Irrigation regimes modulate non-structural carbohydrate remobilization and improve grain filling in rice(Oryza sativa L.)by regulating starch metabolism

Recently developed ‘super’ rice cultivars with greater yield potentials often suffer from the problem of poor grain filling, especially in inferior spikelets. Here, we studied the activities of enzymes related to starch metabolism in rice stems and grains, and the microstructures related to carbohydrate accumulation and transportation to investigate the effects of different water regimes on grain filling. Two ‘super’ rice cultivars were grown under two irrigation regimes of well-watered(WW) and alternate wetting and moderate soil drying(AWMD). Compared with the WW treatment,the activities of ADP glucose pyrophosphorylase(AGPase), starch synthase(StSase) and starch branching enzyme(SBE), and the accumulation of non-structural carbohydrates(NSCs) in the stems before heading were significantly improved, and more starch granules were stored in the stems in the AWMD treatment. After heading, the activities of α-amylase, β-amylase, sucrose phosphate synthase(SPS) and sucrose synthase in the synthetic direction(SSs)were increased in the stems to promote the remobilization of NSCs for grain filling under AWMD. During grain filling, the enzymatic activities of sucrose synthase in the cleavage direction(SSc), AGPase, StSase and SBE in the inferior spikelets were increased, which promoted grain filling, especially for the inferior spikelets under AWMD.However, there were no significant differences in vascular microstructures. The grain yield and grain weight could be improved by 13.1 and 7.5%, respectively, by optimizing of the irrigation regime. We concluded that the low activities of key enzymes in carbon metabolism is the key limitation for the poor grain filling, as opposed to the vascular microstructures, and AWMD can increase the amount of NSC accumulation in the stems before heading, improve the utilization rate of NSCs after heading, and increase the grain filling, especially in the inferior spikelets, by altering the activities of key enzymes in carbon metabolism.

The auxin transporter OsAUX1 regulates tillering in rice(Oryza sativa)

Tillering is an important agronomic trait of rice(Oryza sativa)that affects the number of effective panicles,thereby affecting yields.The phytohormone auxin plays a key role in tillering.Here we identified the high tillering and semi-dwarf 1(htsd1)mutant with auxin-deficiency root characteristics,such as shortened lateral roots,reduced lateral root density,and enlarged root angles.htsd1 showed reduced sensitivity to auxin,but the external application of indole-3-acetic acid(IAA)inhibited its tillering.We identified the mutated gene in htsd1 as AUXIN1(OsAUX1,LOC_Os01g63770),which encodes an auxin influx transporter.The promoter sequence of OsAUX1 contains many SQUAMOSA PROMOTER BINDING PROTEIN-LIKE(SPL)binding sites,and we demonstrated that SPL7 binds to the OsAUX1 promoter.TEOSINTE BRANCHED1(OsTB1),a key gene that negatively regulates tillering,was significantly downregulated in htsd1.Tillering was enhanced in the OsTB1 knockout mutant,and the external application of IAA inhibited tiller elongation in this mutant.Overexpressing OsTB1 restored the multi-tiller phenotype of htsd1.These results suggest that SPL7 directly binds to the OsAUX1 promoter and regulates tillering in rice by altering OsTB1 expression to modulate auxin signaling.

A Farmer’s Approach to Detecting Photoperiod Sensitivity in Rice (Oryza sativa ssp. indica) Landraces

Most indigenous rice landraces are sensitive to photoperiod during short day seasons,and this sensitivity is more pronounced in indica than in japonica landraces.Attempts to identify photoperiod sensitive(PPS)cultivars based on the life history stages of the rice plant,and several models and indices based on phenology and day length have not been precise,and in some cases yield counterfactual inferences.Following the empirical method of traditional Asian rice farmers,the author has developed a robust index,based on the sowing and flowering dates of a large number of landraces grown in different seasons from 2020 to 2023,to contradistinguish PPS from photoperiod insensitive cultivars.Unlike other indices and models of photoperiod sensitivity,the index does not require the presumed duration of different life history stages of the rice plant but relies only on the flowering dates and the number of days till flowering of a rice cultivar sown on different dates to consistently identify photoperiod sensitive cultivars.

