Physiological characteristics of recent Canada western red spring wheat cultivars: nitrogen uptake and remobilization
Genetic yield gains have been difficult to achieve within the CWRS wheat class because of stringent quality requirements, and a growing-season environment of low precipitation and high temperatures. Understanding the physiological basis of yield gains may provide breeders with better insight as to the selection of parents, or provide screening tools to identify desirable genotypes. The objective of the present study was to compare four new CWRS wheat cultivars, which averaged higher yields than Neepawa in three years of multi-location testing within registration trials, both as a group and individually while maintaining or even increasing protein content, with two older cultivars, Neepawa and Marquis, in terms of N uptake and N remobilization. Results indicated that new cultivars had higher N uptake and/or higher N remobilization than old cultivars. Low tissue N concentration at maturity could be a criterion for selecting high-yielding and high-protein cultivars.Grain protein concentration (GPC) is an important trait of major interest in breeding of bread wheat (T. aestivum L.), because it determines both baking and nutritional properties. Breeding for both high yield and high GPC is very difficult as a negative relationship between yield and GPC was found by many studies (Simmonds 1995; McNeal, et al., 1972; Whitehouse, 1973; Bhatia, 1975; Costa and Kronstad, 1994). Simmonds (1996), therefore, concluded that high yield and high GPC were unattainable simultaneously. However, Kibite and Evans (1984) indicated that the negative relationship between yield and GPC was not primarily driven by genetic factors, but mainly by environmental factors. Cox et al. (1985) found that negative correlations between yield and GPC for some wheat lines were low, although significant, which indicated that simultaneous increase in yield and GPC could be achieved by selection. This is supported by some studies (Davis et al., 1961; Terman et al. 1969; Johnson, 1978; McKendry et al. 1995). Jenner et al. (1991) indicated there is no fundamental conflict on physiological grounds in selecting cultivars for high carbohydrate yield at acceptable, even high, levels of GPC. GPC is determined by plant total nitrogen (N) uptake and N remobilization to the grain. Many studies found genetic differences in N uptake (Löffler, et al. 1985; Van Sanford and MacKown, 1986; Le et al. 2000; Desai and Bhatia, 1978; McKendry, et al. 1995), while Oscarson et al. (1995) did not find any major differences in NO3 uptake capacity among wheat grown hydroponically. A positive correlation between N uptake and GPC was found by Beninati and Busch (1992) and McKendry et al. (1995), but not by others (McNeal et al. 1966; Johnson et al. 1967; Desai and Bhatia 1978). Cultivar difference in N remobilization was also found by some authors (Seth et al. 1960; Johnson et al. 1968; Van Sanford and MacKown, 1987). However, the relationship between plant N metabolism and GPC was not clear. Some reported that N partitioning was associated with GPC (Johnson et al. 1968; Cox et al. 1986; McKendry et al. 1995), but others (McNeal et al. 1972; Woodruff 1972; Van Sanford and MacKown 1987; May et al. 1991) did not support this. Nitrogen harvest index was (grain N at maturity/maximum N uptake, %) used as a selection criterion by some authors (Desai and Bhatia 1978; Cregan and Berkum 1984; Löffler et al. 1985; Jenner et al. 1991; McKendry et al. 1995). Borghi et al. (1987) suggested that both higher biomass yield and efficiency of N remobilization are important traits to overcome the negative relationship between grain yield and GPC. Some studies suggested to use tissue N (Rostami and Giriaei 1998; Rostami and O'Brien 1996; Sylvester-Bradley 1990) or tissue protein concentrations (Noaman and Taylor 1990; Noaman et al. 1990) as selection criteria for increasing GPC because they were positively correlated with GPC. However, Jenner et al. (1991) indicated that from a physiological point of view, there is little logic in using grain protein percentage as a selection criterion. Delzer et al. (1995) also pointed that selection for grain protein only is questionable because the higher GPC is often associated with lower grain yield. Although there are not short of studies on N mechanisms, there are lack of consistencies in the results. Clarke et al. (1990) indicated that unless greater variation in N utilization parameters among cultivars can be demonstrated, there seems to be little justification for selection for parameters other than grain yield and protein concentration. Some recently developed bread wheat cultivars in western Canada have significantly increased yields, while maintaining or increasing percent protein content, relative to earlier cultivars (Wang et al. 2002). These cultivars can be used to study the physiological basis for these genetic improvements in N utilization. A better understanding of these improvements may allow breeders to design more efficient screening methods to develop future high yield and high GPC cultivars. This information may also assist agronomists and producers design soil and crop management practices that will permit full expression of these improved traits. The objective of this study was to estimate the characteristics of these new cultivars in N utilization in comparison with older cultivars and to identify potential criteria for selection of high yield and high GPC cultivars in the western Canadian semiarid prairie.
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