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.     

Response of Contrasting Rice Genotypes to Zinc Sources under Saline Conditions

Abiotic stresses are among the major limiting factors for plant growth and crop productivity. Among these, salinity is one of the major risk factors for plant growth and development in arid to semi-arid regions. Cultivation of salt tolerant crop genotypes is one of the imperative approaches to meet the food demand for increasing population. The current experiment was carried out to access the performance of different rice genotypes under salinity stress and Zinc (Zn) sources. Four rice genotypes were grown in a pot experiment and were exposed to salinity stress (7 dS m⁻¹), and Zn (15 mg kg⁻¹ soil) was applied from two sources, ZnSO₄ and Zn-EDTA. A control of both salinity and Zn was kept for comparison. Results showed that based on the biomass accumulation and K⁺/Na⁺ ratio, KSK-133 and BAS-198 emerged as salt tolerant and salt sensitive, respectively. Similarly, based on the Zn concentration, BAS-2000 was reported as Zn-in-efficient while IR-6 was a Zn-efficient genotype. Our results also revealed that plant growth, relative water content (RWC), physiological attributes including chlorophyll contents, ionic concentrations in straw and grains of all rice genotypes were decreased under salinity stress. However, salt tolerant and Zn-in-efficient rice genotypes showed significantly higher shoot K⁺ and Zn concentrations under saline conditions. Zinc application significantly alleviates the harmful effects of salinity by improving morpho-physiological attributes and enhancing antioxidant enzyme activities, and the uptake of K and Zn. The beneficial effect of Zn was more pronounced in salt-tolerant and Zn in-efficient rice genotypes as compared with salt-sensitive and Zn-efficient genotypes. In sum, our results confirmed that Zn application increased overall plant’s performance under saline conditions, particularly in Zn in-efficient and tolerant genotypes as compared with salt-sensitive and Zn efficient rice genotypes.     

Differential responses of antioxidant system and photosynthetic characteristics in four rice cultivars differing in sensitivity to sodium chloride stress

To keep pace with ever growing global population, progressive and sustained increase in rice production is necessary, especially in areas with extremely variable climatic conditions, where rice crop suffers from numerous abiotic stresses including salinity. Designing an effective phenotyping strategy requires thorough understanding of plant survival under stress. The investigation was carried out with four rice cultivars namely FR13A, IR42, Rashpanjor, and Pokkali that differed in salinity tolerance. The study showed that a genotype with initial vigour had some advantage in preserving shoot biomass under salt stress. Though both FR13A and IR42 showed sensitivity to salinity, FR13A with higher initial biomass maintained greater dry weight under saline condition. Increase of Na⁺:K⁺ ratio under salinity, due to accelerated absorption of Na⁺ and lesser absorption of K⁺ compared to control, was considerably higher in susceptible (118–200 %) than in tolerant (33–48 %) genotypes. While Na⁺ concentration in shoot increased significantly in both tolerant and susceptible genotypes, decrease in shoot K⁺ content was noticed only in susceptible genotypes. The imbalance of Na⁺ and K⁺ contents led to increased H₂O₂ production, causing greater peroxidation of membrane lipids and reduction in chlorophyll content and CO₂ photosynthetic rate. Certain chlorophyll fluorescence parameters could distinguish between salinity tolerant and sensitive genotypes. To protect the plant from oxidative damage, several enzymatic and nonenzymatic antioxidants such as ascorbate were involved. The genotypes with capacity to assemble antioxidant enzymes in time could detoxify the reactive oxygen species more efficiently, leading to greater protection and reduced impact of salt stress.     

