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Published in Crop Sci. 44:2187-2192 (2004).
© 2004 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA

FORAGE & GRAZING LANDS

Russian Wildrye Seedlings Are Sensitive to Acidic Soil

A. A. Hopkinsa,*, D. P. Malinowskic, H. Zhangd and D. W. Walkerb

a Forage Improvement Division, Samuel Roberts Noble Foundation, Inc., Ardmore, OK 73401
b Administrative Division, Samuel Roberts Noble Foundation, Inc., Ardmore, OK 73401
c Texas A&M Agricultural Research and Extension Center, Vernon, TX 76384
d Dep. of Plant and Soil Sciences, Oklahoma State Univ., Stillwater, OK 74078

* Corresponding author (aahopkins{at}noble.org)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Russian wildrye [Psathyrostachys juncea Nevski] is potentially useful as a cool-season forage grass for the southern Great Plains. Our objective was to determine the effect of acidic and limed soils on seedling growth of tetraploid and diploid Russian wildrye germplasm. Seedlings of Mandan R1983 and Tetra-1 (tetraploids), as well as ‘Bozoisky Select’ and ‘Mankota’ (diploids), were grown in limed and nonlimed acidic sand, sandy loam, and silt loam soils in the greenhouse at Ardmore, OK. Seedlings were harvested following approximately 60 d of growth in soil. Root-bound Al was extracted with a solution of 0.5 M citric acid. Regardless of soil type, seedling shoots grew significantly (P < 0.05) larger because of lime application, averaging 13.1 cm taller and 100% heavier for limed versus nonlimed acidic soils. Tiller number, root length, and root weight also increased with lime application, though not as consistently. Root-bound Al did not differ between tetraploids and diploids. In most cases, tetraploids and diploids had an equally favorable response to liming, indicating that Russian wildrye seedlings are susceptible to soil acidity regardless of ploidy level. Because of consistency of response and ease of measurement, seedling shoot weight should be useful for screening Russian wildrye germplasm for possible tolerance to soil acidity. Russian wildrye cultivars with improved seedling tolerance to acidic soils, which occur extensively in the southern Great Plains, might well be needed.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
FORAGE NEEDS during the fall to spring months in the southern Great Plains are primarily met by feeding hay or grazing cool-season annual grasses. Use of cool-season perennial forages in the region could result in substantial economic benefits through decreased production costs, and environmental improvements through reduced soil erosion. However, currently available cultivars of cool-season perennial grasses are generally not well adapted to the region, perhaps because of insufficient tolerance to heat and drought stress (Malinowski et al., 2003). Russian wildrye is noted for substantial drought tolerance (Asay and Jensen, 1996) and as a result may have potential for use as a cool-season perennial forage in the southern Plains. A preliminary germplasm screening in Texas identified a number of accessions with promising levels of grazing tolerance and persistence (Hopkins and West, 2002).

Stand establishment of Russian wildrye can be difficult and limits use of this species. Cultivars with improved establishment ability, such as Bozoisky Select (Asay et al., 1985) and Mankota (Berdahl et al., 1992), have been developed. Greater establishment ability has been observed for tetraploid (2n = 4x = 28) versus diploid (2n = 2x = 14) germplasm (Asay et al., 1996; Berdahl and Ries, 1997), including the tetraploid cultivar Tetracan (Lawrence et al., 1994). Tetraploids have been reported to have larger seeds and leaves than diploids (Berdahl and Ries., 1997; Asay et al., 1996; Jefferson, 1993), longer coleoptiles at cool temperatures (7–16°C, Berdahl and Ries., 1997; Berdahl and Ries, 2002) and longer roots (Jefferson, 1993), all of which could contribute to improved establishment. Tetraploids also appear to have greater water use efficiency compared with diploids, on the basis of both direct measurement under drought stress (Frank and Berdahl, 1999) and indirect measurement such as carbon isotope discrimination (Asay et al., 1996).

Soils in the southern Great Plains generally fall within a pH range of 6 to 8. However, strongly acidic soils exist in many areas, particularly where small grains have been grown continuously for several years, and/or where lime has not been applied regularly (Westerman, 1987; Zhang et al., 1998). In these cases, acidity is generally confined to the upper 30 cm of soil. Aluminum toxicity can be a problem in some of these acidic soils, particularly at the seedling stage.