Influence of Epistasis and QTL×Environment Interaction on Heading Date of Rice(Oryza sativa L.)

QTLs for heading date of rice （Oryza sativa L.） with additive, epistatic, and QTL × environment （QE） interaction effects were studied using a mixed-model-based composite interval mapping （MCIM） method and a double haploid （DH） population derived from IR64/Azucena in two crop seasons. Fourteen QTLs conferring heading date in rice, which were distributed on ten chromosomes except for chromosomes 5 and 9, were detected. Among these QTLs, eight had single-locus effects, five pairs had double-locus interaction effects, and two single-loci and one pair of double-loci showed QTL × environment interaction effects. All predicted values of QTL effects varied from 1.179 days to 2.549 days, with corresponding contribution ratios of 1.04%-4.84%. On the basis of the effects of the QTLs, the total genetic effects on rice heading date for the two parents and the two superior lines were predicted, and the putative reasons for discrepancies between predicted values and observed values, and the genetic potentiality in the DH population for improvement of heading date were discussed. These results are in agreement with previous results for heading date in rice, and the results provide further information, which indicate that both epistasis and QE interaction are important genetic basis for determining heading date in rice.

Advances in Physiological-biochemical and Genetic Mechanisms of Seed Aging in Rice(Oryza sativa L.)

Reasons causing or accelerating seed aging are mainly damage of mem- branes, DNA and proteins, decline of protein synthesis capacity and excessive ac- cumulation of reactive oxygen species. With the application of natural aging or artifi- cial aging methods, it was reported that quantitative trait loci （QTLs） of seed stora- bility in rice were widely distributed on the chromosomes except the 10th chromo- some. In this paper, we reviewed the progresses in the research on physiological- biochemical and genetic mechanisms of seed aging, and analyzed the existing problems and developing prospect in molecular breeding of rice with improved seed storability, in order to provide reference for the basic research and genetic improve- ment of rice seed storabUity.

Peculiarities of nuclear DNA multiplication during embryogenesis in rice(Oryza sativa ssp.japonica)embryo cells

The study was to determine the Feulgen-DNAcontents of the nuclei in the cells of globular,

The response of roots and the rhizosphere environment to integrative cultivation practices in paddy rice

Integrative cultivation practices(ICPs)are essential for enhancing cereal yield and resource use efficiency.However,the effects of ICP on the rhizosphere environment and roots of paddy rice are still poorly understood.In this study,four rice varieties were produced in the field.Each variety was treated with six different cultivation techniques,including zero nitrogen application(0 N),local farmers’practice(LFP),nitrogen reduction(NR),and three progressive ICP techniques comprised of enhanced fertilizer N practice and increased plant density(ICP1),a treatment similar to ICP1 but with alternate wetting and moderate drying instead of continuous flooding(ICP2),and the same practices as ICP2 with the application of organic fertilizer(ICP3).The ICPs had greater grain production and nitrogen use efficiency than the other three methods.Root length,dry weight,root diameter,activity of root oxidation,root bleeding rate,zeatin and zeatin riboside compositions,and total organic acids in root exudates were elevated with the introduction of the successive cultivation practices.ICPs enhanced nitrate nitrogen,the activities of urease and invertase,and the diversity of microbes(bacteria)in rhizosphere and non-rhizosphere soil,while reducing the ammonium nitrogen content.The nutrient contents(ammonium nitrogen,total nitrogen,total potassium,total phosphorus,nitrate,and available phosphorus)and urease activity in rhizosphere soil were reduced in all treatments in comparison with the non-rhizosphere soil,but the invertase activity and bacterial diversity were greater.The main root morphology and physiology,and the ammonium nitrogen contents in rhizosphere soil at the primary stages were closely correlated with grain yield and internal nitrogen use efficiency.These findings suggest that the coordinated enhancement of the root system and the environment of the rhizosphere under integrative cultivation approaches may lead to higher rice production.

