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Published in Crop Sci. 44:1976-1981 (2004).
© 2004 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA

CROP BREEDING, GENETICS & CYTOLOGY

Agronomic Performance of Hard Red Spring Wheat Isolines Sensitive and Insensitive to Photoperiod

J. A. Dycka, M. A. Matus-Cádizb, P. Huclb,*, L. Talbertc, T. Huntd, J. P. Dubuce, H. Nassf, G. Claytong, J. Dobbh and J. Quicki

a UGG Ltd., Morden, MB Canada R6M 1C2
b Dep. of Plant Sciences and Crop Development Centre, Univ. of Saskatchewan, 51 Campus Dr., Saskatoon, SK, Canada S7N 5A8
c Dep. of Plant Sciences, Montana State Univ., Bozeman, MT, USA 59717-3140
d Dep. of Plant Agriculture, Univ. of Guelph, Guelph, ON, Canada N1G 2W1
e Retired, Soils and Crops Research and Development Center, Agriculture and Agri-Food Canada, Sainte Foy, QC Canada G1V 2J3
f Crops and Livestock Research Centre, Agriculture and Agri-Food Canada, Charlottetown, PE, Canada C1A 4N6
g Lacombe Research Centre, Agriculture and Agri-Food Canada, Lacombe, Alberta, Canada T4L 1W1
h Retired, British Columbia Ministry of Agriculture, Food and Fisheries Food Industry Branch, Dawson Creek, BC, Canada V1G 4J2
i Dep. of Soil and Crop Sciences, Colorado State Univ., Fort Collins, CO, USA 80523-1170

* Corresponding author (hucl{at}sask.usask.ca)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
An increasing number of western Canadian hard red spring wheat cultivars (Triticum aestivum L.) are photoperiod insensitive, in part, to accommodate short day winter nurseries within breeding programs. The objective of this study was to compare the agronomic performance of near-isogenic photoperiod sensitive (PS) and insensitive (PI) hard red spring wheat lines over 21 environments (1996–1998) to determine if insensitivity had an effect on agronomic performance. Eight PS and eight PI isogenic lines within each of three genetic backgrounds including AC Minto, CDC Makwa, and SWP5304 were evaluated. The dominant allele Ppd-D1 conferred insensitivity to PI lines. The experimental design was a randomized complete block design with three replications. Testing environments included Fort Vermillion, AB (58°N), Dawson Creek, BC (55°N), Saskatoon, SK (52°N), Elrose, SK (51°N), Elgin, MB (49°N), Bozeman, MT (45°N), Ste. Foy, QC (46°N), Charlottetown, PE (46°N), Guelph, ON (43°N), and Akron, CO (40°N). Measurements were made on 11 traits including final leaf number, days to heading and maturity, plant height, grain yield, kernel weight, spikelets per spike (total, fertile, and sterile), seeds per spike, and yield per spike. Generally, PS lines were later in heading and maturity, taller, initiated more leaves and spikelet primordia, and 5% higher yielding. Genetic backgrounds differed significantly in all traits, except final leaf number and grain yield. Significant, noncrossover, photoperiod response type x genetic background interactions were observed only for fertile spikelets per spike and seeds per spike. Our results suggest that photoperiod sensitivity may be advantageous in the northern latitudes of North America.

Abbreviations: BKG, genetic background • CWRS, Canada Western Red Spring • EXP, experiment • PI, photoperiod insensitive response type • PPD, photoperiod response type • PS, photoperiod sensitive response type


