N ha^-1 in 45–60 days of growth) of which the major portion (about 80%) is derived from biological N₂ fixation. The average amounts of N accumulated by green manures can entirely substitute for mineral fertilizer N at current average application rates. With similar N use efficiencies, green manure N is less prone to loss mechanisms than mineral N fertilizers and may therefore contribute to long-term residual effects on soil productivity.Despite a high N₂-fixing potential and positive effects on soil physical and chemical parameters, the use of green manure legumes for lowland rice production has declined dramatically world-wide over the last 30 years. Land scarcity due to increasing demographic pressure and a relatively low price of urea N are probably the main determining factors for the long-term reduction in pre-rice green manure use. Post-rice green manures were largely substituted for by high-yielding early-maturing grain legumes. Unreliability of green manure performance, non-availability of seeds, and labor intensive operations are the major agronomic constraints. The recognition and extrapolation of niches where green manures have a comparative advantage may improve an often unfavorable economic comparison of green manure with cash crop or fertilizer N. Socio-economic factors like the cost of land, labor, and mineral N fertilizer are seen to determine the cost-effectiveness and thereby farmers' adoption of sustainable pre-rice green manure technology. Hydrology and soil texture determine the agronomic competitiveness of a green manure with N fertilizers and with alternative cash crops. In general, the niches for pre-rice green manure are characterized by a relatively short time span available for green manure growth and a soil moisture regime that is unfavorable for cash crops (flood-prone rainfed lowlands with coarse-textured soils).Given the numerous agronomic and socio-economic constraints, green manure use is not seen to become a relevant feature of favourable rice-growing environments in the foreseeable future. However, in environments where soil properties and hydrology are marginal for food crop production, but which farmers may be compelled to cultivate in order to meet their subsistence food requirements, green manures may have a realistic and applicable potential.     

Submergence tolerance of rice – A new glasshouse method for the experimental submergence of plants

A new method is described for evaluation of submergence tolerance of rice ( Oryza sativa L.) plants. Responses of a range of cultivars corresponded with known differences in field performance. The method 1) allows fast and effective determination of submergence tolerance, 2) allows screening of many plants in a small glasshouse area, 3) provides for recovery of superior plants for seed collection, 4) allows manipulation of many environmental variables to mimic the natural submergence environment, and 5) uses simple, inexpensive, readily available equipment. Physiological studies performed with this method gave results similar to those obtained from field studies and showed that submergence tolerance increased in older plants; it decreased with increasing depth, increasing temperature and with high or low light levels. The system is ideal for the rapid evaluation of rice germplasm under controlled conditions and physiological studies on the mechanism of rice submergence tolerance.     

Submergence tolerance in relation to variable floodwater conditions in rice

Flash floods adversely affect rice productivity in vast areas of rainfed lowlands in South and Southeast Asia and tropical Africa. Tolerant landraces that withstand submergence for 1–2 weeks were identified; however, incorporation of tolerance into modern high-yielding varieties through conventional breeding methods has been slow because of the complexity of both the tolerance phenotype and floodwater conditions, and the ensuing discrepancies encountered upon phenotyping in different environments. Designing an effective phenotyping strategy requires a thorough understanding of the specific floodwater characteristics that most likely affect survival during flooding. We investigated the implications of floodwater temperature and light penetration, caused by artificial shading, seasonal variation, or water turbidity, for seedling survival after submergence. Three field experiments were conducted using rice genotypes contrasting in their tolerance of submergence: FR13A and Kusuma (tolerant); Gangasiuli (intermediate); Sabita, CRK-2-6 and Raghukunwar (elongating/avoiding types); and IR42 (sensitive). We tested the hypotheses that warmer floodwater decreases plant survival and that turbid water augment plant mortality by causing effects similar to those caused by shading, by reducing light penetration. Plants survive better when water is cooler, and survival decreased at about 8% per unit increase in water temperature above 26 °C. Lower intensity of light and warmer temperatures seem to reduce biomass and increase mortality under flooding. An increase in the concentrations of O₂ and CO₂ and a decrease in water pH did not improve survival in clear unshaded water. Turbid floodwater was more damaging to rice as plant mortality increased as the percentage of silt increased, and the effects of water turbidity cannot be explained by the reduction in light penetration alone. Even the most tolerant rice cultivar, FR13A, experienced higher mortality when flooded with turbid floodwater. Correlation studies revealed that cultivars with the capacity to maintain higher biomass, higher chlorophyll, and non-structural carbohydrate concentrations after submergence had higher survival. These findings help to understand the variation observed in submergence tolerance when screening is done under different environments. The study could have implications for designing proper screening strategies and assessing the damage submergence causes across different rice-growing regions.     