Combined effects of waterlogging and salinity on electrochemistry, water-soluble cations and water dispersible clay in soils with various salinity levels

The combination effects of waterlogging and salinity on redox potential (Eh), pH, electric conductivity (EC), water-soluble cations (NH₄ ⁺, K⁺, Na⁺, Ca²⁺, Mg²⁺, Fe²⁺, and Mn²⁺) and water-dispersible clay (WDC) were studied in six soils collected near salt lakes in western Australia. The soils with various salinity levels were incubated under a waterlogged condition at 30 °C for 12 weeks. The Eh, pH, EC, and cations of soil solutions were monitored over the waterlogged period. The Eh values generally dropped to the lowest point within 12 days of waterlogging, then increased slightly, and reached equilibrium after 4 weeks of waterlogging. Increasing salinity levels increased soil Eh. While waterlogging increased soil pH in the first 3–4 weeks, increasing salinity level decreased soil pH during the entire waterlogging period. Waterlogging increased the EC values in the first 2 weeks, partly due to dissolution of insoluble salts. The concentrations of water-soluble NH₄ ⁺ were significantly increased with salinity level and waterlogging, and reached maximum values at week 2, and then declined to the initial level. Waterlogging and salinity increased the concentrations of water-soluble K⁺, Ca²⁺, Mg²⁺, Fe²⁺, and Mn²⁺ ions, but the magnitudes of changes were greatly affected by soil properties. Increases in water-soluble K⁺, Ca²⁺ and Mg²⁺ were attributed to increased solubility of insoluble salts, and increased competition for the adsorption sites of the soil exchange complex due to elevated concentrations of Na⁺, Fe²⁺ and Mn²⁺. Increases in water-soluble Fe²⁺ and Mn²⁺ induced by waterlogging were attributed to the dissolution of Fe and Mn oxides under reduced conditions. Waterlogging increased, but salinity decreased, the amounts of water-dispersible clay in the soils of low EC value. The higher salinity level can counteract the adverse effect of waterlogging on clay flocculation.     

Genome-wide association mapping of salinity tolerance in rice (Oryza sativa)

Salinity tolerance in rice is highly desirable to sustain production in areas rendered saline due to various reasons. It is a complex quantitative trait having different components, which can be dissected effectively by genome-wide association study (GWAS). Here, we implemented GWAS to identify loci controlling salinity tolerance in rice. A custom-designed array based on 6,000 single nucleotide polymorphisms (SNPs) in as many stress-responsive genes, distributed at an average physical interval of <100 kb on 12 rice chromosomes, was used to genotype 220 rice accessions using Infinium high-throughput assay. Genetic association was analysed with 12 different traits recorded on these accessions under field conditions at reproductive stage. We identified 20 SNPs (loci) significantly associated with Na⁺/K⁺ ratio, and 44 SNPs with other traits observed under stress condition. The loci identified for various salinity indices through GWAS explained 5–18% of the phenotypic variance. The region harbouring Saltol, a major quantitative trait loci (QTLs) on chromosome 1 in rice, which is known to control salinity tolerance at seedling stage, was detected as a major association with Na⁺/K⁺ ratio measured at reproductive stage in our study. In addition to Saltol, we also found GWAS peaks representing new QTLs on chromosomes 4, 6 and 7. The current association mapping panel contained mostly indica accessions that can serve as source of novel salt tolerance genes and alleles. The gene-based SNP array used in this study was found cost-effective and efficient in unveiling genomic regions/candidate genes regulating salinity stress tolerance in rice.     

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.     

Leaf cell membrane stability‐based mechanisms of zinc nutrition in mitigating salinity stress in rice