OsNRAMP2 facilitates Cd efflux from vacuoles and contributes to the difference in grain Cd accumulation between japonica and indica rice

Cadmium(Cd)accumulation in rice grain is of health concern.Identifying genes involved in grain Cd accumulation and performing molecular breeding may reduce it.In this study,knockout of OsNRAMP2,a member of the NRAMP family,reduced grain Cd concentrations by more than 38%,and overexpressing OsNRAMP2 increased grain Cd concentrations by more than 50%.Physiological experiments showed that OsNRAMP2 facilitated Cd translocation from root to shoot by positively regulating Cd efflux from the vacuoles.At filling stage,OsNRAMP2 was highly expressed in all tissues except for husk,suggesting its role in Cd remobilization.Changes in OsNRAMP2 expression affected the concentrations of Fe,Mn,Zn,and Cu in grain and also affected rice growth.Phylogenetic analysis showed that the distribution of OsNRAMP2 haplotypes between japonica and indica was different.Among the four haplotypes of OsNRAMP2,Hap 1,with a 6-bp nucleotide insertion in exon 1,had grain Cd concentration at least 45.3%lower than any of the other three haplotypes.Almost all(99.3%)japonica accessions but rare indica accessions(4.44%)from the 3K sequenced rice genomes carry Hap 1 of OsNRAMP2.Our study sheds light on the molecular mechanism of grain Cd accumulation and provides a promising target for low-Cd rice breeding.

Calcineurin B-like interacting protein kinase OsCIPK23 functions in pollination and drought stress responses in rice (Oryza sativa L.)

Drought is very harmful to grain yield due to its adverse effect on reproduction, especially on pollination process in rice. However, the molecular basis of such an effect still remains largely unknown. Here, we report the role of a member of CBL （Calcineurin B-Like） Interacting Protein Kinase （CIPK） family, OsCIPK23, in pollination and stress responses in rice. Molecular analyses revealed that it is mainly expressed in pistil and anther but up-regulated by pollination, as well as by treatments of various abiotic stresses and phytohormones. RNA interference-mediated suppression of OsCIPK23 expression significantly reduced seed set and conferred a hypersensitive response to drought stress, indicating its possible roles in pollination and drought stress. In consistent, overexpression of OsCIPK23 induced the expression of several drought tolerance related genes. Taken together, these results indicate that OsCIPK23 is a multistress induced gene and likely mediates a signaling pathway commonly shared by both pollination and drought stress responses in rice.

Bulked Segregant Analysis to Detect QTL Related to Heat Tolerance in Rice(Oryza sativa L.) Using SSR Markers

The study was undertaken to assess the genetic effect of quantitative trait loci （QTLs） conferring heat tolerance at flowering stage in rice. A population consisting of 279 F2 individuals from the cross between 996, a heat tolerant cultivar and 4628, a heat-sensitive cultivar, was analyzed for their segregation pattern of the difference of seed set rate under optimal temperature condition and high temperature condition. The difference of seed set rate under optimal temperature condition and high temperature condition showed normal distribution, indicating the polygenic control over the trait. To identify main effect of QTL for heat tolerance, the parents were surveyed with 200 primer pairs of simple sequence repeats （SSR）. The parental survey revealed 30% polymorphism between parents. In order to detect the main QTL association with heat tolerance, a strategy of combining the DNA pooling from selected segregants and genotyping was adopted. The association of putative markers identified based on DNA pooling from selected segregants was established by single marker analysis （SMA）. The results of SMA revealed that SSR markers, RM3735 on chromosome 4 and RM3586 on chromosome 3 showed significant association with heat tolerance respectively, accounted for 17 and 3% of the total variation respectively. The heat tolerance during flowering stage in rice was controlled by multiple gene. The SSR markers, RM3735 on chromosome 4 and RM3586 on chromosome 3 showed significant association with heat tolerance respectively, accounted for 17 and 3% of the total variation respectively. The two genetic loci, especially for RM3735 on chromosome 4, can be used in marker-assistant-selected method in heat tolerance breeding in rice.