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
THE DEVELOPMENT of photoperiod insensitive (PI) wheat cultivars has become popular within breeding programs of the northern USA and Canada with the increased use of winter nurseries in southern latitudes to accelerate breeding programs. Hucl (1995) reported that before the mid-1980s virtually all Canada Western Red Spring (CWRS) wheat cultivars were photoperiod sensitive (PS). Currently, about 20% of currently grown CWRS cultivars in western Canada (Saskatchewan Agriculture, Food, and Rural Revitalization, 2003) are described as PI (CDC Teal, Hughes and Hucl, 1993; AC Eatonia, de Pauw et al., 1994; AC Elsa, Clarke et al., 1997; AC Intrepid, de Pauw et al., 1999; AC Abbey, de Pauw et al., 2000). Photoperiod response is of interest to plant breeders in northern latitudes as PS cultivars have provided good yield stability, local adaptation, and high productivity in northern areas of North America (Busch et al., 1984; Knott, 1986). Photoperiod types are classified as PS, which require long days for timely flowering, or PI, which can be grown successfully in long or short day environments. Scarth and Law (1984) reported that three genes control photoperiod in wheat including Ppd-D1 located on the long arm of chromosome 2D, Ppd-B1 on the short arm of 2B, and Ppd-A1 located on the long arm of 2A. The dominant alleles Ppd-A1, Ppd-B1, and Ppd-D1 (formerly Ppd3, Ppd2, and Ppd1, respectively) confer insensitivity to photoperiod whereas the recessive alleles (ppd-A1, ppd-B1, and ppd-D1; formerly ppd3, ppd2, and ppd1, respectively) confer sensitivity. The potency of the group 2 photoperiod genes for insensitivity has been ranked in the order Ppd-D1 > Ppd-B1 > Ppd-A1 (Worland, 1996).

Research conducted in Europe and North America indicates that (i) plant development patterns in wheat generally differ between PS and PI types and (ii) environmental conditions for a particular location, year, or geographical area can contribute to the yield advantage of one photoperiod type over the other. In Europe, PI cultivars, all likely carrying Ppd-D1, were shown to have a 30% yield advantage over PS cultivars in southern regions, 15% in mainland regions, and no yield advantage in the UK (Worland et al., 1994; Worland 1996). In all environments, Ppd-D1 tended to accelerate heading and maturity, reduce tillering, reduce plant height, and reduce spikelets per spike. Increased spikelet fertility compensated for the shortened life cycle and reduced tillering and spikelet number. Interestingly, the environmental conditions prevailing during the period from grain set to maturity had a significant effect on grain yield. In southern and mainland Europe, PI cultivars benefited from the hot and dry summers as these early genotypes were able to fill their grain before the hot desiccating summer conditions. In the UK, PS cultivars performed best in their traditional cool damp summers as the extended grain filling period allowed later flowering genotypes to produce larger grains and higher yields. In contrast, PI cultivars performed best in the non-traditional warmer, drier summers that sometimes occur in the UK. Thus, the Ppd-D1 allele aids PI cultivars in avoiding the heat stress within consistent and variable environments. Heat and moisture stresses during grain filling have been associated with abortion of tillers and reduced kernel weight in wheat (Fischer and Maurer, 1976; Musick and Dusek, 1980). Moisture stress before anthesis and at maturity has reduced grain yield, seeds per spike, and kernel weight in wheat (Entz and Fowler, 1988). Currently, most wheat varieties grown in southern and mainland Europe are PI because of their wider adaptation, relative to PS cultivars, while varieties in the UK are mostly PS (Foulkes et al., 2004).

Photoperiod genes have a direct influence on photosynthetic potential and components of yield (Muira and Worland, 1994). For example, Pugsley (1965)(1966) reported that delays in maturity with PS genotypes caused an increase in both leaf and spikelet numbers. Wall and Cartwright (1974) reported that PS lines were later heading and produced a greater number of spikelets per spike. In North America, two reports have concluded that the PI trait could be used without agronomic penalty in the spring wheat growing region of the USA (Busch et al., 1984; Marshall et al., 1989). Busch et al. (1984) compared the performance of 10 near-isogenic hard red spring wheat pairs derived from two crosses over three environments in Minnesota. For grain yield averaged over years, PI lines were equal to PS lines at one location and higher yielding than PS lines at two locations by an average of 9%. Marshall et al. (1989) compared the performance of near-isogenic PI and PS hard red spring wheat lines derived from 11 different parents in the upper midwestern USA (44–48°N). Generally, this comparison indicated that PI lines were (i) earlier to head (2.5 d); (ii) earlier to ripen (1.0 d); (iii) longer in grain-filling period (1.4 d); (iv) 3% higher yielding (91 kg ha–1); (v) lower in protein (–2 g kg–1); (vi) shorter (–27 mm); and (vii) similar in test weight. In all parental backgrounds and environments tested, PI lines yielded as high as or higher than PS lines.