• Excess salt affects about 955 million ha of arable land worldwide, and 49% of agricultural land is Zn‐deficient. Soil salinity and zinc deficiency can intensify plant abiotic stress. The mechanisms by which Zn can mitigate salinity effects on plant functions are not well understood.
• We conducted an experiment to determine how Zn and salinity effects on rice plant retention of Zn, K⁺ and the salt ion Na⁺ affect chlorophyll formation, leaf cell membrane stability and grain yield. We examined the mechanisms of Zn nutrition in mitigating salinity stress by examining plant physiology and nutrition. We used native Zn‐deficient soils (control), four salinity (EC ) and Zn treatments – Zn 10 mg·kg^?1 (Zn₁₀), EC 5 dS ·m^?1 (EC ₅), Zn₁₀+EC ₅ and Zn₁₅+EC ₅, a coarse rice (KS ‐282) and a fine rice (Basmati‐515) in the study.
• Our results showed that Zn alone (Zn₁₀) significantly increased rice tolerance to salinity stress by promoting Zn/K⁺ retention, inhibiting plant Na⁺ uptake and enhancing leaf cell membrane stability and chlorophyll formation in both rice cultivars in native alkaline, Zn‐deficient soils (P  <  0.05). Further, under the salinity treatment (EC ₅), Zn inputs (10–15 mg·kg^?1) could also significantly promote rice plant Zn/K⁺ retention and reduce plant Na⁺ uptake, and thus increased leaf cell membrane stability and grain yield. Coarse rice was more salinity‐tolerant than fine rice, having significantly higher Zn/K⁺ nutrient retention.
• The mechanistic basis of Zn nutrition in mitigating salinity impacts was through promoting plant Zn/K⁺ uptake and inhibiting plant Na⁺ uptake, which could result in increased plant physiological vigour, leaf cell membrane stability and rice productivity.

     

Different mechanisms of ion homeostasis are dominant in the recretohalophyte <Emphasis Type="Italic">Tamarix ramosissima</Emphasis> under different soil salinity

Total ion (Na⁺, K⁺, Ca²⁺, SO₄ ^2? and Cl^?) accumulation by plants, ion contents in plant tissues and ion secretion by salt glands on the surface of shoots of Tamarix ramosissima adapted to different soil salinity, namely low (0.06 mmol Na⁺/g soil), moderate (3.14–4.85 mmol Na⁺/g soil) and strong (7.56 mmol Na⁺/g soil) were analyzed. There are two stages of interrelated and complementary regulation of ion homeostasis in whole T. ramosissima plants: (1) regulation of ion influx into the plant from the soil and (2) changing the secretion efficiency of salt glands on shoots. The secretion efficiency of salt glands was appraised by the ratio of ion secretion to tissue ion content. Independent of soil salinity, the accumulation of K⁺ and Ca²⁺ was higher than the contents of these ions in the soil. Furthermore, the accumulation of K⁺, Ca²⁺ and SO₄ ^2? ions by plants was maintained within a narrow range of values. Under low soil salinity, Na⁺ was accumulated, whereas under moderate and strong salinity, the influxes of Na⁺ were limited. However, under strong salinity, the accumulation of Na⁺ was threefold higher than that under low soil salinity. This led to a change in the Na⁺/K⁺ ratio (tenfold), an increase in the activity of salt glands (tenfold) and a reduction in plant growth (fivefold). An apparently high Na⁺/K⁺ ratio was the main factor determining over-active functioning of salt glands under strong salinity. Principal component analysis showed that K⁺ ions played a key role in ion homeostasis at all levels of salinity. Ca²⁺ played a significant role at low salinity, whereas Cl^? and interrelated regulatory components (K⁺ and proline) played a role under strong salinity. Proline, despite its low concentration under strong salinity, was involved in the regulation of secretion by salt glands. Different stages and mechanisms of ion homeostasis were dominant in T. ramosissima plants adapted to different levels of salinity. These mechanisms facilitated the accumulation of Na⁺ in plants under low soil salinity, the limitation of Na⁺ under moderate salinity and the over-activation of Na⁺ secretion by salt glands under strong salinity, which are all necessary for maintaining ion homeostasis and water potential in the whole plant.     

Differences in physiological and biochemical characteristics in response to single and combined drought and salinity stresses between wheat genotypes differing in salt tolerance

