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Dep. of Plant Sciences and Plant Pathology, Montana State Univ., Bozeman, MT 59717
* Corresponding author (usslt{at}montana.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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Past experimental data shows correlations between yield and measurable agronomic traits in wheat, including harvest index (Calderini et al., 1995; Donmez et al., 2001; Feil, 1992; Hucl and Baker, 1987; Nass, 1973; Perry and D'Antuono, 1989; Reynolds et al., 1994; Reynolds et al., 1999; Siddique et al., 1989; Wang et al., 2002), number of grains/m2 (Calderini et al., 1995; Feil, 1992; Reynolds et al., 1994, 1999; Wang et al., 2002), long grain fill periods (Reynolds et al., 1994, 1999; Wang et al., 2002), number of grains/head (Calderini et al., 1995; Feil, 1992; Hucl and Baker, 1987; McNeal, 1960; Nass, 1973; Perry and D'Antuono, 1989; Siddique et al., 1989; Wang et al., 2002), number of grains/spikelet (Feil, 1992; McNeal, 1960; Siddique et al., 1989), number of tillers (Reynolds et al., 1999; Sedgley, 1991; Wang et al., 2002), kernel weight (Hucl and Baker, 1987; McNeal, 1960; McNeal et al., 1978; Wang et al., 2002), leaf size and posture (Sedgley, 1991), and photosynthetic ability (Nass, 1973). Thus, there are clearly plant characteristics that are related to yield potential. However, relatively little work has been done to determine the degree of correlation for these characters measured in space-planted versus densely seeded situations.
Syme (1972) conducted a study to determine the correlation between single-plant characteristics and field yield performance of 49 spring wheat cultivars released by 16 countries. The cultivars were grown in pots in an unheated glasshouse, and 16 characteristics were measured for their possible correlation with yield. These measurements were then correlated with the mean yields of the varieties when they were grown under field plot conditions. This study found that days to heading, height, and 100-grain weight are highly correlated between single plants and field plots, but there was no correlation between single plants and field plots for grain yield. A stepwise regression was performed to determine the mean yields of the field plots on the basis of single plant variables. Harvest index was the most important characteristic to enter into the regression, explaining 71.7% of the variation in cultivar mean yields. Days to emergence of Leaf 7 and 100-grain weight were also in the equation that explained 78.5% of the variation in cultivar final mean yields.
Fischer and Kertesz (1976) obtained correlations between space-plants and solid seeded plots for 40 genotypes. In general, grain weight per spaced plant was positively correlated with plot grain weight (r = 0.31). However, correlations were often higher for specific yield-related traits in spaced vs. solid seeded situations, including shoot harvest index (r = 0.66), plant harvest index (r = 0.56), spikes per plant (r = 0.59), spikelets per spike (r = 0.86), grains per spikelet (r = 0.59), kernel weight (r = 0.92), and grains per square meter (r = 0.45). These data suggest that performance in spaced and densely seeded situations is similar for some traits.
The goal of the present experiment was to determine the correlation of genotype performance in space-planted and densely seeded plots for a set of agronomic and physiological parameters in spring wheat. The results may provide direction for selection targets in breeding programs that impose severe selection pressure on space-planted nurseries.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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In 2002, the irrigated mid-dense and dense plots were planted 24 April, and the nonirrigated mid-dense and dense plots were planted 25 April. Spaced plots were planted on 26 April. All plots were planted 12 May 2003. Nonirrigated and irrigated plots were harvested 30 Aug. and 9 Sept. 2002 and 19 and 23 Aug. 2003, respectively. Mid-dense and densely seeded treatments were harvested with a plot combine. Spaced seeded plots were hand harvested. In addition, a 1-m section was cut, weighed, and threshed from a single row of the mid-dense and dense seeded plots.
Precipitation from May through July in 2002 was 200 mm. An additional 152.4 mm of water was applied to the irrigated plots during June and July. The available soil nitrogen for 2002 was 24.4 kg ha–1, and N–P–K 9.7/1.2/0 kg ha–1 was applied and tilled into the soil before planting. Precipitation from May through July in 2003 was 121 mm. An additional 159 mm of water was applied to the irrigated plots during June and July. The available soil nitrogen for 2003 was 23.6 kg ha–1, and N–P–K 17.2/7.2/7.2 kg ha–1 was applied and tilled into the soil before planting.