In Canada, Knott (1986) reported that limited research had been conducted on the Canadian prairies with respect to the agronomic effect of incorporating photoperiod insensitivity into locally adapted cultivars. Knott (1986) compared the performance of near-isogenic PI and PS hard red spring wheat lines derived from one cross in Saskatoon, SK (52°N) and Elrose, SK (51°N) in a 2-yr study. Sensitive lines significantly out-yielded PI lines by an average of 2.1%. An important consideration for determining the effect of photoperiodism on spring wheat is the range of latitudes tested. Lebsock et al. (1973), Busch and Chamberlain (1981), Busch et al. (1984), Marshall et al. (1989), and Knott (1986) used test locations within a relatively small geographical area within the upper midwestern USA or Canada. To date, research studying the effect of photoperiod on the agronomic performance of spring wheat over a broad range of environments within Canada is lacking. In the present study, spring wheat lines near isogenic for photoperiod response were tested over a wide range of latitudes from Colorado and Montana to the northern edge of western Canada's grain belt. The objective of the study was to compare the agronomic performance of near-isogenic PS and PI hard red spring wheat lines over 21 environments in 1996 to 1998 to determine the effect of photoperiod response on agronomic traits at the higher latitudes of North America.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Near-isogenic lines differing in photoperiod response were developed by six cycles of backcrossing. Photoperiod sensitive spring wheat parents including Canada Western Red Spring (CWRS) cv. AC Minto (Townley-Smith et al., 1993), CWRS cv. CDC Makwa (Hughes and Hucl, 1992), and experimental line SWP5304 were each crossed with the PI parent CWRS cv. Laura (de Pauw et al., 1988). The hard red spring wheat cultivar Laura, registered in western Canada in the mid 1980s, is PI and thought to carry the dominant Ppd-D1 allele for photoperiod insensitivity, based on parentage from CIMMYT (de Pauw et al., 1988). CDC Makwa is earlier maturing and slightly higher yielding than AC Minto. SWP5304, developed by the Saskatchewan Wheat Pool, has a glaucous leaf epidermis thought to increase tolerance to drought (Clarke et al., 1993). From the first backcross generation (B1) with Laura, the progeny segregated into the two photoperiod response phenotypes. Under a 10-hr photoperiod in a growth cabinet, only short-day or PI plants were selected for the next cycle of backcrossing. Conditions were set to 23/19°C (day/night) with 12 h light and a photosynthetically active radiation level of 350 µmol m–2 s–1. Under these conditions PS plants headed out 15 to 20 d later than PI plants. After 6 cycles of backcrossing, segregating sensitive and insensitive BC6F1 plants were allowed to self-pollinate. Progeny tests using the above test conditions were used to confirm that the genotypes with contrasting photoperiod response were homozygous. Sixteen lines, eight sensitive and eight insensitive, were selected from each of the three genetic backgrounds for use in testing agronomic performance.