The combined drought and salinity stresses pose a serious challenge for crop production, but the physiological mechanisms behind the stresses responses in wheat remains poorly understood. Greenhouse pot experiment was performed to study differences in genotype response to the single and combined (D + S) stresses of drought (4% soil moisture, D) and salinity (100 mM NaCl, S) using two wheat genotypes: Jimai22 (salt tolerant) and Yangmai20 (salt‐sensitive). Results showed that salinity, drought and/or D + S severely reduces plant growth, biomass and net photosynthetic rate, with a greater effect observed in Yangmai20 than Jimai22. A notable improvement in water use efficiency (WUE) by 239, 77 and 103% under drought, salinity and D + S, respectively, was observed in Jimai22. Moreover, Jimai22 recorded higher root K⁺ concentration in drought and salinity stressed condition and shoot K⁺ under salinity alone than that of Yangmai20. Jimai22 showed lower increase in malondialdehyde (MDA) accumulation, but higher activities of superoxide dismutase (SOD, EC 1.15.1.1) and guaicol peroxidase (POD, EC 1.11.1.7), under single and combined stresses, and catalase (CAT, EC 1.11.1.6) and ascorbate peroxidase (APX, EC 1.11.1.11) under single stress. Our results suggest that high tolerance of Jimai22 in both drought and D + S stresses is closely associated with larger root length, higher Fv/Fm and less MDA contents and improved capacity of SOD and POD. Moreover, under drought Jimai22 tolerance is firmly related to higher root K⁺ concentration level and low level of Na⁺, high‐net photosynthetic rate and WUE as well as increased CAT and APX activities to scavenge reactive oxygen species.     

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.     

Thai jasmine rice cultivar KDML105 carrying <Emphasis Type="Italic">Saltol</Emphasis> QTL exhibiting salinity tolerance at seedling stage

Saltol, the major salinity tolerance quantitative trait loci (QTL) in rice, was introgressed from IR66946-3R-230-1-1 (FL530) into Khao Dawk Mali 105 (KDMl105) by two rounds of marker-assisted backcrossing (MAB). Twenty-eight BC₂F₂ introgression lines (BILs) with positive Saltol allele (BIL^+Saltol) and 19 BILs with negative Saltol allele (BIL^?Saltol) were validated for the effect of Saltol as key salinity tolerant trait at seedling stage. A hydrophonic system with salt stress of 12 dS m^?1 (130 mM Na⁺) was conducted, and significant differences between BILs^+Saltol and BILs^?Saltol were observed for the period of plant survival (PPS), total K⁺ (T-K⁺) and Na⁺ (T-Na⁺) concentration, whole plant Na⁺-K⁺ ratio (T-Na⁺/K⁺), shoot Na⁺ (S-Na⁺) and K⁺ (S-K⁺) concentration, and shoot Na⁺-K⁺ ratio (S-Na⁺/K⁺). BILs^+Saltol displayed higher PPS, uptake less Na⁺ (T-Na⁺; 43.4 ppm), and more K⁺ (T-K⁺; 30.9 ppm), while the BILs^?Saltol uptake more Na⁺ (T-Na⁺; 45.7 ppm) and less K⁺ (T-K⁺; 28.2 ppm). Direct effects on PPS and salt injury score (SIS) were observed, indicating Na⁺/K⁺ homeostasis mechanism by the Saltol under hydrophonic salt stress. All BILs^+Saltol recovered KDML105 cooking quality profile such as low apparent amylose content (AAC), high score of alkaline spreading value (ASV), intermediate gel consistency (GC), and strong fragrance. However, variation in agronomic traits was observed. The possibility of lowering S-Na⁺/K⁺ ratio under salt stress at seedling stage in KDML105 by introgression of the Saltol was demonstrated. Currently, BC₂F₇ of the BIL^+Saltol selected lines are being tested for salinity tolerance in the salt-affected areas in the northeast of Thailand.     

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.     

Comprehensive physiological analyses and reactive oxygen species profiling in drought tolerant rice genotypes under salinity stress

