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a USDA-ARS, U.S. Dairy Forage Research Center, 1925 Linden Dr. West, Madison, WI 53706-1108
b Dep. of Agronomy, Univ. of Wisconsin, Madison, WI 53706-1597
* Corresponding author (mdcasler{at}facstaff.wisc.edu)
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Various strategies have been used to improve establishment potential of forage crops. Seed size is a heritable trait that has responded to selection and is often positively correlated with seedling vigor (Berdahl and Barker, 1984; DeHaan et al., 2001; Trupp and Carlson, 1971; Twamley, 1974). However, selection for larger seeds may be insufficient as a sole selection criterion to improve seedling vigor (McLean and Nowak, 1997). Seedling vigor and/or germination rate have responded to selection in several species (DeHaan et al., 2001; Lawrence, 1977; Townsend, 1974; Voigt and Brown, 1969).
If seeds are placed in a stressful environment, selection pressure can be simultaneously applied for seedling vigor and stress tolerance. Selection for emergence from deep planting has improved establishment capacity for field plantings under dryland conditions (Berdahl and Barker, 1984; Lawrence, 1977). Selection for germination and seedling vigor at suboptimal temperatures resulted in faster seedling growth under field conditions (Klos and Brummer, 2000a, 2000b). Because reed canarygrass must survive and grow in an environment dominated by fast-growing annuals, seed size or seedling vigor under competitive conditions may be the most important traits for improving establishment.
Among the perennial grasses, establishment rate is inversely proportional to perenniality (Casler et al., 1999; Undersander et al., 2001). A tradeoff most likely occurs between shoot growth (seedling vigor) and root growth, a structural prerequisite for perenniality. Long-term perennial grasses, such as reed canarygrass, appear to devote resources early in their life cycle to root and rhizome development at the expense of seedling vigor and shoot growth (Undersander et al., 2001). Conversely, perennial grasses with superior seedling vigor tend to be short-term perennials, less capable of long-term survival under competitive or stressful conditions (Undersander et al., 2001).
The objectives of this study were (i) to select reed canarygrass plants and populations for establishment capacity in the presence of annual weeds, (ii) to evaluate progeny for progress from selection, and (iii) to determine mechanisms for improved establishment potential in selected populations.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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During the remainder of 1993 and all of 1994, weeds were controlled only by clipping. The weed population consisted primarily of the annuals crabgrass [Digitaria sanguinalis (L.) Scop.], green foxtail [Setaria viridis (L.) Beauv.], barnyardgrass [Echinochloa crus-galli (L.) Beauv.], fall panic [Panicum dichotomiflorum Michx.], lambsquarter (Chenopodium album L.), and redroot pigweed (Amaranthus retroflexus L.), with each species uniformly distributed across the experimental area to form a uniform and dense ground cover. The entire experiment was clipped to a stubble height of 9 cm whenever the weed canopy reached an average height of approximately 20 cm. The experiment was clipped six times in 1993 and seven times in 1994. Fertilizer was not applied to this experiment. Clipping was accomplished with a flail chopper to remove all clipped forage. Soil tests revealed adequate levels of P and K.
Maximum plant diameter of each plant's crown was measured in early spring 1994 and 1995. Percentage mortality of each plot was determined from the plant diameter data. Data were analyzed by analysis of variance, assuming cultivars to be fixed and blocks to be random.
Forty-five plants were selected for high plant diameter in spring 1994 and 1995 combined with high spring vigor in May 1995. Plant diameter was used as a selection criterion because it measured a plant's ability to compete with weeds for soil surface area. Spring vigor (visual evaluation of plant canopy height when most plants were approximately 15 to 20 cm tall) was used as an indicator of stored carbohydrate reserves for rapid spring growth. These 45 plants were planted to a polycross block in May 1995, arranged in a randomized complete block design with four replicates. Plants were fertilized with 56 kg N ha–1 in early spring 1996 and 1997 and polycross seed was produced in July of 1996 and 1997. Seed was bulked in equal quantities for each polycross family produced in each year. This population of 45 polycross families was named WR97.
Also in May 1995, 500 plants each of five reed canarygrass cultivars (Bellevue, Palaton, Rival, Vantage, and Venture) were transplanted into isolated crossing blocks at Arlington, one block for each cultivar. Ten-week-old seedlings were transplanted by hand into rows that were 0.9 m apart with a 0.3-m spacing within rows. Each block was isolated by a minimum of 100 m from other reed canarygrass. Weeds were controlled by pre-emergence herbicide applications and hand weeding. Plants were fertilized with 56 kg N ha–1 in early spring 1996 and 1999 and seed was harvested in July of 1996 and 1999. Seeds were bulked in equal quantities for each plant within each block. Seed harvested from these five crossing blocks was used to represent the original cultivars in all remaining studies.
