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a Semiarid Prairie Agricultural Research Centre, Agriculture and Agri-Food Canada, Swift Current, SK, Canada S9H 3X2
b Dep. Plant Science, Univ. of Manitoba, Winnipeg, MB, Canada R3T 2N2
* Corresponding author (angadis{at}em.agr.ca)
Received for publication April 2, 2001.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Abbreviations: Aurora, ‘AC-Aurora’ • DAS, days after seeding • Sierra, ‘AC-Sierra’ • SW-101, ‘Sunwheat-101’ • RCBD, randomized complete block design • SW-103, ‘Sunwheat-103’
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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On the Canadian Prairie, potential evapotranspiration exceeds precipitation for the growing season (Ash et al., 1992), and crops depend heavily on stored soil moisture for their water requirements. However, even in the driest regions of western Canada, water from deeper soil layers is often left unused (Hurd, 1974). Therefore, a crop like the standard height sunflower, capable of extracting water from deeper layers, will have a distinctive advantage over shallower rooting annual crops like canola (Brassica napus L.) and wheat. However, adoption of the standard height sunflower in western Canada is limited by the short growing season.
Literature on the intraspecific variations in root system of crop plants is limited (O'Toole and Bland, 1987). Fereres et al. (1986) observed that sunflower cultivars adapted to a more arid climate were less sensitive to water stress than the cultivars developed for a humid climate. In rice (Oryza sativa L.) and soybean, better adaptability of cultivars to drier growing conditions was attributed to deeper rooting (Boyer, 1996). Genotypic variation in rooting depth has been observed in sunflower, and greater rooting depth is usually associated with longer growth duration (Fereres et al., 1986; Schneiter, 1992).
Recent dwarf sunflower cultivars are gaining popularity with producers due to ease of cultivation and the reduced growth duration (Johnston et al., 1995). The development of dwarf sunflower follows that in cereals. In cereals, the influence of dwarfing genes on rooting characters have been observed (Ehdaie and Waines, 1996; Entz et al., 1992; Grant et al., 1991). In wheat, the changes in root traits depend on host cultivar (Blum and Sullivan, 1997; Ehdaie and Waines, 1996). Information on root systems of different stature sunflower cultivars is lacking, and the limited number of studies on water extraction in relation to sunflower plant height have produced contradictory results (Sadras et al., 1991; Schneiter, 1992; Zaffaroni and Schneiter, 1989). Therefore, information on the root system and water use pattern of dwarf sunflower cultivars in comparison with standard height cultivars, especially under western Canadian conditions, is urgently needed.
The first objective of this study was to compare the root systems and water depletion patterns of standard height compared with dwarf sunflower cultivars. No previous studies have considered this comparison. The second objective was to compare the rooting characteristics of short-season dwarf hybrids with those of a new class of open-pollinated dwarf cultivars (Sunolas) bred for short-season production areas. Information from this study will increase our understanding of soil water dynamics of sunflower as influenced by genotypic variation. The information will also provide an understanding of how these different sunflower genotypes will perform in a cropping system. Crop growth, yield, yield components, water use efficiency, and water relations results are presented in previous papers (Angadi and Entz, 2002a, 2002b).
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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In the present study, sunflower cultivars from dwarf open pollinated, dwarf hybrid, and standard height hybrid classes were used. Sunola cultivars (Aurora and Sierra; Western Grain Seed Corp., Saskatoon, SK, Canada) represent the dwarf open pollinated cultivars developed for short growing season areas. The second class of dwarf hybrid was represented by two sunwheat hybrids, SW-101 and SW-103 (SeedTec Int. Inc., Woodland, CA). The two standard-height hybrids were IS-6111 (Inter State Hybrid Seed Co., Fargo, ND) and SF-187 (Cargill, Minneapolis, MN).
Both dwarf hybrids and dwarf open pollinated cultivars used in the present study were 
40% shorter compared with the traditional standard height cultivars (Angadi and Entz, 2002a). Compared with the standard height cultivars, the dwarf open pollinated sunola cultivars have reduced leaf size, longer petiole, and reduced head diameter (Beckie and Brandt, 1996), while sunwheats have more compressed plant architecture with shorter internodes (Angadi, 2001). Under Canadian conditions, the growth duration from seeding to physiological maturity of IS-6111, SF-187, SW-101, SW-103, Aurora, and Sierra is reported to be 118, 118, 109, 106, 98, and 101, respectively (Description of Variety, Agriculture Canada, Food Production and Inspection Branch).