Studies on the Mechanism of Single Basal Application of Controlled-Release Fertilizers for Increasing Yield of Rice (Oryza sativa L.)

This paper was to explore the mechanism of single basal application of controlled-release fertilizers for increasing yield of rice （Oryza sativa L.）. Pot trials and cylinder trials were carried out from 2002 to 2005 to study the influences of single basal application of 3 controlled-release fertilizers on the changes of soil available N, root development, senescence and lodging resistance at late growth stages. Results showed that at 30 days after fertilization, single basal application of controlled-release fertilizers coated with vegetal-substance （CRF1） and polymer materials （CRF3） increased soil available N to 12.0 and 147.9%, respectively, in comparison to split fertilization of rice-specific fertilizer （RSF1）. Treatments of the two CRFs obviously benefited the development of root system, resulting in greater rice root weights with extensive distribution and higher root activity. In addition, the two CRF treatments, in comparison to RSF1, enhanced chlorophyll consents of the flag leaves to 9.5 and 15.5%, and soluble protein up to 89.7 and 108.0% respectively. Application of the two CRFs also made the base of rice stems strong and large, declined the proportion of shoot and root, increased root depth index. Though relatively low K rate, single basal application of the CRF3 coated with NH4MgPO4 could also promote the development of root system, enhance root activity and some physiological functions of flag leaves. Based on these results, it was concluded that major mechanisms for increasing rice yield by single basal application of the CRFs should be attributed to grater soil available N supply, superior development of root systems, better nutrient absorption capacity, slower senescence and enhancement of lodging resistance at late stages.

Genotypic Difference in the Responses of Seedling Growth and Cd Toxicity in Rice(Oryza sativa L.)

The experiment was carried out to study the genotypic difference in the responses of seed germination, growth and physiological characters of rice seedlings to Cd toxicity. The result showed that the germination was slightly stimulated under low Cd concentration （0.01-1.5 mM Cd）, while severely depressed under higher Cd concentration （2.0 mM）. Rice seedlings exposed to 0.01 mM Cd showed slight increases in plant height, root volume, biomass and chlorophyll concentration. These parameters were significantly reduced when Cd level in the medium was increased to 0.5 mM, and meanwhile corresponding increase in superoxide dismutase （SOD） and peroxidase （POD） activities, and MDA （malondialdehyde） content was observed. However, SOD and POD activities declined when plants were exposed to 1 mM Cd when compared with those under 0.5 mM Cd. Cadmium addition lowered Fe, Cu and Mn concentrations in roots and shoots. There was significant genotypic difference in the response of these parameters to Cd stress. Under Cd stress, Xiushui 110 had the least inhibition of growth and increase in MDA content, higher shoot Cd concentration, and greatest increase in POD and SOD activities, indicating its higher tolerance to Cd toxicity, while Bing 9914 had the greatest reduction of growth, and Zn, Cu, Fe, and Mn contents, but greatest increase in MDA content, and least increase in activities of antioxidative enzymes, indicating its sensitivity to Cd toxicity.

Biomass-Based Rice (Oryza sativa L.) Aboveground Architectural Parameter Models

To quantify the relationships between rice plant architecture parameters and the corresponding organ biomass, and to research on functional structural plant models of rice plant, this paper presented a biomass-based model of aboveground architectural parameters of rice （Oryza sativa L.） in the young seedling stage, designed to explain effects of cultivars and environmental conditions on rice aboveground morphogenesis at the individual leaf level. Various model variables, including biomass of blade and blade length, were parameterized for rice based on data derived from an outdoor experiment with rice cv. Liangyou 108, 86You 8, Nanjing 43, and Yangdao 6. The organ dimensions of rice aboveground were modelled taking corresponding organ biomass as an independent variable. Various variables in rice showed marked consistency in observation and simulation, suggesting possibilities for a general rice architectural model in the young seedling stage. Our descriptive model was suitable for our objective. However, they can set the stage for connection to physiological model via biomass and development of functional structural rice models （FSRM）, and start with the localized production and partitioning of assimilates as affected by abiotic growth factors. The finding of biomass-based rice architectural parameter models also can be used in morphological models of blade, sheath, and tiller of the other stages in rice life.