Characteristics Evaluated
Traits evaluated included leaf length, leaf width, tillers/meter and tillers/plant, days to heading, days to maturity, grain fill period, harvest index, plot weight, test weight, single kernel weight, single kernel diameter, spikelets/head, seeds/head, Haun maturity (Haun, 1973), and chlorophyll content. Leaf lengths and widths were measured on three random flag leaves in each plot with a clear ruler placed over the leaves. Lengths were measured from the collar to the tip of the leaf and were averaged. Leaf width was measured across the center (widest part) of the flag leaves; the measurements were then averaged. The total number of tillers was counted on 10 plants in each of the space planted plots. The measurements were averaged to arrive at the average number of tillers per plant. Also, the total number of tillers was counted in 1 m of a row in each of the mid-dense and dense seeded plots. Heading dates were taken on each of the plots by assigning a day of the year (DOY) when 75% of the heads were completely emerged. A DOY was assigned to the planting date and was subtracted from the DOY of heading to obtain the number of days to heading. Physiological maturity was rated by assigning a DOY to the plot when 75% of the plot exhibited heads with glumes showing complete loss of green color (Hanft and Wych, 1982; Singh et al., 1984). The DOY that was assigned to the planting date was then subtracted from the DOY of physiological maturity to obtain the number of days to maturity. The duration of the grain fill period was determined by subtracting the DOY for heading from the DOY for physiological maturity. Haun development scale measurements were taken once per week, beginning at leaf emergence, on each plot. These measurements were taken randomly on three plants within each plot, according to J.H. Haun (Haun, 1973). Chlorophyll readings were taken twice per week, after heading. Readings were taken on the center of five random flag leaves in each plot with a hand held Minolta chlorophyll meter (Minolta Camera Co., Ltd), and average chlorophyll content was obtained for each plot.
Harvest index was determined by dividing grain weight by bundle weight for five plants from each spaced plot and from a 1-m section of each mid-dense and dense plot. After each plot was combined, the seed was weighed to determine the raw grain weight of each plot. After the seed was weighed for plot weight, the seed from each plot was cleaned and reweighed on a Seedburo (Chicago, IL) test weight scale to determine the test weight of each plot. A subsample of seed was taken from each plot and was analyzed in the Single Kernel Characterization System 4100 (Perten, Huddinge, Sweden) to determine seed weight and diameter. Before harvest, 10 heads were randomly removed from each plot. Each head was individually counted for the number of spikelets it contained. The heads were then threshed and seed from each individual head was counted. The measurements of each of the 10 heads per plot were then averaged.
Statistical Analysis
Data for each response variable were analyzed by PROC MIXED in SAS (SAS Institute Inc., 1997) via analysis of variance using a randomized block split model combined over years and water levels. All factors except replications and years were considered fixed. Means for water regime and genotype were obtained, and differences were assessed with LSD. Correlations were computed in Minitab (Minitab Inc., 2000) from the cultivar means averaged over years and water levels.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Table 1 illustrates differences in mean performance based on water regime, planting density, and genotype for several traits measured in the present study. As expected, irrigation increased plot weight. Because moisture level was tested against its interaction with year, differences between the two moisture levels were often not statistically significant because of relatively few degrees of freedom in this error term. Plot weight increased, while harvest index decreased with increasing plant density. Increasing planting density resulted in a trend toward earlier heading and maturity, with the greatest impact being on days to maturity. Planting density also affected seed weight and test weight, where seed weight decreased, but test weight increased, with planting density. Cultivar means for plot weight demonstrated progress from breeding efforts as more recent cultivars such as Reeder, Scholar, and McNeal approached twice the yield of the ancestral cultivar Marquis. Harvest index ranged from 0.28 (‘Sawtana’) to 0.43 (‘Hi-Line’ and ‘Westbred 926’). Days to heading ranged from 59.7 (Westbred 926) to 67.5 (‘Era’). Reeder and Westbred 926 had the longest grain fill duration (45 d). ‘Thatcher’ and ‘Shortana’ had the lowest seed weight (29.5 mg), while ‘Fortuna’ had the highest seed weight (41.1 mg). Leaf width ranged from 1.35 cm (‘Pilot’) to 1.85 cm (Westbred 926). These data illustrate a wide range of phenotypic variation for a diverse array of characteristics encountered in a breeding program.