Phosphorus cycling in rainfed lowland rice ecosystems on sandy soils

Phosphorus cycling in rainfed lowland rice ecosystems is poorly understood. Soil drying and grazing of rice straw during the long dry season, the growth of volunteer pastures during the early wet season, and intermittent loss of soil-water saturation while the rice crop is growing are important distinguishing characteristics of the rainfed lowlands in relation to P cycling. We studied P cycling in an acid sandy rainfed lowland soil that covers about 30% of the rice growing area of Cambodia. Soils with similar properties in comparable rainfed sub- ecosystems occur in Laos and northeast Thailand. We developed a general schema of P pools and fluxes in the crop and soil for rice-based cropping systems in the rainfed lowlands of Cambodia. The schema was derived from a number of field experiments carried out over five consecutive cropping seasons to quantify the residual value of P fertiliser, P mass balances, soil P fractions, the effect on subsequent rice crops of crop residues and volunteer pastures incorporated into the soils, and the dynamics of P turnover in the soil. With a single rice crop yielding 2.5–3 t ha⁻¹, application of 8–10 kg P ha⁻¹ maintained yields and a small positive P balance in the soil. However, the soil P balance was sensitive to the proportion of rice straw returned to the soil. Volunteer pastures growing during the early wet season accumulated significant amounts of P, and increased their P uptake when soils were previously fertilised with P. These pastures recycled 3–10 kg P ha⁻¹ for the succeeding rice crops. While inorganic soil P pools extractable with ion exchange resins and 0.1 M NaOH appeared to be the main source of P absorbed by rice, microbial and organically-bound P pools responded dynamically to variation in soil water regimes of the main wet, dry and early wet seasons. The schema needs to be developed further to incorporate site-specific conditions and management factors that directly or indirectly affect P cycling, especially loss of soil-water saturation during the rice cropping cycle. The paper discusses the application of results for acid sandy soils to other significant rice soils in the rainfed lowlands of southeast Asia.     

Changes in cell wall polysaccharides in the internodes of submerged floating rice

In excised stem segments of floating rice (Oryza sativa L.), as well as in intact plants, submergence greatly stimulates the elongation of internodes. The differences in the composition of cell wall polysaccharides along the highest internodes of submerged and air-grown stem segments were examined. The newly elongated parts of internodes that had been submerged for two days contained considerably less cellulosic and noncellulosic polysaccharides than air-grown internodes, an indication that the cell walls of the newly elongated parts of submerged internodes are extremely thin. In the young parts of both air-grown and submerged internodes, the relative amounts of noncellulosic polysaccharides were equal to those of -cellulose, whereas the relative amounts of -cellulose were higher than those of noncellulosic polysaccharides in the upper, old parts. In the cell-elongation zones of both air-grown and submerged internodes, glucose was predominant among the noncellulosic neutral sugars of cell wall. The relative amount of glucose in noncellulosic neutral sugars decreased toward the upper, old parts of internodes, whereas that of xylose increased.     

