Leaf gas exchange of upland and lowland rice cultivars

Leaf gas exchange of upland and lowland rice cultivars were measured during late vegetative and during grain filling stages in the field under upland and lowland growth conditions. The rate of photosynthesis and water use efficiency (the rate of photosynthesis/the rate of transpiration) under upland conditions decreased with ageing, but generally varied little among four cultivars. At mid-grain filling under lowland conditions, upland cultivars showed lower rates of photosynthesis and transpiration than the lowland cultivars with concomitant reduction in whole plant conductance. At this stage, water use efficiency was higher under upland conditions than under lowland conditions, particularly in the upland cultivars. Water stress reduced the rate of photosynthesis without altering water use efficiency.     

The effects of root-induced pH changes on the depletion of inorganic and organic phosphorus in the rhizosphere

A new method allowing control of rhizosphere pH and mineral nutrition was applied to study depletion of various organic and inorganic phosphorus fractions extractable sequentially with 0.5M KHCO₃ (pH 8.5), 0.1M NaOH and residual P extractable with 6M H₂SO₄ from the rhizosphere soil.Soil pH was affected about 2 mm from the root mat. Depletion zones of inorganic P (KHCO₃-P_i) extractable with 0.5M KHCO₃ extended up to about 4 mm but the depletion zones of all other P fractions were about 1 mm only. The root-induced decrease of soil pH from 6.7 to 5.5 increased the depletion of total P from all fractions by 20% and depletion of KHCO₃-P_i and residual P by 34% and 43%, respectively. Depletion of organic P (KHCO₃-P_o) extractable with 0.5M KHCO₃ was not affected by a change in rhizosphere pH. With constant or increased pH, depletion of inorganic P (NaOH-P_i) was 17% and organic P (NaOH-P_o) was 22% higher than with decreased pH. Only 54–60% of total P withdrawn from all fractions was from KHCO₃-P_i. Substantial amounts of KHCO₃-P_o and NaOH-P_o were mineralized and withdrawn from the rhizosphere within 1 mm from the root mat, as 11–15% of total P withdrawn originated from the organic P fractions. A remaining 11–16% was derived from NaOH-P_i, and 15–18% from residual P fractions likely to be rather immobile. Thus, 40–46% of the P withdrawn near the root mat of rape originated from non-mobile P fractions normally not included in 0.5M NaHCO₃ extraction used to obtain an index of plant-available soil P.     

Bioavailability of soil inorganic P in the rhizosphere as affected by root-induced chemical changes: a review

In most soils, inorganic phosphorus occurs at fairly low concentrations in the soil solution whilst a large proportion of it is more or less strongly held by diverse soil minerals. Phosphate ions can indeed be adsorbed onto positively charged minerals such as Fe and Al oxides. Phosphate (P) ions can also form a range of minerals in combination with metals such as Ca, Fe and Al. These adsorption/desorption and precipitation/dissolution equilibria control the concentration of P in the soil solution and, thereby, both its chemical mobility and bioavailability. Apart from the concentration of P ions, the major factors that determine those equilibria as well as the speciation of soil P are (i) the pH, (ii) the concentrations of anions that compete with P ions for ligand exchange reactions and (iii) the concentrations of metals (Ca, Fe and Al) that can coprecipitate with P ions. The chemical conditions of the rhizosphere are known to considerably differ from those of the bulk soil, as a consequence of a range of processes that are induced either directly by the activity of plant roots or by the activity of rhizosphere microflora. The aim of this paper is to give an overview of those chemical processes that are directly induced by plant roots and which can affect the concentration of P in the soil solution and, ultimately, the bioavailability of soil inorganic P to plants. Amongst these, the uptake activity of plant roots should be taken into account in the first place. A second group of activities which is of major concern with respect to P bioavailability are those processes that can affect soil pH, such as proton/bicarbonate release (anion/cation balance) and gaseous (O₂/CO₂) exchanges. Thirdly, the release of root exudates such as organic ligands is another activity of the root that can alter the concentration of P in the soil solution. These various processes and their relative contributions to the changes in the bioavailability of soil inorganic P that can occur in the rhizosphere can considerably vary with (i) plant species, (ii) plant nutritional status and (iii) ambient soil conditions, as will be stressed in this paper. Their possible implications for the understanding and management of P nutrition of plants will be briefly addressed and discussed.     