Experiment 2: Cycle 1 of Within-Family Selection
Seed of each of the 45 polycross families was tested for germination according to the Association of Official Seed Analysts standards (AOSA, 1998). Germination was computed separately for each of two 0.25-g seed samples per family. Seeding rates for each family were adjusted for germination percentage and seed mass to a constant number of pure live seeds (PLS) per unit area.
The 45 polycross families and the five cultivars (represented by seed from the 1996 Arlington seed increases) were planted in four four-replicate randomized complete block experiments in April 1998 at Arlington, WI. The four experiments consisted of three seeding rates without companion crop (64, 128, and 256 PLS m–2) and one seeding rate (256 PLS m–2) with a red clover (Trifolium pratense L.) companion. The three seeding rates were equivalent, on average, to seeding rates of 0.6, 1.2, and 2.4 kg ha–1 (approximately 5, 10, and 20% of the normal recommended rate). The red clover was planted at a rate of 7 kg ha–1 with a cultipacker seeder over the top of the seeded reed canarygrass plots. Plots were 0.9 by 1.4 m and consisted of five drilled rows, spaced 15 cm apart. Each of the five cultivars was seeded to nine plots per replicate to improve statistical precision of the comparison between cultivars and polycross families. This and all other field experiments followed a previous year crop of soybean [Glycine max (L.) Merr.] and the seedbed was prepared by chisel plowing and cultimulching.
Management was similar to that for Experiment 1—weeds were controlled only by clipping when the weed canopy reached a height of approximately 20 cm. The weed population was relatively uniform, largely consisting of the species described above. The experiment was clipped three times in 1998. Fertilizer was not applied to this experiment. Soil tests revealed adequate levels of P and K.
In late April 1999, tiller density of each plot was determined by counting all emerged reed canarygrass tillers within two 0.1-m2 sampling frames placed at random positions within each plot. Maximum reed canarygrass tiller height was 15 cm. There were no weeds to interfere with tiller counts. Red clover was killed with an early spring application of 0.45 kg a.i. ha–1 2,4-D [(2,4-dichlorophenoxy) acetic acid]. Tiller density data were analyzed by analysis of variance assuming all effects to be random. Nearest neighbor adjustment was used to adjust family and cultivar means for spatial variation (Casler, 1999). Comparisons between cultivars and families were made using contrasts. Cultivar means were compared using the protected LSD (Carmer and Walker, 1982). Heritability of tiller density, on a half-sib family mean basis, was computed as h2 = s2F/(s2F + s2e/r), where s2F = the family variance component, s2e = the error variance component, and r = number of replicates.
Experiment 3: Selection Evaluation
Immediately after tiller counts were made, Experiment 2 was fertilized with 56 kg N ha–1 and left undisturbed until the initiation of seed shattering. The experiment had been treated with 1.12 kg ha–1 alachlor [2-chloro-N-92,6-diethylphenyl)-N-(methoxymethyl)-acetamide] and 0.07 kg ha–1 imazethapyr {(±)-2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-5-ethyl-3-pyridinecarboxylic acid} for weed control. Seed was harvested individually from every plot in Experiment 2. All seeds harvested from the 45 polycross families and the five check cultivars in Experiment 2 were separately bulked for each family and cultivar. These seeds represented the culmination of one cycle of natural selection within cultivars and families for establishment capacity. For the check cultivars, this represented a single cycle of selection. For the polycross families, this seed represented a second cycle of selection, albeit with a different protocol and marginally different selection criterion than that of the first cycle (Experiment 1). This selection method can be described as phenotypic selection within maternal lines, without pollen control. Seed of the 45 polycross families was bulked in equal quantities and named WR99.
The 10 polycross families with the highest establishment capacity, measured by tiller density, were identified from Experiment 2 and labeled within the original 45-clone polycross block from Experiment 1. The polycross block was fertilized with 56 kg N ha–1 in early May 1999. Shortly after heading, the remaining 35 clones within this polycross block were mowed to prevent pollination and seed set. Seed was harvested on four replicates of the selected 10 clones, threshed, cleaned, and bulked in equal quantities for the 10 clones. This population was named WR00.
