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a Dep. of Botany and Plant Sci., Univ. of California, Riverside, CA 92521-0124
b Dep. of Soil Sci., Faculty of Agriculture, Tishreen Univ., Lattakia, Syria
c Gryndlscot Farms, RR 9, Dunnville, Ontario N1A 2W8 Canada
* Corresponding author (bahman.ehdaie{at}ucr.edu)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Abbreviations: WSC, water-soluble carbohydrates
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Large variation reported for the amount of WSC accumulation and mobilization in wheat stems is partly due to differences in environmental conditions, genotypes, and the demand by developing grains for carbohydrates (Evans and Wardlaw, 1996). Another possible cause of the variability could be the sampling of whole stems. It was reported there are differences among internodes in the amount of WSC that are accumulated and mobilized (Wardlaw and Willenbrink, 1994; Shakiba et al., 1996).
Developmentally, potential stem reserve accumulation and subsequent mobilization in wheat depends on stem length and stem specific weight (stem weight/stem length). Storage increases with longer stems and greater specific weight (Blum et al., 1994). The Rht-B1b and Rht-D1b (previously known as Rht1 and Rht2, respectively) dwarfing genes of wheat were found to reduce reserve storage by 35 and 39%, respectively, as a consequence of 21% reduction in stem length (Borrell et al., 1993).
Stem internodes including the peduncle, penultimate, and the lower internodes are components of the stem in wheat. The first paper of this series demonstrated substantial genotypic variation for stem length and stem weight in different internodes of the main stem (Ehdaie et al., 2006). Dry matter accumulation and mobilization varied along the stem in well-watered and droughted field conditions. Partitioning of stem length into different internodes had a bearing on reserve accumulation before and after anthesis. The contribution of the lower internodes as a segment to stem mobilized dry matter, on average, was 55%, that of the penultimate was 27% and that of the peduncle was 18%. The relatively modern semidwarf and dwarf bread wheat cultivars examined such as ‘Express’, ‘Anza’, and ‘Yecora Rojo’ had relatively short lower internodes as a stem segment that limited their capacity to store reserves before anthesis. Ehdaie et al. (2006) concluded that balanced partitioning of stem length into upper and lower internodes should improve accumulation and mobilization of stem reserves in wheat.
There are two component traits involved in the extent of contribution of stored reserves to grain yield in wheat (Ehdaie and Waines, 1996). The first component is the ability to store assimilates in the stem and the second component is the efficiency with which the stored reserves are mobilized and translocated to grain.
The amounts of accumulated and mobilized stem reserves are either estimated indirectly by monitoring the changes in internode dry weight (Hunt, 1978; Pheloung and Siddique, 1991; Borrell et al., 1993; Shakiba et al., 1996; Cruz-Aguado et al., 2000), or directly by measuring internode WSC content during the grain-filling period (Davidson and Chevalier, 1992; Kiniry, 1993; Blum et al., 1994, Shakiba et al., 1996). In previous studies only a few wheat cultivars or isogenic lines for plant height were used that did not represent the genotypic variation for WSC accumulation and mobilization (Davidson and Chevalier, 1992; Kiniry, 1993; Blum et al., 1994; Wardlaw and Willenbrink, 1994; Shakiba et al., 1996). Therefore, little is known about the extent of genotypic variation for stem WSC accumulation in the internodes and of the efficiency with which stored WSC are mobilized and transported to grain in wheat. In the first paper of this series, we reported the genotypic variation in postanthesis changes in internode dry weight and specific weight for a diverse set of 10 bread wheat cultivars including spring and winter types and tall, semidwarf, and dwarf genotypes along with a semidwarf spring durum wheat (T. turgidum L. var. durum) (Ehdaie et al., 2006). In this study, we tested the hypotheses that different internodes of the main stem in wheat accumulate and mobilize different amounts of WSC, that internodes respond differently to drought with regard to these traits, and that there is significant genotypic variation among wheat cultivars for WSC-related traits.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Field experiments were sown on 19 Dec. 1997 and on 15 Jan. 1999 in a Ramona Type A sandy loam soil (fine-loamy, mixed, thermic Typic Haploxeralf) at the Moreno Farm of the University of California Agricultural Experiment Station, Moreno Valley, CA. The 11 genotypes were sown in a split-plot design with four replicates (blocks). The main plots consisted of two irrigation treatments, namely well-watered and droughted treatment. The split-plot consisted of the genotypes. Plants in well-watered treatment were irrigated with sprinklers to minimize water shortage until they reached physiological maturity. Irrigation was terminated for plants in droughted treatment when plants in 50% of plots reached late booting stage on 23 Mar. 1998 and on 15 Apr. 1999. In the 1997 season, plants in the well-watered treatment received 496 mm of water (132 mm irrigation + 364 mm rain) and those in the droughted treatment received 430 mm of water (66 mm irrigation + 364 mm rain). In the 1999 season, plants in the well-watered treatment received 332 mm of water (278 mm irrigation + 54 mm rain) and those in the droughted treatment received 270 mm of water (216 mm irrigation + 54 mm rain). After irrigation was terminated in the droughted treatment, 63 and 30 mm of rain fell during early grain-filling period in the 1997 season and in the 1999 season, respectively.
