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Crop Science 42:152-159 (2002)
© 2002 Crop Science Society of America

CROP PHYSIOLOGY & METABOLISM

Water Relations of Standard Height and Dwarf Sunflower Cultivars

S. V. Angadi*,a and M. H. Entzb

a Semiarid Prairie Agricultural Research Centre, Agriculture and Agri-Food Canada, Swift Current, SK, Canada S9H 3X2
b Dept. Plant Science, University of Manitoba, Winnipeg, MB, Canada R3T 2N2

* Corresponding author (angadis{at}em.agr.ca)


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Development of early maturing short stature sunflower (Helianthus annuus L.) cultivars followed the success of semi-dwarf cereals. Although a number of breeding programs have recently released dwarf sunflower cultivars for production in western Canada, the drought adaptability of dwarf cultivars has not been studied. Therefore, field studies were conducted under rainfed conditions at two Manitoba locations during 1994 and 1995 to compare seasonal water relations of ‘Sunwheat-103’ (SW-103), a dwarf hybrid, and ‘AC-Aurora’ (Aurora), a dwarf open pollinated cultivar, with ‘IS-6111’, a standard height hybrid. The dwarf hybrid, SW-103, experienced less stress (up to 0.44 MPa higher leaf water potential) than the standard height hybrid, IS-6111. Genotypic variation for osmotic adjustment was observed in both 1994 and 1995. SW-103 and Aurora respectively had up to 0.42 and 0.39 MPa lower osmotic potential at full turgor compared with IS-6111. Aurora displayed greater osmotic adjustment per unit decrease in leaf water potential than other cultivars. All cultivars maintained positive pressure potential during the study and whenever significant differences in pressure potential were observed, SW-103 usually had higher pressure potential than other cultivars. IS-6111 maintained higher photosynthetic rate and stomatal conductance than the dwarf cultivars. These results suggest that under conditions of adequate water supply, the tall hybrid had the highest productivity despite experiencing the lowest leaf water potential and osmotic adjustment. The drought tolerance advantages of shorter statured cultivars identified here may be more important to productivity maintenance under limited soil water conditions.

Abbreviations: {psi}l, leaf water potential • {psi}{pi}, leaf osmotic potential • {psi}p, leaf pressure potential • {psi}{pi}100, leaf osmotic potential at full turgor • PAR, photosynthetically active radiation


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
THE SUCCESS OF SEMI-DWARF GENES in cereals has prompted scientists to consider height reduction in many other crops including sunflower (Helianthus annuus L.). While the genetics of plant height and agronomic benefits of dwarf sunflowers have been reported (Miller, 1992; Johnston et al., 1995), the implications of dwarfing genes for plant water relations in sunflower have not been previously explored. The conventional interpretation with cereal crops is that shorter plants are best suited to highly productive environments, while taller plants are best suited for yield stability under adverse conditions, although critical evidence for this is lacking (Blum et al., 1988). In wheat (Triticum aestivum L.), a limited number of studies indicated a reduction in water stress experienced by reduced stature plants (Kirkham and Smith, 1978; Blum and Sullivan, 1997), while others reported no effect of plant stature on plant water stress (Entz and Fowler, 1990). In the northern Great Plains of Canada and the USA, short-season hybrid and open-pollinated sunflower cultivars are grown in short-season areas that are also characterized by water deficits. Information on the effects of varying stature on sunflower drought response is important for adoption of short stature sunflower in stress prone environments of North America and elsewhere.

The effect of dwarfing genes on the morphology and physiology of cereals depends on the genetic traits or genetic background of the cultivar (Ehdaie and Waines, 1996; Blum and Sullivan, 1997). In sunflower, semidwarf and dwarf phenotypes have been recently developed (Schneiter, 1992). Plant architecture in sunflower varies with breeding program and cultivar, indicating the role of different genes (Miller, 1992; Miller and Hammond, 1991) or differences in the genetic background of the cultivar (Ehdaie and Waines, 1996; Blum and Sullivan, 1997). A new group of open-pollinated dwarf sunflower cultivars, referred to as Sunola, have been developed in western Canada and have been registered for cultivation since 1992. Sunola plants are characterized by reduced plant height, leaf size, head diameter, and thin stems compared with standard height sunflower (Beckie and Brandt, 1996). The canopies of Sunola cultivars are more open due to smaller leaves and longer petioles. Such an open canopy can alter radiation penetration and air movement (Nobel, 1999), which may affect water flux through the soil-plant-air continuum.