Forty-eight lines, including sensitive (n = 8) and insensitive (n = 8) lines from each of the three genetic backgrounds, were evaluated in 21 field experiments over 3 yr (1996–1998). The experimental design was a randomized complete block design (RCBD) with three replications. Twenty-one station-years of field data collected from 10 locations in 1996 to 1998 were used to evaluate the photoperiod response of spring wheat. In 1996, the lines were tested at five locations (Bozeman, MT [45.40°N 111.00W]; Guelph, ON [43.34°N 80.16W]; Ste. Foy, QC [46.47°N 71.18W]; Saskatoon, SK [52.10°N 106.40W], and Elrose, SK [51.12°N 108.01W]). In 1997, testing was conducted at an additional five locations, including Akron, CO (40.09°N 103.13W); Charlottetown, PE (46.14°N 63.09W); Dawson Creek, BC (55.44°N 120.15W); Elgin, MB (49.26°N 100.15W); and Fort Vermillion, AB (58.22°N 115.59W). In 1998, testing was conducted at six locations (Bozeman, MT; Saskatoon, SK; Elrose, SK; Charlottetown, PEI; Elgin, MB; and Fort Vermillion, AB). The expansion of sites in 1997 and 1998 provided a broader range of latitudes and growing conditions. Individual plots consisted of four rows 3.0 m long at Bozeman, MT; six rows 3.0 m long at Guelph, ON; four rows 3.0 m long at Ste. Foy, QC; four rows 3.6 m long at Saskatoon and Elrose, SK; four rows 4.8 m long at Akron, CO; four rows 3.0 m long at Charlottetown, PE (rows spaced 0.2 m apart); six rows 5.0 m long at Dawson Creek, BC; four rows 3.6 m long at Elgin, MB; and four rows 7.0 m long at Fort Vermillion, AB. Rows were spaced 0.3 m apart unless otherwise specified. Plots were sown at a rate of 250 seeds m–2. Seeds were treated with the systemic fungicide Vitavax Single Solution (Uniroyal Chemical Ltd., Elmira, ON; active ingredient carbathiin) at the recommended rate. Fertilizer was drilled in with the seed at recommended rates to supply sufficient levels of N-P-K. Eleven traits were measured, but not all traits were measured in each environment. Number of leaves on the main stem was determined as described by Wang et al. (1995). Data were collected for days to heading (50% spike emergence), days to maturity (95% physiological maturity), and plant height. At maturity, 10 spikes were collected from the upper canopy of each plot to determine spikelets per spike, seeds per spike, and yield per spike. Plots were combine-harvested at maturity and grain samples were dried with forced air driers. Data were collected on grain yield and 1000-kernel weight.

Statistical analyses were conducted by Minitab Version 12 (Minitab Inc, State College, PA). The structured treatment design used in the field study was a factorial design with two photoperiod response types, three genetic backgrounds, and eight lines within each photoperiod and background. For ANOVAs, the statistical model included sources of variation due to experiment (EXP), replication within EXP, photoperiod response type (PPD), genetic background (BKG), lines within PPD and BKG, and all interactions among EXP, PPD, and BKG. Experiment represents the testing at individual sites in individual years. Experiment, replication, and experimental line were considered random effects. Genetic background and photoperiod response type were fixed effects. Bartlett's test (P = 0.05) was used to test for the homogeneity of variances. Average differences between photoperiod response types were tested for significance at 0.05 and 0.01 by paired t tests.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Photoperiod response types differed significantly in final main-stem leaf number, days to heading, days to maturity, plant height, grain yield, total spikelets per spike, and fertile spikelets per spike (Table 1). Grand means for agronomic traits of PI and PS response types indicated that the PS lines were later heading (3.1 d) and later maturing (2.1 d), taller (5.5 cm), and higher yielding (170 kg ha–1) than PI lines (Table 2). In addition, PS lines possessed significantly increased final leaf number (0.3 leaves), total spikelets per spike (0.5 spikelets), and fertile spikelets per spike (0.4 spikelets).


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  		Table 1. Mean squares for agronomic data of near-isogenic spring wheat lines.

 

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  		Table 2. Agronomic data for two photoperiod response types and three recurrent genetic backgrounds.

 
Genetic backgrounds differed significantly in heading, maturity, plant height, spikelets per spike, seeds per spike, yield per spike, and kernel weight (Table 1). Grand means for agronomic traits of genetic backgrounds indicated that the SWP5304 background, compared to AC Minto, was earlier heading (0.6 d), later maturing (0.7 d), similar in plant height, had more total spikelets per spike (0.8 spikelets), more fertile spikelets per spike (0.4 spikelets), more sterile spikelets per spike (0.4 spikelets), more seeds per spike (1.4 seeds), higher yield per spike (0.1 g), and higher kernel weight (1.9 mg) (Table 2). The SWP5304 background, compared with CDC Makwa, was similar in time to heading, later maturing (0.9 d), taller (2.5 cm), had more total spikelets per spike (0.7 spikelets), more sterile spikelets per spike (0.4 spikelets), similar fertile spikelets per spike, similar seed number per spike, greater yield per spike (0.1 g), and higher kernel weight (4.4 mg).

