Historical changes in population structure during rice breeding programs in the northern limits of rice cultivation

Key message

The rice local population was clearly differentiated into six groups over the 100-year history of rice breeding programs in the northern limit of rice cultivation over the world.

Abstract

Genetic improvements in plant breeding programs in local regions have led to the development of new cultivars with specific agronomic traits under environmental conditions and generated the unique genetic structures of local populations. Understanding historical changes in genome structures and phenotypic characteristics within local populations may be useful for identifying profitable genes and/or genetic resources and the creation of new gene combinations in plant breeding programs. In the present study, historical changes were elucidated in genome structures and phenotypic characteristics during 100-year rice breeding programs in Hokkaido, the northern limit of rice cultivation in the world. We selected 63 rice cultivars to represent the historical diversity of this local population from landraces to the current breeding lines. The results of the phylogenetic analysis demonstrated that these cultivars clearly differentiated into six groups over the history of rice breeding programs. Significant differences among these groups were detected in five of the seven traits, indicating that the differentiation of the Hokkaido rice population into these groups was correlated with these phenotypic changes. These results demonstrated that breeding practices in Hokkaido have created new genetic structures for adaptability to specific environmental conditions and breeding objectives. They also provide a new strategy for rice breeding programs in which such unique genes in local populations in the world can explore the genetic potentials of the local populations.     

Iron plaque formation and its effect on arsenic uptake by different genotypes of paddy rice

Background and aims

Iron plaque on roots has been hypothesized to be an effective restraint on the uptake of arsenic (As) by rice plants. Evaluating the formation of iron plaque and its effect on As uptake by various rice cultivars is valuable because selecting low As uptake rice cultivars results in reduced risks associated with rice consumption. This study examines iron plaque formation and its effect on As uptake by different genotypes of rice cultivars.

Methods

Hydroponic cultures were conducted in phytotron at day 25/night 20°C and the rice seedlings in fifth-leaf age were treated with Fe (II) at the levels of 0 and 100 mg L^?1 in the Kimura B nutrient solutions for 14 days. The amount of iron plaque formation of 28 rice cultivars was determined by using the DCB extractable Fe of roots. Four cultivars representing high and low iron plaque formation capability, from indica and japonica respectively, were selected out of the 28 cultivars and processed for Fe and As treatments. After Fe treatments for 4 days, the seedlings were fed with As (III) at levels of 0, 0.5, and 1 mg L^?1 for another 10 days. We were thus able to determine the amounts of iron plaque formation and the As content in iron plaque, roots, and shoots of the four tested cultivars.

Results

Iron plaque formation capability differed among tested twenty-eight rice cultivars. Feeding As to four tested cultivars enhanced iron plaque formation on roots; the As uptake by roots and shoots was decreased by the addition of Fe. Both the retention of As on iron plaque and the decrease of As uptake by the addition of Fe varied among tested cultivars and were not correlated with the iron plaque formation capability.

Conclusions

Iron plaque can sequestrate As on the roots and reduce rice’s As uptake. However, other factors also influence the As uptake, namely the differences in binding affinity of iron plaque to As, the existent As species in the rhizosphere, and the uptake capability of various As species by rice plants. These factors should also be considered when selecting low As uptake rice cultivars.     

Wheat cultivars form distinctive communities of root-associated arbuscular mycorrhiza in a conventional agroecosystem

Background and aims

The effect of plant species on their root-associated arbuscular mycorrhizal (AM) fungi is well studied, but how this effect operates at the cultivar level remains poorly understood. This study investigates how wheat cultivars shape their AM fungal communities.

Methods

Twenty-one new wheat cultivars were traditionally cultivated in a dryland of northwestern China, and their agronomic traits, soil characteristics and the abundance and community composition of AM fungi were measured.

Results

Both spore community in soils and AM fungal phylotypes inside roots were significantly influenced by cultivar even though hyphal abundance, spore density and AM fungal diversity were similar across cultivars. Three out of 16 AM fungal phylotypes interacted with most cultivars, whilst some phylotypes preferred to colonize cultivars with similar agronomic traits. Six wheat cultivars, all which had hosted 6 AM fungal phylotypes, seemed to be generalists. Nestedness analysis and stochastic model fitting revealed that the AM fungal communities colonizing roots were codetermined by deterministic and stochastic processes.

Conclusions

A complex pattern of cultivar-AM fungal interactions was observed in this study, and our results highlight that the host effect on the community assembly of AM fungi could be operating on the level of plant cultivar.     

