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EFFECT OF SALINITY STRESS ON AMARANTH SEED GERMINATION AND SEEDLING GROWTH
Good plant stands are difficult to obtain in saline environments due to poor germination and seedling emergence. The response of germination to salinity stress varies with species and variety, salt type, salt concentration and environmental conditions. Salinity tolerance during germination and early seedling growth was examined for six genotypes of amaranth namely Amaranthus tricolor, Accession ’83, A. dubius, A. hypochondriacus, A. cruentus and A. hybridus Ten salt treatments, 0, 25, 50, 100, and 200 mM NaCl or Na2SO4 were applied and germination was carried out in petri dishes at 27oC for 10 days. Enhancement of germination was observed at 25 mM NaCl in A. tricolor, A. hypochondriacus, A. cruentus, and at 25 mM Na2SO4 in A. hybridus and A. dubius. The strongest inhibition of germination occurred at the highest salt concentration (200 mM), where only 17% of A. tricolor and 24% of Accession ’83 seeds were able to germinate in NaCl. No genotype germinated at 200 mM Na2SO4. Accession ’83 had the highest final germination while A. hybridus showed the least. A seedling emergence and growth experiment was conducted in a greenhouse, in plastic pots containing sand. Four genotypes (A tricolor, Accession ’83, A. cruentus and A. hypochondriacus) were exposed to NaCl and Na2SO4 at concentrations of 0, 25, 50, 100 mM. Emergence and seedling survival were reduced by increasing salt concentrations. There was no emergence at 100 mM Na2SO4. Stem and root lengths as well as shoot fresh mass were reduced by increasing salt stress. A. tricolor was the most sensitive genotype, with the seedlings surviving only in the control and 25 mM Na2SO4 treatments, while A. hypochondriacus was the most tolerant with 100% and 95% survival at 25 and 50 mM Na2SO4 respectively.
An essential step in growing a successful crop is obtaining an adequate plant population, as yield is reduced by sub-optimal plant densities and uneven stands. Salinity of soil and irrigation water is a continuing threat to economic crop production especially in arid and semiarid regions of the world (Kayani et al., 1990). The ability of seed to germinate in saline environments, the cotyledons to break through a soil crust while emerging, and seedlings to survive in saline conditions are crucial for crop production in saline soils (Maranon et al., 1989).
Several investigations of seed germination under salinity stress have indicated that seeds of most species attain their maximum germination in distilled water and are very sensitive to elevated salinity at the germination and seedling phases of development (Khan and Ungar, 1996a, b; 1997; Keiffer and Ungar, 1997; Ghoulam and Fares, 2001). Plant responses to salinity also depend on the anion associated with sodium. For example, crop species such as Hordeum vulgare (Huang and Redmann, 1995a) and Triticum aestivum (Hampson and Simpson, 1990) were found to be inhibited more by sodium sulfate than by sodium chloride. For other species such as Brassica napus, the reverse was found (Huang and Redmann, 1995a). The detrimental effect of salinity occurs because of osmotic stress and specific ion toxicity (Ungar, 1995). The interaction of specific ion and osmotic effects induce a reduction in the number of seeds germinated and a retardation in the rate of germination.
Germination and seedling development is very important for early establishment of plants under stress conditions. Selecting cultivars for rapid and uniform germination under saline conditions can contribute towards early seedling establishment. Owing to its high nutritive value and a wide adaptability to diverse environments, amaranth has been considered a promising crop for marginal lands and semi arid regions (Cunningham et al., 1992; Allemann et al., 1996). Salinity is one of the major limiting factors in crop production in such areas. It is necessary to understand the response of amaranth to salinity stress if cultivation in saline areas is considered. Little information on the effect of salinity stress on amaranth seed germination and seedling establishment is available.
The research objectives were to:
- assess the response of amaranth seed germination and seedling growth to different salts, and levels of salinity stress, and
- evaluate genetic differences in germination and seedling development.
