Published online 1 March 2007
Published in Crop Sci 47:491-494 (2007)
© 2007 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
CROP BREEDING & GENETICS
Relationship of Flag Leaf Characteristics to Economically Important Traits in Two Spring Wheat Crosses
N. K. Blake,
S. P. Lanning,
J. M. Martin,
J. D. Sherman and
L. E. Talbert*
Dep. of Plant Sciences and Plant Pathology, Leon Johnson Hall, Montana State Univ., Bozeman, MT 59717
* Corresponding author (usslt{at}montana.edu).
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ABSTRACT
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Selection in unreplicated head rows for traits that affect yield potential is an objective for many wheat (Triticum aestivum L. ) breeding programs. Flag leaf characteristics provide a potential target for selection. In this experiment, we determined flag leaf and yield characteristics for recombinant inbred lines (RIL) from two crosses, McNeal/Thatcher and McNeal/Reeder. McNeal is intermediate between Thatcher and Reeder for green flag leaf duration after heading. Flag leaf characteristics had heritability greater than 0.70 for all traits. Heading date was positively associated with yield for one cross in dryland and irrigated environments but tended to be negatively associated with grain volume, kernel weight, and kernel protein in both crosses. The duration of green leaves after heading was positively correlated with yield, grain volume, and kernel weight in both sets of environments in the McNeal/Thatcher RIL. This trait was positively correlated with grain volume, kernel weight, and grain protein in dryland environments in the McNeal/Reeder RIL. Our results suggest that selection against late heading and for a long duration of green flag leaves after heading should result in gains in yield potential, grain volume, and kernel weight for spring wheat.
Abbreviations: FLS, flag leaf senescence • GLDAH, green leaf duration after heading • RIL, recombinant inbred lines.
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INTRODUCTION
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WHEAT BREEDING PROGRAMS often impose intense selection on unreplicated head rows for traits believed to be related to high yield potential. Given the necessity to screen many thousands of head rows, yield-associated traits that can be quickly judged before harvest are attractive targets. Past experimental data show correlations between yield and measurable agronomic traits in wheat cultivars showing different yield potential. Traits determined to be correlated to yield include harvest index (Feil, 1992; Hucl and Baker, 1987; Nass, 1973; Reynolds et al., 1994; Siddique et al., 1989; Wang et al., 2002), number of seed/m2 (Calderini et al., 1995; Feil, 1992; Reynolds et al., 1994, 1999; Wang et al., 2002), long grain-fill period (Reynolds et al., 1994; Wang et al., 2002; Hansen et al., 2005), number of seed/head (Calderini et al., 1995; Feil, 1992; Hucl and Baker, 1987; McNeal, 1960; Nass, 1973; Wang et al., 2002), number of seed/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; Hansen et al., 2005), and photosynthetic ability (Nass, 1973). Thus, there are clearly plant characteristics that are related to yield potential. Many of these characteristics, such as harvest index, seed per unit area, grain per head, and grain size require harvest and are not amenable to selection among many thousands of genotypes in the field. However, others, such as heading date and time of senescence, may be assessed without harvest and thus may be useful selection targets.
Studies with other cereals have shown that an extended period between heading and senescence (physiological maturity) may be beneficial in conditions of late-season water stress. This time may be used as an estimate of grain-fill duration and has been found to be positively correlated with grain yield in corn (Zea mays L.) (Daynard et al., 1971; Daynard and Kannenberg, 1976; Ottaviano and Camussi, 1981), especially under conditions of terminal drought stress. However, Hartung et al. (1989) were unable to show any relationship. Borrell et al. (2000) investigated the impact of green leaf area duration on grain yield in sorghum using a set of nine hybrids placed under three different water regimes. They found a high correlation (r = 0.75) between these traits under conditions of terminal water deficit where the crop depended primarily on stored soil moisture for growth and maturation. There was no association between green leaf area duration after heading and grain yield under well-watered conditions.
Hansen et al. (2005) compared agronomic and physiological traits for 20 hard red spring wheat cultivars adapted to Montana. The cultivars were tested in both space-planted and solid-seeded conditions in four separate replicated experiments in Montana. Number of days between head emergence and flag leaf senescence was highly correlated with final grain yield (r = 0.60–0.75). Flag leaf width was also highly correlated with final grain yield (r = 0.67–0.75). Leaf length was not correlated with final plot weight. This experiment suggested that maturity and size characteristics of flag leaves may be related to yield potential. Given that these traits may be measured or estimated quickly, they may be suitable targets for selection among unreplicated head rows. However, the study of Hansen et al. (2005) did not address genetic relationships between flag leaf characteristics and economically important traits. A few experiments using genetically defined populations have investigated genetic associations between maturity and senescence-related traits and yield in wheat. Sharma (1994) showed that long grain-fill period was highly correlated with grain yield in six wheat populations. Talbert et al. (2001) showed that an extended grain-fill period (as measured by the time between heading and glume senescence) tended to have positive correlations with test weight and grain protein. Other experiments using cross populations have shown inconsistent results regarding the relationship between grain-fill period and yield (Mou and Kronstad, 1994; Talbert et al., 2001).