Epigenetic predisposition to expression of TIMP1 from the human inactive X chromosome

Background

Cultivated rice (Oryza sativa L.) is endowed with a rich genetic variability. In spite of such a great diversity, the modern rice cultivars have narrow genetic base for most of the agronomically important traits. To sustain the demand of an ever increasing population, new avenues have to be explored to increase the yield of rice. Wild progenitor species present potential donor sources for complex traits such as yield and would help to realize the dream of sustained food security.

Results

Advanced backcross method was used to introgress and map new quantitative trait loci (QTLs) relating to yield and its components from an Indian accession of Oryza rufipogon. An interspecific BC₂ testcross progeny (IR58025A/O. rufipogon//IR580325B///IR58025B////KMR3) was evaluated for 13 agronomic traits pertaining to yield and its components. Transgressive segregants were obtained for all the traits. Thirty nine QTLs were identified using interval mapping and composite interval mapping. In spite of it's inferiority for most of the traits studied, O. rufipogon alleles contributed positively to 74% of the QTLs. Thirty QTLs had corresponding occurrences with the QTLs reported earlier, indicating that these QTLs are stable across genetic backgrounds. Nine QTLs are novel and reported for the first time.

Conclusion

The study confirms that the progenitor species constitute a prominent source of still unfolded variability for traits of complex inheritance like yield. With the availability of the complete genome sequence of rice and the developments in the field of genomics, it is now possible to identify the genes underlying the QTLs. The identification of the genes constituting QTLs would help us to understand the molecular mechanisms behind the action of QTLs.     

<Emphasis Type="Italic">Agrobacterium-</Emphasis>mediated genetic transformation of commercially elite rice restorer line using <Emphasis Type="Italic">npt</Emphasis>II gene as a plant selection marker

Transformation of commercially important indica cultivars remains challenging for the scientific community even though Agrobacterium-mediated transformation protocols for a few indica rice lines have been well established. We report successful transformation of a commercially important restorer line JK1044R of indica rice hybrid JKRH 401. While following existing protocol, we optimized several parameters for callusing, regeneration and genetic transformation of JK1044R. Calli generated from the rice scutellum tissue were used for transformation by Agrobacterium harboring pCAMBIA2201. A novel two tire selection scheme comprising of Geneticin (G418) and Paramomycin were deployed for selection of transgenic calli as well as regenerated plantlets that expressed neomycin phosphotransferase-II gene encoded by the vector. One specific combination of G418 (30 mg l^?1) and Paramomycin (70 mg l^?1) was very effective for calli selection. Transformed and selected calli were detected by monitoring the expression of the reporter gene uidA (GUS). Regenerated plantlets were confirmed through PCR analysis of nptII and gus genes specific primers as well as dot blot using gus gene specific as probe.     

Identification of agronomically important QTL in tetraploid potato cultivars using a marker–trait association analysis

Key message

Nineteen tuber quality traits in potato were phenotyped in 205 cultivars and 299 breeder clones. Association analysis using 3364 AFLP loci and 653 SSR-alleles identified QTL for these traits.

Abstract

Two association mapping panels were analysed for marker–trait associations to identify quantitative trait loci (QTL). The first panel comprised 205 historical and contemporary tetraploid potato cultivars that were phenotyped in field trials at two locations with two replicates (the academic panel). The second panel consisted of 299 potato cultivars and included recent breeds obtained from five Dutch potato breeding companies and reference cultivars (the industrial panel). Phenotypic data for the second panel were collected during subsequent clonal selection generations at the individual breeding companies. QTL were identified for 19 agro-morphological and quality traits. Two association mapping models were used: a baseline model without, and a more advanced model with correction for population structure and genetic relatedness. Correction for population structure and genetic relatedness was performed with a kinship matrix estimated from marker information. The detected QTL partly not only confirmed previous studies, e.g. for tuber shape and frying colour, but also new QTL were found like for after baking darkening and enzymatic browning. Pleiotropic effects could be discerned for several QTL.     

Genetic dissection of grain traits in Yamadanishiki,an excellent sake-brewing rice cultivar

Key message

The grain traits of Yamadanishiki, an excellent sake-brewing rice cultivar in Japan, are governed by multiple QTLs, namely, a total of 42 QTLs including six major QTLs.

