Variability of root and physiological traits of different maturity groups of maize (Zea mays L.)

Maize (Zea mays L.) is a major commercial crop, with high potential for production due to high solar radiation and low night temperature in sub-Saharan Africa. It is also the second most susceptible to drought among cereals, although phenotypic traits can be altered to improve drought resistance. Pot and field experiments were conducted to study the variability in root and physiological traits in different maturity groups of maize. Genotypes used were Sammaz 14, Sammaz 29, 2009 EVDT, 2009 TZE–W, TZE COMP-5 and 2009 TZEE, laid out in a Randomized Complete Block Design with 3 replications. The results obtained revealed no significant difference among the genotypes. However, the genotypes showed a good response to leaf temperature, canopy temperature, stomatal conductance and chlorophyll content. Variability was observed in three traits; days to anthesis, silking and anthesis silking interval. There was a significant correlation in leaf temperature in relation to fresh root weight, fresh shoot weight, dry shoot weight, dry root weight and shoot length. Root traits had positive relationship with grain yield. The genotypes had good rooting pattern development and combine with their physiological response they could be hybridized to develop drought tolerant varieties.


INTRODUCTION
Achieving food security; the first step towards poverty alleviation, is one of the biggest challenges facing developing countries.In most of Africa, food production is supplemented with imports to minimize the impact of shortages.Taking a cue from the most agriculturally advanced countries, it could be hypothesized that agriculture, hence food security in sub-Saharan Africa, will develop on a grain base.In West and Central Africa this crop is likely to be maize, which has evolved from a backyard crop to a major commercial crop providing food, animal feed and industrial raw materials (Badu-Apraku et al., 2009).
In general, average yields in tropical and sub-tropical regions are far lower than in temperate ones, with sub-Saharan Africa way below other regions with average values across countries of around 1 t ha -1 .This is in spite the fact that maize is one of the main crops in these regions, where the effects of climate change including rising temperatures, evapotranspiration losses and eventually, decreasing rainfall are expected to be particularly negative (World Bank, 2007).The possibilities for alleviation of water stress are limited.The majority of tropical maize is grown under rain-fed conditions and poor farmers from these regions are unable to implement crop management strategies that might at least mitigate some constraints (Araus et al., 2012).
Maize (Zea mays L.) has high potential for production and productivity in the savanna ecology of sub-Saharan Africa due to high solar radiation and low night temperatures.It is mostly grown under rain-fed conditions and among the cereals, it is the second most susceptible to drought next to rice.Annual maize yield loss due to drought is estimated to be 15% in West and Central Africa and losses may be higher in the marginal areas where the annual rainfall is below 500 mm and soils are sandy or shallow (Edmeades et al., 1995).Drought resistance might be increased by improving the ability of the crop to extract water from the entire soil profile (Wright and Nageswara, 1994).Awal and Ikeda (2002) reported that chlorophyll concentration, stomatal conductance, photosynthesis and relative growth rate were increased after re-watering (Jogloy et al., 2010).Therefore, of all phenotypic traits that can be altered to improve drought resistance of cereal crops, increased penetration and extension of root systems probably offers the greatest potential (Passioura, 2007).By penetrating deeper into the soil, crop roots potentially access and exploit a greater volume of stored water (McKenzie et al., 2009).
The ability to grow deep roots is currently the most accepted target trait for improving drought resistance, but genetic variation has been reported for a number of traits that may affect drought response.Roots are the principal plant organ for nutrient and water uptake.Therefore, improving our understanding of the interaction between root function and drought in maize could have a significant impact on global food security (Henry et al., 2011).
The effect of selection under stress on yield performance of genotypes under optimal conditions and vice versa has been an ongoing debate among plant breeders for decades.Secondary traits can improve the precision with which drought tolerant genotypes are identified, compared with measuring only grain yield under drought stress.Secondary traits such as canopy temperature, stomata conductance, ears per plant and anthesis silking interval have been found to possess strong correlations with grain yield under drought conditions and have been used to select for higher levels of tolerance to drought (Badu-Apraku et al., 2011).There is therefore a need to evaluate for differences in root, shoot and physiological traits of different maturity groups of maize.Each maturity group of maize has its unique advantages and disadvantages with respect to climatic conditions (e.g.rainfall pattern).

