Journal of Heredity Advance Access originally published online on October 22, 2007
Journal of Heredity 2007 98(7):655-665; doi:10.1093/jhered/esm084
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Genetic Analysis of Salt Stress Responses in Asparagus Bean (Vigna unguiculata (L.) ssp. sesquipedalis Verdc.)
From the Key Laboratory of Ministry of Education for Plant Developmental Biology, College of Life Sciences, Wuhan University, Wuhan 430072, Hubei, China (Chen and Ding); and the Laboratory of Germplasm and Genetics, College of Life Sciences, Jianghang University, Wuhan, 430056, Hubei, China (Chen, Tao, and Peng)
Address correspondence to Y. Ding at the address above, or e-mail: yiding{at}whu.edu.cn.
Salt stress responses of 23 asparagus bean cultivars were evaluated using 14-day-old seedlings after 15-day exposure to 75 mM NaCl in a hydroponics culture system. Salt-induced changes in plant growth and morphology, photosynthetic capacity, cell membrane integrity, and cellular protection enzyme systems as well as other physiological and biochemical traits were investigated to identify genotypic variability in salt response. This study also analyzed heredity parameters and correlation of the salt response index (SRI). Salt stress suppressed seedling growth and simultaneously reduced leaf area, content of chlorophyll and soluble proteins, net assimilation rate, as well as dry matter accumulation. In contrast, leaf blade cell membrane relative permeability, content of malondialdehyde, and antioxidant enzyme activity were elevated after salt treatment. Analysis of the heredity parameters has identified that the 18 investigated traits have different genotypic variance of the SRI values. Based on Ward's distances estimated for the sum of square variance in the SRI values, all the cultivars were classified into 2 discrete salt-tolerant and salt-sensitive clusters. Findings from this study will provide theoretical bases for identification and breeding of salt-tolerant cultivars in asparagus bean.
Cowpea (Vigna unguiculata (L.) Walp.) is an important legume species that is widely growing as animal feed, vegetable, or grain crop (Wests and Francois 1983). Asparagus bean (V. unguiculata (L.) ssp. sesquipedalis Verdc.) is one of its major cultivated subspecies. The plants produce tender pods consumed as an important vegetable in China, Southeast Asia, and some subtropical regions. Salinity is one of the most severe abiotic stresses affecting production of the legumes worldwide (Bayuelo-Jiménez et al. 2002; Wang et al. 2003). Plant growth and development are constrained by salt stress, which results in reduced production of dry matter and yield (Aslam et al. 1993). Because of its importance, extensive researches have been conducted to investigate salt tolerance and the associated mechanisms in tomato, cucumber, rice, and many other plant species (Sreenivasulu et al. 2000; Kaya et al. 2001a, 2001b, 2002; Jumberi et al. 2002; Baba and Fujiyama 2003; Lopez Aguilar et al. 2003).
The cowpea is commonly considered as intermediate salt tolerant, having salt tolerance higher than corn but lower than barley, wheat, and cotton (Hall and Frate 1996). In selection and breeding for elite cultivars, salt tolerance, in addition to superior quality and high yield, has been considered as one of the important characteristics for asparagus bean crop. However, most of the reported works limit to morphological, physiological, biochemical, and some molecular descriptions. Very few studies have been conducted on genetic analysis of the phenotypic traits. Salt response index (SRI) is the ratio between the observed values with and without salt treatment; it is a measurement of change for plant traits caused by salt stress. There is a dearth of information on using SRI as an indicator of salt tolerance in asparagus bean cultivars.
Salt tolerance is controlled by multiple genes and regulated by different types of proteins (Bohnert and Jensen 1996). Salt stress can damage or reduce nearly all functions of the plant (Greenway and Munns 1980). It interferes with water absorption and enhances accumulation of Na+, which will lead to imbalance of mineral elements and disturbance of cellular biochemical reactions. The cellular metabolism, physiological biochemical, as well as photosynthetic activities are all adversely affected by salt stress. In addition, salt stress can also affect seed germination and inhibit elongation of germinated seeds (Greenway and Munns 1980; Bohnert et al. 1995; Dash and Panda 2001; Sairam and Tyagi 2004).
