Comparative Metabolite Profiling of Two Rice Genotypes with Contrasting Salt Stress Tolerance at the Seedling Stage

Background

Rice is sensitive to salt stress, especially at the seedling stage, with rice varieties differing remarkably in salt tolerance (ST). To understand the physiological mechanisms of ST, we investigated salt stress responses at the metabolite level.

Methods

Gas chromatography-mass spectrometry was used to profile metabolite changes in the salt-tolerant line FL478 and the sensitive variety IR64 under a salt-stress time series. Additionally, several physiological traits related to ST were investigated.

Results

We characterized 92 primary metabolites in the leaves and roots of the two genotypes under stress and control conditions. The metabolites were temporally, tissue-specifically and genotype-dependently regulated under salt stress. Sugars and amino acids (AAs) increased significantly in the leaves and roots of both genotypes, while organic acids (OAs) increased in roots and decreased in leaves. Compared with IR64, FL478 experienced greater increases in sugars and AAs and more pronounced decreases in OAs in both tissues; additionally, the maximum change in sugars and AAs occurred later, while OAs changed earlier. Moreover, less Na⁺ and higher relative water content were observed in FL478. Eleven metabolites, including AAs and sugars, were specifically increased in FL478 over the course of the treatment.

Conclusions

Metabolic responses of rice to salt stress are dynamic and involve many metabolites. The greater ST of FL478 is due to different adaptive reactions at different stress times. At early salt-stress stages, FL478 adapts to stress by decreasing OA levels or by quickly depressing growth; during later stages, more metabolites are accumulated, thereby serving as compatible solutes against osmotic challenge induced by salt stress.     

Biochemical,physiological and molecular evaluation of rice cultivars differing in salt tolerance at the seedling stage

Changes in the antioxidant enzymes, lipid peroxidation, sodium and potassium, chlorophyll, H₂O₂ and proline content were monitored in the leaves of 42 rice varieties which were not yet well-documented for the salinity tolerance under different salinity levels. The tolerant varieties (FL478, Hassani, Shahpasand, Gharib and Nemat) showed signs of tolerance (lower Na⁺/K⁺ ratio, high proline accumulation, less membrane damage, lower H₂O₂ production, and higher superoxide dismutase and catalase activity) very well. The positive relationship between the level of salt tolerance and the amount of proline accumulation in the rice varieties support the important role of proline under the salt stress. The varieties were genotyped for 12 microsatellite markers that were closely linked to SalTol QTL. The results of association analysis indicated that RM1287, RM8094, RM3412 and AP3206 markers had the high value of R² for the regression models of the studied traits. It shows the important role of SalTol in controlling physio-biochemical traits. The results can be used in the future marker assisted selection (MAS) directly, if the results are confirmed.     

Evaluation of salt tolerance at the seedling stage in rice genotypes by growth performance, ion accumulation, proline and chlorophyll content

Aims

The objectives of this study were to evaluate salt tolerance level of rice genotypes using the well-established screening criteria; the salt injury score, survival percentage and ratio between Na⁺ and K⁺, as well as the contents of proline and chlorophyll, and to identify the relationship between salt tolerance and physiological characters.

Methods

One hundred and six rice genotypes were grown in hydroponic solutions subjected to salt stress and evaluated for salt tolerance ability and the physiological parameters. Multivariate cluster analysis was performed based on salinity tolerance scores (ST scores; score 1 being the most tolerant, score 9 the most sensitive), survival percentage and Na⁺/K⁺ ratio.

Results

ST scores based on salt injury symptoms were negatively correlated with survival percentage and chlorophyll concentration in the stressed seedlings but positively correlated with Na⁺/K⁺ ratio and proline content. Rice genotypes were classified into five salt tolerance groups: tolerant (T), moderately tolerant (MT), moderately sensitive (MS), sensitive (S) and highly sensitive (HS). The means of ST scores were significantly different among the five tolerance groups indicating that the ST score was the most reliable index for identifying salt tolerance. The means of Na⁺/K⁺ ratio and proline content in stressed seedlings were distinctively different between the extreme T and HS groups, but the means among the intermediate groups (MT, MS and S) were not significantly different. Chlorophyll content, on the other hand, was not related to the levels of salt tolerance.

