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Published online 22 January 2007
Published in Crop Sci 47:123-131 (2007)
© 2007 Crop Science Society of America
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CROP ECOLOGY, MANAGEMENT & QUALITY

Growth, Gas Exchange, Chlorophyll Fluorescence, and Ion Content of Naked Oat in Response to Salinity

G. Q. Zhaoa, B. L. Mab,* and C. Z. Renc

a College of Grassland Science, Gansu Agricultural Univ., Lanzhou, 730070, Gansu Province, China
b Eastern Cereal and Oilseed Research Centre (ECORC), Agric. and Agri-Food Canada, Central Experimental Farm, 960 Carling Ave., Ottawa, ON, Canada, K1A, 0C6
c Baicheng Academy of Agricultural Science, Baicheng, 137000, Jilin Province, China

* Corresponding author (mab{at}agr.gc.ca)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Understanding of the physiological responses of crop plants to salinity stress is of paramount importance for selection of genotypes with improved tolerance to salt stress. Two naked oat (Avena sativa L.) genotypes, ‘VAO-7’ and ‘VAO-24‘, were subjected to different salt concentrations (0, 50, 100, 150, 200, and 250 mM NaCl) to determine the effects of salt levels and stress duration on seedling growth, ion content, and photosynthetic productivity. Relative growth rate (RGR) and leaf chlorophyll were determined at weekly intervals after salinity was imposed. Total leaf area, plant dry weight, photosynthetic parameters, and plant tissue ion concentrations were determined at 25 d after salinity application. Under salt stress conditions, germination rates varied greatly among the genotypes. The differences between VAO-7 and VAO-24 for most parameters measured were significant after 2 wk of stress introduction at 200 and 250 mM NaCl. Salt stress at the lowest level (50 mM) reduced total leaf area by 35% and plant dry matter by 52%. At 25 d after salt stress, plants treated with the 250 mM NaCl accumulated 36-fold more Na+, 79% more Ca2+, and 2.4-fold less K+ than the control. Salt treatment resulted in the reduction of almost all the growth parameters and coincident increases in plant Na+ and Ca2+ concentrations. Our results indicate that there is great variability for salt tolerance among naked oat germplasms, and greater photosynthesis capacity, higher RGR, and relatively lower tissue Na+ accumulation at high salt concentrations appeared to be associated with salt tolerance in naked oats.

Abbreviations: A, photosynthetic rate • DASA, days after start of salt application • F'v/F'm, efficiency of light harvesting of photosystem II • gs, stomatal conductance • qP, photochemical quenching • RGR, relative growth rate


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
SALINITY is one of the most important abiotic stresses limiting crop production in arid and semiarid regions, where soil salt content is naturally high and precipitation can be insufficient for leaching. Salinity affects many morphological, physiological, and biochemical processes, including seed germination, plant growth, and water and nutrient uptake (Willenborg et al., 2004). Several studies demonstrated that germination of brassica (Brassica spp. L.), wheat (Triticum aestivum L.), and other species is delayed and reduced by lower water potential (Hao and De Jong, 1988; Rao and Dao, 1987), high salinity (Romo and Haferkamp, 1987), and combinations of these factors (Hao and De Jong, 1988; Livingston and De Jong, 1990; Willenborg et al., 2004). Salinity affects plants by osmotic stress, mineral deficiencies, and physiological and biochemical perturbations (Hasegawa et al., 2000; Munns, 2002; Neumann, 1997).

The RGR of a plant is the function of net assimilation rate and leaf area ratio and can be used in comparison among species or genotypes in response to saline conditions (Cramer et al., 1994). Net assimilation rate is an index of the photosynthetic capacity of the plant per unit leaf area, while the leaf area ratio is an index of the leafiness of the plant (El-Hendawy et al., 2005; Hunt, 1990). Therefore, at the whole plant level, these growth parameters can be used to study genotypic variations in salt tolerance.

Photosynthesis is an important parameter used to monitor plant response to abiotic stress. A close association was found between growth and photosynthetic rate in sunflower (Helianthus annuus L.) (Ashraf, 1999) and wheat genotypes (El-Hendawy et al., 2005) differing in salt tolerance. Salinity stress results in the reduction of plant photosynthesis through stomatal and nonstomatal factors, the latter are not yet fully understood (Dionisio-Sese and Tobita, 2000; Sharma and Hall, 1991). El-Hendawy et al. (2005) found that the reduction in stomatal conductance of salt sensitive wheat cultivars caused by saline conditions was significantly greater than that of salt tolerant cultivars.