For this experiment, we were interested to determine the genetic relationship between easy-to-assess flag leaf characteristics, including duration of green color after heading, leaf width, and leaf length, on economically important traits in two populations of recombinant inbred lines. Populations were developed from the crosses McNeal/Thatcher and McNeal/Reeder. Reeder has a long period of green flag leaf duration after heading, Thatcher has a relatively short period, and McNeal is intermediate (Hansen et al., 2005). Our results address the potential of selection for flag leaf characteristics in unreplicated rows to achieve increased yield potential.
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MATERIALS AND METHODS
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Progeny lines were derived by single seed descent, starting with the F2 generation from the crosses McNeal (Lanning et al., 1995)/Thatcher (CI 10003) and McNeal/Reeder (PI 613586). A total of 88 McNeal/Thatcher and 50 McNeal/Reeder recombinant inbred lines (RIL) with semidwarf growth habit were developed from individual F5 plants. The RIL along with two entries of each parent were planted in 12 x 12 lattice experiments with three replications. RIL in both populations were segregating for height because of parental differences at the Rht loci, and only semidwarf lines were included in this study. The experiment was grown with and without irrigation in separate, adjacent experiments in 2004 and 2005 at the Arthur H. Post Field Research Laboratory near Bozeman, MT. Each plot consisted of a single row that was 3 m long, seeded at a rate of 60 seed/m for dryland trials and 90 seed/m for irrigated trials.
In 2004, the irrigated trial was planted 16 April, and the nonirrigated plots were planted 21 April. All plots were planted 5 May in 2005. Nonirrigated and irrigated plots were harvested with a plot combine on 31 and 25 Aug. 2004 and 22 and 25 Aug. 2005, respectively. Precipitation from April through June in 2004 was 7.82 cm. An additional 10.16 cm of water was applied to the irrigated plots through June. The available soil nitrogen for 2004 was 156 kg ha–1, and 67.2–44.8–22.4 kg ha –1 N-P-K was applied and tilled into the soil before planting. Precipitation from April through June in 2005 was 7.55 cm. An additional 8.89 cm of water was applied to the irrigated plots through June. The available soil nitrogen for 2005 was 212.8 kg ha –1, and 33.6–22.4–22.4 kg ha –1 N-P-K was applied and tilled into the soil before planting.
Characteristics Evaluated
Traits evaluated included leaf length, leaf width, days to heading, days to flag leaf senescence, green leaf duration after heading, grain yield, single kernel weight, and grain volume. Leaf lengths and widths were measured on three random flag leaves in each plot using a clear ruler placed over the leaves at heading. 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. Heading dates were taken by assigning a day of the year when 50% of the heads were completely emerged. A day of the year was assigned to the planting date and was subtracted from the day of the year of heading to obtain the number of days to heading. Senescence date of flag leaves was rated by assigning a day of the year to the plot when 75% of the plot exhibited flag leaves showing complete loss of green color (Hansen et al., 2005). The day of the year that was assigned to the planting date was then subtracted from the day of the year of flag leaf maturity to obtain the number of days to flag leaf senescence. The duration of green leaf period after heading was determined by subtracting the day of the year for heading from the day of the year for flag leaf senescence. 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 grain volume 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 single kernel hardness, weight, and diameter.
Data Analysis
Analysis of individual experiments showed that the lattice design provided no gain in precision over a randomized block design. Therefore, an analysis of variance was obtained for all traits for each cross using a model for a randomized block combined over all environments with PROC GLM in SAS (SAS Institute, Inc., 2004). All factors except environments were considered random. Narrow sense heritability values were computed for each trait-cross combination on an entry mean basis, and 95% confidence intervals were computed as described in Knapp et al. (1985). Correlations among traits were computed for the irrigated and non-irrigated environments using the entry means from each environment.
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RESULTS
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Parents were selected on the basis of divergent characteristics for flag leaf and maturity characteristics (Tables 1 and 2). Significant differences between McNeal and Thatcher (Table 1) included a later heading date for Thatcher and longer green leaf duration after heading for McNeal. McNeal has wider flag leaves, while Thatcher has longer flag leaves. McNeal has higher grain yield, higher grain volume, higher kernel weight, and higher grain volume than Thatcher. Significant differences between McNeal and Reeder (Table 2) included earlier heading date, later flag leaf senescence, and a longer green leaf duration after heading for Reeder. Grain yield characteristics were not significantly different between these parents.