Conclusions

In addition to the commonly used Na⁺/K⁺ ratio, proline content is suggested to be another useful criterion to differentiate salt-tolerant from salt-sensitive rice. This study also identified several Thai improved and local cultivars with the level of salt tolerance and physiological characters comparable to Pokkali, the standard salt-tolerant donor and may be utilized as alternative sources of salt tolerance alleles.     

Salt tolerance in rice in vitro: Implication of accumulation of Na+, K+ and proline

The implication of accumulation of both inorganic (Na⁺, K⁺) and organic (proline) solutes were evaluated in unadapted and NaCl-adapted callus of a salt-sensitive (Basmati 370) and a salt-tolerant (SR-26B) cultivar of rice (Oryza sativa L.) after a NaCl shock. Accumulation of Na⁺,K⁺ and/or proline in callus was co relatable and the relative presence of these components in tissues after shock treatment was found to be important factors to support differential regrowth capacities of the shock treated calluses. Presence or retention of K⁺ in rice callus was a key factor for salt tolerance as it was observed to be positively correlated with growth in both the varieties. The results indicated that K⁺ was the first candidate to counteract the negative water potential of outside milieu, while proline was probably the last metabolic device that rice calluses opted for when exposed to salt stress.     

Differential growth and yield responses of salt-tolerant and susceptible rice cultivars to individual (Na<Superscript>+</Superscript> and Cl<Superscript>−</Superscript>) and additive stress effects of NaCl

Negative impacts exerted by sodium (Na⁺) and chloride (Cl^?) ions individually as well their possible additive effects (under NaCl) were evaluated on growth and yield reductions in rice, besides investigating whether salt-tolerant genotypes respond differentially than their sensitive counterparts. Though both Na⁺ and Cl^? ions get accumulated in plant tissues under NaCl stress, most research has historically been aimed to decipher harmful effects induced by Na⁺ ions. Accordingly, physiological and molecular mechanisms involved in Cl^? toxicity are not clearly understood in crop plants. To address these issues, 65-day-old plants of two rice cultivars, Panvel-3 (tolerant) and Sahyadri-3 (sensitive) were subjected to Cl^?, Na⁺ and NaCl (each with 100 mM concentration and electrical conductivity of ≈10 dS m^?1) stress using soil-based systems. Stress conditions were maintained till harvesting of mature (128-day-old) plants. All three treatments induced substantial antagonistic effects on growth, dry mass, yield components (number of grains per panicle, length, width, thickness and weight of grain, along with the percentage of grains filled) and overall crop yield, with greater impacts under NaCl than its constituent ions. Salinity treatments caused an imbalance in reducing sugars, protein, starch and proline contents, with the greatest magnitude under NaCl. A negative correlation between Cl^?/Na⁺ accumulation and crop yield was witnessed, with higher severity on the sensitive cultivar. The overall magnitude of toxicity was observed highest in NaCl followed by Na⁺ and Cl^?, respectively, suggesting additive effects of constituent ions under NaCl. Both cultivars responded similarly; however, the tolerant cultivar, unlike the sensitive one, kept Na⁺:K⁺ ratio <1.0 and accumulated proline in response to salinity treatments used in this study.     

Microsatellite based linkage disequilibrium analyses reveal <Emphasis Type="Italic">Saltol</Emphasis> haplotype fragmentation and identify novel QTLs for seedling stage salinity tolerance in rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.)