Two types of field experiments, space planted trials and agronomy trials, were used to assess the performance of sunflower cultivars.
Space Planted Trials
In these trials, Aurora, SW-103, and IS-6111 were evaluated under uniform, low interplant competition conditions. The space planted trials were conducted during 1994 and 1995 at both Carman and Winnipeg. In 1994 at Carman, SW-101 and Sierra were included in the trial to compare the variations within the dwarf height classes. These trials were hand seeded between 23 May and 2 June in 5- by 8-m plots with a 1- by 1-m spacing between plants. The experiments were overseeded with three to four seeds at each hill. At the two- to four-leaf stage, plants were thinned to one plant per hill. The experimental design in each case was a randomized complete block design (RCBD) with four replications. An additional space planted study was conducted in 1994 at Carman to study the root systems of Aurora, SW-103, and IS-6111 under field conditions.
Agronomy Trials
In agronomy trials, commercial crops of two cultivars from each of the three height classes were assessed for variations in water extraction traits within and between height classes. Standard height cultivars were planted with a row spacing of 0.75 m and a plant density of 5.5 plants m-2, dwarf hybrids were planted with a row spacing of 0.30 m and a plant density of 10 plants m-2, and open pollinated dwarfs were planted with a row spacing of 0.30 m and a plant density of 17 plants m-2. These are the agronomic practices recommended for the respective classes of cultivars in western Canada (W. Dedio, personal communication, 1993). Plot dimensions were 8 by 3.6 m for dwarf cultivars and 8 by 5.4 m for standard height hybrids. Experimental plots were seeded using a Fabro small plot seeder (Swift Manufacturing Co., Swift Current, SK, Canada). The experiments were overseeded and hand-thinned to the recommended plant population at the four- to six-leaf stage. Agronomy trials conducted at Winnipeg in 1994 and at Carman in 1995 were seeded on 30 May and 13 May, respectively. The experimental design was RCBD with four replicates.
Each experiment was fertilized according to the soil test recommendations (Norwest Lab, Winnipeg). The preceding crop at each location was a cereal grain [wheat or barley (Hordeum vulgare L.)]. Wherever necessary, preseeding applications of glyphosate [N-(phosphonomethyl) glycine] followed with hand weeding were used to maintain weed-free plots. Sunflower beetles [Zygogramma exclamationis (Fabricius)] were controlled with periodic spraying of systemic insecticides.
Experimental Observations
In general, space planted trials were used to assess the root system and water use patterns of cultivars representing each of the three height classes of sunflower cultivars, while the agronomy trials were used to assess the inter- and intra-height class variations in rooting depth and water use patterns. Weather conditions in all field trials were monitored between May to September with weather stations located less than 500 m away from the research plots. Daily values of minimum and maximum air temperatures were used to calculate growing degree-days, using a base temperature of 6.7°C (Kandel, 1995). In 1994, because of malfunctioning of the rain gauge at Winnipeg, rainfall data from the nearby Glenlea Research Station (15 km away from plots) were used.
The root architecture of sunflower cultivars was studied in a space planted trial located at Carman in 1994. The profile wall method (Bohm, 1979) was used to expose root systems of four plants each of Aurora, SW-103, and IS-6111 at 90 d after seeding (DAS). Two vertical trenches, 2.5-m deep and 0.15 m from the row, were dug using a backhoe. The face of the trench was trimmed with shovels and trowels to get a perfect vertical face (checked using a plumb line), 0.01 m away from the tap root. The final 0.01 m of soil was washed using water under pressure (0.25 MPa) (Entz et al., 1992). A handgun with a teejet nozzle was used to wash the root system. Care was taken to wash all root systems in the same way (i.e., using the same spatial washing pattern and total washing time). The exposed root system was traced on a clear plastic sheet and the root length density was determined by using the line intercept method (Tennant, 1975). In addition to the total root length density in the upper square meter, vertical and horizontal distribution of roots in 0.20-m increments was also determined. The top portion of root systems of all three height classes of sunflower were excavated using a spade at 100 DAS in the space planted trial at Winnipeg in 1994 and root diameter was measured in 0.05-m increments between 0.05- and 0.25-m depths.