Despite significant education efforts, farmers in Oklahoma have been reluctant to apply lime to fully correct soil acidity. Surveys of soil samples submitted to the Soil, Water, and Forage Analysis Lab at Oklahoma State University in the mid-1980s to mid-1990s (Zhang et al., 1998) and continuing to the present (Zhang, unpublished data, 2004) indicate that the percentage of samples with pH of 5.5 or less has remained constant or increased in a number of counties. High cost and inadequate availability of lime in some areas, as well as short-term land lease arrangements, contribute to the limited application of lime.

Given these circumstances, tolerance of Russian wildrye seedlings to acidic soils in the southern Plains may be a key trait. If current germplasm is not sufficiently tolerant, then selection and breeding for improved tolerance to acidic soil may be justified. Breeding for acidic soil–Al tolerance has been successful in various plant species, including maize (Zea mays L., Granados et al., 1995), sorghum [Sorghum bicolor (L.) Moench, Duncan et al., 1992], tall fescue (Festuca arundinacea Schreb., Foy and Murray, 1998), wheat (Triticum aestivum L., Smith et al., 1997), and white clover (Trifolium repens L., Voigt and Staley, 2004). Development of Russian wildrye cultivars with improved tolerance to acidic soil will require reliable phenotyping procedures, and sufficient heritability in populations with useful levels of tolerance.

Enhanced exudation of organic acids from roots has been linked to increased Al tolerance in several plant species (Miyasaka et al., 1991; Delhaize et al., 1993; Pellet et al., 1995). Aluminum is sequestered from root tips (Tang et al., 2002) through the chelating action of organic acids such as malate. A solution of citric acid can be used to remove this chelated or "root-bound" Al (Malinowski and Belesky, 1999), with the caveat that a portion of the Al remains in root mucilage (Archambault et al., 1996). Those plants with greater amounts of root-bound Al may have an enhanced ability to sequester, and thus be more tolerant of, Al. Accordingly, determination of root-bound Al may shed light on possible tolerance mechanisms should differences occur in susceptibility of Russian wildrye germplasm to acidic soil–Al toxicity.

The degree of susceptibility of Russian wildrye seedlings to acidic soil–Al toxicity is not known. Mortality of Bozoisky Select seedings was observed in previous field trials near Ardmore, OK. Subsequent analyses showed these soils to be moderately acidic, with pH levels less than 6 (Hopkins, unpublished data, 2001). These observations sparked our interest in determining if soil acidity could be a factor in poor stand establishment of Russian wildrye. In addition, further information is needed regarding the relative response of tetraploid versus diploid Russian wildrye populations to acidic soils. The objective of this research was to determine the effect of acidic and limed soils on seedling growth of tetraploid and diploid Russian wildrye germplasm. Our rationale was that if Russian wildrye seedlings proved susceptible then breeding for improved tolerance to acidic soil might be justified.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 REFERENCES
 
Two runs of this experiment were conducted in the Noble Foundation Research Greenhouse Facility at Ardmore, OK. Except where noted, the same procedures were used for both runs. For Run 1, supplemental light from high-pressure sodium lamps was provided as needed to maintain a minimum light level of 35 µmol m–2 s–1, for a total photoperiod of 24 h from 27 Jan. to 8 Feb. and 16 h from 9 Feb. to 28 Mar. 2003. In this research, all light level measurements were in the photosynthetically active range. Maximum light level, ranging from 123 to 1210 µmol m–2 s–1, exceeded 600 µmol m–2 s–1 most days. Lighting scheme for Run 2 largely duplicated that of Run 1, with 16 h daylengths from 9 Feb. to 9 Apr. 2003. Maximum daily light levels exceeded 750 µmol m–2 s–1 on most days and ranged from 123 to 1261 µmol m–2 s–1. Temperature and relative humidity levels were measured hourly at the bench top level with a HOBO data logger (Onset Computer Corp., Bourne, MA). Temperature ranged from 13 to 42°C and 13 to 46°C for Runs 1 and 2, respectively. Relative humidity averaged approximately 25% for both runs, and rarely fell outside the range of 22 to 40%.

