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CROP BREEDING, GENETICS & CYTOLOGY

Genetic and Environmental Effects on Seed Weight and Seed Yield in Switchgrass

Plant Science Dep., South Dakota State Univ., Brookings, SD 57007-2141

* Corresponding author (arvid_boe{at}sdstate.edu)

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
A positive correlation exists between seed weight and seedling vigor. The objective of this study was to determine genetic and environmental effects on seed weight and seed yield within two switchgrass (Panicum virgatum L.) cultivars. Thirty half-sib families of ‘Sunburst’ (large seeded) and ‘Summer’ (small seeded) were evaluated for 3 yr in Kentucky bluegrass (Poa pratensis L.) sod (competitive) and tilled (noncompetitive) spaced-plant nurseries at Brookings, SD, and in a tilled spaced-plant nursery at Highmore, SD. Parents were evaluated in a separate nursery. Significant differences were found among families within both cultivars for 100-seed weight, but significant family x environment interactions indicated nonuniformity in responses of families to environmental variability. Narrow-sense heritability estimates for 100-seed weight were 0.88 for Sunburst and 0.58 for Summer. Seed weight increased in response to increased ambient moisture, but was similar for competitive and noncompetitive environments (grand means were 101 and 175 mg 100 seeds^-1 for Summer and Sunburst, respectively) at Brookings. However, mean plasticity for seed weight was evident from markedly reduced seed weights for both cultivars at Highmore. Magnitude and direction of plasticity for seed weight differed for the two cultivars. Plastic families of Summer generally had heavier seeds in the competitive environment, whereas those of Sunburst had heavier seeds in the noncompetitive environment. Significant differences were found among families within both cultivars for seed yield. Summer had 70% higher mean seed yields than Sunburst, and annual seed-yield means varied by greater than 80%. Seed weight was highly responsive to temporal and spatial environmental variation, but genetic variation was adequate to expect progress from selection within both cultivars.

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
SWITCHGRASS is a highly variable species (Nielsen, 1944). It occurs in two cytotypes designated as upland or lowland ecotypes that can be differentiated by chloroplast DNA polymorphism (Hulquist et al., 1996, 1997). Variability in phenology (Quinn, 1969; Hopkins et al., 1995), plant height (Nielsen, 1944; Eberhart and Newell, 1959; Quinn, 1969), forage yield (Eberhart and Newell, 1959; Hopkins et al., 1995), forage quality (Hopkins et al., 1995), seed yield index (Eberhart and Newell, 1959), disease resistance (Cornelius and Johnston, 1941; Eberhart and Newell, 1959; Hopkins et al., 1995), and winter survival (Nielsen, 1947) has been described among populations from various localities throughout the range of the species. However, among- and within-population variability for seed yield and seed weight, an important factor in germination and emergence rates (Kneebone and Cremer, 1955; Zhang and Maun, 1991) and rate of early seedling development (Zhang and Maun, 1991; Smart and Moser, 1999) of switchgrass, have received little attention (Boe and Johnson, 1987; Bortnem and Boe, 1993).

The advantage of large seed on seedling growth of switchgrass has been shown to last only up to about 10 wk after emergence when planted at shallow depths in pots in the greenhouse (Zhang and Maun, 1991) and in conventionally prepared seedbeds in the field (Smart and Moser, 1999). However, a primary advantage of large seeds for stand establishment of perennial grasses is the ability of the associated seedlings to emerge from relatively deep planting depths (e.g., Rogler, 1954) and outcompete seedlings derived from smaller seeds (Peters, 1985). In the future, to avoid the potential for erosion associated with tillage and preparation of conventional seedbeds, much of the switchgrass grown for biomass in the northern Great Plains will likely be no-till planted into crop residues or herbicide-suppressed sods where planting depth and seedbed firmness will be less precise than in conventional seedbeds. Consequently, selection for increased seed weight may become an important selection criterion for new cultivars developed for biomass in the northern Great Plains. Therefore, the objective of this study was to determine the importance of genetic and environmental influences on seed weight and seed yield for two switchgrass cultivars adapted to the northern Great Plains.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
During May 1987, progenies of 30 open-pollinated genotypes of Summer (Alderson and Sharp, 1994) and 28 open-pollinated genotypes of Sunburst (Boe and Ross, 1998) switchgrass were transplanted on 1-m centers to two nurseries at Brookings, SD. Both cultivars are upland types (Moser and Vogel, 1995) that are well suited for long-term biomass production in the northern Great Plains. Summer is a tetraploid (Riley and Vogel, 1982). The ploidy level of Sunburst has not been determined, but its cellular DNA content is similar to several hexaploid upland cultivars (Hulquist et al., 1996).