氮素形态对专用小麦中后期根际土壤微生物和酶活性的影响

采用盆栽方法研究了酰胺态氮、铵态氮和硝态氮对强筋小麦(Triticum aestivum L.)"豫麦34"、中筋小麦"豫麦49"和弱筋小麦"豫麦50"生育中后期根际微生物和土壤酶活性的影响.结果表明,专用小麦根际真菌、细菌、放线菌数量和土壤脲酶、蛋白酶、硝酸还原酶活性以及根际pH值对氮素形态的反应不同."豫麦34"施用硝态氮,对根际土壤真菌、细菌(除成熟期外)和放线菌数量均具有明显的促进作用;"豫麦49"施用铵态氮,根际土壤细菌和放线菌数量最大,根际真菌数量在孕穗期和开花期以酰胺态氮处理最大,而成熟期以硝态氮处理最大;"豫麦50"施用硝态氮,对根际土壤真菌、细菌和放线菌数量均具有明显的促进作用.不同专用小麦品种均表现为在酰胺态氮处理下,根际土壤脲酶活性最高;在铵态氮处理下,根际土壤蛋白酶活性最高;在硝态氮处理下,根际土壤硝酸还原酶活性和pH值最高.     

高产磷高效水稻磷素吸收利用特征

通过正常供磷的大田试验（2011年）,以产量和磷籽粒生产效率为指标,将27份中熟水稻亲本材料划分为4个类型,再通过正常和低磷处理的土培试验（2012年）,筛选出高产磷高效水稻材料,并探讨各种磷效率对产量的贡献率.结果表明： 结合两年的试验结果,供试材料的产量和磷利用效率均存在显著的基因型差异,筛选出GR泸17/矮TTP//泸17_2（QR20）为高产磷高效材料.在两个供磷水平下,QR20的产量和磷利用效率均显著高于低产磷低效材料玉香B,其产量分别是玉香B的1.96和1.92倍.大田和土培试验结果均表明,磷积累量对产量的贡献率均高于磷籽粒生产效率和磷收获指数.正常供磷条件下,磷积累量和磷籽粒生产效率对产量的贡献率差异不大,低磷条件下差异较大（66.5%和26.6%）,磷收获指数对产量的贡献率最低,最高仅为11.8%（土培）.土培试验中,正常供磷条件下,拔节-抽穗阶段的磷积累量对产量和磷收获指数的贡献率最高,分别为93.4%和85.7%,对磷籽粒生产效率的贡献率为41.8%;在低磷条件下,分蘖-拔节阶段的磷积累量对产量和磷籽粒生产效率的贡献率最高,分别为56.9%和20.1%,对磷收获指数的贡献率为16.0%.土培正常供磷条件下,水稻QR20的产量、磷积累量和磷收获指数相对于低磷处理分别增加了20.6%、18.1%和18.2%,差异显著.综上,磷效率对水稻产量的贡献率大小依次为磷吸收效率＞利用效率＞转运效率;正常供磷条件下,拔节-抽穗阶段的磷积累量对产量的贡献率最高,低磷胁迫下,分蘖-拔节阶段的磷积累量对产量的贡献率最高,这两个阶段可能是水稻高产磷高效协调统一的关键时期.
     

Effect of nitrogen forms on growth and chemical changes in the rhizosphere of strawberry plants

The experiment was set up to examine the influence of different nitrogen forms: (NH₄)₂SO₄, Ca(NO₃)₂ or NH₄NO₃ on growth response, root induced pH changes in the rhizosphere, root-borne acid phosphatase activity in strawberry plants cv. Senga Sengana. The plants grown on sandy mineral soil were fertilized with 3 forms of nitrogen, in concentrations of 46 mg N·kg⁻¹ soil. The plants were grown in rhizoboxes with removable plexiglass lids. To ensure the root growth along the plexiglass lids, the rhizoboxes were placed at an angle of about 50° with the lid on the lower side. In case of ammonium supply, the nitrification inhibitor DIDIN was added (10 mg·kg⁻¹ of moist soil) to prevent conversion of ammonium into nitrate. The growth response (roots and shoots) of strawberry plants were determined after 11 weeks of treatment with different N forms. The best development of the root system and shoots (root and shoot dry weight and root length) was obtained, when ammonium nitrate was supplied. It is suggested therefore, that NH₄NO₃ stimulates vegetative growth of strawberry plants cv. Senga Sengana. However, there were no statistical differences in a leaf and flower number of the plants grown under different forms of N-fertilization. Determination of rhizosphere pH, and acid phosphatase activity were executed using non-destructive techniques, which enabled weekly measurement of chemical changes in the rhizosphere. The results revealed that the form of nitrogen supplied had a predominant effect on chemical changes in the rhizosphere of strawberry plants. The highest pH values (average pH 6.8) were measured in the rhizosphere of individual plants supplied with Ca(NO₃)₂. Whereas the lowest pH values (average pH 5.8) were detected in the presence of (NH₄)₂SO₄. The curve of rhizosphere pH measured along individual roots of the plants treated with Ca(NO₃)₂ represents the highest pH values whereas the curve of rhizosphere pH under (NH₄)₂SO₄ treatment had the lowest pH values. The highest activity of acid phosphatase were observed in the rhizosphere of strawberry plants grown in the presence of (NH₄)₂SO₄, at pH 5.8.     