Nonsignificant photoperiod response type x genetic background interactions (PPD x BKG) were observed for all 11 traits, except for fertile spikelets per spike and seeds per spike (Table 1). Significant non-crossover PPD x BKG interactions were observed for fertile spikelets per spike and seeds per spike. The interactions were significant because of changes in the magnitude of the response of PPD across BKG. The interactions were noncrossover because the ranking of PPD remained constant across BKG (Baker, 1988). For fertile spikelets per spike, significant changes in magnitude occurred between PPD across CDC Makwa (0.7 spikelets) and SWP5304 (0.6 spikelets) backgrounds (Table 3), indicating that PS lines had consistently more fertile spikelets per spike relative to PI lines as shown in Table 2. For seeds per spike, significant differences between PPD indicated that the PS lines had consistently more seeds per spike, relative to PI lines, within the CDC Makwa background (1.4 seeds per spike) (Table 3). Only crossover interactions are of a concern to plant breeders (Baker, 1988), and thus within this study, as a significant crossover PPD x BKG interaction would have indicated that the PPD changed in ranking or were instable across BKG for a given trait.


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  		Table 3. Agronomic data for photoperiod sensitive (PS) and photoperiod insensitive (PI) lines derived from three genetic backgrounds.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
In Europe, the USA, and Canada, research on the effects of the Ppd-D1/ppd-D1 alleles have indicated that there are plant development differences between photoperiod response types. In our study, there was a significant reduction in the initiation of leaf and spikelet primordia by the PI allele (Table 2). The final leaf number and total spikelets per spike of insensitive lines were reduced relative to sensitive lines. Similarly, PI alleles have been reported to reduce the initiation of primordia in the developing plant apex (Worland, 1996). The reduced number of primordia for lines carrying Ppd-D1, especially with respect to leaf initiation, translated into a shorter development time (days to heading, 3.2 d; days to maturity, 2.1 d), although not necessarily a faster development rate (Table 2). Hay and Ellis (1998) have reported a similar association between final leaf number versus days to heading and final leaf number versus days to maturity. In our study, PI lines were shorter by 5.5 cm (Table 2). Similarly, photoperiod insensitivity, specifically the Ppd-D1 allele, has been reported to reduce plant height (Marshall et al., 1989; Worland, 1996). Seeds per spike and kernel weight were not significantly different between the photoperiod response types (Table 2). Other studies have detected differences between photoperiod types for kernel weight (Busch et al., 1984; Marshall et al., 1989), while others have not detected differences between photoperiod response types (Knott, 1986). Worland (1996) reported that the insensitive Ppd-D1 allele reduced the number of initiated primordia, but increased spikelet fertility. In contrast, our study found that the PI lines had reduced final leaf number (0.3**), total spikelets per spike (0.5**), and fertile spikelets per spike (0.6**) compared with PS lines (Table 2).

Averaged over all environments tested, PS lines out-yielded PI lines by 170 kg ha–1 or 4.9% (n = 20 environments). A preliminary ANOVA analysis indicated that the PS-PI differences in grain yield among sites were not significantly related to latitude (P = 0.16; data not shown). The increase in grain yield of the PS near-isogenic lines likely resulted, at least in part, from delayed heading and an increase in total spikelets per spike and fertile spikelets per spike (Table 2). Rawson (1971) also reported that later heading was associated with higher spikelets per spike and grain yield. Marshall et al. (1989) suggested that the photoperiod response by temperature interaction had a large influence on grain yield. Busch and Chamberlain (1981) and Worland (1996) reported that the avoidance of stress at key plant development times could be important in affecting potential grain yield. Weather data was not available in our present study and, thus, the avoidance of stress at key plant development times by PS lines may or may not have provided sensitive types with a yield advantage over PI lines. Further research is needed in the higher latitudes of North America to determine if the Ppd-B1 and Ppd-A1 alleles, in near-isogenic lines derived from diverse genetic backgrounds, result in a lower yield penalty relative to that imposed by the Ppd-D1 allele in our present study.