A major locus for submergence tolerance mapped on rice chromosome 9

Submergence stress is a widespread problem in rice-growing environments where drainage is impeded. A few cultivars can tolerate more than 10 days of submergence, but the genes conferring this tolerance have not been identified. We used randon-amplified polymorphic DNA (RAPD) and restriction fragment length polymorphism (RFLP) markers to map submergence tolerance in 169 F₂ plants and the resulting F₃ families of a cross between a tolerant indica rice line, IR40931-26, and a susceptible japonica line, PI543851. IR40931-26 inherited strong submergence tolerance from the unimproved cultivar FR13A. Eight-day old F₃ seedlings were submerged for 14–16 days in 55-cm deep tanks, and tolerance was scored after 7 days recovery on a scale of 1 (tolerant) to 9 (susceptible). The tolerant and susceptible parents scored 1.5 and 8.4, respectively, and the F₃ means ranged from 1.6 to 8.9. Two bulks were formed with DNA from F₂ plants corresponding to the nine most tolerant and the nine most susceptible F₃ families. Of 624 RAPD primers used to screen the bulks, five produced bands associated with either tolerance or susceptibility. These markers were mapped to a region of chromosome 9 by linkage to RFLP markers. A submergence tolerance quantitative trait locus (QTL), here designatedSub1, was located ca. 4 cM from the RFLP marker C1232 and accounted for 69% of the phenotypic variance for the trait.     

Zinc-deficiency symptoms in lowland rice as induced by modified-urea materials applied at different rates of nitrogen in calcareous soil

Summary Studies revealed that Zn-deficiency symptoms were induced markedly by the levels of nitrogen and its source in rainfed lowland rice grown on calcareous soil. Visual Zn-deficiency symptoms recorded 3 weeks after transplanting showed that increased supply of nitrogen at puddling resulted in significant increase in the extent of deficiency symptoms of this nutrient element. Zn-deficiency symptoms got aggravated with Mussorrie Rock Phos-coated urea (MRPCU) followed by sulphur-coated urea (SCU). Zn-deficiency symptoms induced by urea supergranules (USG) and prilled urea (PU) supplying two-thirds of nitrogen as basal were found to be of moderate level. No symptoms of Zn-deficiency were noted with no-nitrogen control.     

水稻萌发耐淹性的遗传分析

水稻(Oryza sativa)萌发耐淹性受到复杂的网络调控, 其分子机制不同于苗期耐淹性的相关机制, 萌发耐淹性的强弱影响着直播稻的成苗率。通过对256份水稻核心种质的萌发耐淹性评估, 发现粳稻和籼稻之间的萌发耐淹性差异并不显著, 都存在广泛的遗传变异。利用以粳稻R0380为供体亲本, 籼稻RP2334为轮回亲本的170个高代回交自交系构建含146个分子标记的连锁图谱, 以低氧胚芽鞘长度为性状指标, 通过复合区间作图法检测到影响萌发耐淹性的4个QTLs(quantitative trait loci), 分别定位于第2(2个)、3(1个)和8号(1个)染色体。贡献率最大的QTL为qGS2.2, 其值为17.34%, 增效等位基因来自轮回亲本籼稻RP2334; 其余3个QTLs的增效等位基因均来自供体亲本粳稻R0380, 贡献率分别为12.86%、9.37%和14.60%。     

Submergence effects on rice genotypes during seedling stage: Probing of submergence driven changes of photosystem 2 by chlorophyll a fluorescence induction O-J-I-P transients

The effects of submergence on chlorophyll (Chl) a fluorescence were compared in seven Oryza sativa (L.) cultivars, namely FR 13A, Khoda, Khadara, Kalaputia (tolerant), Sabita, and Hatipanjari (avoiding type), and IR 42 (susceptible). Seedlings were submerged for 4 d under complete darkness. Oxygen concentration of flood water decreased with the period of submergence with concomitant increase in concentration of carbon dioxide. Submergence caused diminution in the amount of total Chl. Genotypic differences were observed for Chl content and survival percentage. Quantification of the Chl a fluorescence transients (JIP-test) revealed large cultivar differences in the response of photosystem 2 (PS2) to submergence. The kinetics of Chl a fluorescence rise showed complex changes in the magnitudes and rise of O-J, J-I, and I-P phases caused by submergence. The selective suppression of the J-I phase of fluorescence especially after 2 d of submergence provided evidence for weakened electron donation from the oxygen evolving complex whereas under severe submergence stress (4 d) both O-J and J-I steps were suppressed greatly with highly suppressed P-step, which resulted in lowering of variable fluorescence. Grouping probability or energetic connectivity between PS2 obtained through JIP-test from the data after 2 d of submergence showed a direct relation with survival percentage, i.e. fluorescence measurements contained the information of the survival chance of a plant under submerged conditions. The information could be used in identifying the submergence tolerant cultivars when the damage is not very severe.     