A set of 84 diverse rice genotypes were assessed for seedling stage salt tolerance and their genetic diversity using 41 polymorphic SSR markers comprising of 19 Saltol QTL linked and 22 random markers. Phenotypic screening under hydroponics identified three indica landraces (Badami, Shah Pasand and Pechi Badam), two Oryza rufipogon accessions (NKSWR2 and NKSWR17) and one each of Basmati rice (Seond Basmati) and japonica cultivars (Tompha Khau) as salt tolerant, having similar tolerance as of Pokkali and FL478. Among the salt tolerant genotypes, biomass showed positive correlation with shoot fresh weight and negative association with root and shoot Na⁺ content. The results indicated repression of Na⁺ loading within the tolerant plants. Linkage disequilibrium (LD) of the Saltol linked markers was weak, suggestive of high fragmentation of Pokkali haplotype, a result of evolutionary active recombination events. Poor haplotype structure of the Saltol region, may reduce its usefulness in marker assisted breeding programmes, if the target foreground markers chosen are wide apart. LD mapping identified eight robust marker-trait associations (QTLs), of which RM10927 was found linked to root and shoot Na⁺ content and RM10871 with shoot Na⁺/K⁺ ratio. RM271 on chromosome 10, an extra Saltol marker, was found associated to root Na⁺/K⁺ ratio. This marker showed a distinct allele among O. rufipogon accessions. There were also other novel loci detected on chromosomes 2, 5 and 10 influencing salt tolerance in the tested germplasm. Although Saltol remained as the key locus, the role of other genomic regions cannot be neglected in tailoring seedling stage salt tolerance in rice.     

Variation in salinity tolerance and shoot sodium accumulation in Arabidopsis ecotypes linked to differences in the natural expression levels of transporters involved in sodium transport

Salinity tolerance can be attributed to three different mechanisms: Na⁺ exclusion from the shoot, Na⁺ tissue tolerance and osmotic tolerance. Although several key ion channels and transporters involved in these processes are known, the variation in expression profiles and the effects of these proteins on Na⁺ transport in different accessions of the same species are unknown. Here, expression profiles of the genes AtHKT1;1, AtSOS1, AtNHX1 and AtAVP1 are determined in four ecotypes of Arabidopsis thaliana. Not only are these genes differentially regulated between ecotypes, the expression levels of the genes can be linked to the concentration of Na⁺ in the plant. An inverse relationship was found between AtSOS1 expression in the root and total plant Na⁺ accumulation, supporting a role for AtSOS1 in Na⁺ efflux from the plant. Similarly, ecotypes with high expression levels of AtHKT1;1 in the root had lower shoot Na⁺ concentrations, due to the hypothesized role of AtHKT1;1 in retrieval of Na⁺ from the transpiration stream. The inverse relationship between shoot Na⁺ concentration and salinity tolerance typical of most cereal crop plants was not demonstrated, but a positive relationship was found between salt tolerance and levels of AtAVP1 expression, which may be related to tissue tolerance.     

The potassium transporter OsHAK21 functions in the maintenance of ion homeostasis and tolerance to salt stress in rice

The intracellular potassium (K⁺) homeostasis, which is crucial for plant survival in saline environments, is modulated by K⁺ channels and transporters. Some members of the high‐affinity K⁺ transporter (HAK) family are believed to function in the regulation of plant salt tolerance, but the physiological mechanisms remain unclear. Here, we report a significant inducement of OsHAK21 expression by high‐salinity treatment and provide genetic evidence of the involvement of OsHAK21 in rice salt tolerance. Disruption of OsHAK21 rendered plants sensitive to salt stress. Compared with the wild type, oshak21 accumulated less K⁺ and considerably more Na⁺ in both shoots and roots, and had a significantly lower K⁺ net uptake rate but higher Na⁺ uptake rate. Our analyses of subcellular localizations and expression patterns showed that OsHAK21 was localized in the plasma membrane and expressed in xylem parenchyma and individual endodermal cells (putative passage cells). Further functional characterizations of OsHAK21 in K⁺ uptake‐deficient yeast and Arabidopsis revealed that OsHAK21 possesses K⁺ transporter activity. These results demonstrate that OsHAK21 may mediate K⁺ absorption by the plasma membrane and play crucial roles in the maintenance of the Na⁺/K⁺ homeostasis in rice under salt stress.     