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Table 1. Mean performance of McNeal, Thatcher, and 88 recombinant inbred lines from a McNeal/Thatcher cross averaged over four trials for flag leaf characteristics and economically important traits.
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Table 2. Mean performance of McNeal, Reeder, and 50 recombinant inbred lines from a McNeal/Reeder cross averaged over four trials for flag leaf characteristics and economically important traits.
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Heritability was lowest for grain volume in both McNeal/Thatcher (0.34) and McNeal/Reeder (0.57). All heritability estimates exceeded 0.0 on the basis of their 95% confidence intervals (Tables 1 and 2). Green leaf duration after heading had heritability values of 0.77 and 0.73 for the McNeal/Thatcher and McNeal/Reeder populations, respectively. These values reflect high heritability values for the component traits heading date and date of flag leaf senescence. Average performances of RIL derived from McNeal/Thatcher and McNeal/Thatcher were different (P < 0.01) for all flag leaf related traits, including leaf width, leaf length, date of flag leaf senescence, and green leaf duration after heading. Grain yield and kernel weight was significantly higher in the McNeal/Reeder population, while there were no significant differences between the two RIL sets for heading date and grain protein. Because of the different mean performances of the RIL sets, correlation analyses were conducted separately for each cross.
Correlations were calculated separately for dryland and irrigated environments as previous work with other cereals has shown that delayed senescence is most likely to be related to yield in dryland environments (Daynard et al., 1971; Borrell et al., 2000). Correlations between flag leaf characteristics and agronomic traits for the McNeal/Thatcher population are shown in Table 3. Heading date was not correlated with yield in either environment. Negative correlations (P < 0.05) were observed between heading date and grain volume in the irrigated environments and between heading date and kernel weight (P < 0.05) in the dryland environment. Date of flag leaf senescence was not significantly correlated with yield but was positively correlated (P < 0.05) with grain volume and kernel weight in the dryland environments. Green leaf duration after heading was positively correlated (P < 0.05) with yield in both sets of environments and was positively correlated (P < 0.01) with grain volume in both sets of environments. Green leaf duration after heading was positively correlated (P < 0.01) with kernel weight in irrigated environments. Leaf width was negatively correlated (P < 0.05) with grain volume and protein in the irrigated environments. There was no significant association between leaf size measurements and yield.
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Table 3. Correlation of flag leaf characteristics and economically important traits in recombinant inbred lines derived from a cross between McNeal and Thatcher in dryland and irrigated experiments grown in 2004 and 2005.
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Table 4 shows correlations between flag leaf characteristics and several agronomic traits for progeny lines from the cross between McNeal and Reeder. Heading date was positively correlated (P < 0.05) with yield in both dryland and irrigated environments. However, heading date was negatively correlated (P < 0.05) with grain volume in the dryland trials and with grain volume, kernel weight, and protein in the irrigated trials (P < 0.05). Date of flag leaf senescence was positively correlated (P < 0.01) with yield in the dryland environments and negatively correlated (P < 0.05) with protein in the irrigated environments. Green leaf duration after heading was not associated with yield in either set of environments but was positively correlated with grain volume (P < 0.01), kernel weight (P < 0.01), and grain protein (P < 0.05) in the dryland environments. The width and length of flag leaves were not correlated with any of the agronomic traits in either dryland or irrigated environments.
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Table 4. Correlation of flag leaf characteristics and economically important traits in recombinant inbred lines derived from a cross between McNeal and Reeder in dryland and irrigated experiments grown in 2004 and 2005.
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DISCUSSION
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Hansen et al. (2005) found that traits related to flag leaf characteristics showed significant associations with grain yield for a set of 20 hard red spring wheat varieties released during the past century. Modern varieties were higher yielding and tended to have wider flag leaves and later flag leaf maturity. The time of flag leaf senescence after heading was longer in modern varieties. The varieties Thatcher, McNeal, and Reeder were evaluated by Hansen et al. (2005) and were used as parents for the present study. Thatcher, McNeal, and Reeder were released in 1934, 1995, and 1999, respectively. Notable differences among these varieties included wider flag leaves in McNeal and Reeder than in Thatcher and longer leaves in Thatcher versus the other two. Thatcher had the latest heading date, yet time of flag leaf senescence was no later than that for McNeal. Reeder headed earlier than the other two varieties, yet flag leaf senescence was later. This gave Reeder a longer period of green leaf duration after heading. On the basis of several years of observations in Montana Spring Wheat Variety Trials, Reeder was remarkable for this trait relative to other widely grown varieties. Additionally, Reeder has shown high yield potential in dryland areas of production in Montana (Plant Sciences and Plant Pathology Department, 2006).