Arabitol dehydrogenase as a selectable marker for rice

Arabitol dehydrogenase has been adapted for use as a plant selectable marker. Arabitol is a five-carbon sugar alcohol that can be used by E. coli strain C, but not by the laboratory K12 strains. The enzyme converts the non-plant-metabolizable sugar arabitol into xylulose, which is metabolized by plant cells. Rice was transformed with a plant-expression-optimized synthetic gene using Biolistic-mediated transformation. Selection on 2.75% arabitol and 0.25% sucrose yielded a transformation efficiency (9.3%) equal to that obtained with hygromycin (9.2%). Molecular analyses showed that the atlD gene was integrated into the rice genome of selected plants and was inherited in a Mendelian manner. This study indicates that arabitol could serve as an effective means of plant selection.     

Crystal structure of nucleoside diphosphate kinase required for coleoptile elongation in rice (Oryza sativa L.)

Nucleoside diphosphate kinase (NDK) is a ubiquitous enzyme found in all organisms and cell types, and catalyzes the transfer of the phosphoryl group from a nucleoside triphosphate to a nucleoside diphosphate. The enzyme is involved in and required for coleoptile elongation in rice as the level of the rice NDK (rNDK) changes during seed germination and the early stages of seedling growth. The expression of rice NDK gene is up-regulated in the growing coleoptiles when the anaerobic stress persists. The rNDK structure determined at 2.5 A resolution consists of a four-stranded anti-parallel beta-sheet, of which the surfaces are partially covered with six alpha-helices; its overall and active site structures are similar to those of homologous enzymes except the major conformation variations of residue 132-138 regions, involving significant structural contacts. The model contains 148 residues of 149 residues in total and averaged 19 water molecules per monomer for 12 molecules in an asymmetric unit. A mold of 12 superimposed molecules shows that the alphaA-alpha2 area has greater variations and higher temperature factors, indicating the flexibility for a substrate entrance. Hexameric molecular packing in both crystal and solution implies that rNDK functions as hexamers. This rNDK structure, which is the first NDK structure from a higher plant system, provides the structural information essential to understand the functional significance of this enzyme during growth and development in both rice and other plants.     

Salt tolerance of rice cultivars

Summary The dry matter production and the concentration of nutrients in rice (Oryza sativa L.) cultivars from soil adjusted to different levels of salinity were evaluated under a greenhouse conditions. Soil salinity levels were produced by applying 0.34 mol l^–1 solution of NaCl which resulted in the following levels, control (0.29), 5, 10 and 15 dS m^–1 conductivity of saturation extract. The effect of salinity on dry matter production varied from cultivar to cultivar.The concentrations of P and K in the tops of rice cultivars decreased with increasing soil salinity. But the concentrations of Na, Zn, Cu and Mn increased.Significant varietal differences were found in relation to salinity tolerance. Based on dry matter yield reduction, rice cultivars were classified as tolerant, moderately tolerant, moderately susceptible or susceptible.     

水稻茎线虫病研究进展

水稻茎线虫病是水稻主要病害之一,能造成水稻严重减产。在我国水稻茎线虫属于检疫对象。该病害在孟加拉国、印度及缅甸等危害严重。本文结合国内外研究现状,从水稻茎线虫病原、形态、分布、症状等方面综述了水稻茎线虫病的研究进展,以期为我国开展检疫和防治提供参考。     

Soil physical behaviour and crop responses to tillage in lowland rice soils of varying clay content