Rice being a staple cereal is extremely susceptible towards abiotic stresses. Drought and salinity are two vital factors limiting rice cultivation in Eastern Indo-Gangetic Plains (EIGP). Present study has intended to evaluate the consequences of salinity stress on selected drought tolerant rice genotypes at the most susceptible seedling stage with an aim to identify the potential multi-stress (drought and salt) tolerant rice genotype of this region. Genotypic variation was obvious in all traits related to drought and salt susceptibility. IR84895-B-127-CRA-5-1-1, one of the rice genotypes studied, exhibited exceptional drought and salinity tolerance. IR83373-B-B-25-3-B-B-25-3 also displayed enhanced drought and salt tolerance following IR84895-B-127-CRA-5-1-1. Variations were perceptible in different factors involving photosynthetic performance, proline content, lipid peroxidation, K⁺/Na⁺ ratio. Accumulation of reactive oxygen species (ROS) disintegrated cellular and sub-cellular membrane leading to decreased photosynthetic activities. Therefore, accumulation and detoxification of reactive oxygen species was also considered as a major determinant of salt tolerance. IR84895-B-127-CRA-5-1-1 showed improved ROS detoxification mediated by antioxidant enzymes. IR84895-B-127-CRA-5-1-1 seedlings also displayed significant recovery after removal of salt stress. The results established a direct association of ROS scavenging with improved physiological activities and salt tolerance. The study also recommended IR84895-B-127-CRA-5-1-1 for improved crop performance in both drought and saline environments of EIGP. These contrasting rice genotypes may assist in understanding the multiple stress associated factors in concurrent drought and salt tolerant rice genotypes.     

Salt sensitivity in chickpea (Cicer arietinum L.): ions in reproductive tissues and yield components in contrasting genotypes

The reproductive phase in chickpea (Cicer arietinum L.) is affected by salinity, but little is known about the underlying cause. We investigated whether high concentrations of Na⁺ and Cl^– in the reproductive structures influence reproductive processes. Chickpea genotypes contrasting in tolerance were subjected to 0, 35 or 50 mm NaCl applied to soil in pots. Flower production and abortion, pod number, percentage of empty pods, seed number and size were evaluated. The concentrations of Na⁺, K⁺ and Cl^– were measured in various plant tissues and, using X‐ray microanalysis, in specific cells of developing reproductive structures. Genotypic variation in reproductive success measured as seed yield in saline conditions was associated with better maintenance of flower production and higher numbers of filled pods (and thus seed number), whereas seed size decreased in all genotypes. Despite the variation in reproductive success, the accumulation of Na⁺ and Cl^– in the early reproductive tissues of developing pods did not differ between a tolerant (Genesis836) and a sensitive (Rupali) genotype. Similarly, salinity tolerance was not associated with the accumulation of salt ions in leaves at the time of reproduction or in seeds at maturity.     

Growth, proline accumulation and ion content in sodium chloride-stressed callus of date palm

Summary Growth and physiological responses of date palm. Phoenix dactylifera L. cv. Barhee, callus to salinity stress were examined. Callus induced from shoot tips of offshoots was cultured on Murashige and Skoog medium supplemented with NaCl at concentrations ranging from 0 to 225 mM, in consective increments of 25 mM. Data obtained after 6 wk of exposure to salt have shown a significant increase in callus proliferation in response to 25 mM NaCl the lowest level tested, beyond which callus weight decreased. At 125 mM NaCl and higher, callus growth was nearly completely inhibited. Physiological studies on callus exposed to salt stress have shown an increase in proline accumulation in response to increased salinity. Proline accumulation was correlated to callus growth inhibition. Furthermore, increasing the concentration of NaCl in the culture medium generally resulted in a steady increase in Na⁺ and reduction in K⁺ concentrations. However, at 25 mM NaCl, the only level at which callus growth was significantly enhanced, an increase in K⁺ content was noted, in comparison to the NaCl free control. In response to increasing external NaCl level, the Na⁺/K⁺ ratio increased The Na⁺/K⁺ ratio was positively correlated to proline accumulation and hence callus growth inhibition. This study provides, an understanding of the response of date palm callus to salinity, which is important for future studies aimed at developing strategies for selecting and characterizing somaclonal variants tolerant to salt stress.     

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.     