Abstract

Japanese rice wine (sake) is produced using brewing rice (Oryza sativa L.) that carries traits desirable for sake-brewing, such as a larger grain size and higher white-core expression rate (WCE) compared to cooking rice cultivars. However, the genetic basis for these traits in brewing rice cultivars is still unclear. We performed analyses of quantitative trait locus (QTL) of grain and days to heading over 3 years on populations derived from crosses between Koshihikari, a cooking rice, and Yamadanishiki, an excellent sake-brewing rice. A total of 42 QTLs were detected for the grain traits, and the Yamadanishiki alleles at 16 QTLs contributed to larger grain size. Two major QTLs essential for regulating both 100-grain weight (GWt) and grain width (GWh) were harbored in the same regions on chromosomes 5 and 10. An interaction was noted between the environment and the QTL associated with WCE on chromosome 6, which was detected in two of 3 years. In addition, two QTLs for WCE on chromosomes 3 and 10 overlapped with the QTLs for GWt and GWh, suggesting that QTLs associated with grain size also play an important role in the formation of white-core. Despite differences in the rate of grain growth in both Koshihikari and Yamadanishiki across 2 years, the WCE in Yamadanishiki remained consistent, thus demonstrating that the formation of white-core does not depend on grain filling speed. These data can be informative for programs involved in breeding better cooking and brewing rice cultivars.

     

Genetic diversity and association mapping of seed vigor in rice (Oryza sativa L.)

Key message

Twenty-seven QTLs were identified for rice seed vigor, in which 16 were novel QTLs. Fifteen elite parental combinations were designed for improving seed vigor in rice.

Abstract

Seed vigor is closely related to direct seeding in rice (Oryza sativa L.). Previous quantitative trait locus (QTL) studies for seed vigor were mainly derived from bi-parental segregating populations and no report from natural populations. In this study, association mapping for seed vigor was performed on a selected sample of 540 rice cultivars (419 from China and 121 from Vietnam). Population structure was estimated on the basis of 262 simple sequence repeat (SSR) markers. Seed vigor was evaluated by root length (RL), shoot length (SL) and shoot dry weight in 2011 and 2012. Abundant phenotypic and genetic diversities were found in the studied population. The population was divided into seven subpopulations, and the levels of linkage disequilibrium (LD) ranged from 10 to 80 cM. We identified 27 marker–trait associations involving 18 SSR markers for three traits. According to phenotypic effects for alleles of the detected QTLs, elite alleles were mined. These elite alleles could be used to design parental combinations and the expected results would be obtained by pyramiding or substituting the elite alleles per QTL (apart from possible epistatic effects). Our results demonstrate that association mapping can complement and enhance previous QTL information for marker-assisted selection and breeding by design.     

Effect of short-term heat exposure on physiological traits of indica rice at grain-filling stage

Heat stress impacts the quantity and quality of rice grains, particularly during grain-filling stage needed for grain development. In this study, the effect of short heat stress (42 °C, 30 min) on indica rice plants at the grain-filling stage (dough grain stage) was found by determining their physiological and growth traits F_v/F_m, ?F/F_m′, chlorophyll content, leaf water potential (LWP), membrane stability, relative leaf area (RLA), relative plant height (RPH), total grain weight per panicle (TGW) and 1000 GW. Thai economic rice cvs. KDML105 and Pathumthani 1 (PTT1) were compared to the heat-tolerant rice cultivars N22 and Dular and to the heat-sensitive rice cultivar IR64. The results showed that short heat stress exhibited effects on physiology and growth greater than the control (35 °C, 30 min) by reducing of F_v/F_m, ?F/F_m′, chlorophyll content, LWP, membrane stability and RLA. This result impacted the TGW and 1000 GW. A higher reduction of physiological traits was shown in IR64, followed by KDML105. In contrast, N22 and Dular were minimally affected by heat stress and were able to adapt and recover based on their grain weight that exhibited less of an effect. PTT1 was also impacted by heat stress similarly to Dular. Thus, short heat stress affected the physiological parameters of five rice cultivars at the dough grain stage. In addition, the five indica rice cultivars were classified into three groups: (1) the heat-tolerant group (N22, Dular and PTT1), (2) the moderately heat-tolerant group (KDML105), and (3) the heat-sensitive group (IR64) by PC-ORD program at 50% of similarity of the 13 physiological traits.     