The influence of various tillage methods on two wetland rice soils in the Philippines is reported. The soils differed principally in clay content, 38% for the clay loam (clayey, mixed isohyperthermic Entic Hapludoll) while 56% for the clay (clayey, mixed noncalcareous, isohyperthermic Andaqueptic Haplaquoll). This had a marked effect on their response to tillage and varying water regime. The clay soil, under field conditions, showed little change in pore size distribution or soil water behaviour with different tillage methods. Crop (Rice, Oryza sativa L., var. IR20) yields were unaffected by tillage.In contrast, tillage effects were very marked in the clay loam soil, which consisted of a greenhouse and a field trial. In the greenhouse, which experienced severe dry periods, wet tillage not only increased the moisture retentivity but also the soil impedance at soil matric potential ()<–0.01 MPa. Seasonal average was <–1 MPa. Root length density decreased by 39% with dry tillage and by 56% with wet tillage compared with zero tillage. Grain yield however, did not vary with soil treatment. In the field, which experienced moderate dry spells, varied between –0.13 and –0.48 MPa. Root length density was significantly reduced at soil impedance >0.75 MPa. Wet tillage increased soil moisture storage which minimized the soil impedance during the dry cycle more effectively than did dry tillage. The crop performed best under wet tillage and least under zero tillage. Wet tillage in this soil was more effective under moderate than under severe water stress conditions.     

A role of <Emphasis Type="Italic">OsGA20ox1</Emphasis>, encoding an isoform of gibberellin 20-oxidase,for regulation of plant stature in rice

Gibberellin (GA) 20-oxidase (GA20ox) is a key enzyme that normally catalyzes the penultimate steps in GA biosynthesis. One of the GA20ox genes in rice (Oryza sativaL.), OsGA20ox2 (SD1), is well known as the Green Revolution gene, and loss-of function mutation in this locus causes semi-dwarfism. Another GA20ox gene, OsGA20ox1, has also been identified, but its contribution to plant stature has remained unclear because no suitable mutants have been available. We isolated a mutant, B142, tagged with a T-DNA containing three CaMV 35S promoters, which showed a tall, GA-overproduction phenotype. The final stature of the B142 mutant reflects internode overgrowth and is approximately twice that of its wild-type parent. This mutant responds to application of both GA3 and a GA biosynthesis inhibitor, indicating that it is a novel tall mutant of rice distinct from GA signaling mutants such as slr1. The integrated T-DNAs, which contain three CaMV 35S promoters, are located upstream of the OsGA20ox1 open reading frame (ORF) in the B142 mutant genome. Analysis of mRNA and the endogenous GAs reveal that biologically active GA level is increased by up-regulation of the OsGA20ox1 gene in B142. Introduction of OsGA20ox1 cDNA driven by 35S promoter into the wild type phenocopies the morphological characteristics of B142. These results indicate that the elongated phenotype of the B142 mutant is caused by up-regulation of the OsGA20ox1 gene. Moreover, the final stature of rice was reduced by specific suppression of the OsGA20ox1 gene expression. This result indicates that not only OsGA20ox2 but also OsGA20ox1 affects plant stature.     

Anoxia tolerance and ethanol sensitivity of rice and oat coleoptiles

Anoxia tolerance and ethanol sensitivity of rice (Oryza sativa L.) and oat (Avena sativa L.) seedlings were evaluated to clarify their growth habit in anoxia. Anoxic stress inhibited elongation and dry weight gain of coleoptiles of the oat and rice seedlings; however, the inhibition of the oat coleoptiles was much greater than that of the rice coleoptiles. Anoxic stress increased endogenous ethanol concentration and alcohol dehydrogenase activity in oat and rice coleoptiles and their increases in the rice coleoptiles were much greater than those in the oat coleoptiles. At concentrations greater than 30 mM and 300 mM, exogenously applied ethanol inhibited the elongation and weight gain for the oat and the rice coleoptiles, respectively, and the inhibition was increased with increasing ethanol concentrations with marked inhibition being achieved on the oat coleoptiles. These results suggest that anoxia tolerance and induction of ethanolic fermentation in anoxia may be greater in rice than oat, and ethanol sensitivity of rice may be lower than that of oat.     