The soil moisture is the most important factor limiting crop productivity on the Canadian Prairie where potential evapotranspiration exceeds precipitation throughout the growing season (Ash et al., 1992) and the crops depend on stored soil moisture for much of their water requirements. Passioura (1994) stated that whole season crop growth is independent of short periods of stress under moderate stress conditions. Drought stress in southern Manitoba is caused by lack of residual soil moisture and unfavorable distribution of rainfall. Under these conditions, drought tolerance is required to maintain crop productivity.

Therefore, a field study was conducted to understand the effects of genetic reduction of plant height on the drought stress tolerance in sunflower. The first objective of the study was to observe variations among different height sunflower cultivars for water potential, osmotic adjustment and pressure potential. The second objective was to study the effect of plant stature on photosynthetic rate and stomatal conductance.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 REFERENCES
 
Experimental Details
Studies were conducted in 1994 and 1995 at Carman (49.5°N, 98.0°W) and Winnipeg (49.8°N, 97.2°W) to monitor sunflower water relations under rainfed conditions in southern Manitoba. The soil at Winnipeg was clay (Riverdale series, Entisol, Cumulic Regosol) with gradual release of water and slow development of stress, while the soil at Carman was sandy clay loam (Denham and Eigenhof series, Udic Boroll, Orthic Black Chernozem) with characteristic quick release of water and rapid development of stress. The available water to 1.20 m depth were 260 and 247 mm at Carman and Winnipeg, respectively. The typical relative water deficit for a long duration crop at Carman is 50 mm more than at Winnipeg (Ash et al., 1992). On the basis of soil test results (Norwest Lab, Winnipeg), 0 to 96 kg N, 27 to 43 kg P2O5, and 9 to 24 kg S were broadcast applied immediately before seeding. Preseeding application of glyphosate and hand weeding were used to keep plots weed free. Sunflower beetles were controlled by spraying systemic insecticides.

Three commercial cultivars, AC-Aurora (an open-pollinated dwarf Sunola cultivar; referred to as Aurora), Sunwheat-103 (a dwarf hybrid; referred to as SW-103) and IS-6111 (a standard height hybrid) were used in the study. Under Canadian conditions the growth duration of IS-6111 is reported to be 11 and 19 d longer than SW-103 and Aurora, respectively (Description of Variety, Agriculture Canada, Food Production and Inspection Branch). All cultivars were hand seeded between 23 May and 2 June in 5 by 8 m plots at a spacing of 1 by 1 m. Plots were over seeded and the extra seedlings were thinned at the two to four leaf stage. The statistical design was a randomized complete block design (RCB) with four replications.

Observations
Weather conditions in all field trials were monitored from May to September with meteorological stations located less than 500 m away from the research plots. Daily values of minimum and maximum air temperatures and rainfall data were recorded from the weather stations. In 1994, the rain-gauge at the Department of Plant Science Field Research Facility at Winnipeg did not function properly. Therefore, rainfall data from the nearby Glenlea Research Station (about 15 km away from plots) was used.

The upper most fully expanded sunlit leaves were used for all water relations measurements, and observations were made between 1100 and 1400 h. All field observations were made in each plot on clear sunny days with midday solar radiation levels, measured with a quantum sensor (Licor Inc., Lincoln, NE), were >=1600 µmol m-2 s-1 Photosynthetically Active Radiation (PAR). Leaf water potential ({psi}l) was measured as the negative of the hydrostatic pressure required to bring the xylem sap to the cut end of the petiole (Turner, 1981) with a Scholander type pressure chamber (Model 1002, PMS Instrument Company, Corvallis, OR). Leaves were sealed in a plastic bag before sampling (Turner, 1981). The chamber was pressurised at a rate of 0.01 to 0.02 MPa s-1. The whole process of cutting the leaf to appearance of xylem sap (balancing pressure) on the cut end took 3 to 5 min for each leaf.