The difference of cadmium accumulation between the indica and japonica subspecies and the mechanism of it

Many studies have shown genotypic differences in Cadmium (Cd) accumulation among rice cultivars, and concentrations in shoots and grains are generally higher in indica rice cultivars than in japonica rice cultivars, but the mechanism remains unknown. The main objective of this study was to investigate differences in heavy metal accumulation between rice subspecies through the analysis of 46 indica cultivars and 30 japonica cultivars. At the seedling stage, the mean Cd concentrations in the shoots of indica subspecies were significantly higher than those in japonica subspecies (1.22-fold), but this pattern was not observed in the roots. At the filling stage, the mean Cd concentrations in the shoots and spikes of indica subspecies were 1.66- and 2.14-fold higher than the respective concentrations in japonica subspecies. At the harvest stage, the mean Cd concentrations in the shoots and brown rice of indica subspecies were 1.61- and 2.27-fold higher than the respective concentrations in japonica subspecies. These results indicate that root-to-shoot and shoot-to-grain translocation, rather than Cd absorption in the roots, may be the key processes that determine the differences in Cd accumulation among rice subspecies. Gene expression analysis revealed that overall, the expression levels of the Cd transporter gene OsNramp1 notably increased (22.46-fold), but the expression levels of OsHMA2, OsHMA3 and OsNRAMP5 were not significantly changed at the seedling stage in the 76 cultivars exposed to Cd; the expression levels of OsNramp1 were positively correlated with the Cd concentrations in spikes at the filling stage. In addition, a significant difference was observed in the expression levels of OsNramp1 between the indica and japonica subspecies, which may explain the higher Cd concentrations in roots but lower Cd concentrations in spikes and brown rice for the japonica subspecies. Together, these results demonstrate that OsNramp1 may be the most important gene among the four selected genes in the promotion of Cd uptake by roots and transfer of Cd into spikes and eventually into brown rice.     

Phosphorus accumulation in grains of japonica rice as affected by nitrogen fertilizer

Background and aims

This study aims to investigate the effect of nitrogen (N) on grain phosphorus (P) accumulation in japonica rice.

Methods

Six cultivars with contrasting agronomic traits were grown for 3 years (from 2008 to 2010) of field experiments under seven N treatments and 1 year (in 2010) of pot experiments with five N treatments to study the effect of N on grain phosphorus accumulation and to explore its physiological foundation.

Results

Grain total P and phytic acid concentration showed a clearly decreasing trend as N rate increased for both field and pot experiments. Pot experiment revealed that application of N increase plant biomass, but tended to lower plant P uptake, especially for the split topdressing treatments. Both harvest index (HI) and P harvest index (PHI) increased with N rate, but PHI was consistently higher than HI, indicating the larger proportion of P translocation to grain than that of dry matter by N. Further, ratio of PHI/HI differed significantly among genotypes, but was stable across contrasting N treatments.

Conclusions

The combination of decreased plant P uptake and dilution effect of increased grain yield by N is proposed as underlying mechanism of the decreased grain P concentration by high N.     

Non-destructive estimation of root mass using electrical capacitance on ten herbaceous species

Background and Aims

Characteristically baseline levels of Sb in the environment are low, but problematic local elevation trends arise from anthropogenic activities such as mining and incineration. Arsenic (analog of Sb) accumulation by rice can be reduced by iron (Fe) plaque. A hydroponic experiment was conducted to investigate whether Fe plaque could reduce the uptake and translocation of different Sb species in different rice cultivars.

Methods

After Fe plaque on rice roots was induced in solution containing 0, 0.2, 0.4, 0.7, 1.2, 2.0?mM Fe²⁺ for 24?h, seedlings were transferred into nutrient solution with 20?μM Sb(V) or Sb(III) for 3?d.

Results

About 60–80% (Sb(III) treatment) and 40–60% (Sb(V) treatment) of the total Sb accumulated in Fe plaque. There was a significant correlation between the concentrations of Sb and Fe on the root surface. A similar relationship was observed in roots and shoots. Cultivar (Jiahua 1) formed the most Fe plaque, had the highest Fe associated Sb sequestration but the lowest Sb concentration in the root interior.

Conclusions

Fe plaque may act as a ‘buffer’ for Sb(V) and Sb(III) in the rhizosphere, and cultivars played an important role in the different species Sb uptake and translocation.     

Application of T-DNA activation tagging to identify glutamate receptor-like genes that enhance drought tolerance in plants

Key message

A high-quality rice activation tagging population has been developed and screened for drought-tolerant lines using various water stress assays. One drought-tolerant line activated two rice glutamate receptor-like genes. Transgenic overexpression of the rice glutamate receptor-like genes conferred drought tolerance to rice and Arabidopsis.