Salt can impose a multifaceted injury to cowpea plants, such as seed germination, vegetative growth, and yield (Imamul and Larher 1983; Wests and Francois 1983; Kannan and Ramani 1988; Maas and Poss 1989; Larcher et al. 1990; Plaut et al. 1990; Fernandes de Melo et al. 1994; Murillo-Amador et al. 2000, 2001, 2006). To improve the reliability and selection efficiency for salt tolerance, it is necessary to identify the salt-induced characteristic changes in multiple traits among different genotypes. This study compared seedling growth and their morphology, photosynthetic capacity, cell membrane characteristics (EC, malondialdehyde [MDA]), and cellular protection enzyme activity (superoxide dismutase [SOD], catalase [CAT], peroxidase [POD], ascorbate peroxidase [APX]), as well as their relationship with salt tolerance. The plants were growing in a hydroponics culture system supplemented with 75 mM NaCl. The SRI value was considered as the indicator for salt tolerance. Through comparing genetics and correlation of SRI, we have obtained critical information about salt tolerance in different cultivars of asparagus bean. Findings from this study will provide theoretical bases and practical guidance for distinguishing salt tolerant germplasm resources and breeding for tolerant cultivars.
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Plant Materials
All the cultivars (Table 1) were maintained in the Laboratory of Germplasm and Genetics, College of Life sciences of Jianghan University. Complete records of the origin and genetic background indicate that these cultivars can represent the genetic diversity of asparagus bean germplasm resources in China.
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Plant Material Preparation and Salt Treatment
Seed Germination and Seedling Growth
Seeds harvested in the previous fall season (2004) were used for this study. A total of 200 healthy seeds with no obvious defects and disease were included for each cultivar (Table 1). After surface sterilization in 70% ethanol for 2 min, the seeds were rinsed twice in deionized water and then placed on water-moistened filter papers in sterilized petri dishes for germination at 28 ± 1 °C and 80% of relative humidity (RH). Seeds germinated in 1 day were transplanted into 9 x 15–cm pots filled with mixtures of same soil and fertilizers and grown in a greenhouse. After the first emergence, the seedlings were watered evenly to ensure that the growth condition was consistent.
Salt Treatment
Salt treatment experiment was conducted during April–June 2005 in a greenhouse located in a geographical region with latitude of 30°38'N, longitude of 114°20'E, and altitude of 23 m a.s.l. (above sea level). Fourteen-day-old seedlings with 2 true leaves and uniform shoot and root sizes were subjected to salt treatment. After removal of cotyledons and soil residues on the root surfaces, the seedlings were wrapped in 2 layers of black kraft paper and placed in clean plastic cups containing 100 ml of Hoagland nutrient solution (EC 2.2–2.5 ms/cm) (Hoagland and Arnon 1950). The pH of the solution was adjusted to 6.5 with 0.1 M KOH or 0.1 M HCl, and 75 mM NaCl was added into the solution for salt treatment. The regular Hoagland nutrient solution was used as control. The experimental design was a 2-factorial set of 23 cultivars and 2 salt levels arranged in a randomized complete block design, with 4 replications of each treatment (10 seedlings per cultivar). The seedlings were growing at 30/20 °C (day/night), 80% of RH, and natural sunlight with a 13:11 h light:dark photoperiod and about 180 µmol/m2/s light intensity. To minimize fluctuation of salt concentration due to absorption and water evaporation, the nutrient solution was replaced every 3 days and the plastic cup was shaken daily to ensure aeration in the root system (Silva et al. 2003). The growth of plants was recorded and photographed daily. At 8 AM of the fifteenth day, leaf blades of the composite leaves were collected for further analysis.
Traits Indicative of Salt Tolerance and the Measurement Methodology
Salt Tolerance Index
Data were collected 15 days after salt treatment. Individual seedling was rated on a scale from 0 to 4 based on the following salt injury grading criteria.