Experiment 3a: Establishment Capacity
The five cultivars (represented by seed from the 1999 Arlington seed increases), their five cycle-1 progeny populations from Experiment 2 (seed produced in 1999), and three experimental populations (WR97, WR99, and WR00) were seeded in April 2001. The experimental design was a randomized complete block with eight replicates at each of four locations. The locations were: Arlington (Plano silt loam; 43°20'N, 89°23'W), Ashland (Ontonagon silty clay loam [very-fine, mixed, active, frigid Haplic Glossudalf]; 46°35'N, 90°54'W), Lancaster (Fayette silt loam [fine-silty, mixed, superactive, mesic Typic Hapludalf]; 42°50'N, 90°47'W), and Marshfield (Withee silt loam [fine-loamy, mixed, superactive, frigid Aquic Glossudalf]; 44°39'N, 90°08'W). Seeds were tested for germination (AOSA, 1998) and the seeding rate was 64 PLS m–2. Plot size was 0.9 by 1.5 m.
Plots were clipped two or three times during 2001 to control annual weeds. Ground cover of reed canarygrass was determined in October 2001 using a 0.75 by 0.75 m grid divided into 25 0.15 by 0.15 m squares, as described by Vogel and Masters (2001). Tiller density was determined in May 2002 using the same grid. Reed canarygrass tillers were counted in four random 0.15 by 0.15 m squares per plot. Establishment potential was defined as the degree to which reed canarygrass covered the ground and tiller density. Data were analyzed by analysis of variance assuming all effects to be random, except populations, which were fixed. Comparisons between cultivars and selected populations were made using contrasts. Population WR99 was compared to population WR97 as a measure of the effect of one cycle of within-family selection for establishment capacity. Population WR00 was compared to population WR97 as a measure of the effect of one cycle of among-family selection for establishment capacity.
Experiment 3b: Forage Yield
These 13 populations were also seeded at Arlington and Marshfield using a normal seeding rate of 1280 PLS m–2, which was approximately equivalent to 11.2 kg ha–1. The experimental design and plot size were identical to that used for Experiment 3a, except for the use of 16 replicates instead of eight. Plots were clipped two or three times during 2001 to control annual weeds. Plots were harvested three times each in 2002 through 2004, in early June, late July, and late October at a 9-cm cutting height. Forage yield was determined for each plot, but dry matter was assumed to be constant across plots and was based on a random dry matter sample. Total forage yield was analyzed by analysis of variance as described for Experiment 2a.
Experiment 4: Establishment Mechanisms
The following traits were hypothesized to have a potential positive effect on improving establishment of reed canarygrass seedings: seed mass, rate of germination, seedling height, shoot fresh and dry mass, root fresh and dry mass, rate of tillering, and tiller number. Three experiments were designed to measure these traits on the 13 populations described above and to determine associations of these traits with establishment capacity. Seed lots used for these experiments were identical to those used for Experiments 3a and 3b.
Experiment 4a: Seed Mass and Emergence
Ten 100-seed lots were counted from each of the 13 populations evaluated in Experiments 3a and 3b. Seed mass was determined for each seed lot, after which they were planted in sand flats in the greenhouse. Flats were filled with sand to a depth of 4 cm and seeds were planted at a 5-mm depth. Each 100-seed lot was planted in a 0.5-m row and adjacent rows were 5 cm apart. Flats were watered and covered with clear plastic covers until the first seedling emerged on the seventh day after planting, after which the covers were removed. The number of emerged seedlings per row was counted daily from 7 to 21 d after planting. The experiment was designed as a randomized complete block with five replicates in each of two runs (January and February 2004). A 16-hour photoperiod with a minimum of 400 µmol m–2 s–1 of photosynthetically active radiation was maintained using high-pressure sodium lamps.
Based on the shape of emergence curves as a function of time, emergence was modeled using the log-linear regression model: Yi = a + b[ln(Xi)], where Yi = emergence on the ith day (Xi). The expected number of days to 50% emergence (D50) was computed as D50 = exp{[(E/2) – a]/b}, where E = emergence on day 21. Seed mass, log-linear slope (b), and D50 for each experimental unit were analyzed by analysis of variance assuming all effects to be random, except populations, which were fixed. Comparisons between cultivars and selected populations were made using contrasts.