Physiological and biochemical parameters for evaluation and clustering of rice cultivars differing in salt tolerance at seedling stage

Salinity tolerance levels and physiological changes were evaluated for twelve rice cultivars, including four white rice and eight black glutinous rice cultivars, during their seedling stage in response to salinity stress at 100 mM NaCl. All the rice cultivars evaluated showed an apparent decrease in growth characteristics and chlorophyll accumulation under salinity stress. By contrast an increase in proline, hydrogen peroxide, peroxidase (POX) activity and anthocyanins were observed for all cultivars. The K⁺/Na⁺ ratios evaluated for all rice cultivars were noted to be highly correlated with the salinity scores thus indicating that the K⁺/Na⁺ ratio serves as a reliable indicator of salt stress tolerance in rice. Principal component analysis (PCA) based on physiological salt tolerance indexes could clearly distinguish rice cultivars into 4 salt tolerance clusters. Noteworthy, in comparison to the salt-sensitive ones, rice cultivars that possessed higher degrees of salt tolerance displayed more enhanced activity of catalase (CAT), a smaller increase in anthocyanin, hydrogen peroxide and proline content but a smaller drop in the K⁺/Na⁺ ratio and chlorophyll accumulation.     

Effect of Salt Stress on Growth,Na+ Accumulation and Proline Metabolism in Potato (Solanum tuberosum) Cultivars

Potato (Solanum tuberosum) is a major crop world-wide and the productivity of currently used cultivars is strongly reduced at high soil salt levels. We compared the response of six potato cultivars to increased root NaCl concentrations. Cuttings were grown hydroponically and treated with 0 mM, 60 mM and 180 mM NaCl for one week. Growth reduction on salt was strongest for the cultivars Mozart and Mona Lisa with a severe senescence response at 180 mM NaCl and Mozart barely survived the treatment. The cultivars Desiree and Russett Burbank were more tolerant showing no senescence after salt treatment. A clear difference in Na⁺ homeostasis was observed between sensitive and tolerant cultivars. The salt sensitive cultivar Mozart combined low Na⁺ levels in root and stem with the highest leaf Na⁺ concentration of all cultivars, resulting in a high Na⁺ shoot distribution index (SDI) for Mozart as compared to Desiree. Overall, a positive correlation between salt tolerance and stem Na⁺ accumulation was found and the SDI for Na⁺ points to a role of stem Na⁺ accumulation in tolerance. In stem tissue, Mozart accumulated more H₂O₂ and less proline compared to the tolerant cultivars. Analysis of the expression of proline biosynthesis genes in Mozart and Desiree showed a clear reduction in proline dehydrogenase (PDH) expression in both cultivars and an increase in pyrroline-5-carboxylate synthetase 1 (P5CS1) gene expression in Desiree, but not in Mozart. Taken together, current day commercial cultivars show promising differences in salt tolerance and the results suggest that mechanisms of tolerance reside in the capacity of Na⁺ accumulation in stem tissue, resulting in reduced Na⁺ transport to the leaves.     

Control of xylem Na+ loading and transport to the shoot in rice and barley as a determinant of differential salinity stress tolerance

Control of xylem Na⁺ loading has often been named as the essential component of salinity tolerance mechanism. However, it is less clear to what extent the difference in this trait may determine differential salinity tolerance between species. In this study, barley (Hordeum vulgare L. cv. CM72) and rice (Oryza sativa L. cv. Dongjin) plants were grown under two levels of salinity. Na⁺ and K⁺ concentrations in the xylem sap, and shoot and root tissues were measured at different time points after stress onset. Salt‐exposed rice plants prevented xylem Na⁺ loading for several days, but failed to control this process in the longer term, ultimately resulting in a massive Na⁺ shoot loading. Barley plants quickly increased xylem Na⁺ concentration and its delivery to the shoot (most likely for the purpose of osmotic adjustment) but were able to reduce this process later on, keeping most of accumulated Na⁺ in the root, thus maintaining non‐toxic shoot Na⁺ level. Rice plants increased shoot K⁺ concentration, while barley plants maintained higher root K⁺ concentration. Control of xylem Na⁺ loading is remarkably different between rice and barley; this difference may differentiate the extent of the salinity tolerance between species. This trait should be investigated in more detail to be used in the breeding programs aimed to improve salinity tolerance in crops.     

K+ Efflux and Retention in Response to NaCl Stress Do Not Predict Salt Tolerance in Contrasting Genotypes of Rice (Oryza sativa L.)