Leaf osmotic potential ({psi}{pi}) was measured by the psychrometric method (Turner, 1981). The sample leaf, similar to the one used for {psi}l, was cut into two pieces, avoiding the midrib, and each piece of leaf lamina approximately 5 by 5 cm in size was placed into a 5 mL disposable syringe. The syringe tips were sealed with parafilm and packed in ice. Within 2 to 4 h, the samples were moved to a freezer at -20°C. At a later date, osmotic potential of the expressed sap was measured with a Wescor Vapor Pressure Osmometer (Model Wescor 5500XR, Logan, UT). No effort was made to account for the dilution effect of apoplastic water. Means of two samples from the same leaf were used for statistical comparisons.

Pressure potential ({psi}p) was calculated by subtracting {psi}{pi} from the {psi}l measured on the same day. Relative water content measured at the same time as {psi}{pi} was used to convert {psi}{pi} to osmotic potential at full turgor ({psi}{pi}100) (Morgan, 1984). The difference in {psi}{pi}100 during various stages of growing season indicates the osmotic adjustment due to active solute accumulation.

Stomatal conductance and photosynthesis were measured on the upper most, fully expanded, illuminated leaves on clear days, with a closed portable photosynthesis system (LI 6200/6050, LI-COR Instruments, Lincoln, NE). At each sampling time, 2 to 3 leaves were sampled in each plot with three readings on each leaf. The selected leaf was clamped into a 1 L leaf chamber with an approximate measurement area of 40 cm2. Measurement on each leaf took about 45 to 60 s. Means of all observations for each plot were used for statistical analysis.

Data collected from different environments (and on different dates) were analyzed separately by an analysis of variance procedure (SAS Institute Inc., 1985). A Fisher's protected LSD test (P = 0.05) was used for mean comparison. The relationships between {psi}l, stomatal conductance, photosynthesis, and {psi}{pi}100 were tested by regression analysis. Mean values were used for regression analysis. Slopes of regression equations were tested for statistical significance with JMP software (Sall and Lehman, 1996).


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
REFERENCES
 
Weather Conditions
Air temperature and rainfall for Carman and Winnipeg during the years of experimentation are presented in Fig. 1 . Long term average (1925 to 1990) rainfall for Carman and Winnipeg are 310 and 340 mm, respectively. In general, the 1994 growing season was cooler and received either above normal (Winnipeg) or slightly below normal (Carman) rainfall compared with the 1995 season, which was warmer with intermittent dry spells. In addition, a greater portion of seasonal rainfall in 1995 was received late in the season. Water relation observations vary in response to short term weather changes. With the exception of a few days, the maximum temperature on the day of observation was above 25°C (range 21.2 to 33.5°C) with a negligible amount of rainfall in the preceding 72 h. A few rainfall events of >25 mm however, occurred 72 h before observation period at the beginning or end of the growing season.



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  		Fig. 1. Weather parameters at Carman and Winnipeg during 1994 and 1995 growing seasons. Solid lines and perforated lines represent maximum and minimum temperature, respectively. The precipitation is represented by vertical solid bars. Seeding dates of the space planted trials are shown with open arrows.

 
Agronomic Performance
Information on the agronomic performance and soil water use pattern of all cultivars have been presented separately (Angadi and Entz, 2002a, b). The average maximum plant height of Aurora and SW-103 were 87 and 81 cm compared with 138 cm for IS-6111 (Angadi and Entz, 2002a). Mean aerial biomass production was 7.4 t ha-1 for IS-6111 compared with 3.3 t ha-1 for SW-103 and 2.7 t ha-1 for Aurora (Angadi and Entz, 2002a). Growing season net water extraction was 32.7 cm for IS-6111, 28.6 cm for SW-103 and 29.8 cm for Aurora (Angadi and Entz, 2002a).