Aluminum access tubes were installed in the center of each plot to a depth of 1.9 m to determine soil moisture at selected dates during the growing season by the neutron attenuation technique. In the space planted trials, the access tubes were 0.15 m from the nearest plant in the center of the plot. In the agronomy trials, the access tubes were placed between two plants in the central portion of the middle row in each plot. The tubes were installed 1 wk after seeding in the space planted trials, and at the two- to six-leaf stage in the agronomy trials. Soil moisture contents between 0.1 and 1.9 m were measured in 0.2-m increments using a field calibrated neutron probe (Model 4330, Troxler labs, Research Triangle Park, NC). Deep percolation, upward soil moisture flux, and runoff were assumed to be negligible.
Soil water depletion for any given date and depth was calculated by subtracting water content in each 0.2-m layer between 0.1 to 1.9 m from the initial water content at the same depth at the beginning of the season. The soil water depletion front was identified according to Entz et al. (1992), and was determined as deepest layer showing significant difference (P < 0.05) in water content between the initial and the observed date. The soil water depletion front velocity is an indirect estimation of root penetration rate. The observation date on which the roots penetrated a particular layer for the first time in field studies, expressed as growing degree days, was plotted against the rooting depth to determine the soil moisture depletion front velocity.
Statistical Analysis
Analysis of variance was conducted by using GLM procedure (SAS Institute, 1985) and the Fisher protected LSD test was used for mean comparisons. Unless otherwise mentioned, the 5% significance level was used for all statistical comparisons. Soil moisture data for all field trials were analyzed separately to assess the effect of cultivar and time on the water depletion at various depths. Regression slopes and intercept of linear relationship between rooting depth and accumulated growing degree days were tested for statistical significance using covariance technique of JMP statistics software (Sall and Lehman, 1996, p. 520).
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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90% of sunflower root length density was present in the top 0.40 m soil layer (Fig. 2A), which is very similar to observations of Sadras et al. (1989) for standard height sunflower cultivars in Argentina. The sharp decrease in root length density with depth was more gradual with IS-6111 than with SW-103. The horizontal distribution data (Fig. 2B) indicated that most of the roots of SW-103 were concentrated in the central 0.20 m along the tap root, while roots of IS-6111 were distributed to the next 0.20 m vertical layer on both sides of the tap root. Visual observation also indicated that root system of IS-6111 was distributed across a larger area compared with dwarf cultivars. These differences in root distribution are consistent with work by Schneiter (1992), who showed greater water depletion by a standard height hybrid from the inter-row space compared with a dwarf open pollinated cultivar. In sunflower, the tap root is the major channel for conducting water from deep in the soil to above ground foliage. The diameter of the tap root is an indicator of water transportation ability of the root system (O'Toole and Bland, 1987). At the 0.05-m depth under field conditions, the tap root diameter of all cultivars were similar (Fig. 3) . However, at lower soil depths the tap root of dwarf cultivars narrowed more rapidly than that of the standard height hybrid. Thus, IS-6111 appeared to have a bigger channel to transport water.
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Soil Water Depletion Front Velocity
The soil water depletion front increased in a linear fashion with growing degree day accumulation for all cultivars tested (Fig. 4)
. The depletion front velocity was higher in 1995 than in 1994 (data not presented). However, expressing the time in accumulated degree days (6.7°C base temperature) instead of calendar days (data not shown) narrowed differences between years significantly, indicating that thermal time more closely described depletion front than calendar time. Temperature has been shown to have a significant influence on the root development in crops (McMichael and Burke, 1996). Although, temperature effect on the root growth of diverse sunflower genotypes has been observed in the laboratory, field observations have been lacking (Seiler, 1998). Therefore, these results present the effect of soil temperature on sunflower root growth under field conditions.