Transgenic rice with low sucrose-phosphate synthase activities retain more soluble protein and chlorophyll during flag leaf senescence

We investigated whether changes in sucrose-phosphate synthase (EC 2.4.1.14, SPS) activity could alter N remobilization during leaf senescence. Transgenic rice (Oryza sativa L. cv. Nipponbare) with low SPS activities and wild-type rice plants were grown with basal N (1.0 mM NH₄NO₃) until the late vegetative stage. Subsequently, half of the plants were transferred to a low N (0.1 mM NH₄NO₃) condition to accelerate leaf senescence, and the others were continuously grown with basal N. With low N supply, the amounts of chlorophyll and soluble protein in flag leaf blades decreased after anthesis in both the low SPS plants and wild-type plants, although the decrease was less in the low SPS plants. Panicle weights were significantly lower in the low SPS plant than in the wild-type plant. These results suggest that the remobilization of N from flag leaves was diminished by suppressing the development of reproductive sinks in the low SPS plant.     

Screening of rice (Oryza sativa L.) genotypes for physiological characters contributing to salinity resistance,and their relationship to overall performance

Summary Phenotypic resistance of salinity is expressed as the ability to survive and grow in a salinised medium. Some subjective measure of overall performance has normally been used in plant breeding programmes aimed at increasing salinity resistance, not only to evaluate progeny, but to select parents. Salinity resistance has, at least implicitly, been treated as a single trait. Physiological studies of rice suggest that a range of characteristics (such as low shoot sodium concentration, compartmentation of salt in older rather than younger leaves, tolerance to salt within leaves and plant vigour) would increase the ability of the plant to cope with salinity. We describe the screening of a large number of rice genotypes for overall performance (using an objective measure based on survival) and for the aforementioned physiological traits. There was wide variation in all the characters studied, but only vigour was strongly correlated with survival. Shoot sodium concentration, which a priori is expected to be important, accounted for only a small proportion of the variability in the survival of salinity. Tissue tolerance (the cellular component of resistance reflecting the ability to compartmentalise salt within leaves) revealed a fivefold range between genotypes in the tolerance of their leaves to salt, but this was not correlated positively with survival. On the basis of such (lack of) correlation, these traits would be rejected in normal plant breeding practice, but we discuss the fallacies involved in attempting correlation between individual traits and the overall performance of a salt-sensitive species in saline conditions. We conclude that whilst overall performance (survival) can be used to evaluate the salt resistance of a genotype, it is not the basis on which parents should be selected to construct a complex character through breeding. It was the norm for varieties which had one good characteristic affecting salt resistance to be unexceptional or poor in the others. This constitutes experimental evidence that the potential for salt resistance present in the rice genome has not been realised in genotypes currently extant. The results are discussed in relation to the use of physiological traits in plant breeding, with particular reference to environmental stresses that do not affect a significant part of a species' ecological range.     

Identification of candidate markers associated with agronomic traits in rice using discriminant analysis

Plant genetic mapping strategies routinely utilize marker genotype frequencies obtained from progeny of controlled crosses to declare presence of a quantitative trait locus (QTL) on previously constructed linkage maps. We have evaluated the potential of discriminant analysis (DA), a multivariate statistical procedure, to detect candidate markers associated with agronomic traits among inbred lines of rice (Oryza sativa L.). A total of 218 lines originating from the US and Asia were planted in field plots near Alvin, Texas, in 1996 and 1997. Agronomic data were collected for 12 economically important traits, and DNA profiles of each inbred line were produced using 60 SSR and 114 RFLP markers. Model-based methods revealed population structure among the lines. Marker alleles associated with all traits were identified by DA at high levels of correct percent classification within subpopulations and across all lines. Associated marker alleles pointed to the same and different regions on the rice genetic map when compared to previous QTL mapping experiments. Results from this study suggest that candidate markers associated with agronomic traits can be readily detected among inbred lines of rice using DA combined with other methods described in this report.N. Zhang and Y. Xu contributed equally to work and considered as first authors.Approved for publication by the Director of the Louisiana Agricultural Experiment Station as paper no. 04-14-0335.