Salinity can cause ion toxicity and ion imbalance in plants (Greenway and Munns, 1980). In saline soils, salinity not only causes high Na+ in plants but also influences the uptake of essential nutrients such as K+ and Ca2+ because of the effect of ion selectivity (Marschner, 1995). Crop plants use K+, rather than Na+, as an important component of osmotic adjustment and an essential macronutrient. However, K+ and Na+ compete to enter plant cells because they are similar in ionic radius and ion hydration energies (Schachtman and Liu, 1999). Consequently, crops growing in saline soils may suffer both Na+ toxicity and K+ deficiency. The reduced K+ in plants, in turn, affects the integrity and functionality of the cell membranes under saline conditions, which has been suggested as an important selection criterion for salt tolerance in wheat (Gorhman et al., 1987). Ion uptake, transport, and selectivity has been well documented in wheat (El-Hendawy et al., 2005; Houshmand et al., 2005), rice (Oryza sativa L.) (Saleque et al., 2005; Yeo and Flowers, 1983), sorghum [Sorghum bicolor (L.) Moench] (Netondo et al., 2004a, 2004b), alfalfa (Medicago sativa L.) (Ashraf et al., 1986), and maize (Zea mays L.) (Cramer et al., 1994).

Naked oat is a widely grown crop both for human food and animal feed, especially in northern China. Although the oat growing area in the world has declined during the past 100 yr, because of the shift from horse power to petroleum powered mechanization in agriculture, oat, including naked oat, has gained renewed interest in recent decades (Burrows, 2005). Naked hulless oat contains higher protein (134 g kg–1), lysine (5.7 g kg–1), fat (66 g kg–1), and ß-glucan than covered oat (Burrows, 2005; Hoekstra et al., 2003; Ronald et al., 1999). In Baicheng, Jilin province of China, field tests showed that some of the recent cultivars are able to tolerate soil pH >9.0 (Ren et al., 2006, unpublished). However, because of reduced oat production and research effort during the last 50 yr (Burrows, 2005), data about the range and genotypic differences for abiotic stress tolerance are limited; these data are essential to develop best management practices in arid and semiarid regions. It is anticipated that the area of cultivated lands with salt problems will spread and intensify because of climate change. Improved naked oat genotypes may play an important role in remediation of saline and other degraded soil conditions. Consequently, we hypothesize that an understanding of the physiological responses of crop plants to salinity stress, and identification of biological traits in oat associated with salt tolerance, will help plant breeders in further genotype improvement and crop physiologists and agronomists develop best management options to cope with environment-induced abiotic stresses.

The objective of this study was to determine the effects of salt levels and stress duration on seedling growth, ion content, and gas exchange of naked oat. In addition, our study attempts to provide a more comprehensive understanding of the role of the physiological processes in salt tolerance in naked oats.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Preliminary Germination Experiment
A preliminary germination test was conducted to choose two genotypes with distinct salt tolerance for further study. Nine naked oat genotypes were subjected to six levels of salinity in a factorial design with four replicates. The tested genotypes were VAO-1, VAO-2, VAO-7, VAO-8, VAO-11, VAO-14, VAO-22, VAO-23, and VAO-24, kindly provided by Dr. V. Burrows, an Emeritus Research Scientist–Oat Breeder, Agriculture and Agri-Food Canada. The six salt (NaCl) concentrations were 0 (control), 50, 100, 150, 200, and 250 mM (corresponding to the following EC: 3.42, 6.74, 9.66, 12.40, 15.04 dS m–1). Treatment solutions were made with calcium chloride (CaCl2) at 5:1 (NaCl:CaCl2) molar concentration. Fifty seeds of each genotype were placed evenly in a Petri dish (90 mm in diameter) covered with filter paper. Treatment solution (20 mL) was added to each Petri dish. The Petri dish was checked daily and additional solution was added if necessary. The number of germinated seedlings was checked daily and the final number of germination was determined at 10 d after treatment, expressed as percentage of germination. A seed with radicle exceeding 2 mm in length was regarded as germinated. At the final count, shoot and root lengths of each seedling were measured. The ratio of shoot to root length was calculated.