Leaf Water Potential
Prevailing midday {psi}1 in the field trials ranged from -0.48 (Aurora at Carman 1994) to -1.74 MPa (IS-6111 at Carman 1994) (Fig. 2 and 3) . Similar ranges of {psi}1 have been reported in sunflower under other agroclimatic conditions (Prasad et al., 1985; Rachidi et al., 1993), although under severe stress {psi}1 can drop below -3.0 MPa (Connor and Jones, 1985; Gimenez and Fereres, 1986; Wise et al., 1990). Therefore, sunflower cultivars in this study experienced only moderate water stress.



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  		Fig. 2. Seasonal pattern of leaf pressure potential ({psi}p), water potential ({psi}l) and osmotic potential at full turgor ({psi}{pi}100) of space planted sunflower cultivars in 1994. The vertical bar represents the LSD value (P = 0.05). Absence of a LSD bar indicates non-significant difference among cultivars.

 


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  		Fig. 3. Seasonal pattern of leaf pressure potential ({psi}p), water potential ({psi}l) and osmotic potential at full turgor ({psi}{pi}100) of space planted sunflower cultivars in 1995. The vertical bar represents the LSD value (P = 0.05). Absence of a LSD bar indicates non-significant difference among cultivars.

 
Significant differences in {psi}l among sunflower cultivars on a number of observation dates indicate that sunflower cultivars experienced different levels of water stress in this study. Whenever cultivar differences were significant, the dwarf hybrid, SW-103, had the highest {psi}1 (i.e., least water stress) among the cultivars compared (Fig. 2 and 3). For example, at Carman in 1994, {psi}1 decreased by 1.19 MPa for IS-6111 during the course of growing season compared with only 0.73 MPa for SW-103. Based on soil water extraction differences between SW-103 relative to IS-6111 (i.e., SW-103 depleted less soil water, due to a shallower root system; Angadi and Entz, 2002b), it would be expected that the dwarf should be under more water stress. However, this was not the case.

An explanation for less water stress by SW-103 vs. IS-6111, despite less soil water extraction by SW-103, may involve canopy characteristic differences between these cultivars. First, IS-6111 produced 127% more aerial dry matter than SW-103 (Angadi and Entz, 2002a), suggesting higher leaf area for transpiration in IS-6111. Giminez and Fererez (1986) reported a significant negative correlation between leaf area and leaf water potential in sunflower. Second, SW-103 plants were 30% shorter than IS-6111 plants in this study (Angadi and Entz, 2002a), while leaf number was similar for IS-6111 and SW-103 (Angadi, 2001). Therefore, SW-103 had a more compact canopy than IS-6111. Greater leaf overlap due to a compact canopy in dwarf versus standard height sunflower has been previously reported by Sadras et al. (1991). Compact crop canopies are known to affect microclimate by reducing wind speed and light penetration and by increasing relative humidity (Nobel, 1999), which in turn reduces transpiration loss from the canopy (Pataki et al., 1998). It was interesting to note that a similar explanation for superior water status (i.e., less negative water potential) for semi-dwarf vs. tall wheat has recently been presented by Blum and Sullivan (1997), who worked with isogenic lines.

The open-pollinated cultivar, Aurora, experienced water stress levels similar to or greater than those of the standard height hybrid (Fig. 2 and 3). A possible explanation for greater water stress in Aurora versus SW-103, even though Aurora extracted more soil water than SW-103 (Angadi and Entz, 2002a), may once again be due to differences in canopy architecture. Aurora has a very open canopy (Becky and Brandt, 1996); plants were taller than SW-103 (87 vs. 81 cm), however above-ground dry matter was less for Aurora than SW-103 (Angadi and Entz, 2002a). Therefore, the canopy of Aurora was much more open than for the dwarf hybrid. An open canopy, with greater light penetration and better air circulation, has the potential to increase the plant water stress (Nobel, 1999). Other factors such as lack of heterosis in Aurora relative to SW-103, however, also may have contributed to observed differences between cultivars. Finally, it is important to note that this research was conducted in space-planted trials. Canopy densities would be expected to be very different where commercial plant populations are used (commercial plant populations higher for Aurora > SW-103 > IS-6111).