In Canada, research into the agronomic benefit of photoperiod insensitivity in wheat is very limited. In western Canada, approximately 28% of currently grown CWRS cultivars have been classified for photoperiod response. Out of the 25 currently grown CWRS cultivars, five are PI (CDC Teal, Hughes and Hucl, 1993; AC Eatonia, de Pauw et al., 1994; AC Elsa, Clarke et al., 1997; AC Intrepid, de Pauw et al., 1999; AC Abbey, de Pauw et al., 2000) while two are described as PS (AC Barrie, McCaig et al., 1996; AC Cadillac, de Pauw et al., 1998). The photoperiod response of the remaining 18 CWRS cultivars has not been described in the literature. Currently, the diversity of PI alleles within Canadian PI cultivars is unclear. Cao et al. (2002) reported that AC Minto and CDC Makwa were closely related based on their Neepawa-involved pedigree (genetic similarity coefficient = 0.80). Wheat cultivars within the Neepawa-involved pedigree subgroup (n = 12) were most related to five cultivars sharing North Dakota germplasm (similarity coefficient = 0.67) and least related to two cultivars which shared CIMMYT-based (Mexico) pedigrees (similarity coefficient = 0.56). Caution must be taken when extrapolating the results within this study to other related or unrelated genetic backgrounds grown in the higher latitudes of North America because the PI trait is currently carried by some CWRS wheat cultivars without agronomic penalty. For example, AC Intrepid (PI) out-yields AC Barrie (PS) by 5% in the central region of Saskatchewan, Canada (Saskatchewan Agriculture, Food, and Rural Revitalization, 2003). At this point, it is premature to make any general recommendations to spring wheat breeders in northern USA and Canada to switch from the PI to the PS trait within their breeding programs. Further research is needed assessing (i) PI and PS near-isogenic lines derived from different genetic backgrounds and (ii) PI near-isogenic lines carrying different PI alleles in multiple and diverse geographical regions within the higher latitudes of North America. Collectively, this research may or may not show that the Ppd alleles, conferring photoperiod insensitivity, are detrimental to grain yield in spring wheat grown in the northern areas of North America.

To date, Canadian research suggests that PS lines are generally higher yielding than PI lines when grown in the higher latitudes of North America. In contrast, Busch et al. (1984) and Marshall et al. (1989) reported that the PI trait could be used without agronomic penalty in the spring wheat growing region of the USA, indicating that further research is needed to determine at which latitude or geographical region the PI genotypes out-perform PS types. In the higher latitudes of North America, the agronomic trade-off appears to be a 2- to 3-d delay in heading and maturity. The risk of grade-loss due to early frost should be balanced against delayed maturity in the northern edge of western Canada's grain belt. The significance of discovering further genes and regulatory pathways controlling flowering and maturity in spring wheat may allow plant breeders to create cultivars with increased environmental adaptability. With regard to breeding for appropriate maturity, genetic loci influencing flowering and development time could be incorporated into backgrounds that carry sensitivity to photoperiod. Earliness per se could also be used directly to influence time to heading in cultivars carrying sensitivity to photoperiod (Hoogendoorn, 1985; Worland, 1996). Chromosome mapping and markers associated with photoperiod, vernalization and earliness genes are becoming increasingly useful as cultivars are tailored to specific environments (Ben Amer et al., 1997; Sourdille et al., 2000).


   ACKNOWLEDGMENTS
 
Appreciation is expressed to K. Jackle and M. Grieman (CDC-U of S; Saskatoon, SK); S. Lanning (MSU; Bozeman, MT); Z. Szlavnics (U of G; Guelph, ON); J. Bourassa (AAFC; Sainte Foy, QC); Kevin McCallum (UGG; Elrose, SK and Elgin, MB); A. Cummiskey (AAFC; Charlottetown, PE); J. Unruh (Lacombe Research Centre, AAFC; Fort Vermillion, AB); and R. Vervoben (Res. Comm. BCGPA; Dawson Creek, BC) for their technical assistance. The authors gratefully acknowledge funding support from the British Columbia Grain Producers for conducting the experiment at Dawson Creek, BC. Thanks are extended to Dr. R.J. Baker (Retired; Dep. of Plant Sciences, U of S; Saskatoon, SK) for his guidance in the statistical analysis of the data.

Received for publication November 4, 2003.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 




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