Salt injury grading criteria—0: no salt injury, 100% survival without damage; 1: mild salt injury, indicated by small area (approximately 1/5) of leaf apex and leaf margin turning yellow; 2: moderate salt injury when 1/2 leaf apex and leaf margin became chlorotic; 3: severe salt injury when majority of apexes and leaf margin turned sallow; and 4: extreme salt injury when leaves fell off, stem became shrunken, and the plant eventually died. The salt tolerance index (SI) was calculated using the following formula:
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Morphological Observations
Each individual plant was measured for its fresh weight, height, stem diameter, surface area of true leaves, and number of leaves. Stem diameter was measured right below cotyledon, and the plant height was from shoot apex to the soil surface.
Net Assimilation Rate
Net assimilation rate (NAR) was measured following the same procedure as described by Radford (1967).
Contents of Chlorophyll and Carotenoids
Contents of chlorophyll and carotenoids was determined according to Arnon (1949)
Total Soluble Protein Content
Total soluble protein (TSP) content was determined spectrophotometrically using Commassie blue G-250 according to the Bradford's (1976) method.
MDA Content
MDA content was assayed spectrophotometrically using thiobarbituric acid test (Ohkawa et al. 1979)
Changes of Cell Membrane Permeability
Changes of cell membrane permeability were measured using an electric conductance meter as described by Dionisio-Sese and Tobita (1998)
Cellular Protection Enzyme Activity
Cellular protection enzyme activity was determined using previously reported methods for each enzyme. SOD activity was measured using the method of Beauchamp and Fridovich (1971). Activity of POD and CAT was assayed according to the method of Chance and Maehly (1955). APX activity was determined following the procedure described by Nakano and Asada (1987) with minor modification.
Data Processing and Statistical Analysis
Salt Response Index
SRI value represents the relative change for each trait caused by salt treatment. It was calculated using the following formula:
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Estimation of Genetic Parameters for SRI Values
The calculation procedure is based on single-factor completely stochastic variance model and using the following formulas:
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Clustering Analysis
Before clustering procedure, all the raw data for each trait of all the cultivars were converted using the subordination function method in fuzzy mathematics. The salt tolerance subordinate index value (F) for each trait was calculated based on its correlation with salt tolerance.
When the trait is positively correlated with salt tolerance,
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When the measured trait is adversely correlated with salt tolerance,
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Fij is the subordination value for cultivar i and trait j, Xij is the ratio between treatment and control for cultivar i and trait j, Xmax and Xmin are the maximum and minimum values. After conversion of the SRI values into the nondimensional data matrices, the Ward's distance between each cultivar was calculated and the cultivars were clustered based on the sum of square variance of Ward's distances with the DPS2000 software.
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SRI Value for Individual Trait and Their Correlation with Salt Tolerance
The SRI value (Table 2) was calculated from the observed phenotypic value of each trait. It was >100% for MDA and cell membrane permeability and <100% for SI, plant height, fresh weight, leaf number, NAR, and contents of chlorophyll and soluble proteins in all the cultivars. The 18 parameters were affected very differently by salt stress, and their SRI value ranged from 10% to 470%.
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Changes of SI
SI can represent comprehensively the degree of salt injury; higher SI value is correlated with more severe salt damage. Figure 1 represents the range of SI values for all the 23 cultivars after their seedlings were exposed to salt stress for 15 days. The SI values were obviously different among cultivars. The lowest one started from 10.0% for cultivar Qingjiachang and the highest one reached 90.0%, with a 9-fold difference for cultivar Yacaodou. This result indicates that capacity of salt tolerance can vary tremendously among cultivars, which is the genetic base for improving salt tolerance through selection and breeding process for asparagus bean.