Sand Culture Experiment
Plant Material and Growth Conditions
On the basis of the result of the germination test, VAO-7 and VAO-24 were chosen for the greenhouse experiment. A 2 x 6 factorial experiment, arranged in a completely randomized design with four replications, was conducted from May to August 2005 and ran for a second time from October 2005 to January 2006. Two-liter pots filled with silica sand were used for the study. Twenty seeds were planted in each pot and thinned to 12 plants per pot after emergence. Pots were irrigated with distilled water for 7 d after emergence and before being fertilized with a salt-free Hoagland's solution. Pots were randomized weekly to minimize any variations inside the greenhouse during the experiment.

At 14 d after seedling emergence, salinity treatments of 0, 50, 100, 150, 200, and 250 mM NaCl (NaCl: CaCl2 = 5:1 molar concentration) were imposed through the Hoagland's solution. The specific treatment solutions (200 mL) were applied to each pot daily; additional solutions were added on hot days if necessary. Care was taken to make sure that each pot received the same volume of the planned solution for every application and that there was no moisture stress. The experiment was conducted in a greenhouse with 25/16°C day/night temperatures, 16-h daylength (photoperiod), and 300 µmol m–2 s–1 illumination. The relative humidity was maintained at about 70%.

Growth Analysis
Dry matter samples were taken at 1, 9, 17, and 25 d after salt application (DASA). Plants were cut at the surface level (above crown), dried at 70°C for 72 h, and dry weight was determined. Roots were harvested at 25 DASA, cleaned thoroughly with tap water, and then dried at 70°C for 72 h to determine the dry weight. The RGR was calculated according to Kingsbury et al. (1984) as:

Formula 1[1]
where w1 and w2 are dry weights of shoots (aboveground dry matter) in milligrams at times t1 and t2 (in days).

At the 25 DASA, all leaf blades were removed from the shoot, and leaf area was measured with a LI-3000 Area Meter (LI-COR, Inc., Lincoln, NE).

Leaf greenness was measured on the second topmost fully expanded leaves of 10 plants per pot with a chlorophyll meter (SPAD-502 Chlorophyll Meter, Minolta Camera Co. Ltd., Japan) and expressed as the average of three readings from the basal, mid, and tip of the leaf blade at weekly intervals after stress application.

Photosynthetic Parameters
An infrared, open gas exchange system LI-6400 (LI-COR) coupled with an integrated fluorescence chamber head (LI-6400–40 leaf chamber fluorometer) was used to measure photosynthetic rate (A) and stomatal conductance (gs) on the same leaf as leaf chlorophyll measurement. The area of each leaf in the photosynthetic meter chamber was determined manually. During the measurement, sufficient light (1000 µmol m–2 s–1) was provided with the leaf chamber. Data were manually logged when gas exchange and chlorophyll fluorescence parameters became stable. Fluorescence parameters were measured on light-adapted leaves by the equations of Genty et al. (1989). The efficiency of energy harvesting by open reaction centers of photosystem II for light-adapted leaves was calculated as:

Formula 2[2]
where F'v is the variable fluorescence, F'0 is the minimal fluorescence of a momentarily darkened leaf, and F'm is the maximal fluorescence during a saturating flash of light >7 mmol m–2 s–1. Photochemical quenching (qP) was calculated as indicated by the manufacturer's manual for the LI-6400–40 leaf chamber fluorometer.

Formula 3[3]
where Fs is the "steady–state" fluorescence.

Ion Concentration and Data Analyses
Oven dried plant samples (aboveground dry matter) harvested at 25 DASA were ground into a fine powder and digested according to Thomas et al. (1967). The Na+, K+, and Ca2+ content were determined with a flame atomic absorption spectroscopy (Spectra AA-220, Varian Australia Pty Ltd, Australia).

All data were subjected to analysis of variances for each run of the experiment using the general linear model procedures of SAS (SAS, 1996). Treatment mean differences were separated by the least significant difference (LSD0.05) test if F tests were significant (P ≤ 0.05).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Preliminary Germination Experiment
Among tested genotypes, only five naked oats germinated well (>90% germination rate) under no salt stress conditions (Fig. 1 ); germination rates of the other four genotypes, VAO-1, VAO-8, VAO-11, and VAO-23, were ≤60% (data not shown), probably because of poor seed quality. These genotypes were therefore excluded in the analysis.