New Phytologist 140 (1998), 709–714

In the December 1998 issue of New Phytologist , we published the research paper entitled 'Natural occurrence of Ampelomyces intracellular mycoparasites in mycelia of powdery mildew fungi' by Levente Kiss ( New Phytol. (1998) 140 , 709–714). Since its publication, an error in the published title has been brought to our attention: this refers to the ampelomyces , but the name should be Ampelomyces , meaning the genus.
We apologise to the author and to our readers for this mistake.     

Effectiveness of Sesbania rostrata and Phaseolus calcaratus as green manure for upland rice grown in acidic soil

Two field experiments on green manuring were conducted under upland acidic soil (pH = 4.35) conditions with the following objectives: (1) to determine the influence of inoculation site, P fertilization, and liming on the biomass production, N content, N accumulation, and N availability of S. rostrata grown in an acidic soil, (2) to compare the effectiveness of S. rostrata, P. calcaratus and urea as N sources for upland rice as affected by liming and N source-sowing time combination, and (3) to assess the effect of liming and N source-sowing time combination on % Ndff (N derived from the fertilizer), % Ndfs (N derived from soil), % FNU (fertilizer N utilization), and FNY or fertilizer N yield (kg N ha^–1) of upland rice grown in acidic soil. At 2 weeks after incorporating S. rostrata (95 days after lime application), liming significantly increased N availability by more than 2-fold suggesting that the decomposition of S. rostrata by soil microflora was stimulated by lime. Liming, phosphorus application, and inoculation site improved significantly the dry biomass production, N content and N accumulation of S. rostrata; thus, enhancing its green manuring potential. Regardless of liming, S. rostrata whether applied at 0 week or 2 weeks before sowing was superior to urea in improving grain and straw yields. P. calcaratus when applied at 2 weeks before sowing also produced higher grain yield than urea. Immediate sowing of upland rice after green manure incorporation did not affect negatively the growth and development of upland rice; hence, farmers could save at least 2 weeks in their cropping calendar. N source-sowing time combination had a highly significant influence on % Ndff, % Ndfs, % FNU, N uptake, and fertilizer N yield of upland rice. However, only N uptake was influenced significantly by liming. The rice plant obtained significantly higher % Ndfs from the soils treated with green manure than those treated with urea regardless of liming. The % FNU and % Ndff from the green manures were 11-37% and 9-25%, respectively. These values are much lower than those obtained under continuously flooded soil conditions possibly because of the differences in the organic matter decomposer populations and N loss mechanisms between sloping upland conditions and continuously flooded conditions.     

The chemistry of the lowland rice rhizosphere

Models and experimental studies of the rhizosphere of rice plants growing in anaerobic soil show that two major processes lead to considerable acidification (1–2 pH units) of the rhizosphere over a wide range of root and soil conditions. One is generation of H⁺ in the oxidation of ferrous iron by O₂ released from the roots. The other is release of H⁺ from roots to balance excess intake of cations over anions, N being taken up chiefly as NH₄ ⁺. CO₂ exchange between the roots and soil has a much smaller effect. The zone of root-influence extends a few mm from the root surface. There are substantial differences along the root length and with time. The acidification and oxidation cause increased sorption of NH₄ ⁺ ions on soil solids, thereby impeding the movement of N to absorbing root surfaces. But they also cause solubilization and enhanced uptake of soil phosphate.     

