1.2 Role of soil nitrogen in maize breeding
1.2.2 Genetic control of maize traits under low nitrogen conditions
The genotype x N interaction has been reported under low N conditions (Bertin and Gallais, 2000; Worku et al., 2007). This interaction would suggest variable performance of maize genotypes under farmers’ production environments since these farmers apply differential marginal rates of N. This could be contributing to low average grain productivity of maize (per plantand per unit area) observable in SSA. The wide variations for genotypes across low regimes of N may suggest that selection of genotypes for differential responses to regimes of N would be possible (Worku et al., 2007). Such variation under low N could suggest the prevalence of additive genetic effects for secondary traits (Betran et al., 2003a, b). Despite the association between low soil N and drought suggested by Bänziger et al. (2000), which would permit use of a correlated response to breeding approach, Falconer (1989) reported on the difficulty of indirect selection in maize. Bänziger et al. (2000) and Betran et al. (2003) reported the preponderance of additive variance (VA) for secondary traits (i.e. EPP, ASI, and leaf chlorophyll concentration) in maize under stress. However, Robinson et al. (1949) had asserted much earlier that both VA and the response to selection diminish with cycles of selection. To the contrary, Meseka et al. (2006) and Medici et al. (2004) reported the preponderance of non-additive genetic effects for most of the traits under low N in maize.
Furthermore, other researchers have reported on the preponderance of epistasis under suboptimal regimes of production environments (Gorsline, 1961; Wolf and Hallauer, 1977;
Ceballos et al., 1998). In crops such as maize, where heterosis is crucial, Kearsey and Pooni (1996) reported that even a trace of non-additive gene effects would cause immense and unexpected heterosis. Therefore, this suggests the need for detailed detection, estimation and interpretation of the genetic parameters to improve maize, particularly under sub-optimal environments, such as experienced with low N. Nevertheless, most studies in tropical maize have not only disregarded the existence and importance of epistasis under optimum production and evaluation environments but they have done so too under sub-optimum conditions.
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However, a few studies had been conducted either on the inheritance of secondary traits and their relationships with yield or interrelationships among secondary adaptive traits, specifically under low N in tropical maize in SSA. This may therefore indicate that either previous efforts to improve maize in SSA had not been that successful or little had been documented. It would be worthwhile to breed for tolerance to low N and report research findings to improve maize productivity under low N conditions.
1.2.3 Correlation between traits under N conditions
Nitrogen is a mobile element, so it diminishes with growth stage and its effects would be more visible at the grain filling stage (Friedrich and Shrader, 1979; Hageman and Lambert, 1996). A negative relationship was reported between yield of dry matter and N at silking stage (r= -0.92), although this association fades with age of grain filling (Bertin and Gallais, 2000). Since N conditions photosynthesis, the genetic variability for photosynthetic activity may be higher at the grain fill stage compared with the vegetative stage (Ahmadzadeh et al., 2004). Hageman and Lambert (1996) who studied the field-grown single crosses that were bred for the period 1930-1970 reported significant differences in genotypic responses of such hybrids to environmental differences with time. Under optimum N conditions, Ahmadzadeh et al. (2004) and Beauchamp et al. (1976) asserted that remobilisation of N to developing kernels is slow during the first two weeks after silking and increases thereafter. The situation would be reversed under LN conditions. Although the maize plant requires more N starting at the mid-vegetative stage, with maximum requirements at silking stage, the maximum yield will depend on initial soil N status, including genotype differences for acquiring and using N (Binder et al., 2000). The lesser the initial soil N, the earlier the N has to be applied (Azeez et al., 2006). The authors studied four maize genotypes at 0, 30, 60, and 90 kg N ha-1 and added that N uptake increased with these rates and with 90 kg N ha-1 about 45.3 and 8.8 g N kg-1 were found in shoots and grain, respectively. Later N application, especially at R3 stage may not recover yield because it promotes photosynthates to source strength rather than strengthening the sink, thus there is a net yield loss (Binder et al., 2000). The challenge has been to ensure kernel set and its maintenance to physiological maturity in efforts to improve yield under low N. However, the literature — whether it pertains to genetic control and/or physiology of photosynthetic potential or machinery against specific grain filling stages onto which to concentrate breeding efforts under low N regime — is silent on this subject. This
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knowledge, however, would help to reduce breeding cycles by not waiting until the final harvest of the crop.
Tsai et al. (1984) described two types of maize hybrids in relation to NUE and concentration of plant N at R stages, based on 201 and 447 kg N ha-1 application rates. A high N fertility hybrid contained about 65% of its final N at the mid-silking stage, regardless of N levels. The remaining portion had to be absorbed subsequently from the soil after the mid-silking stage.