Each plot consisted of six rows, 5 m in length. Interrow spacing was 20 cm and interplant spacing was 3 cm. The land was fallowed the previous year and 112 kg ha–1 urea fertilizer was incorporated into the soil before planting.
In each plot, 30 to 40 main tillers from the two middle rows next to the guard rows were tagged as spikes emerged from the flag leaf sheaths. Three main tillers were harvested at random as each genotype reached anthesis and at 10-d intervals after anthesis until maturity. The main tillers were harvested from the soil surface. After each harvest, leaf blades were removed and main tillers were immediately dried in a forced-air drier at 80°C for 48 h. Then, each main tiller was divided into spike and stem; then leaf sheaths were removed from the stem. Each stem was divided into three segments, namely peduncle (first internode below the spike including the distal node), penultimate internode (the internode below the peduncle including the distal node), and the lower internodes. The length and weight of each segment was measured. The segments were chopped separately into fragments of about 2 to 4 mm and transferred into plastic scintillation vials to which 5 mL of distilled water were added. The vials were sealed and placed in a hot water bath (90°C) for 1 h. After cooling, the samples were filtered into tubes and kept refrigerated until analysis. To determine total WSC content, 2 mL of 1% resorcinol in ethanol and 0.75 mL of 30% HCl were added to each tube and the standard tubes; then tubes were incubated at 80°C for 8 min. After chilling the tubes in a cold water bath, the WSC content of each sample was determined (Roe, 1934). Two runs were made for each sample and the mean was used in the statistical analyses. The WSC content of each stem segment was divided by its length to calculate WSC specific content (WSC linear content, mg WSC cm–1 length) and WSC concentration was calculated by dividing WSC content of each segment to its weight (mg WSC g–1 dry weight).
The magnitude of mobilized WSC in each internode segment was estimated as the difference between postanthesis maximum and minimum WSC content. Mobilization efficiency of WSC in each internode segment was estimated by the proportion (%) of postanthesis maximum WSC content of that segment that mobilized. An approximate standard deviation for mobilization efficiency (a ratio) was determined by assuming the numerator and denominator are independent. In this case, the estimates will be conservative since the numerator (amount of WSC mobilized) and the denominator (postanthesis maximum WSC content) are positively correlated.
Dates of heading, anthesis, and physiological maturity were, respectively, determined from the four middle rows in each plot when 50% of spikes partially emerged from the flag leaf sheaths, when 50% of the spikes had extruded anthers, and when 50% of the spikes lost their green color.
Analysis of variance (ANOVA) was performed for each character measured or calculated for each year (Steel et al., 1997). The mean of the three samples in each harvest was used in the statistical analysis. The combined ANOVA was also performed across years, treating irrigation, genotype, and date of harvest as fixed effects and year, replication, and their interactions with irrigation, genotype, and date of harvest as random effects. Tests of significance of fixed effects were accomplished by using appropriate mean squares (Steel et al., 1997). Associations between characters were examined by correlation analysis. Means were compared using the LSD test (Steel et al., 1997). The variance components for stem maximum WSC content, measured at 20 d after anthesis, were estimated using the combined ANOVA across years and irrigation regimes (Miller et al., 1958). Broad-sense heritability for stem maximum WSC content was calculated based on genotypic means (Fehr, 1987).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The significant two-way interactions observed were mainly due to differences in changes of magnitude of genotype means (non-crossover interactions) rather than in ranking of the genotypes (crossover interactions) in different years and in different irrigation regimes. Among the three-way interactions, the patterns of genotype x irrigation interactions and the patterns of genotype x date of harvest interactions were similar in both years. Only genotype x date of harvest showed different patterns over irrigation treatments. Based on these observations, means averaged across years and irrigation regimes and the patterns of genotypic variation across date of harvest will be reported.