MATERIALS AND METHODS
Seeds of six amaranth genotypes, namely: Amaranthus tricolor, A. hybridus, A. dubius, Accession ’83, A. hypochondriacus, and A. cruentus were supplied by Agricultural Research Council – Roodeplaat Vegetable and Ornamental Plant Institute, South Africa in May 2002, and stored at 4oC until use. Germination experiments were carried out during July and August 2002.
The trials were conducted at the Experimental Farm of the University of Pretoria. Seeds were germinated in covered, sterilized, disposable petri dishes containing Whatman No. 3 filter paper moistened with either distilled water (control), or 25, 50, 100 or 200 mM of either NaCl or Na2SO4 solutions. The high rates of NaCl and Na2SO4 were included to ensure a range of germination reactions. Petri dishes were sealed with parafilm to prevent evaporation of water, thus minimizing changes in concentration of solutions. Three replicates of 50 seeds each were used for all treatments. Seeds were incubated in a growth chamber at 27oC and were considered germinated with the emergence of the radicle. Germinated seeds were determined every day until the end of germination period of 10 days. Every three days, the germinated seeds were removed from the petri dishes. The first three seeds to germinate in each replicate were retained for measurements of radicle and hypocotyl lengths at the end of the experiment. In order to maintain adequate moisture 5 ml of the original salt solutions were added to each petri dish every three days.
The rate of germination was estimated by using a modified Timson index of germination velocity = ∑G/t, where G is percentage of seed germination at 2 day intervals and t is total germination period (Khan and Ungar, 1984). On the 10th day radicle and hypocotyl lengths were determined.
Seedling emergence and growth
This experiment was carried out in August 2002 to evaluate the effects of irrigation with various saline solutions on emergence and seedling growth. Four amaranth genotypes, namely A. tricolor, Accession ’83, A. cruentus, and A. hypochondriacus were compared. Ten seeds for each treatment were planted at a uniform depth of 5 mm in one liter plastic pots filled with acid washed sand. The pots were placed on benches in a heated greenhouse at a temperature range of 16 to 24oC (mean minimum and maximum) for 21 days and irrigated every other day with NaCl or Na2SO4 solutions at concentrations of 0, 25, 50, and 100 mM. High humidity was maintained by covering the pots with transparent plastic bags. The bags were removed as soon as seedlings started to emerge.
The number of emerged seedlings was noted every day. After 21 days the seedlings were assessed for survival, and harvested. Shoot and root lengths, number of lateral roots and shoot fresh mass were determined. All treatments were replicated three times in a completely randomized design.
Data were submitted to Bartlett’s test for the homogeneity of variance. Square root transformations of percent germination and emergence data were necessary to achieve homogeneity of variance and compare data from the early and late germinations. Data were analyzed by the SAS (Statistical Analysis System) (SAS Institute Inc. Cary, NC, USA 1996 Copyright) method and means were compared using Tukey’s t-test at P≤0.05. Percentages without transformation are reported.
RESULTS AND DISCUSSION
The main effects of genotype, salt type, and concentration were significant, but due to significant interactions between them, only interactive effects between the treatment combinations are presented. Seed germination in control treatments varied with genotype, from 80% in A. hybridus to 97% in Accession ’83. With NaCl treatments the germination of A. hypochondriacus and A. cruentus was enhanced by 25 mM NaCl (Figure 2.1a). In A. tricolor, Accession ’83 and A. hybridus there was no significant difference in percent germination between control and seeds submitted to 25 mM NaCl. Germination was progressively inhibited with increased NaCl concentrations. The greatest inhibition occurred with the highest salt concentrations of 100 and 200 mM. For example, at 100 mM NaCl the reduction in germination ranged from 8% in Accession ’83 to 24% in A. hypochondriacus. Only 17% of A. tricolor and 24% of Accession ’83 were able to germinate at 200 mM NaCl, with no germination in the other genotypes.