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Seedling Growth and Morphological Characteristics
Salt-induced reduction in plant height varied significantly among different cultivars. Plants of adzuki bean, HangKong pink bean, Dujiang, Black long bean, and Guilinheizi were retarded after 6 days, whereas the Zaocui, Xiangyabai, Qingjiachang, Zikui, Thailand bean, Zhijiang 19, and so on, showed similar symptom after 9 days. Other morphological traits including fresh weight, stem diameter, surface area of true leaves, and leaf number also exhibited cultivar-specific changes. Yacaodou had the most significant reduction in fresh weight (Figure 2a), and the salt-treated plants weighted only 41.4% compared with the control (SRI = 0.414). The Zaocui, Zhijiang 28-2, and Qingjiachang were slightly affected. The Thailand bean experienced only minimal inhibition (Figure 2b), and plant fresh weight was 93.7% of the control (SRI = 0.937). In addition, salt stress also caused different degree of reduction in leaf surface area (Figure 2). The adzuki bean leaf area was reduced by 50.0% (Figure 2c), and the Qingjiachang was only reduced by 4.5% (Figure 2d). Similar difference in plant height retardation was also observed in these 2 cultivars. To the complete contrary, the stem diameter, leaf number, and root growth were promoted by salt treatment for some cultivars (Table 2). For example, Qingjiachang showed an increased stem diameter, and the leaf number was 133.3% compared with the control. In addition, salt treatment induced bigger root systems for Qingjiachang (Figure 2f), Black long bean, and Dujiang, whereas it was inhibitory in Gangtouzhanyang (Figure 2e), Dujiang, and Philipine cowpea 7. In the case of Qingjiachang, stem diameter, root mass, and true leaf number all showed some increases after salt treatment. Similar phenomenon can also be observed in other traits (Table 2). Based on these results, the salt stress–induced changes are not limited to specific tissues, but it is more associated with the characteristic of the cultivars. Thus, the observed intercultivar difference in salt response is not a random event but an indication of real genetic phenomenon.
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Characteristics of the Photosynthetic Apparatus
Salt-induced restriction in water supply can cause stomata closure, which will in turn lead to decreased absorption of CO2 and eventually result in reduction of photosynthesis (Delfine et al. 1999; Sultana et al. 1999). Chlorophyll content is associated directly with light harvesting potential and is normally considered as one of the important components in photosynthetic capacity (Delfine et al. 1999). In the current study, salt stress caused a significant reduction in the contents of chlorophyll and carotenoids, especially Chl a decreased more obviously than Chl b that resulted in a smaller ratio of Chl a/b (Table 2). The decreased content of chlorophyll is a combinatorial result of reduced synthesis of chlorophyll molecules and enhanced degradation of the pigments. We did see big difference among a few cultivars, such as No. 5 asparagus bean, Zhijiang 28-2, Qingjiachang, Black long bean, and Thailand bean. The chlorophyll content in these cultivars showed higher SRI values, indicating that the plants had experienced lower injury and the cultivars should have higher tolerance to salt stress. The SRI value decreased for the other cultivars that were more severely affected. The net photosynthetic rate was reduced by 10.0–47.0%; the highest cultivar was Philippine cowpea 7 (47.0%) and the lowest one was Zaocui (10.0%). These observations indicate that salt injury as indicated by reduction in photosynthetic capacity and leaf surface area varied significantly among the tested cultivars.
Soluble Protein Content
Salt treatment induced significant reduction in the content of TSP for all the cultivars. The most severely affected was Dujiang, which was reduced by 31.1% (SRI = 0.31). Aihu and the other 6 cultivars all showed significant salt inhibition. The No. 5 asparagus bean experienced only minimal change (SRI = 0.83) (Table 2).
Cell Membrane Characteristics
Alteration in cell membrane structure can lead to extensive electrolyte leakage from intracellular spaces. The degree of electrolyte leakage is correlated closely with the cell membrane injury and is represented by relative membrane permeability. Salt stress induced different degrees of increase in membrane permeability, and cultivars with higher SRI value experienced more severe membrane injury (Table 2). Cultivars including Dujiang, HangKong pink bean, and adzuki bean had the most severe membrane injury (SRI > 3), whereas Zaocui, Zhijiang 28-2, and No. 5 asparagus bean only experienced very minor damage (SRI < 1).