The three soils used in this experiment included a Konsil loamy fine sand (fine-loamy, siliceous, thermic Ultic Paleustalfs), a Wilson silt loam (fine, montmorillonitic, thermic Vertic Ochraqualfs), and a Teller fine sandy loam (fine-loamy, mixed, active, thermic, Udic Argiustolls). The Konsil and Wilson soils were collected from two locations near Ardmore, in south central Oklahoma, whereas the Teller soil was collected near Perkins, in north central Oklahoma. Following addition of a sufficient amount of finely ground hydrated lime (Texas Lime Co., Cleburne, TX) to bring soil pH to near neutral (Table 1), a portion of each limed soil was moistened with tap water and incubated at 32°C, for approximately 17 d. Soil exchangeable Al was extracted by 1.0 M KCl and quantified by a Spectro CirOs ICP (inductively coupled plasma) spectrometer at the Oklahoma State University Soil, Water, and Forage Analysis Lab. All other soil analyses were performed by Ward Laboratories, Inc. (Kearney, NE) using standard soil test procedures. Medium (5 x 18 cm) Conetainers (Stuewe and Sons, Inc., Corvallis, OR) were filled to near full volume with a given soil, totaling 300 g dry weight per Conetainer for the Konsil soil and 275 g for the Wilson and Teller soils, and brought to field capacity with a nutrient solution containing N, P, and K according to soil test recommendations (Table 1). Field capacity was determined as the weight-to-weight ratio of water needed to bring a given oven dried soil type to saturation. Soils were maintained at approximately 75% of field capacity by periodically weighing Conetainers during Runs 1 and 2 and adding deionized water as needed. Seeds were placed in Petri dishes on moistened blotter paper, put into a germinator equipped with a clear front to allow for ambient room lighting and kept at room temperature, which in this report was about 22°C. Two of the resulting 7-d-old seedlings were transplanted to each Conetainer. A limited number of the young seedlings died because of transplant failure, so data were subsequently expressed on a per plant basis. Entries consisted of the cultivars Bozoisky Select and Mankota (diploids), and the populations Tetra-1 (Jensen et al., 1998) as well as Mandan R1983 (tetraploids). A split-plot design consisting of eight replications was used with liming treatment (limed or acidic) and soil type (Konsil, Teller, or Wilson) as main plots, and entries as sub-plots (Fig. 1) . The experimental unit was an individual Container containing one or two seedlings. Seedling transplant and harvest dates in 2003 were 27 January and 28 March for Run 1, and 5 February and 10 April for Run 2, respectively.


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  		Table 1. Amendments added to soils used to evaluate susceptibility of Russian wildrye to acidic soil.

 


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  		Fig. 1. Arrangement of liming and soil type treatments applied to seedlings of four Russian wildrye entries. Gray cells indicate an individual row of a given acidic soil; white cells indicate a row of a given limed soil. One replication is shown.

 
Seedlings were harvested following 60 (Run 1) or 64 (Run 2) d of growth in soil. Roots were washed free of soil with tap water, placed in 50 mL of a 0.5 M solution of citric acid for 30 min at room temperature to extract root-bound Al (Ohman, 1988), and rinsed a second time with tap water. Roots and shoots were measured, separated at the crown, dried for approximately 3 d in an oven at 50°C, and weighed. At the conclusion of a run, soil samples were composited across replications and entries for each lime x soil combination for subsequent analyses.

After collection, the root extract solution was filtered through a Whatman #1 filter paper and frozen until later analysis. Soluble Al concentration in root extracts (i.e., root-bound Al) was analyzed with a Spectro CirOs ICP spectrometer at a wavelength of 167.08 nm. Where available, root extracts of all entries grown in the Teller and Konsil soils were analyzed from Replications 5, 7, and 8 from Run 1 and Replications 2, 5, and 6 from Run 2, for a total of 91 samples.