Dissection of two quantitative trait loci for grain weight linked in repulsion on the long arm of chromosome 1 of rice(Oryza sativa L.)

Grain weight is a key determinant of grain yield in rice. Three sets of rice populations with overlapping segregating regions in isogenic backgrounds were established in the generations of BC2 F5, BC2 F6 and BC2 F7, derived from Zhenshan 97 and Milyang 46, and used for dissection of quantitative trait loci(QTL) for grain weight. Two QTL linked in repulsion phase on the long arm of chromosome 1 were separated. One was located between simple sequence repeat(SSR) markers RM11437 and RM11615, having a smaller additive effect with the enhancing allele from the maintainer line Zhenshan 97 and a partially dominant effect for increasing grain weight. The other was located between SSR markers RM11615 and RM11800, having a larger additive effect with the enhancing allele from the restorer line Milyang 46 and a partially dominant effect for increasing grain weight. When the two QTL segregated simultaneously, a residual additive effect with the enhancing allele from Milyang 46 and an over-dominance effect for increasing grain weight were detected. This suggests that dominant QTL linked in repulsion phase might play an important role in heterosis in rice. Our study also indicates that the use of populations with overlapping segregating regions in isogenic backgrounds is helpful for the dissection of minor linked QTL.

Enclosed stigma contributes to higher spikelet fertility for rice(Oryza sativa L.) subjected to heat stress

With global warming, rice plants may be subjected to heat stress more regularly during the heatsensitive flowering stage, causing spikelet sterility and grain yield loss.Stigma exsertion is considered to increase pollen reception and promote female reproductive success.The aim of this study was to investigate the role of stigma exsertion on spikelet fertility at high temperatures.Five rice cultivars(Liangyoupeijiu, Shanyou 63, Huanghuazhan, Nagina 22, and IR64) with differing degrees of stigma exsertion were cultivated and exposed to high temperature at anthesis.Heat-tolerant cultivars did not always show a high percentage of spikelets with exserted stigmas, and vice versa.Irrespective of the presence of more pollen grains on exserted stigmas, spikelets with exserted stigmas did not show greater spikelet fertility than spikelets with fewer exserted stigmas or hidden stigmas under heat stress.GA3 application augmented the percentage of spikelets with exserted stigmas;however, it did not increase spikelet fertility under heat stress.Spikelet fertility of whole panicles was negatively correlated with the percentage of spikelets with exserted stigmas, but positively with that with hidden stigmas.Viability of the hidden stigmas was less reduced than that of exserted stigmas under heat stress, suggesting that hidden stigmas have an advantage in maintaining viability.Heat stress delayed anther dehiscence and reduced the viabilities of both exserted stigmas and pollens, thereby causing low spikelet fertility.Together, these results suggest that high spikelet fertility does not depend on stigma exsertion and that enclosed stigma generally contributes to higher spikelet fertility and heat tolerance under high-temperature conditions during flowering in rice.

Validation of qGSIO, a quantitative trait locus for grain size on the long arm of chromosome 10 in rice (Oryza sativa L.)

Grain size is a major determinant of grain weight and a trait having important impact on grain quality in rice. The objective of this study is to detect QTLs for grain size in rice and identify important QTLs that have not been well characterized before. The QTL mapping was first performed using three recombinant inbred line populations derived from indica rice crosses Teqing/IRBB lines, Zhenshan 97/Milyang 46, Xieqingzao/Milyang 46. Fourteen QTLs for grain length and 10 QTLs for grain width were detected, including seven shared by two populations and 17 found in one population. Three of the seven com- mon QTLs were found to coincide in position with those that have been cloned and the four others remained to be clarified. One of them, qGSIO located in the interval RM6100-RM228 on the long arm of chromosome 10, was validated using F2：3 populations and near isogenic lines derived from residual heterozygotes for the interval RM6100-RM228. The QTL was found to have a considerable effect on grain size and grain weight, and a small effect on grain number. This region was also previously detected for quality traits in rice in a number of studies, providing a good candidate for functional analysis and breeding utilization.

