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a USDA-ARS, Land Manage. and Water Conserv. Res. Unit, Pullman, WA 99164
b USDA-ARS, North Central Soil Conserv. Res. Lab., Morris, MN 56267
* Corresponding author (sharratt{at}wsu.edu)
Received for publication December 11, 2003.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: MCFA, medium-chain fatty acids • WUE, water use efficiency
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Cuphea spp. (family Lythraceae) produce seed enriched in MCFA (Miller et al., 1964; Graham, 1989). Cuphea spp. are native to North, Central, and South America and until recently remained wild (Knapp, 1993). Several of the 260 species of Cuphea identified to date are found in temperate climate regions (Graham, 1989) and exhibit favorable agronomic characteristics (Hirsinger, 1985) that could result in a domestic and renewable source of MCFA.
Cuphea spp. largely thrive in tropical climates, but many species are also well adapted to regions that are moist and temperate. Indeed, the small taproot characteristic of these species likely limits their adaptation to wetter environments and is perhaps one morphological trait that induces wilting in the absence of water (Graham, 1989). A few species of Cuphea have been found in more arid environments; thick leaves or large taproots allow these species to cope with drought stress (Graham, 1989). Other morphological traits such as seed shattering and seed dormancy are wild-type traits that have hindered commercial production of Cuphea spp. However, germplasm lines of Cuphea that are nonshattering, nondormant, and self compatible have been developed over the past decade (Knapp, 1993).
In one of only two known field experiments to identify the best agricultural management practices for Cuphea spp. production, Gesch et al. (2002)(2003) assessed the impact of sowing date and row spacing on growth and seed yield of Cuphea spp. in the northern Corn Belt of the United States. Although plant population confounded the results of their study, Cuphea spp. produced more seed when sown in May rather than in April or June. Poor emergence resulted in low yield of Cuphea spp. sown in mid-April while enhanced interplant competition for available light, water, and nutrients contributed to low yield of Cuphea spp. sown in June. The researchers also found that Cuphea spp. sown in wide rows compensated for seed yield by producing more branches and seedpods than that sown in narrow rows. The other known experiment examined the impact of depth and rate of sowing on seedling emergence and seed yield of Cuphea spp. in Iowa (Roath, 1998). Emergence, and thus seed yield, was favored by sowing at a higher rate and at a shallower depth. Seed yield varied considerably across years; a crop failure resulted in no seed production in 2 of the 7 yr of the study. For the other 5 yr in which Cuphea spp. produced seed, yield (averaged across all treatments) varied from 52 to 705 kg ha–1. No explanation was given for the interannual variation in seed yield. Roath (1998) did, however, suggest that intense rainfall events after sowing and seed shatter late in the season can cause poor seed yield.
Although Cuphea spp. have been grown successfully in the Corn Belt, availability of soil water appears to be one environmental factor that may limit growth and seed production of Cuphea spp. (Arndt, 1985; Graham, 1989). Little information, however, exists that describes rooting characteristics and associated water use of Cuphea spp. Therefore, the objective of this study was to evaluate the influence of sowing date and row spacing on soil water use and rooting characteristics of Cuphea spp.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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0.5% slope in 2002. The field site was previously in Cuphea spp. and soybean before establishment in 2001 and 2002, respectively. One day before sowing, 110 kg N ha–1, 12 kg P ha–1, and 30 kg K ha–1 were broadcast and incorporated across the site. Cuphea spp. were sown on 4, 18, and 30 May 2001 and 30 April, 14 May, and 28 May 2002. These dates represent an early, normal, and late sowing date for crops typically grown in the region. Seed was sown at a depth of 13 mm and at an interrow spacing of 200, 400, and 600 mm using a grain drill. Seed was sown at 0.85 g seed m–1 row, or an equivalent rate of 42, 21, and 14 kg ha–1 for the respective 200-, 400-, and 600-mm row-spacing treatments. The soil surface was compacted with a roller packer after sowing to ensure good contact between the seed and soil. Main treatments (sowing date) were established in 6- by 18-m plots and split to accommodate secondary treatments (row spacing). Individual plots were 6 by 6 m. Three replications were included in the experimental design. At the time of emergence (i.e., no change in plant population according to observations made every other day), each plot was thinned by hand to establish a population of 400000 plants ha–1, or an equivalent intrarow plant spacing of about 120, 60, and 40 mm for the respective 200-, 400-, and 600-mm row-spacing treatments.