Membrane lipid peroxidation happens when plants are growing under stress conditions. The product MDA can severely disrupt biological membrane integrity and function. The lipid peroxidation as measured by MDA increased from 7.3% to 367.7% under salt stress (Table 2). Cultivars including Guilinheizidou, adzuki bean, and Huangjia had the highest elevation of MDA content (SRI > 3), whereas cultivars including Gangtouzhanyang, Wanqing 512, Thailand bean, and Zhijiang 28-2 maintained at low level (Table 2).
Cellular Protection Enzyme Activity
All the cultivars showed different fluctuations in the protection enzyme activity (Table 2). In total, 80% cultivars had some elevation in APX activity, 66% of the cultivars increased in SOD activity, and 60% of the cultivars increased in CAT and POD activity. Thus, the panel of the total enzyme activity comprising of these 4 enzymes should be induced by salt stress based on the principle of vector overlay. When the trend of the 4 enzymes was further examined, POD showed very minor difference among cultivars, whereas SOD, CAT, and APX varied more significantly. It suggests that SOD, CAT, and APX may be more sensitive than POD to salt stress in the leaf tissues of the seedlings.
Heredity Parameters of the SRI Values
Table 3 summarizes the estimation of all the heredity parameters for the SRI of the 18 investigated traits. The genotypic variance of SRI was the highest for SI, contents of chlorophyll and carotenoids, injury rate, and activity of APX, SOD, CAT, and POD enzymes, followed by plant fresh weight, leaf number, leaf surface area, and relative membrane permeability. All the other traits had very low genotypic variance. The salt injury index, plant height, plant fresh weight, stem diameter, leaf area, leaf number, contents of chlorophyll and carotenoids, and soluble protein had the highest broad heredity of SRI values, which indicates that these traits have high genetic stability and are not much affected by environmental factors. The broad heredity for the SRI value was at an intermediate level for the NAR, MDA content, relative membrane permeability, and activity of APX, SOD, CAT, and POD. Plant height, stem diameter, leaf surface area, and NAR had the lowest relative genetic gain of SRI values.
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Correlation of SRI
Correlation relationship among the 18 traits as estimated by their SRI values appeared very complex. SI was closely correlated with the relative membrane permeability and salt injury rate (Table 4). However, SI was significantly but reversely correlated with plant height, fresh weight, leaf surface area, content of chlorophyll, carotenoids, and soluble protein, as well as CAT activity. SRI of chlorophyll content was positively correlated with contents of soluble protein and caroteniods, as well as CAT activity. The result suggests that cultivars with delayed reduction in chlorophyll content can exhibit similar pattern for soluble protein, carotenoids, and CAT activity. For those cultivars with fast degradation of chlorophyll molecules under salt stress, the decreased photosynthetic capacity will cause immediate reduction in the accumulation of organic substances and soluble proteins.
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SRI of MDA is positively correlated with that of membrane permeability but below significant level. Previous research evidences have pointed out that lipid peroxidation can cause disruption of membrane structure and function, which is further approved by the result from this study. The SRI values of CAT, POD, and SOD are all intercorrelated significantly. In some cultivars, the CAT activity was higher but POD and SOD activity was relatively lower, suggesting that these 2 enzymes can function synergistically to minimize salt damage. SOD is an important enzyme in removing the reactive oxygen species (ROS), and the POD and CAT function to remove H2O2. The balanced regulation in the activity of these enzymes can be induced by ROS directly or indirectly (Jiang 1999).