Sudden elevations in external sodium chloride (NaCl) accelerate potassium (K⁺) efflux across the plasma membrane of plant root cells. It has been proposed that the extent of this acceleration can predict salt tolerance among contrasting cultivars. However, this proposal has not been considered in the context of plant nutritional history, nor has it been explored in rice (Oryza sativa L.), which stands among the world’s most important and salt-sensitive crop species. Using efflux analysis with ⁴²K, coupled with growth and tissue K⁺ analyses, we examined the short- and long-term effects of NaCl exposure to plant performance within a nutritional matrix that significantly altered tissue-K⁺ set points in three rice cultivars that differ in salt tolerance: IR29 (sensitive), IR72 (moderate), and Pokkali (tolerant). We show that total short-term K⁺ release from roots in response to NaCl stress is small (no more than 26% over 45 min) in rice. Despite strong varietal differences, the extent of efflux is shown to be a poor predictor of plant performance on long-term NaCl stress. In fact, no measure of K⁺ status was found to correlate with plant performance among cultivars either in the presence or absence of NaCl stress. By contrast, shoot Na⁺ accumulation showed the strongest correlation (a negative one) with biomass, under long-term salinity. Pharmacological evidence suggests that NaCl-induced K⁺ efflux is a result of membrane disintegrity, possibly as result of osmotic shock, and not due to ion-channel mediation. Taken together, we conclude that, in rice, K⁺ status (including efflux) is a poor predictor of salt tolerance and overall plant performance and, instead, shoot Na⁺ accumulation is the key factor in performance decline on NaCl stress.     

Mapping of QTLs Controlling Na+, K+ and CI− Ion Concentrations in Salt Tolerant Indica Rice Variety CSR27

Soil salinity and sodicity are major constraints to rice production in about twenty per cent of the irrigated crop land. Inbuilt genetic tolerance to salinity is the most economical and environmentally sustainable way to solve this problem. A mapping population of 200 F₂ plants and their corresponding F₃ families, derived from a cross between a salt tolerant indica rice variety CSR27 and a salt sensitive variety MI48 were used to map OTLs for salt tolerance. Seventeen different parameters, including seedling salt injury score, Na⁺, K⁺, CI^? concentrations and Na⁺/K⁺ ratio in leaf and stem tissues at vegetative and reproductive stages were mapped. A framework linkage map was constructed using 79 SSR and EST markers distributed over the twelve rice chromosomes at an average interval of 20.7cM and total map distance of 1634.5 cM. Twenty five major OTLs, each explaining more than ten per cent of the trait phenotypic variance, were mapped on chromosomes 1, 2, 3 and 8. These included one OTL for seedling salt injury score, nine for Na⁺ concentration, three for K⁺ concentration and four for Cl^? concentration in leaf and stem tissues at vegetative and reproductive stages. The Na⁺/K⁺ ratio, an important ion balancing parameter for the salt tolerance, was controlled by eight OTLs explaining phenotypic variance in the range of 42.88–52.63%. Four OTL intervals were robust with major effect and having OTLs for multiple salt tolerance parameters that might be governed by common or tightly linked genes. One major OTL for multiple salt tolerance parameters on chromosome 8 and three major OTLs for CI^? ion concentration are novel for this study. The OTLs identified here will serve as a base for fine mapping, gene tagging and marker assisted selection for salt tolerance in rice.     

Over-expression of a vacuolar Na<Superscript>+</Superscript>/H<Superscript>+</Superscript> antiporter gene improves salt tolerance in an upland rice

To develop a salt-tolerant upland rice cultivar (Oryza sativa L.), OsNHX1, a vacuolar-type Na⁺/H⁺ antiporter gene from rice was transferred into the genome of an upland rice cultivar (IRAT109), using an Agrobacterium-mediated method. Seven independent transgenic calli lines were identified by polymerase chain reaction (PCR) analysis. These 35S::OsNHX1 transgenic plants displayed a little accelerated growth during seedling stage but showed delayed flowering time and a slight growth retardation phenotype during late vegetative stage, suggesting that the OsNHX1 has a novel function in plant development. Northern and western blot analyses showed that the expression levels of OsNHX1 mRNA and protein in the leaves of three independent transgenic plant lines were significantly higher than in the leaves of wild type (WT) plants. T₂ generation plants exhibited increased salt tolerance, showing delayed appearance and development of damage or death caused by salt stress, as well as improved recovery upon removal from this condition. Several physiological traits, such as increased Na⁺ content, and decreased osmotic potential in transgenic plants grown in high saline concentrations, further indicated that the transgenic plants had enhanced salt tolerance. Our results suggest the potential use of these transgenic plants for further agricultural applications in saline soil.     

