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USDA-ARS, Forage and Range Research, Utah State Univ., Logan, UT 84322-6300. Cooperative investigations of the USDA-Agricultural Research Service and the Utah Agricultural Experiment Station, Logan Utah 84322
* Corresponding author (kevin{at}cc.usu.edu)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Several grasses under moderate irrigation (30–40 cm) have been proposed for reclaiming saline-alkali sites in the Intermountain West. The most prominent of these is tall wheatgrass [Thinopyrum ponticum (Podp.) Barkworth & D.R. Dewey], which also has proven to be a valuable parent in hybridization programs to transfer salinity resistance to wheat (Triticum aestivum L.) (Sharma and Gill, 1983). In addition to its resistance to salinity, tall wheatgrass is productive and resistant to drought; however, it tends to become coarse and unpalatable when mature (Hafenrichter et al., 1968). Previously reported genetic variation among wheatgrasses (Dewey, 1962a, 1962b; Hunt, 1965; Moxley et al., 1977) suggests that breeding for improved persistence under saline conditions would be feasible.
Excess salinity adversely affects plants through ion toxicity and by decreasing the uptake of water, which is usually limited under rangeland conditions. Plants resist salinity through mechanisms conditioning avoidance or tolerance (Levitt, 1980; Johnson, 1991). Avoidance is the exclusion of toxic ions from internal plant tissues, whereas tolerance is the capability of a plant to withstand the presence of salt ions in the tissues. A complete review of breeding and genetics of salt tolerance was done by Shannon (1984).
Selection at the seedling stage for increased salinity tolerance was reported in crested wheatgrass [Agropyron desertorum (Fisch. ex Link) Shultes] (Dewey, 1962b). However, there was no association between increased salinity tolerance in crested wheatgrass seedlings and performance under saline conditions at later growth stages (Dewey, 1962a). Artificial selection for resistance to salinity has been effective in other species as well. Ashraf et al. (1987) obtained narrow-sense heritabilities in excess of 50% in alfalfa [Medicago sativa L.] and red clover [Trifolium pratense L.] and concluded that positive responses could be expected from recurrent selection for seedling tolerance to NaCl on these species. Salinity tolerance of alfalfa also has been altered at the mature plant stage through selection (Nobel et al., 1984).
Development of salt tolerant plant materials will likely require selection at several stages of plant growth. Dewey (1962a) proposed a breeding scheme to improve salinity tolerance in perennial grasses that included selection during germination with a subsequent selection taking place at a later growth stage. Selection for salt-tolerance in the field has proven to be very difficult and often not effective because of lack of uniformity of most salt affected fields. Peel et al. (2004) described a greenhouse protocol to screen large numbers of genotypes on the basis of their ability to survive at the advanced seedling stage under saline conditions that should be useful to improve salinity tolerance.
NewHy-RS hybrid wheatgrass resulted from hybridization between quackgrass and bluebunch wheatgrass [Pseudoroegneria spicata (Pursh.) Á. Löve]. NewHy combines the vigor, productivity, salinity tolerance, and persistence of quackgrass with the drought resistance, bunch growth habit, and seed and forage quality of bluebunch wheatgrass (Asay et al., 1991). Rhizome development in NewHy is comparable to intermediate wheatgrass [Thinopyrum intermedium (Host) Barkworth & D.R. Dewey]. This hybrid cultivar is recommended for range sites and pastures with moderate salinity problems that receive at least 35 cm of annual precipitation (Asay et al., 1991). Under intensive grazing management, NewHy requires at least 25 d between grazing events. However, during hotter portions of the summer, rest periods should be extended to 35 d (Jensen et al., 2001). If the persistence of NewHy under saline conditions could be increased, it would provide a source of forage with increased quality over that of tall wheatgrass (Asay et al., 1991) and greater salt tolerance than other irrigated pasture grasses. Objectives of this study were to evaluate (i) improvement in persistence under increased salinity levels of NewHy after three cycles of recurrent selection and (ii) corresponding changes in seedling vigor and germination.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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A study comparing the relative ability of NewHy (Foundation seed; Cycle-0), Cycle-1, Cycle-2, and Cycle-3 plants to persist under increasing salt concentrations was conducted in a greenhouse at Logan, UT, in 2002 and 2003 using the methods described by Peel et al. (2004). Also included in the study was a seed collection from northern Utah, which represented the seed source of the quackgrass parent involved in developing NewHy. Briefly, seeds from the same seed lots in both years were planted 1.5 cm deep in 3.8- x 21-cm Ray Leach Cone-tainers (Stuewe and Sons, Corvallis, OR) filled with 70 mesh (particle size = 0.10 mm) silica sand and watered daily with tap water until seedlings emerged. There was one seed sown per Cone-tainer. When seedlings reached the 2-to-3 leaf stage, all irrigations were made by immersing flats (98 Cone-tainers) into a complete nutrient solution twice weekly.