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Relationships between depletion front velocity and thermal time presented in this study will be useful to crop modelers, as rooting depth, at least for the three cultivars tested here, was positively related to thermal time. Furthermore, the use of heat accumulation to correct seasonal differences in depletion front velocities may have implications for modeling root development in different environments. Some of the differences reported in the literature on depletion front velocities can be attributed to temperature differences. For example, Thomas et al., (1995) observed 30% reduction in depletion front velocities of chickpea (Cicer arietinum L.) and barley by winter seeded crops compared with spring seeded crops in Australia. The depletion front velocity of the standard height hybrid of 29.3 mm d-1 in 1995 (data converted to mm d-1 for comparison) was lower than 44 mm d-1 observed for sunflower by Dardanelli et al. (1997), but comparable with 36 mm d-1 reported by Meinke et al. (1993). Greater depletion front velocity for sunflower in Argentina compared with the present study may be related to a higher mean temperature [20 to 24°C in the Argentina study compared with the present study where the temperatures were frequently lower than 20°C (Fig. 1)] (Dardanelli, et al., 1997). This indicates that when factors like soil moisture, nutrients, and soil physical properties are relatively similar, temperature differences across agroclimatic conditions can be corrected by using heat unit accumulation. However, additional data from above-mentioned studies or from other similar studies should be used to establish the relationship between depletion front velocity and temperature.
Soil Water Depletion Front
The soil water depletion front has been used to assess the rooting depth in a number of crops including sunflower. A close relation between the rooting depth and the depletion front has been observed in wheat in western Canada (Entz et al., 1992). Therefore, rooting depths in field trials were assessed indirectly by measuring the depletion front.
Space Planted Trials
Among cultivars, IS-6111 had the deepest depletion front (1.0 m at Carman in 1994 to 1.8 m at Winnipeg in 1995), which was 0.2 to 0.6 m deeper than SW-103 (Fig. 5 and 6)
. The rooting depth of 1.80 m is similar to a rooting depth of 1.88 m reported for a standard height sunflower observed using minirhizotrons in Mandan, ND (Merrill et al., 1994). The depletion front of Aurora was similar to IS-6111 in all trials, except at Carman in 1995, where the soil water depletion front of IS-6111 was 0.2 m deeper than Aurora. Genotypic variation for depletion front was observed at the beginning of flowering (60 to 70 DAS) and it was maintained until maturity. All space planted sunflower cultivars reached their maximum rooting depth between 85 and 95 DAS in most environments.
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Agronomy Trials
Similar to space planted trials, SW-103 had a shallower depletion front than IS-6111 in agronomy trials, while contrary to the space planted trials, Aurora had shallower depletion front than IS-6111 (Fig. 7, 8)
. The standard height hybrids had 0.2 m (at Winnipeg in 1994) to 0.4 m (at Carman in 1995), and 0.2 m (at Winnipeg in 1994) to 0.8 m (at Carman in 1995) deeper depletion front than dwarf hybrid and dwarf open pollinated cultivars, respectively. Intermittent dry spells that prevailed during the 1995 season (Fig. 1) might be responsible for the deeper depletion front of standard height hybrids compared with dwarf sunflower cultivars. Although additional cultivars in each height class presented smaller interclass variations, overall ranking of height classes remained unchanged. Thus, the results of agronomy trials imply that 0.2 to 0.8 m of the soil profile remain unexplored by using dwarf cultivars vs. standard height sunflower cultivars.
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2 wk earlier compared with standard height hybrids in the present study (data not presented), rooting depth did not appear to be related to growth duration alone. For example, higher depletion front velocities for IS-6111 (tall and long growth duration cultivar) and Aurora (shorter statured, shorter growth duration cultivar) compared with SW-103 (shorter statured, shorter growth duration cultivar) indicated that neither the plant stature nor the growth duration regulated depletion front completely. Therefore, this study identified genetic variation for depletion front velocity and rooting depth in sunflower.
Soil Water Depletion
At all locations, the profile was wet in the beginning of the season, and a gradual extraction of soil moisture was observed with development of the crop. Rainfall during the growing season, especially at the later stages of plant development, saturated top layers and confounded water extraction observations from different depths and at different developmental stages. Therefore, soil moisture depletion, as determined by the difference in soil moisture between two observation dates, were used for comparison among cultivars.