For this experiment, we wished to determine the genetic relationship between flag leaf characteristics and traits of economic importance to wheat growers. Flag leaf characteristics are an attractive target for selection, in that they can be assessed quickly and without the need for harvest. This is important in large breeding nurseries which may contain many thousands of unreplicated head rows. In addition to being related to yield or other economically important traits, the high heritability of these traits is important for selection efforts to be effective. In this experiment, heritability values for green leaf duration after heading were at least 0.73 for both crosses, reflecting high heritability values for both heading date and date of leaf senescence. These values are consistent with previous results that showed heritability values of 0.89 for heading date, 0.64 for physiological maturity (estimated as time of glume senescence), and 0.40 for the period between heading and glume senescence based on several spring wheat crosses (Talbert et al., 2001).
Heading date was positively correlated with grain yield in the McNeal/Reeder cross for both dryland and irrigated environments. However, within this cross, correlations tended to be negative with grain volume, kernel weight, and protein. Heading date was not correlated with yield in the McNeal/Thatcher cross but also showed negative correlations with grain volume and kernel weight in dryland and irrigated environments, respectively. These data suggests that late heading is not a good selection goal in that any gain in yield may be offset by losses in other traits of economic importance. Additionally, there is a risk with late heading in the Great Plains of extending flowering into the hottest part of the summer, potentially resulting in loss of fertility and seed set.
The time of flag leaf senescence was positively associated with yield in the McNeal/Reeder population grown under dryland conditions. However, late flag leaf senescence was associated with low protein for this cross grown under irrigation. Delayed flag leaf senescence was not associated with increased yield in the McNeal/Thatcher population but was associated with increased grain volume and kernel weight in dryland environments. In general, these data suggest that selection for late flag leaf senescence alone may have a small impact on yield but may be accompanied by risk in regard to other traits. Additionally, delayed senescence is correlated with late heading (r = 0.76), which itself may be deleterious for some traits.
Time between heading and flag leaf senescence may be a superior selection target to senescence alone in that the effect of late heading per se should be mitigated. In this experiment, the duration of green leaves after heading was positively correlated with several of the economically important traits we measured. No correlation was observed with yield in the McNeal/Reeder cross. However, highly significant positive correlations were observed with test weight, kernel weight and grain protein in the dryland environments. Significant positive correlations were observed between duration of green leaves after heading date with yield and grain volume in the McNeal/Thatcher cross in both dryland and irrigated environments. There was a significant positive correlation with kernel weight in the irrigated environment. These results show that an extended period of time between heading and flag leaf senescence tends to be positive in these crosses, and positive effects are not mitigated by negative effects on other traits.
These results may be compared with previous data from Talbert et al. (2001), who studied the relationship of heading date, time of glume senescence (physiological maturity), and the time between glume senescence and heading date with grain yield, grain volume, and grain protein for 12 spring wheat crosses grown in three Montana environments. The parents of these crosses had a fairly narrow range for the time between heading and flag leaf senescence (Talbert et al., 2001). Talbert et al. (2001) found that late heading and late physiological maturity tended to be negatively correlated with grain volume and protein. Positive correlations were observed between heading date and grain yield for four of 12 crosses and between physiological maturity date and grain yield for three of 12 crosses. However, significant negative correlations were observed between heading date and grain volume for 10 of 12 crosses and between physiological maturity and grain volume for nine of 12 crosses. Additionally, correlation between these traits and grain protein was often negative. The time between heading and glume senescence (estimated as grain-fill period) was not significantly associated with yield potential in these crosses. However, this trait was positively associated with grain volume and grain protein in three of 12 and five of 12 crosses, respectively.
The primary difference between the two experiments is in the selection of parents for the crosses. Talbert et al. (2001) used parents within a narrow range of heading to senescence time periods. In this experiment, we included a line within this range (McNeal), an older line with relatively fewer days between heading and senescence (Thatcher), and a recent release that is remarkable for its delayed time of senescence (Reeder). Additionally, in the current experiment, we measured senescence of the flag leaf rather than glumes as in the previous experiment. Flag leaves were reported in this paper because the ease of measurement lends itself more to rapid evaluation of many lines. Results tended to show a stronger positive effect of long green leaf duration after heading in the current experiment, perhaps because of the greater differences among the lines. The cumulative results of both experiments suggest that selection for a normal heading date along with delayed leaf senescence in head row nurseries should result in higher yielding lines to evaluate in future generations of selection. The positive effect of long green leaf duration after heading is pronounced in dryland environments, which characterize most areas of spring wheat production in the northern Great Plains.
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ACKNOWLEDGMENTS
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Research was supported by awards from USDA-CREES CAP (2006-55606-16629) and the Montana Board of Research and Commercialization.
Received for publication May 3, 2006.
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ABSTRACT
INTRODUCTION