The cultivated nursery (hereafter referred to as the noncompetitive environment) was on tilled Lismore silt loam (fine-loamy, mixed, pachic udic haploborolls) that had been fallow for 2 yr. The sod nursery (hereafter referred to as the competitive environment) was on a grass sward dominated by Kentucky bluegrass (Poa pratensis L.). The sward had been undisturbed for at least 20 yr. The soil that supported the sward was Vienna silt loam (fine-loamy, mixed, udic haploborolls).

The Kentucky bluegrass sward was chosen as a competitive environment because of its consumption of soil moisture while switchgrass is still dormant in the early spring and its ability to provide competition during early stages of new tiller growth in switchgrass in May and June when growing season precipitation is generally highest in the northern Great Plains. During the rest of the growing season, higher air temperatures and reduced precipitation would generally be expected to cause stress in switchgrass, even in the absence of interspecific competiton.

Experimental designs were randomized complete blocks with five replicates of five-plant plots for each of the 58 families. Sod plugs 15 cm in diameter and depth were removed to provide transplant sites for seedlings in the competitive environment. Tillage and hand weeding were used to control weeds in the noncompetitive environment. Interplant spaces in the competitive environment were maintained at a height of about 10 cm by mowing during the growing seasons.

During Oct. 1988, 1989, and 1990, panicles from each plant were hand harvested in two replications of each environment. At those same times during 1989 and 1990, the favorableness of the two environments for vegetative growth of switchgrass was estimated by measuring heights of individual plants from ground level to the apex of the panicle of the longest culm. To obtain enough seeds for seed-weight analysis, it was necessary to harvest all of the panicles from plants in the competitive environment. Plants in the noncompetitive environment were substantially more vigorous, and adequate numbers of seeds for seed-weight analyses were obtained by harvesting about 10 panicles from each.

In addition, seed was also harvested during 1988, 1989, and 1990 from (i) individual open-pollinated parental plants of each family located in a cultivated spaced-plant (1-m centers) nursery located about 100 m from the competitive environment nursery and (ii) a cultivated spaced-plant (1-m centers) nursery established during 1987 on a Glenham (Typic Argiustoll, fine-loamy, mixed, mesic) loam about 250 km west of Brookings at Highmore, SD. The parental plant nursery was established during 1982 and contained about 600 plants of each cultivar. In addition to 1988, 1989, and 1990, seed was also collected from the parents during 1983, 1984, and 1987. The nursery at Highmore contained unreplicated plots of the same families in the Brookings nurseries. They were randomly arranged in 5-plant plots with 1-m interplant spacings. Nurseries at both locations were burned to remove the previous year's biomass in late winter or early spring before initiation of new growth. No fertilizers were applied during the study.

Harvesting was done at full seed maturity before any appreciable shattering. The two cultivars have similar phenologies, and harvesting at any environment was completed in less than two days. Panicles were excised with pruning shears, threshed on a rubber rub-board, and screened by hand to remove fragments of rachises and panicle branches. Fertile florets were separated from the remaining inert matter with a South Dakota-type seed blower. Besides being lighter, empty florets were generally less terete and paler in color than fertile florets. However, separating empty from fertile florets with a seed blower was difficult for many samples of Summer from Highmore because of shriveled caryopses in the fertile florets. In addition, a substantial fraction of the unfertilized florets contained unexserted anthers, which added tereteness and weight. These difficulties were not encountered in any of the samples from the larger seeded Sunburst. Seed weights for individual plants in all of the nurseries described above were determined as means of two 100-fertile floret (hereafter referred to as 100-seed) samples that were randomly selected, counted by hand, and weighed on an analytical balance.

During early Oct. 1992 and 1993, seed yield on a family-plot basis was determined by harvesting bulk seed from each family x replication plot for five replications in the competitive environment at Brookings. Harvesting, threshing, and cleaning were as described above for 100-seed weight analyses.