Experiment 4b: Root and Shoot Vigor
Twelve 100-PLS lots were counted from each of the 13 populations using emergence on day 21 from Experiment 4a as an estimate of PLS for each seed lot. Flats were filled with a 50:50 (v:v) sand/silt loam soil mixture to a depth of 4 cm and seeds were planted at a 5-mm depth. Each 100-PLS lot was planted in a 0.5-m row and adjacent rows were 5 cm apart. Flats were watered daily. The experiment was designed as a split-plot in a randomized complete block with two replicates in each of two runs (March and April 2004)—harvest dates were whole plots and populations were subplots. Harvest dates were 16, 23, and 30 d post-emergence, corresponding approximately to the two-leaf, three-leaf, and four-leaf morphological stages.
At harvest, the height from soil level to the longest fully extended leaf blade within each row was measured within each row. Shoots were clipped at soil level, washed free of sand and soil, blotted dry with paper towels, and weighed. Roots were removed and washed free of sand and soil using a 5-mm screen and a high-pressure water jet, blotted dry with paper towels, and weighed. Shoots and roots were dried at 60°C for 3 d then weighed again to determine dry mass for each experimental unit. The slopes of the linear regressions of shoot and root fresh and dry mass on number of days post-emergence were computed for each replicate and run. All variables were analyzed separately for each harvest date using analysis of variance and assuming all effects to be random, except populations, which were fixed. Comparisons between cultivars and selected populations were made using contrasts.
Experiment 4c: Tillering
The 13 populations were planted into 5-cm-square containers that were 5 cm deep. Each container was planted with two to four seeds in late November 2004, which were thinned immediately after emergence to one seedling per cell. Emergence date of each seedling was recorded. An experimental unit consisted of five cells which were filled with a 50:50 (v:v) peat moss/silt loam soil mixture. The experiment was designed as a randomized complete block with 12 replicates (60 total seedlings per population). A 16-hour photoperiod with a minimum of 400 µmol m–2 s–1 of photosynthetically active radiation was maintained using high-pressure sodium lamps.
The date of emergence of the first new tiller was recorded for each seedling. At 35 d post-emergence, plants were clipped to a 7-cm stubble height and fertilized with the equivalent of 80 kg N ha–1. The number of tillers was counted on each seedling 42 d post-emergence. Days to tiller initiation and tiller number were analyzed using analysis of variance and assuming all effects to be random, except populations, which were fixed. Comparisons between cultivars and selected populations were made using contrasts.
Synthesis
All variables with significant differences among the 13 population means (P < 0.01) were entered into a multiple regression analysis to explain variation in ground cover or tiller density of Experiment 3a. Backward stepwise elimination with P = 0.01 was used to eliminate variables from the regression model.
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Polycross families were highly variable in tiller density at all seeding rates and for the mixture with red clover. Narrow-sense heritability for tiller density was moderate for all establishment treatments, ranging from 0.45 to 0.51. The phenotypic correlation was positive for polycross family means across the four establishment treatments, ranging from 0.34 to 0.61, indicating a general continuity in phenotypic variability across the four seeding rates and companions.
The initial selection of spaced plants with high crown diameter and spring vigor following 2 yr of frequent mowing under annual weed competition appears to have been successful in increasing establishment capacity (Table 2). The mean of all selected polycross families exceeded the mean of the cultivars by 34 to 47% across the four establishment treatments (P < 0.01). Furthermore, the mean of the polycross families was numerically higher than the mean of Vantage for each establishment treatment. Polycross families derived from Vantage were highest in tiller density at all four establishment treatments, while those from Rival were lowest in tiller density for three of four treatments. This general trend indicated a positive correlation between parental performance in the original spaced-plant nursery and progeny performance under the four establishment treatments, confirming that tiller density is a heritable trait and that it has a positive genetic correlation with crown diameter and plant survivorship.
Experiments 3a and 3b: Selection Evaluation
The initial cycle of selection for establishment capacity, based on low seeding rates, numerically increased both ground cover in October of the establishment year and tiller density in May of the following year for all five cultivars (Table 3). This numerical increase in ground cover or tiller density was significant for nine of the 10 comparisons made for the five cultivars. Averaged over all five cultivars, this represented a 28% increase in ground cover (23.1 vs. 18.0%) and a 36% increase in tiller density (93.8 vs. 68.8 tillers m–2) from a single cycle of selection within cultivars (P < 0.01). These results confirm those from the first two experiments, indicating that establishment capacity is a heritable trait in reed canarygrass, there is a tremendous amount of genetic variability for this trait, and it is relatively easy to make progress.