Abstract

Rice (Oryza sativa) is a multi-billion dollar crop grown in more than one hundred countries, as well as a useful functional genetic tool for trait discovery. We have developed a population of more than 200,000 activation-tagged rice lines for use in forward genetic screens to identify genes that improve drought tolerance and other traits that improve yield and agronomic productivity. The population has an expected coverage of more than 90 % of rice genes. About 80 % of the lines have a single T-DNA insertion locus and this molecular feature simplifies gene identification. One of the lines identified in our screens, AH01486, exhibits improved drought tolerance. The AH01486 T-DNA locus is located in a region with two glutamate receptor-like genes. Constitutive overexpression of either glutamate receptor-like gene significantly enhances the drought tolerance of rice and Arabidopsis, thus revealing a novel function of this important gene family in plant biology.     

Physiological and genetic characterization of rice nitrogen fixer PGPR isolated from rhizosphere soils of different crops

Aims

We aimed to identify plant growth-promoting rhizobacteria that could be used to develop a biofertilizer for rice.

Methods

To obtain plant growth-promoting rhizobacteria, rhizosphere soils from different crops (rice, wheat, oats, crabgrass, maize, ryegrass, and sweet potato) were inoculated to rice plants. In total, 166 different bacteria were isolated and their plant growth-promoting traits were evaluated in terms of colony morphology, indole-3-acetic acid production, acetylene reduction activity, and phosphate solubilization activity. Moreover, genetic analysis was carried out to evaluate their phylogenetic relationships based on 16S rRNA sequence data.

Results

Strains of Bacillus altitudinis, Pseudomonas monteilii, and Pseudomonas mandelii formed associations with rice plants and fixed nitrogen. A strain of Rhizobium daejeonense showed nitrogen fixation activity in an in vitro assay and in vivo. Strains of B. altitudinis and R. daejeonense derived from rice rhizosphere soil, strains of P. monteilii and Enterobacter cloacae derived from wheat rhizosphere soil, and a strain of Bacillus pumilus derived from maize rhizosphere soil significantly promoted rice plant growth.

Conclusions

These methods are effective to identify candidate species that could be developed as biofertilizers for target crops.     

Comparison of As sequestration in iron plaque and uptake by different genotypes of rice plants grown in As-contaminated paddy soils

Background and aims

Limited information is available on comparing the iron plaque formation capabilities and their effect on arsenic (As) uptake by different rice plant genotypes grown in As-contaminated soils. This study investigates the effect of iron plaque on As uptake in different rice genotypes grown in As-contaminated soils from the Guandu Plain of northern Taiwan.

Methods

Twenty-eight rice genotypes including 14 japonica and 14 indica genotypes were used in this study. Rice seedlings were grown in As-contaminated soils for 38 days. The iron plaque formed on the rice roots were extracted using dithionite–citrate–bicarbonate. The concentrations of As, Fe, and P in soil solutions, iron plaque, and plants were measured. The speciation of As in the root’s iron plaque was determined by As K-edge X-ray absorption near-edge structure spectroscopy (XANES).

Results

The amounts of iron plaque formation on roots were significantly different among 28 tested rice genotypes, and 75.7–92.8 % of As uptake from soils could be sequestered in iron plaque. However, there were no significant negative correlations between the amounts of Fe or As in the iron plaque and the content of As accumulated in rice plants of tested genotypes. XANES data showed that arsenate was the predominant As species in iron plaque, and there were difference in the distribution of As species among different rice genotypes.

Conclusions

The iron plaque can sequester most of As uptake from soils no matter what rice genotypes used in this study. However, the iron plaque alone did not control the extent of As accumulation in rice plants from As-contaminated soils among 28 tested rice genotypes. Low As uptake genotypes of rice selected from this study can be recommended to be grown in the As-contaminated soils.     

Meta-expression analysis of unannotated genes in rice and approaches for network construction to suggest the probable roles

Key message

This work suggests 2020 potential candidates in rice for the functional annotation of unannotated genes using meta-analysis of anatomical samples derived from microarray and RNA-seq technologies and this information will be useful to identify novel morphological agronomic traits.