Analyses of variance were conducted on family-plot means for plant height and 100-seed weight and on family-plot totals for seed yield. All effects, other than environment, were considered to be random. Approximate F-tests were performed according to expected mean squares (Satterthwaite, 1946). When justified by significant F-tests, means were separated by Fisher's protected least significant difference.

Narrow-sense heritability estimates for 100-seed weight were determined by doubling the linear regression coefficient obtained from regression of progeny means on parental means (Nguyen and Sleper, 1983). Progeny means were obtained by averaging across three years (1988–1990) and two environments (competitive and noncompetitive) at Brookings. Means for individual parental plants were obtained by averaging across six years (1983, 1984 and 1987–1990) in a separate spaced-plant nursery adjacent to the competitive nursery at Brookings.

Phenotypic plasticity was determined at the cultivar level as the difference between means in different spatial and temporal environments (Via, 1993) at Brookings and Highmore. A significant (P = 0.05) main effect of environment indicated the cultivar expressed phenotypic plasticity for that particular trait. Phenotypic plasticity within cultivars was determined from the evaluation of half-sib families for three years in two environments at Brookings. A significant (P = 0.05) family x environment mean square indicated genetic variation for phenotypic plasticity within a cultivar (Via, 1993). Families were considered to be plastic for a trait if the difference between environment means exceeded the LSD at P = 0.05.

	RESULTS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
For both cultivars, significant (P < 0.01) differences were found between environments, between years, and among families for plant height at Brookings. Summer had a mean height of 173 cm in the noncompetitive environment compared with 110 cm in the competitive environment. Mean height for Sunburst was 169 cm in the noncompetitive environment compared with 114 cm in the competitive environment. Ranges in family means were from 119 to 152 cm for Sunburst and 126 to 152 cm for Summer. Family x environment, family x year, and family x year x environment interaction variances were nonsignificant (P > 0.05) for both cultivars. The environment x year interaction was significant (P < 0.01) because of 35% greater mean height in the competitive environment in 1990 compared with 1989 in contrast to similar mean heights in the noncompetitive environment in both years. Precipitation from May through August was about 60% greater in 1990 than 1989 (Table 1).

Significant differences for seed weight were also found among years for both cultivars in the progeny nurseries at Brookings (Table 2). Summer and Sunburst responded similarly by producing heavier seeds each successive year from 1988 through 1990 (Table 3), with mean 100-seed weights of both cultivars over 30% greater in 1990 than in 1988. Similarly in the parent nursery at Brookings, the mean 100-seed weights of both cultivars were about 50% greater in 1990 compared with 1988.

Parent-progeny regression analyses gave narrow-sense heritability estimates for 100-seed weight of 0.88 ± 0.13 for Sunburst and 0.58 ± 0.20 for Summer. These estimates indicated progress from selection could be expected within both cultivars, but would likely be greater per cycle for Sunburst.

Highly significant (P < 0.01) differences were found between years and among families within both cultivars for seed yield at Brookings. The family x year interaction mean square was significant (P < 0.05) for Sunburst but not (P > 0.05) for Summer. However, in general the five highest and five lowest yielding families of Sunburst ranked similarly in both years. Annual means for Summer were 42 kg ha^-1 in 1992 and 75 kg ha^-1 in 1993. Annual means for Sunburst were 31 kg ha^-1 in 1992 and 40 kg ha^-1 in 1993. Family means ranged from 30 to 98 kg ha^-1 for Summer and from 20 to 66 kg ha^-1 for Sunburst (Table 6). These estimates of seed yield are necessarily low because they were obtained from a density of about 1 plant m^-2, which is much lower than densities in rows or swards managed for seed production. Student's t tests indicated Summer had significantly (P < 0.01) higher seed yields than Sunburst in both years. Taking into account differences between the two cultivars for 100-seed weight, Summer produced about three times as many seeds per hectare as Sunburst.

	DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

Both cultivars produced plastic responses similar in magnitude and direction for 100-seed weight in response to temporal variation in precipitation at Brookings and Highmore (Table 1) and in response to macroenvironmental differences between Brookings and Highmore. The 50% reduction in mean 100-seed weight of Summer and 28% reduction in mean 100-seed weight of Sunburst at Highmore compared to Brookings (significant at P = 0.05 by t tests) revealed plasticity in both cultivars that was not evident between the two environments at Brookings. Presumably, this large difference between environmental means was a reflection of greater stress imposed by lower precipitation (Table 1) and higher temperatures during the growing seasons at Highmore.