Cluster Analysis Based on the SRI
All the cultivars were placed into 2 clusters on the basis of Ward's distance range, and each cluster was further divided into 2 subclusters (Figure 3). Thailand bean and American dwarf cowpea were first grouped together at a Ward's distance of 0.69 and then placed into IA together with Zaocui, No. 5 asparagus bean, and Qingjiachang at 1.54. Huarong and Xiangyabai grouped into IB at 0.91. IA and IB formed cluster I of salt-tolerant cultivars, of which the IA cultivar subgroup had higher salt tolerance. Dujiang, adzuki bean, and some other cultivars were placed into IIA at 1.67, Aihu, Yacaodo, and Phillipine cowpea 7 were grouped into IIB at 1.76, and IIA and IIB were clustered together at 2.29 forming the salt-sensitive cultivar cluster II. The 2 cultivar clusters merged into one big cluster at 8.30. The cultivars of each cluster are listed below:
- Salt-tolerant cultivar cluster I comprises the following cultivars:
- IA: Zaocui, Black long bean, and Thailand bean; No.5 asparagus bean; American dwarf cowpea; Zhijiang 19; Zhijiang 28-2, Qingjiachang, and Zikui;
- IB: Huarong and Xiangyabai.
- Salt-sensitive cluster II comprises the following cultivars:
- IIA: Yinyan, Dujiang, Huangjia, Guilinheizidou, and adzuki bean;
- IIB: Aihu, Baipangzi, Wanqing 512, Gangtouzhanyang, HangKong pink bean, Yacaodou, and Philippine cowpea 7.
- IA: Zaocui, Black long bean, and Thailand bean; No.5 asparagus bean; American dwarf cowpea; Zhijiang 19; Zhijiang 28-2, Qingjiachang, and Zikui;
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Cellular Protection Enzyme Activity and Salt Tolerance in Asparagus Beans
The imbalance between antioxidant capacity and ROS accumulation can cause oxidative damage when plants are exposed to environmental stresses (Scandalios 1993). Plants have evolved a set of mechanisms to minimize oxidative injury to cellular and subcellullar organelles. Regulation of different antioxidant enzymes such as SOD, CAT, POD, and so on has been observed in many plant species. Salt stress can induce an elevation of total antioxidant enzyme activity in wheat-germinated seeds (Meneguzzo et al. 1999), pea (Pisum sativum L.) (Hernandez et al. 1999), and Cassia auriculata L. (Agarwal and Pandey 2004). It can also induce POD activity in Mesembryanthemum crystallinum L. (Miszalski et al. 1998), Ipomoea pes-caprae, L. (Venkatesan and Chellappan 1999), and chickpea (Sheokand et al. 1995). In the callus tissues of Citrus limon Burm. f., salt stress can enhance POD activity, whereas CAT activity maintains stable (Pigueras et al. 1996). All these research evidences indicate that stable or increased antioxidant enzymes are often associated with salt stress. The current study observed that whereas antioxidant enzyme activity can be characterized as salt inducible in most cases, some cultivars exhibited the opposite property.
This discrepancy can be caused by selection of the genotypes and the growth stages of the tested plants. Gomes-Filhom et al. (1996) and Franco et al. (1999) suggested that salt stress can impose injury to both plant growth and development and plant response to salt damage is associated with developmental stages. Rice, barley, corn, and wheat are most sensitive to salt injury at seedling stage but become more tolerant as plants grow (Maas 1986). Some cowpea varieties respond to salt stress very differently when plants are at certain developmental stages. Most of cowpea varieties are sensitive to salt during seed germination and become highly tolerant during early vegetative growth and early flower bud formation stages (Maas and Poss 1989; Hall and Frate 1996). The accumulative salt injury as measured by total death rate (Murillo-Amador et al. 2006) can be higher than accumulation of individual stages at seed germination and seedling emergence in cowpea (Murillo-Amador et al. 2000, 2001). However, it is impossible to distinguish the salt response at different developmental stages when samples are collected at the same time in some cases. The current study initiated salt stress on 14-day seedlings, and the samples were collected after 15 days of salt treatment. The growth and development for different cultivars are not following exactly the same rhythm, which may explain the variance in the enzyme activity for the samples collected at the same time. In addition, some phenotypic changes such as increase of stem diameter and root length under salt stress may also be associated with the developmental stages of the cultivars when the samples were collected.