Figure 1
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  		Fig. 1. Effects of increasing NaCl concentration on germination rate of naked oat. Bars with different letters are significantly different by an F-protected LSD0.05 test.

 
Salinity inhibited seed germination in all tested genotypes in the experiment. Germination rate, seedling growth, and shoot-to-root length ratio all declined with the increase of NaCl concentration (data not shown). In the 250 mM salt treatment, germination of VAO-24 was completely inhibited, while germination rate for VAO-7 was reduced only to 50% (Fig. 1). Under 200 and 250 mM saline conditions, reduction in germination of VAO-22 was also significantly smaller than that of VAO-2, VAO-14, and VAO-24.

Salinity stress not only affected germination rate but also delayed the germination process. Under salt stress, it took more days to reach the germination peak (when majority of seeds were germinating). The shoot-to-root length ratio was reduced under NaCl stress (data not shown), suggesting that the plants attempted to develop a longer root system to cope with the stress situation.

Sand Culture Experiment
For most parameters measured in the greenhouse study, effects of genotype and saline treatments in the second run experiment followed the same pattern as in the first run, although the numerical differences in the second run were sometimes smaller than those in the first run. This was primarily due to lower radiation during the winter times and slower rate of growth and development in the second run of the experiment. There was no significant run x genotype or run x treatment effects. Unless otherwise specified, the following presentation is focused on the first run of the experiment.

Response of Plant Growth to Salinity
Genotype and saline treatment main effects were both significant (P < 0.01) for relative growth rate (RGR) starting from wk 1 after the salt stress was imposed. There was significant genotype x salinity interaction at Week 1 and Week 2 out of 3 wk of measurements (Table 1). In the second run experiment, however, significant genotype x salinity interactions for RGR did not occur until Week 3, while the main effect of both genotype and salinity remained the same as those in the first run of the experiment. The RGR was greatly reduced as the NaCl concentration and stress duration increased, the greatest reduction being at the third week of stress (Fig. 2 ). Differences among salinity levels were also significant with prolonged stress. Compared with the control, the treatment of 250 mM NaCl caused a reduction in RGR of 87% at Week 3 and of 81 and 74% at Week 1 and Week 2, respectively (Fig. 2). Significant differences between the two genotypes were observed from Week 2 with the salinity level of 200 mM or higher (Fig. 2). Plant growth was greatly inhibited under higher salt stress. Relative growth rate was decreased more for VAO-24 than for VAO-7 during Week 2 and Week 3 in the first run and during all three measurements in the second run of the experiment. With 200 mM salt stress, RGR for VAO-24 was reduced by 31% at the end of Week 2, compared with that of VAO-7 in the first run of the experiment. On average, greater reduction (67%) of RGR in VAO-24 was observed at the same salinity level. With the 250 mM NaCl treatment, RGR for VAO-24 was reduced by 92% at 25 DASA, in comparison with VAO-7 (Fig. 2).


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  		Table 1. Mean square of different variation sources and coefficient of variation for parameters determined on Day 25 after saline stress application.

 

Figure 2
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  		Fig. 2. Effects of increasing NaCl concentration on the relative growth rate (RGR) of shoots averaged over two naked oat varieties. Bars with different letters are significantly different by an F-protected LSD0.05 test.

 
Total leaf area of naked oat was significantly decreased with increased NaCl concentration (Table 1). For example, at 25 DASA, a 35% reduction in green leaf area was observed at 50 mM NaCl; leaf area was further reduced by 85 and 91%, respectively, at salinity levels of 200 and 250 mM. Although genotype effect was overall not significantly different in the first run of the experiment, large differences in total leaf area between the two genotypes were observed at 200 and 250 mM salinity levels (Fig. 3 ). Total leaf area of VAO-24 was 36% smaller than that of VAO-7 at the 200 mM NaCl salinity treatment.


Figure 3
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  		Fig. 3. Effects of increasing NaCl concentration on total leaf area and plant shoot dry weight (DW) of two naked oat cultivars 25 d after salt application. Within each variety, bars with different letters are significantly different by an F-protected LSD0.05 test.

 
Plant dry weight was also greatly reduced with increased NaCl concentration (Fig. 3). Similar to total leaf area, significant reduction (52%) was obtained at 50 mM NaCl. Under higher salinity levels, plant growth was further inhibited, causing an 84 and 86% decrease at the 200 and 250 mM NaCl treatments at 25 DASA. A significant difference between the two genotypes was not observed until stress reached 200 and 250 mM. Compared with VAO-7, plant dry weight of VAO-24 was decreased by 46 and 61% under 200 and 250 mM salinity stress, respectively.

