Stress is any phenomenon or process that reduces the yield potential of a genotype (Hill et., 1998). Nitrogen does not only fulfil a regulatory role in metabolic processes, but it also regulates DMA and the uptake and utilisation of other mineral nutrients. In connection with this, Hageman and Lambert (1996) reported that variance of yield could be a good measure of a genotype’s response to N. Pending the crucial role of N in maize (Lafitte, 1994), tropical
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maize in SSA is produced under LN conditions, therefore this subsection reviews stresses related to low N and genetic characters that confer tolerance to these conditions. The relationships of such characters and their outward contribution to final yield are also reviewed.
The effects of drought, low N and high plant density stresses are severe at silking and grain filling stages in maize and these may cause barrenness in the case of drought and/or poorly developed ears and thick plant stands (Ribaut et al., 1996; Bänziger et al., 2000; Sangoi, 2001; Magorokosho et al., 2003; Lal et al., 2010). These authors agree on the compounding effects of (LN) with other stresses being short reduced plant stature, greater leaf area reduction of pre- and post-flowering, especially at R1, enhanced floral protandry, lower pre- and post-leaf chlorophyll content, lower KPP, kernel weight, HI, and enhanced per plant-yield variability. However, Boomsma et al. (2009) and Lal et al. (2010) reported on the lack of genetic efforts to improve maize genotypes for tolerance to such multiple stresses and simultaneous focus on NUE and N stress tolerance. Detecting genotypes that reduce the effects of these stresses, such as continuous growth of reproductive parts, is crucial, and this would further translate into kernel set and their maintenance to physiological maturity.
1.5.2 Physiological basis of prolonged leaf chlorophyll concentration character
Extended leaf chlorophyll content (SG) is a function of the balance between demand by and supply to the grain of N during grain filling. Genotypes vary for this trait. Conceptually, such a trait has a time component and plant and soil N status, and is relevant at post-flowering growth stages. At the pre-anthesis stage, more N has been found to be proportionally allotted to leaves of non-senescent genotypes (SG) in a sorghum variety (Borrell and Hammer, 2000). The possible explanation could be leaf structure differences in both SG and senescent genotypes. Borrell and Hammer (2000) added that the leaves for SG genotypes are thicker than their senescent counterparts, so creating more demand for N at anthesis in SG genotypes. Essentially, senescence (due to normal ageing and/or N deficiency) is the result of more demand than supply of N from the plant source and the environment. Moisture stress, in fact, may accelerate senescence. A larger portion of N is remobilised, as opposed to being drawn from the soil. Delayed remobilisation from leaves prolongs photosynthetic machinery, which becomes a plus to yield (Subedi and Ma, 2005; Hawkins et al., 2007).
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However, other studies indicate that SG genotypes have well ramified root systems to meet the further demands of N from the soil during grain filling periods, as opposed to senescent genotypes (Pan et al., 1986; Bänziger et al., 1999; Hawkins et al., 2007). Besides the advantages of the SG trait, its physiological and genetic bases are not clear, even in crops where it has been mostly studied, such as sorghum and horticultural crops.
The leaf N in SG genotypes remains higher than in non SG because in SG:
i) the leaf N level at anthesis was higher, ii) N uptake during grain filling was higher, and
iii) the remobilisation of N from leaves of SG during grain filling was less (Borrell and Hammer, 2000).
Host-pathogen relationships at grain filling may influence the SG profiles but debate has been inconclusive as to whether the green colouration in that relationship is due to retention, regreening, and/or new synthesis. This argument has been reported mainly in fungal pathotypes in cereals (Scholes and Farrar, 1987). Basra and Goyal (2002) reported on the nitrate ions (NO3-
) as a reservoir of leaf N, where excess NO3-
stored in leaf vacuoles is remobilised when the N supply from the soil is depleted. Therefore, accumulation of NO3-
in leaves during the vegetative phase under low N may act as a marker for selecting genotypes with enhanced yield potential under such conditions. Very little systematic research has been conducted to evaluate whether the SG character would still be beneficial during late season stress i.e. foliar diseases, low N and drought. It is also not clear whether the SG hybrids require additional N fertiliser, let alone in which growing conditions. A study on SG sorghum has indicated a penalty to yield and yield components at low N and drought stress environments (Borrell and Hammer, 2000). Although the SG trait in maize is crucial for grain filling and kernel maintenance, especially under late season stress, the inheritance of the trait, especially when measured at different growth stages, has not been established.
Knowledge on the profiles of leaf chlorophyll content across kernel fill stages will help determine the rate of DMA and the specific stage in maize where N is needed the most.
Various studies support the SG character being a function of soil N. The late season test at ear leaf (around ¼ milk line) by Chlorophyll Meter (Model SPAD-502 Minolta Camera, Japan)
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has been proven reliable and economical in separating cultivars with adequate N from those that are deficient in N, also the same trend may apply for N sites (Piekielek et al., 1995).
Although leaf N in maize stabilises with age (Coe and Nueffer, 1979; Binder et al., 2000), Piekielek et al. (1995) reported that the stability of leaf N with age was inherent to the genotype’s leaf chlorophyll content, consequently affecting the late season SPAD-502 tests.
However, Bertin and Gallais (2000) counter-argued by reporting that chlorophyll content was affected by N stress early in plant development. The general trend was that early season SPAD test would not be reliable due to soil temperatures and hybrid vigour imposed by selection. Crafts-Brandner and Poneleit (1987), Piekielek et al. (1995), Mahalakshmi and Bidinger (2002) and Hawkins et al. (2007) suggested that experiments on SG should involve plants with uniform phenology, sowing dates, and periods of maturity, since N varies according to genotype and environment. This suggests that experiments on SG should be designed to maximise precision, thereby enabling interpretation and practical application of the results.