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As a result of selection for increased establishment capacity, forage yield increased in one of five cultivars, but was not significantly changed in the remaining cultivars or in WR99 or WR00 (Table 4). Despite this lack of significance in the individual comparisons, probably due to insufficient replication, the general trend was for an increase in forage yield, which averaged 2.6% (9.98 vs. 9.73; P = 0.01) across the five cultivars. Because of the high seeding rates for this study, all plots established well during the seeding year. Furthermore, the difference between selections and cultivars was consistent across locations and years, showing statistical significance (P < 0.05) for five of six location–year combinations. Thus, the correlated responses in forage yield were not a result of short-term establishment effects, but appeared to represent improvements in forage yield potential. Selection for increased seed size or speed of germination did not affect forage yield of two other perennial grasses (Lawrence, 1977; Jessen and Carlson, 1985).
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First, owing largely to their consistency across selection events, these observations may indicate an alternative strategy to large seeds for enhancing seedling vigor and the probability of seedling establishment. Perennial grass seedlings undergo a phase change, transitioning from a heterotrophic phase, in which they are dependent on seed reserves, to an autotropic phase when they become dependent on photosynthesis from leaves and water uptake from roots (Whalley et al., 1966). If small seeds make this transition faster than large seeds, their seedlings may have a fitness advantage over seedlings from larger seeds. Such a fitness advantage might be manifested in more rapid root and/or shoot development, regulated by earlier initiation of adventitious roots and/or new shoots. Phase changes in plants are genetically regulated (Poethig, 2003) and genetic variability for timing of phase changes have been observed in several species (Jordan et al., 1999; Scott et al., 1999; Vega et al., 2002).
Second, because seed mass is highly sensitive to environmental effects (Boe, 2003), these may be environmental effects. For the five cultivars, seeds of the original cultivars were produced from spaced plants, while seeds of the selected populations were produced in situ from Experiment 2, conditions which resulted in sparse swards with 149 to 288 tillers m–2 (Table 2). Differential plant spacing may have an environmental effect on seed mass. For the composite populations, seed was produced in three different years (1997, 1999, and 2000). If seed mass of reed canarygrass is positively related to seedling vigor and establishment capacity, as in numerous other species (Kitchen and Monsen, 1994; Smart and Moser, 1999), reductions in seed mass for all selected lines, due to environmental effects, would result in a downward bias to any estimates of genetic gain from selection. Because all experiments were planted with a constant number of pure live seeds across all populations, differences between selected and unselected lines for other traits may be considered to represent genetic gains due to selection. The potential downward bias to these effects, from possible environmental effects of reduced seed mass, should be recognized.
Third, they may be a result of drift during the selection process. Drift could have a reasonably uniform effect across selected populations and may not affect all traits evaluated on these populations, depending on their sensitivity to inbreeding depression (Falconer, 1989). In switchgrass (Panicum virgatum L.), seed size had no effect on establishment capacity once a seedling had two or more adventitious roots (Smart and Moser, 1999). Thus, an effect of variation in seed mass may not be observed after a critical stage of seedling development, potentially including a phase change from heterotropic to autotrophic.
Rate of seedling emergence was consistently higher in all selected populations compared to their respective parent population, although none of these differences were significant due to insufficient replication (data not shown). Averaged across the five cultivars, this effect was significant, averaging an 18.1% increase in seeding emergence rate (26 vs. 22 seedlings ln[d]–1; P = 0.05). Thus, it appears that an increased rate of seedling emergence may be a factor in the increased establishment rate of the selected populations. Similarly, selection for increased germination rate increased field emergence and seedling height in alfalfa, Medicago sativa L. (Klos and Brummer, 2000b). Variation among the 13 population means was not significant for the number of days to 50% emergence (D50).
Seedling height did not differ among the 13 populations for any of the three harvest dates and was excluded from all further analyses. Fresh and dry mass were highly correlated with each other for both shoots and roots (r = 0.91 to 0.99: P < 0.01), so all conclusions were identical based on analyses of either fresh mass or dry mass. Fresh mass of both shoots and roots were chosen for presentation and use in all further analyses.