Instrumentation to measure soil water content and soil water potential was installed in each plot immediately after sowing. Soil water content was assessed weekly in each plot by neutron attenuation and at the beginning and end of the season by gravimetric sampling. Neutron access tubes were installed both within and between seed rows. Soil water content was determined at 0.3-m depth increments to a depth of 0.9 m. Soil matric potential was measured using tensiometers placed at a depth of 1.0 and 1.25 m in one replication of each sowing date treatment and all plots with 400-mm interrows (total of 15 plots). These measurements, made weekly, aided in determining the direction and magnitude of water flow below the root zone.
Water use (WU) was determined by:
 | [1] |
SW is change in soil water content, RO is water loss due to lateral surface water flow (i.e., runoff), and WFBR is water flow below the root zone. Runoff was assumed negligible due to few intense rainfall events (three events in 2001 and four events in 2002 exceeded 30 mm d–1), no visual rills or washing of debris at the soil surface immediately following these rainfall events, and nearly level topography. Precipitation, soil water content, and water flow below the root zone were measured from emergence to harvest. Precipitation was measured daily at a nearby microclimate station (100 m from the experimental plots). Water flow below the root zone was determined according to:
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h is the difference in hydraulic potential (cm) over the depth interval z (cm). Hydraulic conductivity was assumed to vary with soil water matric potential according to Campbell (1985):
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e is the air entry matric potential (cm) equivalent to the intercept of the natural log of the water retention curve, is the matric potential (cm), and b is the slope of the natural log of the water retention curve. Saturated hydraulic conductivity was determined using the constant head method (Klute and Dirksen, 1986) on soil cores samples extracted from a depth of 1.00 to 1.05 and 1.20 to 1.25 m in all plots sown in mid-May. These same samples were used to determine water retention characteristics using a pressure plate apparatus (Klute, 1986). Root length density was assessed on 26 July 2001 and 5 Aug. 2002 for the early sowing, 1 Aug. 2001 and 9 Aug. 2002 for the mid-May sowing, and 15 Aug. 2001 and 16 Aug. 2002 for the late-May sowing. These dates correspond to the onset of seed filling (seeds visible in the oldest pods). Soil core samples (32-mm diam.) were extracted to a depth of 0.6 m. Excavations made in 2000 (the year before the initiation of this study) indicated that Cuphea spp. had a small taproot with few roots penetrating below 0.4 m. Soil core samples were collected within and between rows at four locations in each plot. Samples were sectioned to ascertain root length density at depths of 0 to 100, 100 to 200, 200 to 300, 300 to 400, and 400 to 600 mm. The within-row and between-row samples collected in each plot were consolidated into one within-row and one between-row sample. Root length density was determined by the line intersect method (Bohn, 1979). This method required soaking the samples in softened water and extracting the root material by gently washing the material through nested sieves (nominal openings of 1.0 and 0.5 mm). Root and other organic material retained by the sieves were placed in a glass dish filled with water. A grid was placed beneath the dish, and root length was then determined by counting the number of roots that intersected each grid line.
Cuphea spp. were harvested on 9 Oct. 2001 and 8 Oct. 2002, which was respectively 3 d after and 1 d before the first killing frost (<–2°C) in autumn. Samples (1 m–2) were clipped at the soil surface, bagged, dried at 60°C until constant weight, and threshed by machine. Seed separated by threshing was dried at 60°C, cleaned using a seed cleaner, and weighed. Final plant populations were obtained from the area harvested within each plot.