Since there was genotypic variation in phenological periods, the correlation between the number of days each genotype was under drought in the droughted treatment until physiological maturity and stem WSC content in the droughted treatment was determined for both growing seasons. The correlation coefficients in the first season (r = –0.40) and in the second season (r = –0.01) were not significant, indicating that stem WSC content was not confounded by the genotypic differences in phenology. Days from sowing to anthesis, on average, ranged from 102 to 122 d in the first season and from 96 to 109 d in the second season. Drought imposed at late boot stage reduced number of days to anthesis, on average, by 1 to 7 d depending on the genotype. Anza (winter) and Wincora showed delayed anthesis by 6 and 11 d compared to their respective spring types, Anza (spring) and Yecora Rojo.
Main Effect
Water-Soluble Carbohydrate Content
Drought, on average, reduced WSC content in the peduncle by 26%, in the penultimate internode by 36%, and in the lower internodes of the main stem by 32% (Table 1). There was significant variation for internode WSC content among the genotypes. Only durum wheat Westbred Turbo consistently had greater WSC content in all segments of the main tiller. Maringa, a relatively tall cultivar had the lowest WSC in peduncle (57.1 mg), whereas Ramona 50, a semi-tall cultivar, had the highest WSC content (95.3 mg) among the bread wheat cultivars. The landraces, No. 14, No. 49, and Chinese Spring, as a group, had greater WSC content in penultimate and the lower internodes than semidwarf and dwarf modern cultivars such as Anza (spring), Express, and Yecora Rojo (Table 1).
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Water-Soluble Carbohydrate Specific Content
Drought reduced WSC specific content (linear content) in the peduncle by 13% and in penultimate and the lower internodes by 27% (Table 2). Genotypic differences in WSC specific content were significant for each segment of the main stem. Among the genotypes, durum wheat Westbred Turbo and bread wheat Yecora Rojo showed the highest means for WSC specific content in the internodes, followed by Ramona 50 and Wincora. The WSC specific content of the landraces such as No.14, No. 49, and Chinese Spring were among the lowest (Table 2).
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Water-Soluble Carbohydrate Concentration
Drought had no significant effect on concentration of WSC in the peduncle, but reduced it by 13 and 21% in the penultimate and lower internodes, respectively (Table 3). Genotypic differences were significant for WSC concentration in each segment of the main tiller. None of the genotypes examined had consistently greater concentration of WSC in all segments. Westbred Turbo had the highest concentration of WSC in the peduncle and penultimate internodes and Yecora Rojo had the highest concentration in the lower internodes (Table 3).
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In peduncle and penultimate internodes the maximum WSC concentration was reached 20 d and in the lower internodes 10 d after anthesis (Table 3). Concentration of WSC in the penultimate internode (409 mg g–1 dry weight) was greater than in the peduncle (342 mg g–1 dry weight) and the lower internodes (340 mg g–1 dry weight).