Mapping and genetic validation of a grain size QTL qGS7.1 in rice(Oryza sativa L.)

Grain size is a major determinant of grain weight, which is one of the components of rice yield. The objective o this study was to identify novel, and important quantitative trait loci(QTLs) for grain size and weight in rice. QTLs were mapped using a BC4F4 population including 192 backcross inbred lines(BILs) derived from a backcross between Xiaolijing(XLJ) and recombinant inbred lines(RILs). The mapping population was planted in both Lingshui(Hainan, 2015) and Fuyang(Zhejiang, 2016), with the short-and long-day conditions, respectively. A total of 10 QTLs for grain length, four for grain width, four for the ratio of grain length to width, and 11 for grain weight were detected in at least one environment and were distributed across 11 chromosomes. The phenotypic variance explained ranged from 6.76–25.68%, 14.30–34.03%, 5.28–26.50%, and 3.01–22.87% for grain length, grain width, the ratio of grain length to width, and thousand grain weight, respectively. Using the sequential residual heterozygotes(SeqRHs) method, qGS7.1, a QTL for grain size and weight, was mapped in a 3.2-Mb interval on chromosome 7. No QTLs about grain size and weight were reported in previous studies in this region, providing a good candidate for functional analysis and breeding utilization.

Comparison and analysis of QTLs for grain and hull thickness related traits in two recombinant inbred line(RIL) populations in rice(Oryza sativa L.)

Grain traits are major constraints in rice production, which are key factors in determining grain yield and market values. This study used two recombinant inbred line（RIL） populations, RIL-JJ（japonica/japonica） and RIL-IJ（indica/japonica） derived from the two crosses Shennong 265/Lijiangxintuanheigu（SN265/LTH） and Shennong 265/Luhui 99（SN265/LH99）. Sixty-eight quantitative trait loci（QTLs） associated with 10 grain traits were consistently detected on the 12 chromosomes across different populations and two environments. Although 61.75% of the QTLs clustered together across two populations, only 16.17% could be detected across two populations. Eight major QTLs were detected on the 9, 10 and 12 chromosomes in RIL-JJ under two environments, a novel QTL clustered on the 10 chromosome, q GT10, q BT10 and q TGW10, have a higher percentage of explained phenotypic variation（PVE） and additive effect; 15 major QTLs were detected on the 5, 8, 9, and 11 chromosomes in RIL-IJ under two environments, a novel clustered QTL, q GT8 and q TGW8, on the 8 chromosome have a higher additive effect. Finally, the analysis of major QTL-BSA mapping narrowed the q TGW10 to a 1.47-Mb region flanked by simple sequence repeat markers RM467 and RM6368 on chromosome 10. A comparison of QTLs for grain traits in two different genetic backgrounds recombinant inbred line populations confirmed that genetic background had a significant impact on grain traits. The identified QTLs were stable across different populations and various environments, and 29.42% of QTLs controlling grain traits were reliably detected in different environments. Fewer QTLs were detected for brown rice traits than for paddy rice traits, 7 and 17 QTLs for brown rice out of 25 and 43 QTLs under RIL-JJ and RILIJ populations, respectively. The identification of genes constituting the QTLs will help to further our understanding of the molecular mechanisms underlying grain shape.