Abstract

Although the genome of rice (Oryza sativa) has been sequenced, 14,365 genes are considered unannotated because they lack putative annotation information. According to the Rice Genome Annotation Project Database (http://rice.plantbiology.msu.edu/), the proportion of functionally characterized unannotated genes (0.35%) is quite limited when compared with the approximately 3.9% of annotated genes with assigned putative functions. Researchers require additional information to help them investigate the molecular mechanisms associated with those unannotated genes. To determine which of them might regulate morphological or physiological traits in the rice genome, we conducted a meta-analysis of expression data that covered a wide range of tissue/organ samples. Overall, 2020 genes showed cultivar-, tissue-, or organ-preferential patterns of expression. Representative candidates from featured groups were validated by RT-PCR, and the GUS reporter system was used to validate the expression of genes that were clustered according to their leaf or root preference. Taking a molecular and genetics approach, we examined meta-expression data and found that 127 genes were differentially expressed between japonica and indica rice cultivars. This is potentially significant for future agronomic applications. We also used a T-DNA insertional mutant and performed a co-expression network analysis of Sword shape dwarf1 (SSD1), a gene that regulates cell division. This network was refined via RT-PCR analysis. Our results suggested that SSD1 represses the expression of four genes related to the processes of DNA replication or cell division and provides insight into possible molecular mechanisms. Together, these strategies present a valuable tool for in-depth characterization of currently unannotated genes.

     

Occluded C in rice phytoliths: implications to biogeochemical carbon sequestration

Aims

Carbon (C) bio-sequestration within the phytoliths of plants, a mechanism of long-term biogeochemical C sequestration, may play a major role in the global C cycle and climate change. In this study, we explored the potential of C bio-sequestration within phytoliths produced in cultivated rice (Oryza sativa), a well known silicon accumulator.

Methods

The rice phytolith extraction was undertaken with microwave digestion procedures and the determination of occluded C in phytoliths was based on dissolution methods of phytolith-Si.

Results

Chemical analysis indicates that the phytolith-occluded C (PhytOC) contents of the different organs (leaf, stem, sheath and grains) on a dry weight basis in 5 rice cultivars range from 0.4 mg?g^?1 to 2.8 mg?g^?1, and the C content of phytoliths from grains is much lower than that of leaf, stem and sheath. The data also show that the PhytOC content of rice depends on both the content of phytoliths and the efficiency of C occlusion within phytoliths during rice growth. The biogeochemical C sequestration flux of phytoliths in 5 rice cultivars is approximately 0.03–0.13 Mg of carbon dioxide (CO₂) equivalents (Mg-e-CO₂) ha^?1?year^?1. From 1950 to 2010, about 2.37?×?10⁸?Mg of CO₂ equivalents might have been sequestrated within the rice phytoliths in China. Assuming a maximum phytoliths C bio-sequestration flux of 0.13 Mg-e-CO₂ ha^?1?year^?1, the global annual potential rate of CO₂ sequestrated in rice phytoliths would approximately be 1.94?×?10⁷?Mg.

Conclusions

Therefore rice crops may play a significant role in long-term C sequestration through the formation of PhytOC.     

Silicon ameliorates manganese toxicity by regulating manganese transport and antioxidant reactions in rice (Oryza sativa L.)

Background and aims

This study aimed to investigate the roles of silicon (Si) in ameliorating manganese (Mn) toxicity in two rice (Oryza sativa L.) cultivars: i.e. cv. Xinxiangyou 640 (XXY), a Mn-sensitive cultivar and cv. Zhuliangyou 99 (ZLY), a Mn-tolerant cultivar.

Methods

Plants were cultured in nutrient solution containing normal Mn (6.7 μM) or high Mn (2.0 mM), both with or without Si supply at 1.5 mM Si.

Results

Plant growth was severely inhibited by high Mn in cv. XXY, but was enhanced by Si supply. In cv. XXY, Si-enhanced tolerance resulted from a restriction of Mn transport, whereas in cv. ZLY Mn uptake was depressed. In cv. XXY, high Mn significantly increased superoxide dismutase (SOD), catalase and ascorbate peroxidase activities but decreased non-protein thiols and glutathione concentrations, leading to accumulation of H₂O₂ and malondialdehyde. The addition of Si significantly counteracted high Mn-elevated malondialdehyde and H₂O₂ concentrations and enhanced plant growth. In cv. ZLY, high Mn considerably raised SOD activities and glutathione concentrations, thus leading to relatively low oxidative damage.

Conclusions

Si-enhanced Mn tolerance was attributed mainly to restricted Mn transport in cv. XXY but to depressed Mn uptake in cv. ZLY. Silicon mainly influenced non-enzymatic antioxidants in these two rice cultivars under high Mn stress.