At 16 d post-emergence, Palaton and Venture both showed large and fairly consistent increases in shoot and root fresh mass (data not shown). Responses were greater for Palaton than for Venture and shoot mass responded more to selection than root mass. Both composite populations responded similarly to selection, showing increases in shoot and root fresh mass; responses were greatest for shoot mass. At 23 d post-emergence, selection responses for shoot and root fresh mass were significant for Palaton, Vantage, and Venture (data not shown). These responses were consistently high, ranging from 37 to 63% increase in fresh mass. Selection responses at 23 d were inconsistent and nonsignificant for the two composite populations.
At 30 d post-emergence, selection responses for shoot fresh mass were still significant for Palaton, Vantage, and Venture, but selection responses for root fresh mass were significant only for Venture (Table 5). Shoot fresh mass at 30 d was increased in both WR99 and WR00, but neither population showed any changes in root fresh mass at 30 d.
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Shoot and root mass both increased linearly with time between 16 and 30 d post-emergence (Fig. 1 ). The 13 populations differed for linear regression coefficients of both shoot and root mass on post-emergence time. Linear responses for shoot mass were numerically increased in four of the five cultivars, but because of insufficient replication, these differences were not significant (Table 6). Averaged over cultivars, selected populations had a 32.3% greater linear response to post-emergence time than the original cultivars (1.48 vs. 1.12 g d–1; P < 0.01; Fig. 1). Differences between cultivars and selected populations in linear response of root mass to post-emergence time followed a similar pattern (Fig. 1), but these differences were too small to be statistically significant (Table 6). There was a progressive increase in linear response of shoot mass for the three composite populations (1.39 g d–1 for WR97, 1.62 g d–1 for WR99, and 1.82 g d–1 for WR00). These responses increased by 0.215 ± 0.003 (P < 0.01) with each incremental increase in selection pressure from WR97 to WR99 to WR00. These three populations did not differ in linear responses of root mass to post-emergence time.
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Mean date of tiller initiation responded inconsistently to selection, decreasing by 3.6 and 6.8% in Vantage and Venture, respectively, but increasing by 5.0% in Rival (Table 7). Tiller number 42 d post-emergence decreased by 6.6 and 11.4% in Bellevue and Venture, respectively, as well as averaged across the five cultivars (3.3%). Neither mean date of tiller initiation nor tiller number responded to selection for the composite populations. In switchgrass, divergent selection for seedling tiller number had no effect on seedling establishment capacity (Smart et al., 2003). Tillering per se was not a factor regulating genetic variability for establishment capacity in our study or the switchgrass study.
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Our study was unique among attempts to improve establishment capacity in that selection was based on the most direct measure of establishment available to us—survival, sexual reproduction, and fecundity of 1-yr-old plants. Harvesting seed from all families and retaining each family utilized 3/4 of the additive genetic variance within the population as a whole (Falconer, 1989). Furthermore, plants were allowed to reproduce and contribute gametes to the next generation in direct proportion to their size or number of flowering tillers, potentially increasing selection pressure for establishment capacity, as dictated by 1-yr-old plant size. Because selection was based directly on establishment capacity, there was no need to rely on strong genetic correlations between establishment capacity and an alternative selection criterion, such as seedling vigor. The observation of genetic gains within all five cultivars and the pooled populations indicated a certain level of repeatability of this selection protocol across populations.
Despite the overall importance of seedling fresh mass at 30 d post-emergence, there was considerable variability among cultivars in the nature of their correlated responses. For Palaton, WR99, and WR00, increased shoot mass and root mass indicated that improved seedling vigor was the mechanism for improved establishment capacity. For Vantage and Venture, increased shoot mass and root mass, combined with faster tiller initiation, suggested that both seedling vigor and tillering rate were responsible for improved establishment capacity. For Bellevue and Rival, there were no significant changes for any seedling trait, suggesting an unknown mechanism was responsible for improved establishment capacity. The variability among these responses and the implication of multiple mechanisms for improving establishment capacity is further evidence that direct selection for some measure of establishment capacity is probably the most efficient selection method to improve this trait of reed canarygrass.
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Received for publication March 8, 2005.
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REFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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  M. D. Casler and E. C. Brummer Theoretical Expected Genetic Gains for Among-and-Within-Family Selection Methods in Perennial Forage Crops Crop Sci., May 1, 2008; 48(3): 890 - 902. [Abstract] [Full Text] [PDF]
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M. D. Casler, D. J. Undersander, and W. E. Jokela Divergent Selection for Phosphorus Concentration in Reed Canarygrass Crop Sci., January 16, 2008; 48(1): 119 - 126. [Abstract] [Full Text] [PDF]
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