The influence of growth hormones under soil N regimes may determine the SG trait and some yield components. Cytokinins have been reported to relate directly with the SG character and the KPP (Robson et al., 2001; Bertin and Gallais, 2000). Further, Bänziger et al. (2000) and Daynard and Duncan (1969) established that Abscic acid (ABA) regulates the number of kernels that reach maturity under multiple stresses, including low soil N. Cultivars that resist leaf photo-oxidation may have extended leaf chlorophyll concentration, thus adapting them to multiple stresses and they produce more dry matter than susceptible genotypes (Robertson, 1975; Britton, 1995; Ping et al., 2005; Joshi et al., 2007). High rates of cytokinin transported from roots to leaves result in SG cultivars, whereas such cultivars block transport of abscic acid from roots to shoots, thus retarding leaf senescence (Ping et al., 2005). Therefore, the physiological basis of extended leaf chlorophyll content in maize could be a function of the genotype, growth hormones, status of N in plant and soil, moisture, and sink-source relationship, among other factors.
1.5.3 Measurement of leaf chlorophyll concentration using the SPAD-502 meter
The Chlorophyll Meter (Model SPAD-502 Camera Minolta Co. Ltd., Japan) has been proven to quantify leaf chlorophyll concentration (Martinez and Guiamet, 2004; Subedi and Ma,
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2005). Other studies indicate that SPAD readings and extractable chlorophyll content are strongly correlated (Dwyer et al., 1991; Subedi and Ma, 2005). The study by spectroradiometer on maize leaves also indicated that N and chlorophyll content are strongly correlated (Ercoli et al., 1993), but good results may be obtained at high and medium concentrations of N. This suggests that SPAD-502 is a reliable meter when it comes to quantifying leaf N, which also reflects on the status of soil-N and the health of the plant. In addition to the Chlorophyll Meter method and laboratory analyses for plant and soil-N, other methods of quantifying LCC include leaf area based indices, leaf area under curve and leaf green-colour scores. A combination of these techniques would help to check for consistency among these methods in response of genotypes to soil-N across management regimes.
However, it is best to use the SPAD-502 to quantify plant (leaf) and soil N if a comparison is made between the meter’s perfect correlations and others recorded through different methods.
The precision of SPAD values is determined by factors such as genotype, environment, meter differences, and human dexterity when recording the LCC data. The lower the irradiance the higher the SPAD values, and vice versa (Hoel and Solhaug, 1998). However, plants adapted to high light intensities are less affected by irradiance variations compared to shade adapted plants. Chlorophyll meter readings may be affected by movements and varying orientations of chloroplasts. In low irradiance, chloroplasts are oriented along the upper and lower cell walls, thereby maximising light absorption, while in high irradiance, they are oriented mainly along the vertical walls parallel to incident irradiance (Hoel and Solhaug, 1998). Robson et al. (2001) reported that shade could retard the cytokinin growth hormone that also reduces photoreceptors phytochromes, so accelerating chlorophyll senescence.
Dwyer et al. (1991) established that high temperatures may inflate SPAD readings.
In addition, leaf spectral properties per se were found to be affected by leaf age, leaf position, and region within a leaf (Dwyer et al., 1991; Earl and Tollenaar, 1997). Young, chlorotic, and senesced leaves have low SPAD values. In mature leaves, the variability of SPAD readings is small. Despite these setbacks, SPAD provides estimates of critical levels of N under both stressful and stress-free conditions (Earl and Tollenaar, 1997; Martinez and Guiamet, 2004; Subedi and Ma, 2005). Based on the various factors that affect SPAD
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readings, Piekielek et al. (1995) suggested the readings be normalised, where the standard N treatment could be considered as a reference to reduced N. Generally, experimental conditions to estimate LCC should be well controlled in order to minimise experimental error.
For example, genotypes should be of relatively equal physiological maturity and equally treated.
1.5.4 Genetics of extended leaf chlorophyll content character
Information on genetic control of extended leaf chlorophyll concentration (SG) in maize is scanty. Few studies indicate that a single dominant gene governs the SG trait in maize (Thomas and Smart, 1993; Ceppi et al., 1987; Gentinatta et al., 1987). On the contrary, Ahmadzadeh et al. (2004) reported a preponderance of additive genetic effects for leaf carbon dioxide exchange rate (CER) late in the season. In sorghum, Kassahun et al. (2010) reported that the onset of senescence (declining LCC) is additively controlled, whereas the slow rate of senescence is governed by complete dominance, as opposed to fast senescence. Still with sorghum, (Walulu et al., 1994) asserted that the SG character is highly influenced by the environment but at the genetic level there are varying levels of dominance.
The trait falls under polygenic control in sunflowers (Cukado-Olmedo and Miller, 1997).
However, all of these studies might be asserting on the obscurity of the inheritance of the trait. Subedi and Ma (2005) added that the genetic and physiological bases of the extended leaf chlorophyll character are not clear. Other workers, to mention but a few, reported that the character is highly variable with the environment (Piekieleki et al., 1995; Robson et al., 2001; Martinez and Guiamet, 2004; Hawkins et al., 2007). Therefore, the inheritance of the character is likely to be unclear, but more work is required to clarify this, especially in tropical maize under low N conditions. The few reports are therefore inconclusive but further investigations may be recommended, such as the present study. Despite these challenges, Robson et al. (2001) believe that varieties that retain high LCC can be bred and they may also remain photosynthetically active.