A low N fertility hybrid contained high percentages of its final N by the mid-silking stage at different N fertiliser levels i.e. 93 and 81% at 201 and 447 kg N ha-1 respectively. This would imply that the benefits of the leaf chlorophyll concentration character are relevant to later- maturing genotypes compared with earlier cultivars (Capristo et al., 2007). The low yield of a short-season genotype would therefore be due to limited sink strength during the grain filling stage, compared to its long-season counterpart. A yield penalty would be serious at low N, compared with high N conditions.
Dry matter (DM) in maize plant is comprised of approximately 1.5% N and 43.6% carbon (C) (Hageman and Below 1990). The high concentration of C demonstrates the predominant role of photosynthesis in achieving maximum yield (Swank et al., 1982; Hageman and Below 1990). Besides its low proportion, N has a vital regulatory role in DM production, in that N and C are closely connected and interrelated (Stulen, 1990). This could be demonstrated by the fact that maize needs for N are lower early in the season, which increases as dry matter production increases later in the season. Carbon provides the skeleton onto which DM is mounted and the role of N would be to regulate DMA since N is the major component of chlorophyll pigment eventually promoting grain yield. In this case, the efficiency of C4 plants, e.g. maize, as DM producers at low water, high temperatures, and high light intensity would be justified (Godwin and Mercer, 1988). However, the relationship between high yield and mineral and protein contents in the grain under N conditions is not clear (Feil et al., 2005).
The capacity of the leaves to produce photosynthates through the first half of the grain filling period has been reported to exceed the needs of ear and/or the capacity of the transport system (Swank et al., 1982). Thus such period may be where N is needed the most in maize.
However, much earlier Hanway (1962) reported that low N reduces dry matter accumulation
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(DMA) but it does not alter partitioning of DM. Furthermore, under low N, the photosynthetic capacity through grain filling period decreases compared with high N conditions, resulting in low yield under low soil N (Swank et al., 1982). This would not only affect the rate and proportion of DMA in kernels but could also influence the kernel moisture content at harvest and the rate of kernel dry-down (KDD). Tsai et al. (1984) asserted that the inefficiency of N uptake after the mid-silking stage and shorter duration in grain filling may, however, have a secondary effect in reducing yield potential of low soil N fertility genotype (Pioneer 3732), as compared with high soil N fertility hybrid (B73 x Mo17). Contrarily, Azeez et al. (2006) found that low-N pool C2 genotype had the highest grain N concentration and a shoot uptake significantly higher than TZB-SR, which is a high N genotype.However, factors that condition grain filling processes are not clear: is it the availability of DM, their translocation or the sink capacity such as EPP or KPE? (Duncan, 1975; Loiva, 1993). Whereas the sink capacity may be observed and quantified under N regimes, the first two factors are not easily measured under field conditions. The relationships among leaf photosynthetic capacity and KDD under low and high N conditions in tropical maize have not been established either.
Prevention of pollination resulted into accumulation of soluble solids in stalk (Hume and Campbell 1972). Ear removal caused same effect (Christiansen et al., 1981). Moreover, the barren plants senesced earlier than one with ears (Christiansen et al., 1981). However, the correlation between leaf chlorophyll concentration status across grain filling stages and EPP under low N in tropical maize is not known.
To recap, the characteristics associated with high yield in maize can therefore be considered as component characters of N uptake, assimilation, translocation, and sink strength (Swank et al., 1982; Tsai et al., 1984; Hageman and Lambert, 1996). Pending the fact that SG genotypes have the greatest DM per plant, carbon exchange rate and grain filling, especially during the last two-to-three weeks of grain fills over senescent genotypes (Hageman and Lambert, 1996), nothing is documented for such facts in tropical maize. However, Bänziger and Lafitte (1997) concluded that while the LCC character may shed information on environmental variation, EPP would discriminate the high yield genotypes under low N.
Another concern could pertain to the source of extra N when soil-N is limiting. The paucity of knowledge concerning the interaction of differential levels of leaf N and other metabolic
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activities at various stages of plant growth and development is another challenge to the maize breeder. Hageman and Lambert (1996) questioned the effect, role and interaction of N metabolism on the size of the plant in lengthening photosynthetic activity of leaves. It has also been difficult to associate physiological traits such as LCC and DMA with final yield.
Experiments to address these challenges may be costly and so require precise yet economical equipment to quantify the LCC character over grain filling stages under low and high soil N. In addition to this, combining knowledge on other traits related to yield and calendar physiological maturity may help to address these challenges. Since yield is the ultimate product of many processes, yield would be the best estimate for superior physiological processes.
1.3 Generation mean analysis and transgressive segregation