Genotype x Date of Harvest Interaction
Water-Soluble Carbohydrate Content
Peduncle
Different patterns of postanthesis changes in WSC content in the peduncle were observed among the genotypes (Fig. 1
). The amount of WSC in the peduncle increased after anthesis and reached a maximum 20 d later for all the genotypes, except for No. 49 which reached maximum 10 d postanthesis. However, the amounts and rates of increase were different. Durum wheat Westbred Turbo was exceptionally different from the bread wheat genotypes. It had the highest WSC content at anthesis (161.3 mg) with a rate of accumulation of 5.5 mg d–1 afterward to reach a maximum of 270.8 mg WSC in the peduncle. Also, the rate of decline in WSC content from peak accumulation to maturity in Westbred Turbo was the sharpest with 7.2 mg d–1 (Fig. 1). The amount of WSC content in the peduncle of bread wheat genotypes at anthesis ranged from 38.9 mg for Anza (spring) to 80.6 mg for Ramona 50. Among the bread wheat genotypes, landrace No. 14 had the highest rate of WSC accumulation (5.8 mg d–1) and rate of decline (4.4 mg d–1). In contrast, Ramona 50 had the lowest rate of WSC accumulation (2.6 mg d–1) and exceptionally slow decline (2.8 mg d–1) (Fig. 1). A positive correlation (r = 0.61, P < 0.05) was found between rate of WSC accumulation and decline in the peduncle.
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Accumulation of WSC per unit length of the penultimate internode continued until 20 d postanthesis for all the genotypes, except for Westbred Turbo and No. 49 which ceased 10 d after anthesis. Westbred Turbo showed the highest accumulation per unit length for the penultimate internode (17.9 mg cm–1) followed by Yecora Rojo (14.7 mg cm–1), and Express (12.5 mg cm–1). Also, these genotypes exhibited similar and faster patterns of decline in WSC specific content in the penultimate internode. The lowest accumulation of WSC per unit of length in the penultimate internode belonged to Maringa, Anza (spring), Anza (winter), and Chinese Spring. No. 49 had the lowest rate of decline in WSC specific content in the penultimate internode.
Westbred Turbo, Maringa, and Chinese Spring attained their maximum WSC specific content in the lower internodes at anthesis, whereas Westbred Turbo, Express, Anza (winter), No. 49, and Anza (spring) attained it 10 d postanthesis; and Yecora Rojo, Ramona 50, and No. 14 reached it 20 d after anthesis. Maximum WSC specific content in the lower internodes belonged to Yecora Rojo (15.7 mg cm–1) followed by Westbred Turbo (15.1 mg cm–1), and Express (13.6 mg cm–1). In contrast, Chinese Spring (6.7 mg cm–1) and Maringa (8.2 mg cm–1) had the lowest accumulation of WSC per unit of these stem segments. Rate of depletion of WSC specific content in the lower internodes was similar for Yecora Rojo and Westbred Turbo and significantly greater than those of other genotypes.
Water-Soluble Carbohydrate Concentration
Different patterns of postanthesis changes in WSC concentration in the peduncle, penultimate and the lower internodes were observed among the genotypes (patterns not shown). At anthesis, concentration of WSC in the peduncle of durum wheat Westbred Turbo (407 mg g–1) was markedly greater than those of bread wheat genotypes which ranged from 167 mg g–1 for Anza (spring) to 309 mg g–1 for No. 49. Concentration of WSC in the peduncle of Westbred Turbo, No. 14, Wincora, Express, and Anza (spring) increased after anthesis, although with different rates, and peaked at 20 d postanthesis. It peaked at 10 d after anthesis in No. 49.
At anthesis, WSC concentration in the penultimate internode of durum wheat Westbred Turbo was the highest (518 mg g–1). It varied among the bread wheat genotypes from 204 mg g–1 for Maringa to 397 mg g–1 for Wincora. Only Westbred Turbo and No. 49 showed maximum WSC concentration in the penultimate internode 10 d after anthesis; the other genotypes attained it 20 d postanthesis.
Concentration of WSC in the lower internodes at anthesis varied from 255 mg g–1 for Ramona 50 to 424 mg g–1 for Yecora Rojo. Yecora Rojo, No. 14, Maringa, and Ramona 50 had a decline in WSC concentration in the lower internodes after anthesis, followed by an increase which peaked at 20 d postanthesis.