Space Planted Trials
In general, the better developed root system of IS-6111 (deeper rooting depth and higher soil water depletion front velocity) was reflected in more water being depleted from the soil profile compared with the dwarf cultivars (Table 1). Separating water depletion into 0.1- to 1.1-m (rooting depths of most annual crops in the western Canada are 1.1 to 1.2 m) and 1.1- to 1.9-m intervals revealed cultivar differences in the 0.1- to 1.1-m layer (Table 1). The higher water depletion by IS-6111 was attributed not only to deeper rooting, but also to the higher depletion efficiency at each depth (Fig. 5 to 6). Water depletion by IS-6111 at Carman in 1995 was significantly higher than that of dwarf cultivars at each 0.2-m layer to a depth of 1.1 m (Fig. 5). A similar trend was observed at other locations, although the differences were not significant at all depths (Fig. 5 and 6). Although, such comparisons can be used to identify the genetic variations, care should be exercised while using absolute values because of differences in rainfall (Dardanelli et al., 1997; Meinke et al., 1993).
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Agronomy Trials
Similar to space planted trials, standard height cultivars depleted more soil water than dwarf cultivars (Table 2). Greater soil moisture depletion by standard height cultivars was due to more efficient water depletion, as well as deeper rooting than shorter statured cultivars. More efficient water depletion by standard height cultivars was evident from significantly higher water depletion by standard height cultivars up to the 1-m depth at Winnipeg (Fig. 7) and up to the 1.40-m depth at Carman (Fig. 8), compared with dwarf hybrids and/or dwarf open pollinated cultivars. Some of these differences were evident at 65 DAS, by which time all cultivars initiated flowering (Fig. 8).
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Although the cultivars were good representatives of their height class, some variations within height classes were observed (Table 2). For example, among dwarf cultivars, SW-101 extracted more water than SW-103 at Carman in 1995, and among standard height hybrids, SF-187 depleted more soil water than IS-6111 at Winnipeg in 1994.
The primary reason for adopting sunflower in drought-prone areas of the world is its ability to maintain water supply during dry periods with the help of its deep and explorative root system, provided soil moisture is available. This ability was documented by Cabelguenne and Debaeke (1998), who observed 70 to 100% exhaustion of available soil water by sunflower in 10 out of 13 yr. Similarly, Rachidi et al. (1993) reported better water extraction by sunflower compared with sorghum. Although no extremely dry years were encountered in the present field studies, rainfall varied, and 1995 was the drier season. In 1995, soil moisture depletion by SF-187 was 134 mm more than that of Aurora at Carman (Table 2). The higher soil hydraulic conductivity of clay loam soil (Jackson et al, 2000) at Carman in 1995 might have contributed to greater water extraction compared with that at Winnipeg in 1994. This indicates that when rainfall failed to supply the water needed, the standard height hybrids managed to obtain that water from the soil profile. The consumptive water use in this study ranged from 272 mm for Aurora at Carman in 1995 to 476 mm for SF-187 at Winnipeg in 1994 (Angadi and Entz, 2002a). Thus, the additional soil moisture depleted by SF-187 accounted for 28% of consumptive water use of the highest water using cultivar SF-187 and 49% of the lowest water using cultivar Aurora. The extra water depleted by tall cultivars was from both 0.1- to 1.1-m, as well as 1.1- to 1.9-m soil layers (Table 2). Although the soil moisture depletion values reported here are <200 to 300 mm depletion reported for sunflower in the literature (Bremner et al., 1986; Zaffaroni and Schneiter, 1989), >100 mm of extra water extracted by the standard height sunflower types will have a significant role in stabilizing yields during dry years.
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SUMMARY AND CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Soil moisture depletion of the standard height hybrid, especially in a year of longer dry cycles similar to 1995, was significantly higher (up to 134 mm) than for any of the dwarf cultivars. The extra water depleted by the standard height cultivar (to a depth of 1.9 m) was due to more efficient water depletion and a greater rooting depth than dwarf cultivars. Longer growth duration for the standard height hybrid may also have contributed to greater water use compared with short statured cultivars.
On the basis of observations in this study, it appears that standard height and dwarf cultivars will have different niches within the cropping system. For example, a dwarf sunflower is suitable where lower dependence on soil moisture is compensated by regular rainfall (or irrigation) or where complete exhaustion of the soil profile is not desirable. In contrast, a standard height sunflower is better adapted to where sunflower is grown on fully recharged soil profile and the crop is expected to rely on soil moisture for a considerable portion of evapotranspiration (long intermittent stress or terminal stress periods) or after shallow rooted crops such as field pea or lentil which do not utilize subsoil water. In addition, because of its ability to extract subsoil water, a standard height sunflower can be used to extract deeply leached nitrate, thereby providing important environmental benefits.
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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