Overexpression of <Emphasis Type="Italic">OsVP1</Emphasis> and <Emphasis Type="Italic">OsNHX1</Emphasis> Increases Tolerance to Drought and Salinity in Rice

Drought and salinity are major abiotic stresses affecting rice production. To improve plant tolerance to salinity and drought, we overexpressed rice Na⁺/H⁺ exchangers (OsNHX1) and H⁺-pyrophosphatase in tonoplasts (OsVP1) in a japonica elite rice cultivar, Zhonghua 11. Compared with our wild-type control, transgenic plants overexpressing both genes incurred less damage when exposed to long-term treatment with 100 mM NaCl or water deprivation. Under high-saline conditions, the transformants accumulated less Na⁺ and malondialdehyde in the leaves, thereby allowing the plants to maintain a low level of leaf water potential and reduce stress-induced damage. Those transgenics also had higher photosynthetic activity during the stress period. Under those conditions, they also showed an increase in root biomass, which enabled more water uptake. These results suggest that OsVP1 and OsNHX1 improve the tolerance of rice crops against drought and salt by employing multiple strategies in addition to osmotic regulation.     

Relative salinity tolerance of rice cultivars native to North East India: a physiological,biochemical and molecular perspective

Salinity is the second most prevalent abiotic stress faced by plants, and rice is not an exception. Through this study, it has been tried upon, to study the relative salinity tolerance of eight local varieties of North East India. Preliminary screening was based on their dose- and time-dependent physiological responses to salinity stress. Among the cultivars, Tampha was found to be relatively more tolerant, whereas MSE9 the most sensitive. To further ascertain their tolerance capacity, MDA and H₂O₂ content was determined, which also confirmed the tolerance level of the two cultivars. Histochemical assays for root plasma membrane integrity and leaf and root H₂O₂ and O₂ ^? content also showed more damage in Tampha in comparison to MSE9. Finally, gene expression analysis for Na⁺/K⁺ co-transporters, OsHKT2;1, OsHKT2;3 and OsHKT2;4, was performed to observe how the expression level of these transporters varies with the tolerance capacity of these two cultivars in leaves and roots under different time frames. The study reveals Tampha to be the most tolerant and MSE9 the most sensitive when compared to the other six screened cultivars for salinity stress.     

短期NaCl胁迫对不同小麦品种幼苗K+吸收和Na+、K+积累的影响

为研究盐胁迫下小麦幼苗生长及Na⁺、K⁺的吸收和积累规律,以中国春、洲元9369和长武134等3种耐盐性不同小麦品种为材料,采用非损伤微测技术检测盐胁迫2 d后的根系K⁺离子流变化,并对植株体内的Na⁺、K⁺含量进行测定。结果表明:短期(2d)盐胁迫对小麦生长有抑制作用,且对根系的抑制大于地上部,耐盐品种下降幅度小于盐敏感品种。盐胁迫下,小麦根际的 K⁺大量外流,盐敏感品种中国春K⁺流速显著高于耐盐品种长武134,最高可达15倍。小麦幼苗地上部分和根系均表现为Na⁺积累增加,K⁺积累减少,Na⁺/K⁺比随盐浓度增加而上升。中国春限Na⁺能力显著低于长武134,Na⁺/K⁺则显著高于长武134。综上所述,盐胁迫下造成小麦组织器官中Na⁺/K⁺比上升的主要原因是根系K⁺大量外流和Na⁺的过量积累,耐盐性不同的小麦品种间差异显著,并认为根系对K⁺的保有能力可能是作物耐盐性评价的一个重要指标。     