Under NaCl stress, leaf chlorophyll content of naked oat was greatly reduced (P < 0.01) in all sampling dates, except for the measurements in Week 2 of the second run experiment (Table 1). With prolonged saline stress, differences between the two genotypes, among salinity concentrations, and the interaction between genotype and salinity became significant at 25 DASA (Table 1). As shown in Fig. 4 , at 25 DASA, leaf greenness in the 250 mM salt treatment was reduced by 44% for VAO-7 and by 72% for VAO-24 compared with the control. The results in Fig. 4 clearly showed that effects were intensified as the treatment was prolonged.


Figure 4
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  		Fig. 4. Effects of increasing NaCl concentration on leaf chlorophyll content of two naked oat varieties. Within each variety, bars with different letters are significantly different by an F-protected LSD0.05 test.

 
Response of Plant Photosynthesis to Salinity
Under salinity stress, leaf photosynthetic rate was reduced significantly (P < 0.01) as NaCl concentration was increased in both runs of the experiment, although there was a significant genotype x salinity interaction (P < 0.05) in the second run (Table 1). Significant reduction in photosynthetic rate was observed even at the low salinity levels (Fig. 5 ). There was no significant difference between 100 and 150 mM NaCl treatments. Photosynthetic rate decreased by 81% for VAO-7 and by 91% for VAO-24 at 250 mM NaCl concentration at 25 DASA. Concurrently, stomatal conductance was also reduced with the increase in salt concentrations (Fig. 5). It was observed that stomatal conductance for VAO-24 was reduced by 67% compared with the control at 100 mM NaCl stress. However, neither photosynthetic rate nor stomatal conductance was reduced significantly at the 150 mM level as compared with 100 mM treatment (Fig. 5).


Figure 5
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  		Fig. 5. Effects of increasing NaCl concentration on photosynthetic rate and stomata conductance of two naked oat varieties at 25 d after salt application. Within each variety, bars with different letters are significantly different by an F-protected LSD0.05 test.

 
Up to the 150 mM NaCl level, the efficiency of light harvesting of photosystem II, as measured by F'v/F'm, was not significantly affected (Fig. 6 ). When salinity concentration increased to 200 mM or higher, a sharp reduction in light harvesting efficiency was observed, with reduction in F'v/F'm by 28% for VAO-7 and 46% for VAO-24 at 250 mM NaCl concentration. Photochemical quenching (qP) showed a similar trend as F'v/F'm, and a significant decrease was observed at 200 and 250 mM NaCl concentrations. Significant differences between the two genotypes were obtained at the 100, 200, and 250 mM salinity levels. Under 100 mM salinity stress, the qP value for VAO-7 was significantly higher than for VAO-24, while similar values were recorded for both genotypes at the 200 mM NaCl concentration, which dropped dramatically at the 250-mM level.


Figure 6
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  		Fig. 6. Effect of increasing NaCl concentration on Fv'/Fm' and qP of two naked oat varieties at 25 d after salt application. Within each variety, bars with different letters are significantly different by an F-protected LSD0.05 test.

 
Response of Ion Content to Salinity
At 25 DASA, plant tissue Na+ concentration increased significantly (P < 0.01) with increasing salinity levels. Compared with the control, plant Na+ accumulation increased about 8-, 15-, 25-, 32-, and 36-fold at 50, 100, 150, 200, and 250 mM NaCl concentrations, respectively. However, there was a significant genotype x salinity interaction in both runs of the experiment (Table 1). Differences in Na+ between the two genotypes were observed at each salt level except at the control and 100 mM (Fig. 7 ). In general, VAO-24 had significantly higher Na+ concentration than VAO-7 at high salt stress treatments.


Figure 7
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  		Fig. 7. Effects of increasing NaCl concentration on shoot Na+, K+, and Ca2+ content of two naked oat varieties at 25 d after salt application. Within each variety, bars with different letters are significantly different by an F-protected LSD0.05 test.