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The primary goal of this experiment was to identify yield-related traits that could be selected in space-planted nurseries for performance in densely seeded situations. Because of the similar means and correlations found in the mid-dense and dense treatments in this analysis, only results with the space-planted and densely seeded treatments are presented in Table 4. Correlation analysis using genotype means of the space-planted and densely seeded treatments showed highly significant correlations with all of the physiological traits, and with most of the agronomic traits. Exceptions were test weight, which was correlated at the 5% level (r = 0.546), and single seed weight, which was not significantly correlated (Table 4). Thus, genotype performance for the vast majority of traits was highly correlated between space-planted and densely seeded treatments based on means over four separate trials.
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Plant growth variables, as indicated by Haun scores, tended to be consistently highly correlated between the space-planted environments and densely seeded environments, as all correlations were highly significant in all environments (Table 4). Chlorophyll content showed consistent, and generally significant, correlations between space and densely seeded environments.
Three criteria may be important for targeting traits to be selected in spaced-planted wheat breeding nurseries, including a correlation between the trait and grain yield, a correlation between genotype performance in space-planted versus densely seeded conditions, and relative ease of assessing the trait in a spaced-planted nursery with tens of thousands of individuals. We observed generally high correlations between genotype performance in space-planted and densely seeded environments for most traits. Many of these have shown evidence of a relationship to yield, both in our experiment and in other experiments. In particular, final plot weight was highly correlated between space-planted and densely seeded treatments. This suggests that high yield may not be related to competitive ability in densely seeded stands, but to a fundamental characteristic related to plant vigor. Harvest index was also highly correlated between space-planted and densely seeded treatments. However, both of these traits are time-consuming to measure, and are probably not reasonable targets for breeding programs dealing with hundreds of thousands of plants for selection.
One easy to measure trait that was highly correlated to plot yield was width of the flag leaf. This observation has been made in other studies (Sedgley, 1991). Genotype performance for this trait was also highly correlated between space-planted and densely seeded environments. Thus, flag leaf width may be a suitable target for selection in space-planted nurseries.
Several traits related to rate of growth were both highly correlated to yield and correlated between space-planted and densely seeded plots. In particular, long grain fill period may be an interesting target for single plant selection, as measurements could be made quickly with plants marked in the field and with no need for recording numbers. Additionally, relatively rapid development to Haun stage 5 was correlated with final yield and between space-planted and densely seeded plots. These data suggest that a desirable genotype may exhibit rapid early season growth up to heading, followed by a long grain fill period. Coupled with the observation that wide flag leaves were associated with increased yield, these data suggest that duration and level of photosynthetic capacity during grain fill are important for maximum yield potential. Selection for these traits should be possible in a space-planted environment, and should be expected to be transferable to a densely seeded commercial situation.
Data from the present studies suggest targets for selection in space-planted wheat breeding nurseries that may result in higher grain yield in densely seeded planting environments. A caveat to the present data is that our measurements on space-planted individuals were replicated not only within a plot but with more than one plot per experiment. This is obviously not the case in a breeding nursery, where each plant is represented at most by a few siblings in a head row, or even as a single genetic individual. Thus, one would expect more error in measurement in an applied breeding setting and correlations to be lower than observed in this study. While progress may be slower because of the lower correlations, our data suggest that selection of plants with fast early season growth, long grain fill period, and wide flag leaves may be expected to lead to higher grain yield when the genotype is transferred to densely seeded environments.
Received for publication March 25, 2004.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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  N. K. Blake, S. P. Lanning, J. M. Martin, J. D. Sherman, and L. E. Talbert Relationship of Flag Leaf Characteristics to Economically Important Traits in Two Spring Wheat Crosses Crop Sci., March 1, 2007; 47(2): 491 - 494. [Abstract] [Full Text] [PDF]
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