Data were analyzed across runs by a split-plot analysis. A mixed model was used, with replications considered random, and soil type, lime treatment, and entry considered fixed effects. Data were subsequently analyzed by soil due to lime x soil interactions. Similarly, ploidy level effects were analyzed, by soil type, by pooling data across entries within a ploidy level. For such analyses, ploidy level was considered a fixed effect. Data were analyzed by the PROC MIXED procedure of SAS (SAS Institute, 1999). Error terms and denominator degrees of freedom were specified by the DDFM = SATTERTH option in the model statement of PROC MIXED, and specific pairs of treatment differences were compared by the LSMEANS PDIFF option. Correlations of root-bound Al with shoot length, root length, shoot weight, and root weight were calculated on a lime x soil basis by the PROC CORR procedure of SAS. Averages in this report were calculated as generalized least square means. Except as noted in tables, significance was declared where P ≤ 0.05.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
REFERENCES
 
Before adding lime, the soils used for this research exhibited a range in chemical properties (Table 2). The Wilson soil could be considered moderately acidic and also had greater Ca and organic matter levels than the other soils, whereas the Teller soil had the greatest level of exchangeable Al. As expected, lime application corrected soil acidity, increased Ca levels, and in almost all cases decreased exchangeable Al.


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  		Table 2. Properties of soils used to evaluate susceptibility of Russian wildrye to acidic soil. Data for initial properties of acidic soils reflect nonamended soil; all other soils were amended as described in the text.

 
Analyses of seedling growth parameters produced similar results when entry or ploidy data were used. It should be noted that significant entry x lime interactions occurred for shoot length on the Wilson and Teller soils, and for root weight on the Teller soil (data not shown). However, these interactions were small compared with liming effects.

Liming led to increased seedling growth as measured by several traits (Table 3). Even modest adjustments in pH, as indicated by the Wilson soil, led to increased seedling growth. Positive responses to liming occurred regardless of soil type except for root length in the Konsil soil as well as tiller number and root weight in the Wilson soil. Differences were most pronounced for the Teller soil, where liming led to greater growth (with percentage increases in parenthesis) as measured by tiller number (98), shoot length (271), root length (613), shoot weight (1500), and root weight (500) (Table 3). After accounting for a limited amount of transplant failure, we did not observe seedling mortality associated with acidic soils. However, many of the seedlings grown in the acidic Teller soil grew very poorly, and almost certainly would not have survived over time. Soil acidity perhaps contributed to mortality of Russian wildrye seedlings that we observed in previous field plantings in the Wilson and Konsil soils (Hopkins, 2001, unpublished data).


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  		Table 3. Generalized least-squares means, ± standard errors, for tiller number per plant (TN), shoot length (SL), root length (RL), shoot weight (SW), and root weight (RW), of diploid and tetraploid Russian wildrye populations grown in limed (L) or acidic (A) Wilson silt loam, Teller fine sandy loam, and Konsil loamy fine sand soils in the greenhouse at Ardmore, OK. Data are averaged across two runs.

 
Shoot weight may prove useful in screening and selecting Russian wildrye seedlings for increased tolerance to soil acidity. Seedling shoot weight and length increased with liming regardless of soil type (Table 3). In addition, shoot weight was the only trait with a nonsignificant lime x soil interaction when data were analyzed across soil types, whether on the basis of ploidy or entry data (data not shown). Further consistency in magnitude as well as direction of response to liming for shoot weight is illustrated by the lack of two and three way interactions when ploidy data were analyzed across soil types (data not shown). Shoot weight can be easily measured by clipping individual seedlings at 1 cm of height, which should allow rescue and recovery of favorable genotypes following data collection. An ample amount of phenotypic variation for shoot weight existed within Russian wildrye populations in this research when seedlings were grown in acidic soil (Table 4), although it is not known if heritability of this trait is sufficiently large to permit effective selection.


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  		Table 4. Generalized least-squares means and ranges for shoot weight of diploid (2x) and tetraploid (4x) Russian wildrye seedlings grown in acidic Wilson silt loam (WA), Teller fine sandy loam (TA), or Konsil loamy fine sand (KA) soils in the greenhouse at Ardmore, OK. Data are averaged across two runs.