Modelling root-induced solubilization of nutrients

Theory for coupled diffusion processes in soil is briefly described and three examples of its application to understand root-induced solubilization of nutrients given. The examples are: (1) solubilization of P through root-induced pH changes in the rhizosphere of rice plants growing in flooded soil; (2) solubilization of P through excretion of organic chelating agents from rice roots growing in aerobic soil; and (3) the effects of root geometry on P solubilization, particularly cylindrical versus planar geometry and the effect of excretion of a solubilizing agent being localized along the root axis. The theory is tested by comparing measured concentration profiles of P near roots with the predictions of the theory made using independently measured parameter values. In the examples given, the agreement between the observed and predicted concentration profiles is very good, indicating that the theory is sound and the processes involved well understood.     

Effect of silicon on the growth and phosphorus uptake of rice

A pot experiment was conducted to measure the effect of silicon on phosphorus uptake and on the growth of rice at different P levels. Rice (Oryza sativa L. cv. Akebono) was cultured in Kimura B nutrient solution without and with silicon (1.66 mM Si) and with three phosphorus levels (0.014 mM P, low; 0.21 mM, medium; and 0.70 mM, high).Shoot dry weight with Si (+Si) in solution increased with increasing P level, while shoot weight without Si (–Si) was maximum at 0.21 mM P, suggesting that +Si raised the optimum P level for rice. +Si increased shoot weight more when P was low or high than when P was medium.The concentration and amount of inorganic P in shoots increased with increasing P level. +Si did not significantly decrease P uptake by rice at 0.014 mM P, however, uptake at 0.21 and 0.70 mM P was 27 and 30 percent less than uptake with –Si, respectively. In –Si with 0.21 and 0.70 mM P, inorganic P in shoots was more than double the concentration in shoots grown in +Si solutions.The Si concentration in shoots decreased slightly with increasing P level, although Si uptake was not significantly affected by P. +Si decreased the uptake of Fe and Mn by an average of 20 and 50 percent, respectively, thus P/Mn and P/Fe ratios increased in the shoot when P was low.From the results above, the beneficial effect of Si on the growth of rice was clearly shown when P was low or high. This effect may have resulted from decreased Mn and Fe uptake, and thus increased P availability within P deficient plants, or from reduced P uptake when P was high.     

淹水条件下籼稻与粳稻苗期根际土壤硝化作用的时空变异

由于硝态氮（NO3^--N）对于水稻的生长起到非常重要的作用,所以发生在水稻根际的硝化作用越来越受到人们的重视.试验采用根盒（3室）--速冻切片技术研究了常规籼稻（扬稻6号）和常规粳稻（农垦57）苗期根际土壤矿质态氮、硝化作用和氨氧化细菌数量的时空变异.结果表明,在淹水条件下,土壤矿质态氮主要为铵态氮（NH4^＋-N）,NH4^＋含量随水稻生育期的推进变化不大,但随着距根区的距离增加其含量随之增加,两个水稻品种之间差异不显著;而NO3-的变化趋势与NH4^＋不一致,NO3^-含量随水稻生育期的延长而显著下降,在培养58d时其平均含量约为0.05 mg kg^-1,同时在整个土体内呈均匀分布,两个水稻品种之间差异显著.土壤的硝化强度随水稻的生长而增强,且两种水稻的硝化强度均为根际土壤最高,然后依次为土体土壤和根区土壤.扬稻6号和农垦57硝化强度最大值分别出现在距根6 mm和2 mm处,最大值分别为0.88 mg kg^-1h^-1和0.73 mg kg^-1h^-1.土壤氨氧化细菌（AOB）数量随水稻生长时间的增加而增加,且其水平变异趋势与土壤的硝化强度一致,根际土壤AOB数量最多,土体土壤次之,根区土壤最少.相关分析结果表明,硝化强度和AOB数量呈显著正相关关系（r=0.86,p＜0.01）.种植扬稻6号的土壤NO3^-浓度、硝化强度以及AOB数量总是高于农垦57.     