Correlation of Salt-Tolerant Traits and Its Implication in Salt-Tolerant Breeding Procedure
Salinity affects 1/3 of the arable land in the world; it is a very severe constraint for growth and production of beans (Abd-Alla et al. 1998), especially in the arid and semiarid regions (Kuznetsov and Shevyakova 1997). Solving the problem of salinity has a global importance. Short-term relief of salt stress can be achieved by water management. However, long-term solution to this problem relies on improvement of salt tolerance for the cultivated crop species (Dalton et al. 2001). Salt tolerance varies greatly among plant genotypes and species (Savvas and Lenz 2000). We have found from this study that salt index had as high as 9-fold differences between some cultivars, indicating the significant variance in the capacity of salt tolerance. On the other hand, it also suggests that a rich genetic resource for salt tolerance exists in different genotypes, and it is feasible to select and breed for salt-tolerant cultivars for the asparagus bean. Breeding and selection scheme should be based on the heredity parameters of the salt response traits. Among the 18 traits investigated in this study, SRI values for SI, fresh weight, contents of leaf chlorophyll and carotenoids, as well as CAT activity showed higher genotypic variance, heredity, and genetic gain. These traits are easy to stabilize; it will be relatively easier to select for these traits and pass them onto offspring. In contrast, leaf area and soluble protein content had SRI value of high heredity but low genotypic variance; it will be very difficult to select for individuals with desirable traits. However, once the desirable traits are obtained, they can be easily stabilized.
SRI of MDA content showed a relatively lower heredity, higher value of genotypic variance, and medieval genetic gain value. Selection of these traits may obtain very desirable individuals, but it will be very difficult to obtain a new cultivar due to the low heredity value. The SRI for NAR, relative membrane permeability, SOD activity all showed low heredity value, low genotypic variance, and genetic gain. In the breeding process, it would be very difficult to improve these traits. However, these 3 traits are highly correlated with salt tolerance, and they may be essential for tolerant cultivars. Based on the hereditary characteristics of the investigated traits, it is suggested that to breed for salt-tolerant cultivars, one should use bigger selection pressure to speedup stability of the important traits. Or, the alternative is to use less strict selection standard but increase selection generations. This procedure can gradually increase heredity value and incorporate the desirable traits into the new cultivars. SRI of CAT activity is significantly correlated with SRI of SOD and membrane permeability, which suggests that the selection of CAT trait may potentially lead to improvement of salt tolerance.
Salt tolerance is regulated by multigenes and proteins (Bohnert and Jensen 1996). Breeding for salt tolerance needs reliable indicators or selection markers (Shannon 1985; Noble and Rogers 1992). The correlation analysis performed in this study indicates that different salt tolerance traits are intercorrelated; it is thus more appropriate to evaluate a breeding strategy comprehensively based on multiple traits. This study analyzed the plant response to salt stress and injury, the genotypic variance, heredity, and genetic gain of different traits. The selection of these traits such as SI is easy to observe and measure, and the results should have very important values in practical breeding for salt tolerance. The fresh weight, similar to SI, had high genotypic variance, broad heredity, and genetic gain in all the cultivars. Together with the results from Greenway and Munns (1980) and Murillo-Amador et al. (2001, 2006), plant fresh weight and production of biomass have shown to be significantly correlated with salt tolerance characteristics and thus should have more consideration in breeding process.
Salt tolerance is generally considered as a complex character. This study analyzed the morphological, photosynthetic, and physiological traits associated with salt stress in asparagus bean. Molecular marker methods have also been used in identifying salt tolerance in legume species (Merchan et al. 2003). Detailed studies on the molecular physiology of salt tolerance will potentially identify the key genes, or proteins for salt tolerance, and may provide a more simple and efficient strategy to improve breeding process for salt-tolerant cultivars.
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National Natural Science Foundation of China (NSFC 30521004); Scientific Research Programs of both Wuhan Municipality and Hubei Province, P.R. China.
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Corresponding Editor: J. Perry Gustafson
Received October 9, 2006
Accepted March 17, 2007
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