Water-Soluble Carbohydrate Mobilization and Efficiency
Peduncle
Drought, on average, reduced postanthesis maximum and minimum WSC content in the peduncle of all genotypes (Table 4). Mean mobilized WSC in the peduncle was similar under well-watered (102 mg) and droughted (105 mg) field conditions. The efficiency of WSC mobilization in the peduncle, on average, was greater under droughted (85%) than well-watered (64%) conditions. However, genotypic reactions to drought with regard to WSC mobilization and efficiency were different. No. 14, No. 49, Ramona 50, and Anza (spring) had greater postanthesis accumulation of WSC in peduncle in droughted than in well-watered conditions. In contrast, Chinese Spring, Maringa, Anza (winter), Express, Wincora, and Westbred Turbo showed a reverse trend; whereas Yecora Rojo had similar maximum WSC content in peduncle under both irrigation regimes (Table 4). Postanthesis minimum WSC content in the peduncle was lowered by drought in all genotypes, except in Yecora Rojo. The amount of WSC mobilized in the peduncle of No. 14, No. 49, Chinese Spring, and Anza (spring) was markedly greater under droughted than well-watered treatment. Ramona 50, Maringa, Anza (winter), Express, Wincora, and Westbred Turbo showed a reverse trend. Mobilized WSC in the peduncle of Anza (winter) and Yecora Rojo was similar in both moisture regimes. All genotypes had greater efficiency in mobilizing WSC under drought, except Ramona 50 that demonstrated a reverse trend; and Yecora Rojo and Westbred Turbo each had similar mobilization efficiency in both irrigation treatments (Table 4).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Our research demonstrated significant genotypic variation in the pattern of accumulation and depletion of WSC in peduncle, penultimate, and the lower internodes of the main stem. The genotypic ranges observed for WSC content, WSC specific content, WSC concentration, and mobilized WSC were greater than those reported by others (Davidson and Chevalier, 1992; Wardlaw and Willenbrink, 1994; Shakiba et al., 1996; Shearman et al., 2005) because a large number and a diverse set of genotypes were used in this study. Previous studies indicated that accumulation of WSC in stem of wheat peaked at anthesis (McCaig and Clarke, 1982; Shakiba et al., 1996), while others reported that WSC accumulated until 7 to 22 d after anthesis (Davidson and Chevalier, 1992; Kiniry, 1993; Wardlaw and Willenbrink, 1994). Our results indicated that different internodes attained their maximum accumulation of WSC either at anthesis or 10 to 20 d postanthesis, depending on the internode and genotype (Fig. 1, 2, 3). Considering all the internodes, stem maximum WSC content was reached 10 to 20 d postanthesis for all the genotypes examined.
The genotypic differences in number of days from sowing to anthesis was not significantly correlated to stem WSC content (r = 0.47), or to stem WSC specific content (r = 0.10), or to stem WSC concentration (r = 0.10). For example, number of days from sowing to anthesis was 107 d for No. 14, Anza (spring), and Express, but stem WSC content for the three genotypes were 404, 240, and 309 mg, respectively. In contrast, Chinese Spring and Ramona 50 reached anthesis 113 and 99 d after sowing, but they had similar stem WSC content, 349 and 359 mg, respectively. Ehdaie et al. (2006) reported that mean stem weight and maximum stem weight for these genotypes were not related to the time of anthesis. Hunt (1978), in a study of 22 wheat cultivars, reported that maximum stem weight was not correlated to the timing of spike emergence. The lack of close association between stem WSC-related traits and time of anthesis indicate that these traits can be manipulated in wheat breeding programs without significantly affecting the phenological periods.
Among the three segments of main stem, the maximum amount of WSC was, on average, accumulated in the lower internodes (294 mg) followed by the penultimate internode (179 mg) and peduncle (137 mg). These results are in agreement with those reported by others (Wardlaw and Willenbrink, 1994; Takahashi et al., 2001). Averaged over irrigation regimes and years, the lower internodes stored 48% of WSC of main stem. The rate of depletion of WSC was faster in the lower internodes (7.0 mg d–1) compared with those in the peduncle (3.3 mg d–1) and penultimate (4.5 mg d–1) internodes, indicating that stored WSC in the lower internodes were mobilized faster than those in the upper internodes. Therefore, despite the greater amounts of WSC accumulated in the lower internodes compared to those in the peduncle and penultimate internodes, the depletion of WSC in the three segments was completed approximately at the same time, 40 d postanthesis or 10 d before physiological maturity (Fig. 1, 2, 3). The postanthesis depletion of WSC content in the peduncle, penultimate, and the lower internodes indicated that all three segments mobilized WSC for grain filling.