The experimental data were analyzed using a split-plot design in analysis of variance. Least significant difference (LSD) was used to separate treatment effects when significant F values (P 0.05) were determined in the analysis of variance.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Aboveground biomass diminished as sowing date was delayed in the spring (Table 1). Biomass production, however, was not influenced by interrow spacing as the probability of a type I error (probability of wrongfully rejecting the equality of treatment means) was 0.65 in 2001 and 0.24 in 2002. This lack of response in production to row spacing was also observed by Gesch et al. (2003), who reported that Cuphea spp. grown in wider rows compensated for yield by producing more branches. Cuphea spp. aboveground biomass declined from 8055 kg ha–1 for the early-May sowing and 7068 kg ha–1 for the mid-May sowing to 5213 kg ha–1 for the late-May sowing in 2001 (LSD = 1384 kg ha–1). In 2002, biomass production ranged from 7816 kg ha–1 for the late-April sowing and 7879 kg ha–1 for the mid-May sowing to 6777 kg ha–1 for the late-May sowing (LSD = 905 kg ha–1). No differences were apparent in biomass production when sown between late April and mid-May in both years. Like aboveground biomass, seed yield was also greater for Cuphea spp. sown in late April or early May than in late May in both years. In 2001, seed yield was 647 kg ha–1 for the early-May sowing, 539 kg ha–1 for the mid-May sowing, and 424 kg ha–1 for the late-May sowing (LSD = 118 kg ha–1). Seed yield in 2002 ranged from about 710 kg ha–1 for the late-April and mid-May sowings to 542 kg ha–1 for the late-May sowing (LSD = 72 kg ha–1). Interrow spacing did not influence seed production of Cuphea spp. as the probability of a type I error was 0.79 in 2001 and 0.38 in 2002. Harvest index, or the ratio of seed yield to aboveground biomass, did not vary with sowing date or row spacing in this study (probability of a type I error was 0.60 for sowing date and 0.37 for row spacing in 2001 and was 0.10 for sowing date and 0.22 for row spacing in 2002). The harvest index ranged from 0.080 in 2001 to 0.087 in 2002. These indices are similar to those derived from data presented by Gesch et al. (2002); those derivations indicate a harvest index that ranged from 0.056 to 0.081 for Cuphea spp. grown in Minnesota.
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Water Use and Water Use Efficiency
Crop water use was assessed in this study by examining water flow dynamics below the root zone (Eq. [1]). Soil water flow below the root zone depended on saturated hydraulic conductivity and water retention characteristics (Eq. [2] and [3]), both of which were found to be similar at the 1.00- to 1.05- and 1.20- to 1.25-m depths. For example, saturated hydraulic conductivity was 1.9 x 10–4 cm s–1 (SE = 1.2 x 10–4) at the 1.00- to 1.05-m depth and 3.7 x 10–4 cm s–1 (SE = 0.7 x 10–4) at the 1.20- to 1.25-m depth. In addition, the slope of the water retention curve (natural log of the relationship between soil matric potential and volumetric water content) varied from 10.05 (SE = 0.59) for the 1.00- to 1.05-m depth to 11.79 (SE = 0.98) for the 1.20- to 1.25-m depth while the intercept of the water retention curve (i.e., air entry matric potential) varied from –1.5 cm or –0.15 kPa (SE = 0.05 kPa) at the 1.00- to 1.05-m depth to –1.6 cm or –0.16 kPa (SE = 0.07 kPa) at the 1.20- to 1.25-m depth. Therefore, water flow was determined using values of saturated hydraulic conductivity and water retention characteristics averaged across the two depth intervals.
Water flow below the root zone did not vary across treatments in 2001 and 2002. For sowing date treatments, net water flow in 2001 was downward at a rate of 0.12 mm d–1 (SE = 0.18) for the early-May sowing and 0.44 mm d–1 (SE = 0.13) for the late-May sowing while water flow was upward at a rate of 0.42 mm d–1 (SE = 0.26) for the mid-May sowing. In 2002, water flow was downward at a rate of 0.30 mm d–1 (SE = 0.14) for the early-May sowing and upward at a rate of 0.27 mm d–1 (SE = 0.47) for the mid-May sowing and 0.15 mm d–1 (SE = 0.37) for the late-May sowing. For row-spacing treatments, net water flow in 2001 was downward at a rate of 0.17 mm d–1 (SE = 0.38) for the 200-mm rows and 0.09 mm d–1 (SE = 0.18) for the 400-mm rows while water flow was upward at a rate of 0.22 mm d–1 (SE = 0.35) for the 600-mm rows. In 2002, water flow was upward at a rate of 0.48 mm d–1 (SE = 0.44) for the 200-mm rows and downward at a rate of 0.05 mm d–1 (SE = 0.20) for the 400-mm rows and 0.54 mm d–1 (SE = 0.67) for the 600-mm rows. Averaged across all treatments, net water flow below the root zone was downward at a rate of 0.047 mm d–1 (equivalent to 6 mm or 2% of total water use) during the 2001 season and upward at a rate of 0.041 mm d–1 (equivalent to 14 mm or 4% of total water use) during the 2002 season. Since water flow did not differ among treatments and water flow contributed little to seasonal water use, water flow below the root zone was excluded from further analysis in comparing seasonal crop water use among sowing date and row-spacing treatments.