Six weeks after germination, plants were subjected to increasing salt concentrations starting at an EC of 6 dS m–1 during the first week of February, and increased 6 dS m–1 increments every 1 to 2 wk until an EC level of 42 dS m–1 was achieved in late May. Salt imbalance in nutrient solutions is a frequent problem in screening studies (Shannon, 1984). To avoid an imbalance in salt-nutrient solution, NaCl and CaCl2 were used in proportions to maintain a sodium adsorption ratio (SAR) of 3.5 (Peel et al., 2004). Actual amounts of Na and Ca for the desired EC are defined in Peel et al. (2004) Eq. [6] and [7]. The actual EC was measured with an Orin Model 120 conductivity meter (Thermo Electron Inc. Beverly, MA). Greenhouse temperatures ranged from 12.0 to 41.8°C in 2002 and from 7.6 to 40.1°C in 2003. Supplemental lighting was measured as the photosynthetically active radiation (PAR) that occurs between 400 to 700 nm. Photosynthetically active radiation at noon on a cloudless day in the greenhouse averaged 383 ± 108 µmol m–2 s–1.
Salt concentration was increased incrementally over time; thus, plant mortality was a dose–time response. To account for both the time exposed to a given EC level and salt concentration, a cumulative linear value was calculated that accounted for salt concentration as measured by EC of the solution and the number of days at each EC concentration. The value, termed "ECdays," was calculated by multiplying the EC concentration by the number of days at that concentration and summed over time, ECdays = 
(EC1 x DEC1 + EC2 x DEC2 +...), where EC1 is the first EC value and DEC1 is the number of days at the first EC value. ECdays were accumulated until plant death occurred (Peel et al., 2004).
A randomized complete block design was used with eight replications. Each flat of 98 cones (7 rows x 14 columns) contained two cycles, each in a 7x7 configuration. The entire outside border of each flat was excluded from scoring, so that only a 5x6 configuration of 30 completely bordered plants were scored for each replicate of each cycle. At the first sign of mortality, plants were recorded as dead or alive every 7 d until 100% mortality was achieved. Probit analysis (SAS Institute Inc., 1999) was used to estimate LD50 for the NewHy, Cycles-1 to -3, and the quackgrass parent. PROC MIXED, with years and cycles as fixed and replications as random effects, was conducted on the entry x year x replication LD50 value. Mean separations were made on the basis of least significant differences (LSD) at the 0.05 probability level (SAS Institute Inc., 1999). Pearsons rank correlation coefficients were calculated on cycle x year means LD50 (ECdays), seed germination, and seedling rate of emergence.
Under nonsaline conditions, seedling vigor on 100 pure live polycrossed seed of Cycles-0, -1, -2, and -3 was assessed on the basis of the ability of the seedling to emerge from a 7.6-cm seeding depth (Asay and Johnson 1980) and measured by the rate of emergence (Maguire 1962). Seed germination of the different cycles followed the procedures described for the wheatgrasses (AOSA, 2002) with four replications.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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The results suggest that salinity tolerance in NewHy likely originated from the quackgrass parent (Table 1), which is highly rhizomatous. Phenotypic selection within each cycle centered on those genotypes with the caespitose growth habit rather than spreading genotypes. However, within each cycle of selection for persistence under increased salinity levels, there was an increased frequency of plants that expressed rhizome development (data not shown). Wheatgrasses and wildryes with strong rhizome development, typically have reduced seed set, germination, and lack seedling vigor (Asay and Jensen, 1996a, 1996b). Therefore of concern was the possible effect that the increased frequency of rhizomes at later cycles was having on seed germination and seedling vigor.
Selection for increased persistence under increased salinity levels had little effect on percent seed germination, which ranged from 85 to 90% in all cycles (Table 1). Cycle-1 had the highest germination rate (Table 1) but was not significantly different from the other cycles. However, selection for increased persistence under increased salinity levels did have a significant (P < 0.05) effect on the ability of seedlings to emerge from a 7.6-cm planting depth (Table 1). All cycles had significantly (P < 0.05) higher seedling emergence from a deep planting depth than the cultivar NewHy. The largest difference was from 1.7 seedling d–1 in NewHy to 3.3 seedling d–1 in Cycle-1 (Table 1). In Cycles-2 and -3, the rate of seedling emergence declined to 2.4 and 2.5 seedlings d–1, respectively. Cycles-2 and -3 were not significantly different. Selection for germination under saline conditions (EC 18 dS m–1) in Cycle-1 appears to have increased the seedling emergence rate from a deep planting depth significantly (Table 1). Without selection for germination under saline conditions in Cycles-2 and -3, rate of seedling emergence from a deep planting depth declined significantly (P < 0.01) from Cycle-1, but remained greater (P < 0.05) than NewHy (Table 1). Associated with increased rate of seedling emergence was increased 100-seed weight (Table 1). Selection for seed germination under saline conditions in Cycle-1 significantly (P < 0.05) increased 100-seed weight compared with Cycles-0, -2, and -3 that were not selected for germination. In Russian wildrye [Psathyrostachys juncea (Fisch.) Nevski], increased 100-seed weight is correlated with increased rates of emergence from a deep planting depth (Berdahl and Barker, 1984). This suggests the importance of selecting for seed germination independently of persistence to increased salinity levels at the immature plant stage (4-to-5 leaf stage). Selection for salinity tolerance should be practiced at both seed germination and seedling stage within NewHy.
In conclusion, we found that the greenhouse salt screening method described by Peel et al. (2004) could be effectively used to select for improved persistence under increased levels of salinity in grasses. An improvement in plant persistence under increased levels of salinity did not negatively affect germination or seedling vigor, and screening for germination under saline conditions appears to have increased 100-seed weight and seedling vigor. Further studies using field evaluation are necessary to validate the application of this screening method for increased plant persistence and forage production to increased levels of salinity in NewHy.
Received for publication August 2, 2004.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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