Considering different internodes, certain genotypes showed consistent patterns for postanthesis changes and maximum values for the WSC-related traits examined. For example, Westbred Turbo and No. 14 had relatively greater amount of WSC content for each internode, while Anza (spring) and Maringa showed relatively smaller values. Westbred Turbo, Yecora Rojo, and Express consistently had greater postanthesis values for WSC specific content, whereas Anza (spring), Maringa, and Chinese Spring had lower values in each internode. Westbred Turbo, No. 14, and Yecora Rojo consistently showed greater values for WSC concentration, whereas Anza (spring) and Maringa had lower values. Also, Ramona 50 and No. 49 showed relatively slow rates of WSC depletion in most cases. These observations indicate that the WSC-related traits studied are under genetic control. This is supported by the maximum values observed for these WSC-related traits being associated more with the modern genotypes than with landraces and old genotypes, with the exception of No. 14. It may be speculated that genetic gain in grain yield of modern spring bread wheat cultivars was partially associated with early maturity and with a reduction in plant height (Ehdaie et al., 2006), but also with an increase in stem WSC specific content and WSC concentration to compensate for shorter plant stature. Recent genetic gains in grain yield of winter wheat cultivars in United Kingdom were attributed to a combination of improved growth rate in the preanthesis period and a larger source for grain filling through increases in stem WSC reserves (Shearman et al., 2005). As far as we are aware, no direct selection for WSC-related traits has been practiced in wheat breeding programs nor has the inheritance of WSC content been studied in wheat. Thus, increased WSC specific content and WSC concentration observed in the modern spring wheat cultivars should be the result of indirect selection via selection for increased grain yield. Improvement in grain yield in wheat was partially attributed to increased number of grains per unit area (Miralles and Slafer, 1995; van Ginkel et al., 1998; Slafer et al., 1999). Apparently, as the number of grains per unit area increased due to selection for higher grain yield and short plant stature, wheat cultivars with greater WSC content per unit stem length and weight were indirectly selected to maintain grain growth and development, especially under stressed environments where current photosynthesis is depressed.
Estimates of variance components and the broad-sense heritability calculated for stem maximum WSC content, measured 20 d after anthesis, indicate that this trait is under genetic control. The genetic component of variance (0.010) was larger than those associated with genotype x year (0.001) and genotype x irrigation regime (0.001) interactions, but smaller than that associated with genotype x year x irrigation regime interaction (0.048). The estimate of broad-sense heritability, based on genotypic means, was 0.42, which indicated that 42% of genotypic variation observed for stem maximum WSC content was due to genetic effects. Because a relatively large variance component was associated with genotype x year x irrigation regime interaction, genotypes should be evaluated under a combination of years and irrigation regimes to obtain a reliable estimate of stem maximum WSC content.
The amount of WSC mobilized from the peduncle was, on average, similar under both irrigation treatments, but the genotypic differences observed were significant in each moisture regime (Table 4). On average, more WSC was mobilized from the penultimate and the lower internodes under well-watered than droughted conditions, though significant differences were found among the genotypes in each moisture treatment (Table 5, 6). The average WSC mobilized in the lower internodes (259 mg) was highest followed by the penultimate (149 mg) and peduncle (103 mg) internodes. The amount of WSC mobilized was highly correlated with postanthesis maximum WSC content ranging from 0.89 to 0.99, depending on internode and irrigation regime.
Among the genotypes examined, relatively high accumulation of WSC was observed in all segments of the main stem only in durum wheat Westbred Turbo. Since this genotype also had relatively high mobilization efficiency, the amount of mobilized WSC was highest in Westbred Turbo ranging from 158 to 557 mg, depending on internode segment and irrigation regime (Table 4, 5, 6). This may be the first report of WSC-related traits in durum wheat. Landraces No. 14, No. 49, and Chinese Spring accumulated greater amount of WSC in their main stems compared to the modern genotypes such as Anza (spring), Express, and Yecora Rojo, but their mobilization efficiency was lower in well-watered conditions. In this study we did not measure the number of tillers per plant. Whaley (2001) reported that wheat stem accumulate more WSC in low than in high plant population density. Since we used a divers set of wheat cultivars, the data reported for WSC content might be confounded by the genotypic variation in number of tillers per plant.