Ectopic expression of PgRab7 in rice plants (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.) results in differential tolerance at the vegetative and seed setting stage during salinity and drought stress

In this work, we have overexpressed a vesicle trafficking protein, Rab7, from a stress-tolerant plant, Pennisetum glaucum, in a high-yielding but stress-sensitive rice variety Pusa Basmati-1 (PB-1). The transgenic rice plants were tested for tolerance against salinity and drought stress. The transgenic plants showed considerable tolerance at the vegetative stage against both salinity (200 mM NaCl) and drought stress (up to 12 days after withdrawing water). The protection against salt and drought stress may be by regulating Na⁺ ion homeostasis, as the transgenic plants showed altered expression of multiple transporter genes, including OsNHX1, OsNHX2, OsSOS1, OsVHA, and OsGLRs. In addition, decreased generation and maintenance of lesser reactive oxygen species (ROS), with maintenance of chloroplast grana and photosynthetic machinery was observed. When evaluated for reproductive growth, 89–96 % of seed setting was maintained in transgenic plants during drought stress; however, under salt stress, a 33–53 % decrease in seed setting was observed. These results indicate that PgRab7 overexpression in rice confers differential tolerance at the seed setting stage during salinity and drought stress and could be a favored target for raising drought-tolerant crops.     

Effect of salt stress on cucumber: Na+–K+ ratio, osmolyte concentration, phenols and chlorophyll content

A pot experiment with 17 diverse genotypes of cucumber with four levels of salt stress viz., 0, 2, 4 and 6 dS m^?1 was carried out during 2006. ANOVA revealed significant differences amongst genotypes and genotype × salt stress interaction indicating the genetic variability and differential response of the genotypes to different salt stress levels. The salt stress adversely affected the biochemical parameters; effects were severe under 4 dS m^?1. No genotype could survive at 6 dS m^?1. Sodium content, Na⁺–K⁺ ratio, proline, reducing sugars, phenol and yield reduction (%) increased significantly as the salt stress increased. Potassium, chlorophyll, membrane stability index and fruit yield decreased significantly under salt stress in all genotypes. However, the genotypes CRC-8, CHC-2 and G-338 showed lower accumulation of sodium, lesser depletion of potassium, lower Na⁺–K⁺ ratio and higher accumulation of proline, reducing sugars, phenols, better membrane stability and lower yield reduction (%) under salt stress, while CH-20 and DC-1 were sensitive to salt stress. Thus, a combination of traits such as higher membrane stability, lower Na⁺–K⁺ ratio, higher osmotic concentration and selective uptake of useful ions and prevention of over accumulation of toxic ions contribute to salt stress tolerance in cucumber. These traits would be useful selection criteria during salt stress breeding in cucumber.     

Salt dynamics in soil profiles during long-term evaporation under different groundwater conditions

To study salt dynamics in soil profiles under different groundwater conditions, a 3-year indoor experiment was carried out under conditions of open-air evaporation. Silt loam soil was treated under three groundwater table depths (0.85, 1.05, and 1.55 m) combined with three groundwater salinities: 0.40 dS m^? 1 (2 g l^? 1), 0.80 dS m^? 1 (4 g l^? 1), and 1.60 dS m^? 1 (8 g l^? 1). A total of nine soil columns (0.14 m internal diameter) were used to simulate different combinations of groundwater depths and salinities. The results obtained showed that salt first accumulated at the bottom of the soil column, and only when soil salinity in this layer had remained relatively stable with time, salt began to accumulate in the adjacent upper soil layers. When all subsoil layers had reached dynamic salinity equilibrium, electrical conductivity (EC) of soils in the surface layer began to increase drastically. With increasing salt accumulation in the surface soil, EC of the subsoil began to rise tardily. The further up the soil layer, the earlier EC started to increase, although the redistribution of salts in the soil profile tended to be homogenous. Groundwater depth did not significantly change subsoil EC values at the same depth; however, it distinctly affected the time needed for the subsoil to reach dynamic salinity equilibrium. Groundwater salinity, on the other hand, did not significantly alter the time point at which soil salinity at the same depth began to increase rapidly or the time period needed to reach dynamic salinity equilibrium. This study explored salt transport processes in the soil profile through a long-term experiment, enabling us to reveal some general laws governing salt dynamics that will be very important to understand the mechanism of soil salinization. The results could be further used to set up strategies to prevent salinization or to improve salt-affected soils.