 
A significant interaction between genotype and salinity treatment was also observed for K+ (Table 1). Increasing salt levels led to a significant reduction in plant K+ concentration (Fig. 7). Compared with the control, plant K+ concentration was decreased by 58% in the 250 mM stress treatments. The K+ concentration of VAO-7 was 55% lower than VAO-24 at 50 mM NaCl concentration (Fig. 7). However, the difference between the two genotypes was small to nonexistent in the 100 and 150 mM salt treatments.

Tissue Ca2+ concentration increased significantly with the increase in salinity concentration. There was a significant genotype x salinity interaction (Table 1). There was no significant difference between 0 and 50 mM salt stress, but the treatment with 100 mM resulted in a 91% increase in plant Ca2+ concentration as compared with the control. Plant Ca2+ accumulation was increased by 2.6-, 3.1-, and 3.7-fold, respectively, when salt levels reached 150, 200, and 250 mM with no difference between 200 and 250 mM treatments. VAO-24 accumulated more Ca2+ than VAO-7 at all salinity levels except for the 150 mM level, at which plant tissue Ca2+ content for VAO-7 was higher (Fig. 7).


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
We have reported the response of two naked oat genotypes, VAO-7 and VAO-24, to various saline concentrations and the salinity duration. VAO-7 was more tolerant to salt stress than VAO-24, as judged from their germination rate, leaf chlorophyll content, photosynthetic productivity, and dry matter accumulation under saline stress conditions.

VAO-7 and VAO-24 have different seed characteristics. VAO-7 has thinner and smaller seeds with lower 1000-grain weight (16.5 g), while seeds of VAO-24 are round and larger with higher 1000-grain weight (24.4 g). This seed size vs. germination relationship under saline stress is consistent with the results of Mian and Nafziger (1992) but contrary to those of Willenborg et al. (2004) who reported that seed size had a significant effect on germination characteristics in oat, and large seed had a higher germination percentage and greater speed of germination. The germination screening in our study also indicated that different genotypes had different sensitivity to salt stress.

We found that plant RGR was reduced greatly with increased salt concentration and stress duration, but the reduction in RGR for the salt tolerant variety was smaller than for the salt intolerant variety. At the whole plant level, the reduction in RGR could be attributed to photosynthesis related morphological changes (Hunt, 1990). The results from the present study indicated that the RGR of both genotypes was related to their photosynthetic rate and leaf area, suggesting that both leaf expansion and photosynthetic rate are the growth limiting factors under salinity conditions. Photosynthetic rate of naked oat dropped significantly as the NaCl stress increased. Total leaf area was also reduced by salinity. A decrease in leaf area may be attributed to early senescence and death, reduced growth rate, or both causes (Bernstein et al., 1993). The negative impact of NaCl on total leaf area development was evident at the lowest salt concentration (50 mM) and increased linearly with increasing salinity. However, El-Hendawy et al. (2005) reported that under salinity stress, the decrease in RGR in wheat was only related to photosynthetic rate, not to total leaf area. A significant reduction in photosynthetic rate and stomatal conductance under salt stress was observed in this study. It was most likely that the decrease in photosynthesis is attributed to NaCl effects on stomatal closure. These results are in agreement with the report by Netondo et al. (2004b), who found a positive correlation between stomatal conductance and CO2 assimilation rate under salinity stress, suggesting stomatal conductance was a primary factor limiting photosynthesis in sorghum under salt stress.

Leaf greenness or chlorophyll content was also affected by salinity. Salinity can affect chlorophyll content through inhibition of chlorophyll synthesis or an acceleration of its degradation (Reddy and Vora, 1986). The chlorophyll content of naked oat decreased with increasing NaCl concentration and stress duration in this study. The difference between varieties was significant at 250 mM NaCl at 25 DASA. Similar results were reported in alfalfa (Winicov and Seemann, 1990), sunflower (Ashraf, 1999), and wheat (El-Hendawy et al., 2005). However, Murillo-Amadot et al. (2002) found that the chlorophyll content of salt-tolerant cowpea [Vigna unguiculata (L.) Walp.] genotypes was increased under salt stress.

Plant dry weight was also reduced significantly by salinity, resulting from reduced RGR. The greatest difference in RGR between the two genotypes was observed at the low salinity level (50 mM). At this level of salinity, a reduction in photosynthetic rate and total leaf area was not as much as with the higher salt stress levels. Reduction in plant dry weight may have resulted from reduced or inhibited tillering at lower salinity levels.