Under well-watered conditions, Anza (winter) and Wincora accumulated more WSC in the main stem probably due to delayed anthesis compared to their respective spring type, namely Anza (spring) and Yecora Rojo; they also mobilized more WSC. However, under droughted conditions, Yecora Rojo and Anza (spring) stored more WSC in the main stem than their respective winter types. Since mobilization efficiency was similar for both spring and winter types under droughted condition, mobilized WSC was greater in Yecora Rojo and Anza (spring) than Wincora and Anza (winter) in droughted conditions (Table 4, 5, 6). It appeared that incorporation of vernalization gene vrn1 in spring wheat cultivars Anza (spring) and Yecora Rojo had an advantage with regard to mobilized WSC in well-watered conditions, but had a disadvantage in droughted conditions.
In regions with Mediterranean climate spring wheat is typically planted in late autumn or early winter and harvested in early summer. Thus, the crop is grown under conditions when rainfall and temperatures are favorable for plant growth before anthesis and then it matures in terminal drought and heat. Under favorable conditions C assimilation rates are higher (Davidson and Chevalier, 1992) and a large portion of assimilates is accumulated in the lower internodes which attain their maximum length before or during anthesis as reported in our previous study (Ehdaie et al., 2006). In the present study, mean maximum WSC content in the lower internodes was 294 mg (Table 6). Of this amount, 219 mg (75%) was accumulated before anthesis and 75 mg (25%) was accumulated during 10 d following anthesis. In comparison, 46 and 56% of WSC found in the peduncle and penultimate internodes were accumulated before anthesis, respectively. Thus, the lower internodes should have appropriate length to store WSC before and after anthesis to become a major source for mobilization of WSC during the grain filling period.
Among the genotypes examined, only three cultivars, namely Express, Yecora Rojo, and Westbred Turbo, demonstrated mobilization efficiency >0.80 for all segments of main stem under both irrigation regimes. However, the capacity for accumulation of WSC in the main stem was relatively low in Express and Yecora Rojo due to their short stems. Main stem length, averaged over both irrigation regimes and years, was 59.7 cm for Express and 47.2 cm for Yecora Rojo compared to 74.9 cm for Westbred Turbo (Ehdaie et al., 2006). Yecora Rojo had the shortest peduncle (20.1 cm), penultimate (11.8 cm), and the lower internodes (15.3 cm) among the cultivars. The lengths of these segments in Express were 23.5, 13.3, and 22.9 cm, respectively. The lower internodes in Yecora Rojo and Express were shorter than those of Westbred Turbo (31.9 cm). Plant height in Yecora Rojo and Express was below the optimum height reported to be between 70 and 100 cm in spring wheat (Ehdaie and Waines, 1994; Miralles and Slafer, 1995) and between 80 and 90 cm in winter wheat (Gent, 1995). Also, the lower internodes in Yecora Rojo and Express were relatively short to accumulate a large amount of WSC before anthesis. Similar conclusions were reached based on postanthesis changes in internode dry weight (Ehdaie et al., 2006). In this study only one durum wheat was used which showed greater WSC accumulation and mobilization than the bread wheats examined. If this is the first report of WSC-related traits in durum wheat, in future studies, several diverse durum, other tetraploid wheats, and bread wheats should be examined to determine if the D genome from Aegilops tauschii Coss. depresses WSC accumulation and mobilization and if so, which chromosomes and genes are involved in their depression.
Among the landraces and old cultivars, only No. 14 possessed WSC-related characteristics close to those expressed by Westbred Turbo, Express, and Yecora Rojo. Crosses between landrace No. 14 and modern bread wheat cultivar Express or Yecora Rojo could broaden the genetic basis for WSC-related traits and may produce offspring with optimum plant height and appropriate internode length with improved accumulation of WSC content and mobilization in stressful environments.
In the third and last paper of this series, interrelationships between postanthesis changes in internode dry matter and WSC and grain yield will be reported.
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ACKNOWLEDGMENTS |
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Received for publication February 23, 2006.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Soil Science Society of America Journal | ||||
| Journal of Plant Registrations | |||||
of main stem internodes for moisture treatment, wheat genotype, and date of harvest means averaged across 2 yr.