With Na2SO4 treatments enhancement of germination was observed in A. dubius and A. hybridus at 25 mM, while there was no difference in germination between the control and 25 mM treated seeds in the rest of the genotypes. A progressive decrease in germination at higher concentrations was observed. Exposure to 100 mM Na2SO4 depressed germination more than NaCl with germination reductions ranging from 18% in Accession ’83 to 45% in A. hybridus. No seed germinated at 200 mM Na2SO4 (Figure 2.1b).
Across the treatment combinations Accession ’83 had the highest germination percentage and A. hybridus the lowest. Significantly higher germination percentages were observed in NaCl than in Na2SO4 treatments, particularly at 50 and 100 mM (Figure 2.1a; 2.1b).
Figure 2.2a and 2.2b illustrate the differences in the trend of amaranth seed germination during the period of incubation in either NaCl or Na2SO4. In both the NaCl and Na2SO4 treatments germination of all the genotypes in the control and 25 mM NaCl treatments commenced after one day of incubation and was mostly completed after 4 days. Germination was delayed at the higher salt concentrations, especially at 100 mM NaCl where the delay in germination was very obvious for A. hypochondriacus, but also occurred in the case of the other genotypes. With the exception of A. tricolor and Accession ’83 none of the genotypes were able to germinate in 200 mM NaCl. A delay in germination of A. tricolor and Accession ’83 seeds at 200 mM NaCl was observed, with the first seeds only germinating on day 4. In most of the genotypes completion of germination at higher salt concentrations was also delayed to 6 to 8 days from the start of incubation.
Most of the genotypes attained more than 50% germination on day one under control and 25 mM NaCl or Na2SO4 treatments. At 50 mM all genotypes had less than 50% germination on day one with the exception of A. hypochondriacus that attained 74 and 57% germination in NaCl and Na2SO4 respectively. Although only 42 and 26 % of seeds of Accession ’83 had germinated on day one in 50 mM NaCl or Na2SO4 over 80% had germinated by day 2. Germination was delayed to day 2 at 100 mM with most genotypes attaining more than 50% in NaCl. On the other hand, exposure to Na2SO4 resulted in 50% germination reached on day four in A. cruentus and A. hypochondriacus. A 50% germination was not attained by A. hybridus in 100 mM Na2SO4 while in NaCl it was attained by day 3 (Figure 2.2a; 2.2b).
LIST OF TABLES
LIST OF FIGURES
CHAPTER LITERATURE REVIEW
1.1 Effects of salinity in agriculture – An overview
1.2 Causes of salinity
1.3 Salinity effects on plants
1.4 Salt tolerance
1.5 Mechanisms of salt stress resistance
1.6 Managing salinity in agricultural production
1.7 The amaranth
CHAPTER 2 EFFECT OF SALINITY STRESS ON AMARANTH SEED GERMINATION AND SEEDLING GROWTH
2.3 Materials and methods
2.4 Results and discussion
CHAPTER 3 SALT TOLERANCE OF AMARANTH AS AFFECTED BY TIMING OF SALINITY STRESS INITIATION
3.3 Materials and methods
CHAPTER 4 DIFFERENCES IN SALINITY STRESS TOLERANCE IN TERMS OF GROWTH AND WATER USE EFFICIENCY AMONG FOUR AMARANTH GENOTYPES
4.3 Materials and methods
CHAPTER 5 INTERACTIVE EFFECTS OF SALINITY AND WATER STRESS ON GROWTH, WATER RELATION AND GAS EXCHANGE IN AMARANTH
5.3 Materials and methods
CHAPTER 6 AMELIORATIVE EFFECTS OF CALCIUM ON MINERAL UPTAKE AND GROWTH OF SALT-STRESSED AMARANTH
6.3 Materials and methods
CHAPTER 7 SALT TOLERANCE OF AMARANTH AS AFFECTED BY SEED PRIMING
7.3 Materials and methods
CHAPTER 8 GENERAL DISCUSSION
8.1 Salinity tolerance at different stages of plant development
8.2 Genotypic variability
8.3 Salinity and water stress interactions
8.4 Amelioration of salinity stress by Ca nutrition and seed priming
8.5 Concluding remarks