Localized supply of phosphorus induces root morphological and architectural changes of rice in split and stratified soil cultures

A localized supply of phosphorus may affect root morphology and architecture, and thereby affect phosphorus uptake by rice plants. In the present study, we attempted to test this hypothesis using two rice cultivars representing upland and lowland ecotypes grown in specially designed split and stratified soil cultures with a low-phosphorus red soil. Our data indicate that a localized supply of phosphorus increased both total root length and root fineness, particularly in the high-phosphorus zone. In split culture, plants roots tended to preferentially grow on the high-phosphorus zone, with about 70–75% of the total root length allocated to the high-phosphorus compartment. The total root length on the high-phosphorus side in the split-phosphorus treatment was significantly longer than that in the homogenously high-phosphorus treatment, implying that a phosphorus-deficiency signal from the low-phosphorus side may stimulate the growth of the roots located in the high-phosphorus zone. In stratified soil culture, changes in root morphology and architecture were also observed as indicated by increased total root length, root fineness and relative root allocation in the high-phosphorus layers, again suggesting altered root morphology and preferential root proliferation in the high-phosphorus regions. The induced changes in root morphology and architecture by localized phosphorus supply may have both physiological significance and practical implications in that plants can meet the demand for phosphorus with parts of the roots reaching the high-phosphorus zone, hence localized fertilization methods such as side dressing or banded application of phosphorus fertilizers may both minimize phosphorus fixation by the soil and increase phosphorus uptake efficiency from the fertilizers.     

水稻覆膜旱作对根叶性状、土壤养分和土壤微生物活性的影响

研究了水稻覆膜旱作、裸地旱作、常规水作处理对根系形态、叶片水势、土壤养分、土壤微生物数量和土壤酶活性的影响。结果表明,覆膜旱作能改善水稻田间的生态环境,提高早季土壤温度1～3℃。与裸地旱作相比,覆膜旱作能增加抽穗期水稻根系的总根长、比根长,而这些指标与常规水作差别不大。在水稻抽穗期,无论是剑叶还是倒2叶的水势,均有如下趋势：水作〉覆膜旱作〉裸地旱作,但均未达到显著水平。不同处理对叶片叶绿素含量的影响在分蘖期差异不大,中后期覆膜旱作明显高于裸地旱作和常规水作,抽穗期分别增加29．44％和15．15％,成熟期则为74．4％、38．9％。覆膜旱作能显著增加水稻的有效穗数,早季和晚季产量比裸地旱作增产10．6％和12．5％,而与常规水作差别不大。与裸地旱作和常规水作相比,覆膜旱作土壤养分含量在分蘖期除了全磷、速效氮显著增加外,其他指标差异不大。而抽穗期则由于覆膜后作物对养分的消耗加大从而使土壤养分含量下降,特别是总磷、速效磷、速效钾与常规水作相比显著下降,分别降低25．9％,31．9％,16．2％。而成熟期则大多指标与常规水作相比有所下降,但未达到显著水平。与常规水作栽培相比,覆膜旱作能增加土壤微生物细菌、真菌、放线菌2．5倍,显著增加土壤过氧化氢酶和蔗糖酶的活性,而土壤脲酶活性则差异不大。土壤过氧化氢酶活性在分蘖期、抽穗期、成熟期分别增加13．8％,81．3％,17．4％,蔗糖酶活性则分别增加42．8％,28．8％和69．9％。     

Effect of silica and magnesium on yield of upland rice in the humid tropics

Grain yield, dry-matter production, and susceptibility to grain discoloration syndrome of upland rice (Oryza sativa L.) in relation to nutrient supplies were studied in Ultisols in Nigeria's humid tropics. Nutrients identified as necessary to produce high dry matter were N, K, Mg, and Si. Among these nutrients, Mg and Si were found to be involved in the protection of rice plants against grain discoloration and their application increased the grain yield of 3 varieties by an average of 34%.     

Soil CO2 venting as one of the mechanisms for tolerance of Zn deficiency by rice in flooded soils

We sought to explain rice (Oryza sativa) genotype differences in tolerance of zinc (Zn) deficiency in flooded paddy soils and the counter‐intuitive observation, made in earlier field experiments, that Zn uptake per plant increases with increasing planting density. We grew tolerant and intolerant genotypes in a Zn‐deficient flooded soil at high and low planting densities and found (a) plant Zn concentrations and growth increased with planting density and more so in the tolerant genotype, whereas the concentrations of other nutrients decreased, indicating a specific effect on Zn uptake; (b) the effects of planting density and genotype on Zn uptake could only be explained if the plants induced changes in the soil to make Zn more soluble; and (c) the genotype and planting density effects were both associated with decreases in dissolved CO₂ in the rhizosphere soil solution and resulting increases in pH. We suggest that the increases in pH caused solubilization of soil Zn by dissolution of alkali‐soluble, Zn‐complexing organic ligands from soil organic matter. We conclude that differences in venting of soil CO₂ through root aerenchyma were responsible for the genotype and planting density effects.