Phenotypic plasticity in 100-seed weight was also due to differential responses of families to the two environments at Brookings. About 60% of the families of Summer were plastic compared with about 40% for Sunburst. All but two families of Summer that exhibited plasticity produced heavier seeds in the competitive environment, whereas all plastic families of Sunburst produced heavier seeds in the noncompetitive environment. Many studies have shown that resource-deprived plants produce fewer and smaller seeds than those adequately supplied with resources (e.g., Sultan and Bazzaz, 1993). However, Sultan (1996) reported although Polygonum persicaria L. plants grown at low light and nutrient levels produced smaller achenes than plants grown under adequate levels of each factor, plants grown in dry soil produced achenes that were 16% heavier than those from plants grown at field capacity.

Seed production of switchgrass occurs across a wide range of environments in the northern Great Plains. Thus, family selection based on across-environment means is logical for increasing seed weight in these cultivars. For example, if the selection threshold was one standard deviation above the cultivar mean averaged across competitive and noncompetitive environments at Brookings, selection for heavy seeds in Summer would favor families that produced heavier seeds in the competitive environment. On the other hand, selection for heavy seeds in Sunburst would favor some homeostatic families and others that produced heavier seeds in the noncompetitive environment.

An alternative strategy of selecting only heavy-seeded homeostatic families because of stability across the two environments at Brookings would result in the two families with the heaviest mean 100-seed weights in the selected group for Sunburst, but none of the top four families would be selected for Summer. In general, the least environmentally sensitive families for both cultivars were the lightest seeded. Six out of the eight lightest-seeded families of both cultivars were homeostatic at Brookings. Thus selecting for large seed would likely increase environmental sensitivity for seed weight in both cultivars, whereas selection for small seed would be expected to decrease environmental sensitivity.

The narrow-sense heritability estimate for seed weight based on parent-progeny relationships in two different environments at Brookings was higher for Sunburst than Summer. In addition, the difference between Sunburst and Summer means was greater at Highmore than at either of the two Brookings locations. Thus, the relative ability of Sunburst to produce heavy seeds was still expressed under moisture and temperature stress at Highmore. This characteristic of Sunburst was also demonstrated in a trial composed of five genotypes each of Sunburst, ‘Blackwell’ (Alderson and Sharp, 1994), and ‘Pathfinder’ (Newell, 1968) conducted during the same three years as the present study at Brookings. In that experiment, Sunburst produced seeds that were about 30% heavier than those of Blackwell and Pathfinder (Bortnem and Boe, 1993).

Large differences between cultivars and among families within cultivars for seed yield in the competitive environment at Brookings indicated inter- and intrapopulation genetic variation for seed yield in switchgrass. Vogel (2000) recently concluded that improved seed production practices developed for switchgrass and other native grasses over the last 20 yr have reduced the need at this time for genetic improvement in seed yield since current demands can usually be met. However, new cultivars developed specifically for biomass may be morphologically different from those previously developed for forage and conservation purposes. Therefore, some attention should be paid to seed yield during selection and evaluation to ensure the seed production capability of new cultivars is adequate to meet the expected demand (Vogel, 2000).

Boe and Johnson (1987) showed mass selection from a bulk lot of switchgrass would be effective for increasing seed weight. However, they did not determine the relative importance of genetic and environmental effects on seed weight in switchgrass. Results from this study indicated additive genetic factors and temporal variation in precipitation had greater influences than did interspecific competition on seed weight at Brookings. However, the impact of macroenvironment on this trait, as demonstrated by the large difference between Brookings and Highmore means, indicated phenotypic plasticity in response to spatial variation may also be influential in determining seed weight and seedling vigor characteristics of different seed lots of switchgrass cultivars produced across diverse environments in the northern Great Plains.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
South Dakota Agric. Exp. Stn. Journal Series No. 3165.

Received for publication September 17, 2001.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 

• Alderson, J., and W.C. Sharp. 1994. Grass varieties in the United States. USDA-SCS Agric. Handb. 170. Washington, DC.
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