Regulation of the uptake and distribution of Na+ in shoots of rice (Oryza sativa) variety Pokkali: role of Ca2+ in salt tolerance response

Soil salinity is a major factor affecting crop productivity worldwide. This study explores mechanisms that contribute to salt tolerance in rice (Oryza sativa L.). Hydroponically grown, 2-week-old salt tolerant and sensitive indica rice varieties, Pokkali and Jaya, respectively, were exposed to a 48-h stress period with NaCl (0–250 mM). When exposed to 200 mM NaCl, micromolar levels of external Ca²⁺ elevated survival of both varieties. The Ca²⁺ levels required were lower for Pokkali than for Jaya, but resulted in significantly higher survival. Estimates of Na⁺ and K⁺ in root and shoot compartments were made by flame photometry, while X-ray microanalysis was used to localize Na⁺ in the extracellular matrix of the shoot. Transpirational bypass flow was estimated using the apoplastic tracer, 8-hydroxypyrene-1,3,6-trisulphonic acid, trisodium salt. Our data demonstrate a Ca²⁺-dependent reduction in Na⁺ transport to shoots, which correlated with a decline in bypass flow and of Na⁺ in the transpirational stream. In addition, the Na⁺ that enters the shoot is partitioned among several distinct compartments. Survival is inversely correlated with Na⁺ levels in the shoot apoplastic fluid, which surrounds the cell and influences cytosolic composition. Pokkali maintained lower Na⁺ in its apoplast compared with the salt sensitive Jaya at the same total shoot Na⁺. Na⁺ in the apoplast appears to be regulated by sequestration into intracellular compartments. This sink supplements the primary response of reducing Na⁺ influx into the shoot and effectively buffers the apoplastic fluid in Pokkali. All of these mechanisms are operational in Jaya as well but are deployed less effectively.     

Physiological changes during development of rice (Oryza sativa L.) varieties differing in salt tolerance under saline field condition

Aims

To investigate the changes in physiological parameters in leaves of field-grown rice genotypes differing in their salt tolerance.

Methods

Thirty rice cultivars classified as tolerant (T; 11), moderately tolerant (MT; 5), moderately sensitive (MS; 7) and sensitive (S; 7) based on the previous screening at the seedling stage were established in a greenhouse. Thirty-day-old seedlings were then transplanted to a rice field, situated in a moderately saline area in northeastern Thailand, where EC slowly increased from 2.03 to 6.46 dS m^?1 from the transplanting date to harvest. Leaf samples (the third leaves from the top or the flag leaves during the vegetative or the reproductive phase, respectively) were collected, at 1 month intervals, when the plants were 60-, 90-, 120- and 150-day-old corresponding to active tillering, early reproductive, late reproductive and harvest stage, respectively. Leaf samples were analyzed for changes in proline, chlorophyll and malondialdehyde (MDA). The harvested panicles were evaluated for the percentage of filled grain weight and the concentration of Na⁺ and K⁺ in the top internode.

Results

The patterns of change and the mean concentrations of most physiological parameters in rice leaves during the course of development were strikingly similar for the four classes of salt tolerance. Proline concentration remained relatively constant throughout the development and finally showed a dramatic increase in the flag leaves at harvest. MDA concentration tended to increase with age reaching the maximum in the flag leaves at harvest. The chlorophyll concentration was higher during the vegetative stage than the reproductive stages. At harvest corresponding to the time of maximum salinity, the T group tended to contain higher proline and stayed green longer than the other groups. Moreover, the T group showed higher percentage filled grain weight which was associated with lower Na⁺/K⁺ ratio in the top internode. The percentage filled grain weight was negatively correlated with Na⁺ concentration and Na⁺/K⁺ ratio in the top internode and proline concentration in flag leaves, but did not correlate with chlorophyll and MDA in flag leaves.

Conclusions

Rice cultivars which are tolerant at the seedling stage also showed higher tolerance in the field condition as shown by higher percentage filled grain weight and lower Na⁺ uptake to the panicles. Tolerant cultivars tended to accumulate less proline in their leaves similar to that found at the seedling stage.