Crop water use from emergence to harvest, and averaged over all sowing date and row-spacing treatments, varied from 328 mm in 2001 to 365 mm in 2002. Soil water extraction accounted for 7% of water use (or 24 mm) in 2001 and 3% of water use (or 11 mm) in 2002. Thus, Cuphea spp. extracted more soil water in years with less rainfall to meet the evaporative demand. Warmer atmospheric conditions did not accentuate water use in 2002 because mean seasonal air temperatures were nearly the same each year (19.5°C in 2001 and 19.6°C in 2002). Water use was likely greater in 2002 due to enhanced biomass production that was caused by more favorable precipitation in 2002 than in 2001. Precipitation from emergence to harvest was 304 mm in 2001 and 354 mm in 2002.
Crop water use was influenced by sowing date each year (Table 2). Water use, however, was not influenced by interrow spacing (probability of a type I error was 0.11 in 2001 and 0.08 in 2002). Water use declined as sowing date was delayed in spring; in 2001, Cuphea spp. sown in early May consumed 355 mm of water, that sown in mid-May used 340 mm of water, and that sown in late May used 289 mm of water (LSD = 6 mm). In 2002, water use varied from 378 mm for the late-April sowing and 383 mm for the mid-May sowing to 333 mm for the late-May sowing (LSD = 4 mm). Although water use declined with sowing date, more water was extracted from the soil profile during the growing season by Cuphea spp. that were sown later in the spring. For example, in 2001, Cuphea spp. sown in early May extracted 20 mm of soil water while that sown in mid-May and late May extracted 26 mm of soil water. In 2002, Cuphea spp. sown in late April extracted 7 mm of soil water while that sown in mid-May and late May extracted 12 and 14 mm of soil water, respectively.
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The low WUE of Cuphea spp. is also likely influenced by the partitioning of dry matter between vegetative and reproductive development. In the present study, mean harvest index was 0.080 and 0.087 in 2001 and 2002, respectively. These values are considerably lower than those reported for other oilseed crops. For instance, the harvest index is around 0.50 for soybean (Ball et al., 2000), about 0.30 for canola (Hocking et al., 1997), and about 0.35 for sunflower (Zaffaroni and Schneiter, 1991). In addition, harvest indices for flax and mustard, which are about 0.20 to 0.25 (Hocking et al., 1997), are more than twofold greater than that of Cuphea spp. Cuphea spp. growth rate is slow during vegetative development but dramatically increases upon entering the reproductive phase (Gesch et al., 2001), which for field-grown plants in Minnesota, occurs around mid- to late July (Gesch et al., 2002). However, partly due to its indeterminacy, even during reproductive phase, Cuphea spp. divert much of their growth into vegetative dry matter accumulation. Hence, less dry matter is diverted into the seed, thereby reducing WUE.
Water use efficiency was influenced by sowing date, but only in 2002. Water use efficiency, however, was not influenced by row spacing either year (probability of a type I error was 0.32 in 2001 and 0.58 in 2002). In 2002, WUE declined as sowing date was delayed in the spring. Water use efficiency was 1.87 and 1.84 kg ha–1 mm–1 for the late-April and mid-May sowings, respectively, and 1.64 kg ha–1 mm–1 for the late-May sowing (LSD = 0.20 kg ha–1 mm–1).
Root Length Density
Root length density was greater in 2001 than in 2002 (Fig. 1) despite the 10% higher biomass production and 22% higher seed yield in 2002 (Table 1). Root length density in the upper 0.2 m of the soil profile, and averaged across all treatments, was 23 km m–3 in 2001 and 21 km m–3 in 2002. Although the 2001 growing season was drier than the 2002 season, our results are similar to those of Hoogenboom et al. (1987) and Merrill et al. (2002), who found that root growth was stimulated by water stress or in drier years. Root length density of Cuphea spp. appears similar to soybean but smaller than other oilseed crops such as canola. Merrill et al. (2002), for example, found that the maximum root length density of oilseed crops in the northern Great Plains of the USA was about 45 km m–3 for crambe (Crambe abyssinica Hochst. ex R.E. Fr.), 35 km m–3 for canola and safflower (Carthamus tinctorius L.), 30 km m–3 for soybean, and 25 km m–3 for sunflower. They also reported considerable interannual variation in root length density that was attributed in part to differences in precipitation across years. Nickel et al. (1995) also reported a maximum root length density of about 30 km m–3 for soybean grown in Minnesota. Root length density, however, is not a constant and does vary as a result of plant genetic characteristics, plant development, soil physical and chemical properties, soil microbial community structure, foliar and root diseases, etc. For example, in contrast to the above root length densities, Johnston et al. (1998) reported a maximum root length density of about 10 km m–3 for soybean and about 100 km m–3 for canola in the upper 0.2 m of the soil profile.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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