Osmotic Adjustment
Osmotic adjustment by solute accumulation, measured as the difference in {psi}{pi}100, is an important drought response in plants (Morgan, 1984). While genotypic variation for osmotic adjustment in sunflower has been reported (Chimenti and Hall, 1993), the effect of plant stature on osmotic adjustment has not been previously investigated. Plant stature had a significant influence on osmoregulation strategies of dwarf and standard height cultivars to counter the water stress effects (Fig. 2 and 3). In general, the dwarf sunflower cultivars, Aurora and SW-103, initiated osmotic adjustment earlier than IS-6111. Whenever cultivar differences were significant, the {psi}{pi}100 of the standard height cultivar, IS-6111, was higher (i.e., less osmoregulation) than that of SW-103 and/or Aurora, indicating that these shorter stature cultivars had a better osmotic response than the standard height cultivar.

Solute accumulation in response to water stress, further depicted the differences among cultivars in osmotic adjustment response (Fig. 4) . Significant differences in slopes (P < 0.05) indicated that solute accumulation in Aurora was 0.25 to 0.29 MPa higher than SW-103 and IS-6111 for each MPa decrease in {psi}1, respectively. Similar to {psi}1, these observations suggest a significant role of genetic background in drought response.



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  		Fig. 4. The relationship between leaf water potential ({psi}l) and osmotic adjustment ({psi}{pi}100) by different height sunflower cultivars in the trials. * and *** indicate significance at 0.05 and 0.001, respectively.

 
The initiation of solute accumulation by IS-6111 was delayed relative to SW-103 (Fig. 1 and 2). This observation was attributed to greater water supply to IS-6111, as Angadi and Entz (2002b) observed faster root penetration and more water extraction in IS-6111 compared with SW-103 in these trials. Blum et al. (1999) reported delayed osmotic adjustment in wheat cultivars capable of extracting more soil water from greater depth. This hypothesis was supported by the observation that the relationship between {psi}{pi}100 and {psi}1 was similar (P > 0.05) between SW-103 and IS-6111 (Fig. 4). Therefore, early initiation of osmotic adjustment in SW-103 relative to IS-6111 in the present study may be attributed to the earlier experience of stress by SW-103. However, the {psi}1 data do not support this as {psi}1 of SW-103 was always similar or higher than that of IS-6111. Therefore, either different threshold levels of {psi}1 or non-hydraulic root signals (Blum and Sullivan, 1997) might have initiated osmotic adjustment in dwarf sunflower cultivars. Further studies under regulated moisture supply, preferably under rainout shelter, are needed to elucidate osmotic adjustment responses. At any rate, both dwarf sunflower cultivars were quicker and/or more efficient in using osmotic adjustment to reduce water stress effects.

Leaf Pressure Potential
All three sunflower height classes maintained positive {psi}p in all field trials (Fig. 2 and 3). Whenever significant differences in {psi}p among height classes were observed, SW-103 had higher {psi}p levels than IS-6111 except at 45 d after seeding at Carman in 1995. Higher levels of {psi}p for SW-103 vs. IS-6111 were not unexpected as SW-103 experienced less water stress (i.e., higher {psi}1) and greater osmotic adjustment. The {psi}p of Aurora was intermediate between SW-103 and IS-6111 and on a few occasions, Aurora experienced higher {psi}p than IS-6111. Thus, a superior osmotic adjustment and/or lower stress level enabled the dwarf cultivars to maintain higher {psi}p than IS-6111 in this study.

Stomatal Conductance and Net Photosynthesis
Seasonal trends in midday stomatal conductance and net photosynthetic rate are presented in Fig. 5 . Due to frequent rainfall, cultivar effects on stomatal conductance and photosynthetic rate were not significant in 1994 (data not presented). Observation periods in 1995 coincided with 21 and 32 d long dry cycles at Carman and Winnipeg (about 7 mm of rainfall received during the period), respectively (Fig. 1). In spite of less rainfall and a longer stress period, effects of the drying cycle at Winnipeg were less obvious than at Carman. The differences in moisture release characteristics of soils at both locations might be responsible for the differences in stomatal conductance between sites. For example, the fine textured soil at Winnipeg supported higher levels of stomatal conductance (up to 5.14 mol m-2 s-1) than coarse textured soils at Carman (up to 1.41 mol m-2 s-1) during the observation period. However, differences in net photosynthesis between locations were small, indicating the insensitivity of photosynthetic rate to water availability as reported by Connor and Hall (1997).