Rice (Oryza sativa L.) nutrient management using mycorrhizal fungi and endophytic Herbaspirillum seropedicae

Integrated nutrient management with biological and chemical fertilizers can improve rice （Oryza safiva L.） productivity, bio-fortification, soil health and fertility. Accordingly, this study was planned to evaluate the combined effects of biological fertilizers including arbuscular mycorrhizal （AM） fungi （Glomus mosseae） and free-living nitrogen-fixing bacteria （Herbaspi- rillum seropedicae）, as well as chemical fertilizers on the yield and nutrient contents of wetland rice under field conditions. Seedlings were inoculated with AM fungi and the bacteria in the nursery and were then transplanted to the field. The experi- ment was carried out as a split factorial design with three replicates. Treatments included three rates of nitrogen （N 1, N2 and N3） and phosphorous （P1, P2 and P3） fertilizers （100, 75 and 50% of the optimum level） in the main plots and mycorrhizal and bacterial treatments in the sub plots. The total of urea （g） used per plot was equal to N1=200, N2=150 and N3=100 at three different growth stages （seeding, tillering and heading） and the total of P （g） per plot used once at seeding using triple super phosphate including P1 =16, P2=13 and P3=10. Plant growth and yield as well as the concentration of nitrogen （N）, phosphorous （P）, potassium （K）, iron （Fe）, and zinc （Zn） were measured in the soil, straw and grains. N-fertilizer and biological fertilizers had significant effects on root, shoot and grain yield of rice, however, P-fertilizer just significantly affected root and shoot dry weights. Interestingly, analyses of variance indicated that biological fertilization significantly affected all the experimental treatments except straw N. AM fungi, N1 and P1 resulted in the highest rate of rice growth and yield. The interactions of chemical and biological fertilization resulted in significant effects on grain Zn, Fe, P, and N as well as soil Fe, K and N. The highest rate of grain nutrient uptake was resulted by the combined use of biological fertilization and the medium level of chemical fertilization. Interestingly, with decreasing the rate of chemical N fertilization, rice nutrient use efficiency increased indicating how biological fertilization can be efficient in providing plants with its essential nutrients such as N. However, the highest rate of soil and straw nutrient concentration was related to the combined use of biologicalfertilization and the highest rate of chemical fertilization. We conclude that biological fertilizer, （mycorrhizal fungi and H. seropedicae） can significantly improve wetland rice growth and yield （resulting in the decreased rate of chemical fertilizer）, espe- cially if combined with appropriate rate of chemical fertilization, by enhancing nutrient uptake （fortification） and root growth.

Responses of Contrasting Rice (Oryza sativa L.) Genotypes to Salt Stress as Affected by Nutrient Concentrations

The study was conducted to investigate the effects of applying different concentrations of the macronutrients K＋,Ca2＋,and Mg2＋ on the responses of contrasting rice（Oryza sativa L.） genotypes under salt stress.A solution culture experiment was conducted in a phytotron at the International Rice Research Institute（IRRI）,under controlled temperature and humidity and natural sunlight.When subjected to salt stress of 100 mmol L-1 using NaCl,the salt tolerant genotypes FL478 and IR651,accumulated less Na＋ and maintained lower ratios of Na＋/K＋,Na＋/Ca2＋,and Na＋/Mg2＋ than the sensitive genotypes IR29 and Azucena.These tolerant genotypes also had higher concentrations of K＋ in their shoots and greater root and shoot biomass and green leaf area.Tolerant genotypes also maintained much lower concentration of Na＋ and lower and more favorable ratios of Na＋/K＋,Na＋/Ca2＋,and Na＋/Mg2＋ in their active and developing tissues.Salt tolerance and shoot and root growth of both tolerant and sensitive genotypes were enhanced considerably when higher concentrations of Ca2＋ and Mg2＋ were applied in culture solution.The concentration of Na＋ and the ratios of Na＋/K＋,Na＋/Ca2＋,and Na＋/Mg2＋ in shoots also declined significantly.The beneficial effects of higher calcium were greater than that of magnesium and application of higher concentration of K＋ seems to have minor effects.Responses to salinity in rice can therefore be considerably enhanced through proper nutrient management,by increasing the concentrations of nutrient elements that have favorable effects such as Ca2＋ and Mg2＋.Calcium is particularly more effective than both magnesium and potassium,and can be applied at relatively larger quantities in salt affected soils.