Compared with gs, the decline in F'v/F'm was minimal when plants were exposed to salinity levels lower than 150 mM. A significant difference in F'v/F'm occurred at higher salt concentrations. This is in agreement with Netondo et al. (2004b), who reported a significant reduction in F'v/F'm for sorghum when plants were subjected to 250 mM salt stress. The change of qP was similar to that of F'v/F'm in naked oat, and significant differences were observed at higher salinity levels (Fig. 6). However, Jiang et al. (2006) reported that although qP showed some differences among barley (Hordeum vulgare L.) genotypes, salinity did not significantly affect this parameter. KrishnaRaj et al. (1993) found significant differences in fluorescence parameters in wheat. However, Lu and Zhang (1998) reported that photosystem II was highly resistant to salinity stress in sorghum, and its thermostability was increased.

As salinity concentrations increase, tissue Na+ content in naked oat increased and no saturation was observed. Although the difference between 200 and 250 mM concentration was not significant, tissue Na+ content at 250 mM was 10% greater than at the 200 mM salt concentration. This is consistent with the reports of El-Hendawy et al. (2005) and Houshmand et al. (2005) in wheat. Although Netondo et al. (2004a) found that in sorghum, tissue Na+ concentration saturated at 150 mM external salt stress after 25 d of salinity application.

Salinity not only caused high Na+ accumulation in plants but also influenced the uptake of essential nutrients such as K+ and Ca2+ through the effects of ion selectivity. High Na+ content strongly inhibited K+ uptake and accumulation. A significant decrease in K+ accumulation was observed even with the lowest salinity concentration in this experiment. As much as a 58% reduction in K+ content was observed at 250 mM treatment. Decline in K+ accumulation because of salinity stress has been widely reported in wheat (Houshmand et al., 2005), sorghum (Netondo et al., 2004a), Swiss chard (Beta vulgaris L.) (Hessini et al., 2005), barley (Jiang et al., 2006), and rice (Saleque et al., 2005).

It is interesting to find that Ca2+ content in naked oat was increased under salinity stress (Fig. 7). This is in contrast with the findings of El-Hendawy et al. (2005) and Netondo et al. (2004a) that salinity stress significantly decreased Ca2+ content in some wheat and sorghum genotypes. An increase of Ca2+ concentration under salinity was also reported in rapeseed (Brassica napus L.) (Procelli et al., 1995), and Alpaslan et al. (1998) found that the concentrations of Ca2+, Cu2+, Zn2+, and Mn2+ were increased by salinity in rice. Houshmand et al. (2005) reported that Ca2+ concentration was not significantly affected by salinity in durum wheat (Triticum durum Desf.). Ca2+ does not always completely ameliorate the inhibition of growth by Na+, and salinity can disturb normal functions without disturbing overall Ca2+ tissue concentrations, especially in the early growth stages. Therefore, Ca2+ content in plants has not typically been proposed as a useful trait for the screening of salt tolerance (Cramer, 2002).

In the present experiment, the varietal response to salinity in ion accumulation was complicated. VAO-24 showed significantly higher values in both K+ and Ca2+ accumulation, but its Na+ content was also significantly higher than VAO-7. The K+/Na+ ratio has been considered as an indicator for salinity tolerance (Gorhman et al., 1987; Houshmand et al., 2005; Saleque et al., 2005). In this study, the varietal differences in K+/Na+ and Ca2+/Na+ were significant at control and low salinity levels, but both varieties showed similar values at higher NaCl concentrations (data not shown).

In summary, salinity stress significantly inhibited naked oat growth by reducing total leaf area, chlorophyll content, photosynthetic rate, stomatal conductance, qP, and F'v/F'm. Both Na+ and Ca2+ content increased with the increasing salinity. Meanwhile, K+ accumulation was decreased significantly. The difference between varieties varied with parameters measured under different salinity levels. The two genotypes differed more in Na+ and Ca2+ accumulation than in K+ accumulation. Our data suggest that selection for greater photosynthetic productivity and higher RGR under saline conditions will be a good strategy for improving naked oat genotypes for better tolerance to salt.


   ACKNOWLEDGMENTS
 
We thank V. Deslauriers, D. Balchin, and L. Evenson for their excellent technical assistance and Dr. V Burrows for providing seeds. Special thanks are extended to Drs. W. Yan, V. Burrows, and M. Morrison who provided us with critical review and valuable suggestions. ECORC Contribution No: 06-673.

Received for publication June 12, 2006.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 





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