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  		Fig. 5. The seasonal trends in stomatal conductance and net photosynthesis rate by different height sunflower cultivars at Carman and Winnipeg in 1995. The amount of precipitation between 66 and 86 d after seeding at Carman and between 47 and 78 d after seeding at Winnipeg were 7.6 and 7.4 mm, respectively. The vertical bar represents the LSD at 0.05. Absence of a LSD bar indicates nonsignificant difference among cultivars.

 
Stomatal conductance is regulated by evaporative demand, controls within the plant system, and water supply capacity of the soil (Tardieu and Simonneau, 1998). Cultivar differences were observed in seasonal trends in photosynthesis and stomatal conductance (Fig. 5). IS-6111, with a deeper and more extensive root system (Angadi and Entz, 2002b), supported higher stomatal conductance than dwarf cultivars under the moderate water-limited conditions of this study. Sadras et al. (1991), also reported higher transpiration rates in standard height compared with semidwarf sunflower cultivars. These observations contrast those in wheat, where comparison of isogenic lines for height under controlled conditions revealed either higher stomatal conductance and photosynthesis with dwarfing genes (LeCain et al., 1989; Morgan et al., 1990; Bishop and Bugbee, 1998) or no change (Blum and Sullivan, 1997).

It was not possible to assess the threshold {psi}1 for stomatal closure in the present study, as it requires a {psi}1 lower than -2.8 MPa (Sadras and Milroy, 1996). Stress levels in the present study were moderate (>-1.74 MPa) and no significant relationship between {psi}1 and stomatal conductance or net photosynthesis was observed (data not presented). Thus, although genetic variation, seasonal trends, and differences between locations were observed for stomatal conductance, as reported in the literature (e.g., Connor and Sadras, 1992), stomata appeared to play a secondary role in regulating stress response in the present study.

Crop productivity is an integration of whole season growth and is independent of short periods of water stress under moderate stress conditions (Passioura, 1994). Blum and Sullivan (1997) reported that, in wheat, the higher production potential of tall cultivars sustains the productivity over the stress tolerant dwarf cultivars under mild stress, while stress tolerance of the dwarf cultivars takes over when stress is severe. This model appears to apply for sunflower observations in the present study. For example, in spite of having the highest {psi}1, SW-103 had the lowest photosynthesis rate, while IS-6111 had the lowest {psi}1, but was found to have a much higher photosynthesis rate than SW-103. In the present study, soil moisture was available at greater depth and the dry cycles were not long enough to exhaust the water (Bremner et al., 1986). Therefore, under conditions of ample soil water supply, IS-6111, the cultivar with the deepest root system, could maintain better water extraction (Angadi and Entz, 2002b) to avoid critical stress at sensitive stages, which enabled the standard height hybrid to maintain photosynthesis rate compared with dwarf cultivars. Therefore, the superior photosynthesis rate of the standard height cultivar in this study was attributed to greater water use.

An interesting discovery was that sunflower cultivars adopted different strategies to maintain positive turgor. The standard height hybrid relied more on water extraction from soil (Angadi and Entz, 2002b), followed by osmotic adjustment and stomatal regulation to maintain positive {psi}p. Strong dependence on soil water extraction for turgor maintenance is often reported in sunflower (Conner and Sadras, 1992). The two dwarf cultivars on the other hand, depended more on osmotic adjustment and stomatal regulation to maintain plant water status. Therefore, this study shows that dwarf sunflower cultivars use osmotic adjustment to maintain tissue water status.


   ACKNOWLEDGMENTS
 
We thank the Canadian Commonwealth Scholarship and Fellowship Plan for supporting the senior author. Additional funding from NSERC and Proven Seed (Division of United Grain Growers) is greatly acknowledged. We are grateful to Keith Bamford for technical assistance and to Drs. B.G. McConkey, H.W. Cutforth, and P. Jefferson for reviewing the manuscript.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Part of the thesis submitted by S.V. Angadi in partial fulfilment of the requirements for Ph.D. degree.

Received for publication February 18, 2001.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 




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