 
Tetraploids performed similarly to diploids for most traits examined. Compared with diploids, tetraploid seedlings had longer shoots when grown in the Wilson and Teller soils, and greater shoot as well as root weights on the Teller soil (data not shown). A significant lime x ploidy interaction occurred for shoot length and root weight on the Teller soil. This was due to an increased advantage of tetraploids over diploids in response to liming in both cases, with the change in magnitude being small relative to the effect of liming. None of the entries were consistently superior with regard to seedling growth. However, compared with the other entries, Tetra-1 had longer shoots on the limed Konsil and Teller soils and the acidic Wilson soil, heavier shoots regardless of liming on the Wilson soil, and heavier roots on the limed Teller soil (data not shown). Consequently, Tetra-1 could prove useful as a germplasm source in breeding Russian wildrye for improved seedling growth in Southern Plains soils, including acidic soils.

Improved establishment ability of tetraploid Russian wildrye has been linked to superior seedling emergence, compared with diploids, when planted at deep seeding depths of 4.5 (Lawrence et al., 1990) to 7.6 cm (Jensen et al., 1998). This is attributed to the longer coleoptiles and leaves of tetraploids (Berdahl and Ries, 2002). In the present research, we transplanted seedlings into soils, and so any effect of emergence from deep seeding depths was not examined. This may account for the similar performance that we observed for tetraploids and diploids.

Tetraploids did not differ from diploids with regard to excluding Al from roots, regardless of soil type or lime treatment (Table 5). This may help explain the lack of any differences in acidic soil–Al tolerance between the tetraploid and diploid germplasm we examined. It should be noted that differences in exclusion between Al susceptible and tolerant genotypes are greatest at the root tip (Rincon and Gonzales, 1992; Pellet et al., 1995), whereas we measured Al desorbed from the entire root system. Also, any tolerance to Al in Russian wildrye may depend on mechanisms other than root exclusion. Root-bound Al was negatively correlated with root weight (r = –0.54) and shoot length (r = –0.50) for the acidic Teller soil and root weight for the acidic Konsil (r = –0.58) soil. All other correlations of root-bound Al with shoot length, root length, shoot weight, and root weight were nonsignificant (data not shown).


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  		Table 5. Generalized least-squares means for concentration of Al in 0.5 M citric acid root extract for diploid and tetraploid Russian wildrye populations grown in limed or acidic soil in the greenhouse at Ardmore, OK. Data are averaged across two runs.

 
Several factors may have led to increased growth of seedlings on limed soil. Aluminum toxicity most likely was the cause of poor seedling growth in the strongly acidic Teller soil. Exchangeable Al levels in the Wilson and Konsil soils may not have been high enough to cause toxicity. However, application of lime to these soils would be expected to increase availability of nutrients such as calcium and phosphorus. This in turn may have led to increased seedling vigor, similar to previous greenhouse research where phosphorus fertilization of Russian wildrye seedlings led to improved growth (Bowman and McGinnies, 1981). Phosphorus availability increases as exchangeable Al decreases (Tisdale et al., 1985), which may have further enhanced the growth of seedlings on the limed Teller soil.

Our research indicates that both diploid and tetraploid Russian wildrye germplasm is sensitive to soil acidity on the basis of positive responses to liming for several seedling growth traits. Field studies will be needed to verify these greenhouse findings, and a broader array of cultivars may need to be examined. Still, producers attempting to establish Russian wildrye in the southern Great Plains would probably be well advised to apply lime as needed to bring soil pH to near neutral. Of the traits we examined, shoot weight appears to respond most consistently to lime application and should be useful in screening Russian wildrye germplasm for tolerance to soil acidity and Al toxicity. On the basis of the results of this research, the Noble Foundation grass breeding program has initiated efforts aimed at estimating heritability and selecting for seedling shoot weight of Tetra-1 in acidic soil.


   ACKNOWLEDGMENTS
 
The authors thank the following individuals for supplying seed: Dr. John Berdahl (Mankota and Mandan R1983), and Dr. Kevin Jensen (Tetra-1).

Received for publication February 13, 2004.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 





This Article
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