MATERIALS AND METHODS
Two experiments were conducted: Field and pot experiment.Both experiments were conducted at the Research and Teaching Farm of Department of Agronomy, Faculty of Agriculture, Bayero University, Kano (Lat 11°58'N, Long 8°25'E and 475 m above sea level).
The materials used for the experiment were six (6) maize genotypes (Table 1) supplied by the Department of Agronomy, Faculty of Agriculture, Bayero University, Kano.The treatments were laid out in a randomized complete block design (RCBD) with three replications.

Field experiment
Land used for the experiment was ploughed and harrowed to a fine tilt.The farm area was marked out into plots and replications.One ridge was used to represent a plot and each ridge was 4 m long.The seeds were sown manually into their respective ridges at the rate of 2 seeds per hole.The seeds were sown at intervals of 75 × 40 cm inter and intra row spacing, respectively.The plants were thinned to leave one plant per stand at 2 weeks after sowing.Weeding was carried out twice, the first weeding was carried out manually using hoe at 2 weeks after sowing, while the second weeding was carried out using animal traction at 4 weeks after sowing.The recommended dose of fertilizer for maize, 120:60:60 -N: K2O: P2O5 kg/ha, respectively were applied at two weeks after sowing, by side placement.Nitrogen was supplied in two split doses, the first dose at two weeks together with phosphorus and potassium and the second dose at 4 weeks after sowing.

Pot experiment
This experiment was conducted in 30 days.The experiment was conducted in buckets with a volume of 6 L. The buckets were arranged in a complete randomized design with three replications and a bucket representing a plot.The buckets were filled with top soil up to the 4 L mark and were irrigated.The maize varieties were then sown.Weeding and irrigation were carried out routinely.

Statistical analysis
Data collected were analyzed using the PROC MIXED statement.The analysis was done using SAS 9.0 (2001).Replication was considered as a random effect and the genotypes were considered as fixed effect.In the pot experiment, analysis of covariance was performed for all traits using shoot length as a covariate to identify the influence of seedling vigor.Simple correlation among trait was calculated using PROC CORR statement.

Mean performance of genotypes for field experiment
The mean performances of the genotypes for some agronomic traits are presented in Table 2.There was no significant difference (P>0.05) for all the traits, except for days to anthesis, silking and anthesis silking interval (ASI).The variability in these traits is due to differences in the maturity group of the genotype.
Table 3 shows the pair wise comparison for the genotypes for some agronomic traits.There was no significant difference (P>0.05) in the comparison, except for days to anthesis, days to silking and ASI in comparison  of SAMMAZ 14 and other genotypes, where there was significant difference (P<0.01).There was a highly significant difference (P<0.01) in comparison among TZEE-COMP 5 and other genotypes for ASI.This variation  also reflects the difference in the maturity groups of the genotypes and also classifies the genotypes into their respective groups.Also the lack of significant difference for other traits measured can be due to low rainfall (water stress) observed during the experimental period (Figure 1).The mean plant height and ear height are within reasonable range compared with the report of Menkir and Akintunde (2001).The mean anthesis -silking interval (ASI) was 3 days for the genotypes.The shortened ASI observed in these cultivars is desirable because it has been reported that low ASI enhance maize tolerance to stresses during flowering and it ensures good grain filling (Edmeades et al., 1993;Bolanos and Edmeades 1996).
The mean performances of some physiological traits are presented in Table 4.There was no significant difference (P>0.05)among genotypes for all the traits measured.The lack of significant difference observed in the physiological traits of the genotypes measured was indications that irrespective of the difference in maturity group the physiological response of the maize genotypes are the same.This also confirms that variability in the traits does not exist between the different maturity groups of the maize genotypes.The maize genotypes showed a good response in terms of improving them towards becoming drought tolerant genotypes.

Mean performance of genotypes for pot experiment
The mean performances of the genotypes for some agronomic traits are presented in Table 5.There was no significant difference (P>0.05)among the genotypes for all the traits measured.There was significant difference (P<0.01)among the genotypes with respect to the shoot length (covariate) for all the traits measured, except for fresh shoot root ratio and root length.This indicated that seedling vigour was detected.Shoot length had effects on the fresh and dry shoot weight, fresh and dry root shoot ratio, as well as, dry root weight.The lack of significant covariate difference observed for fresh root shoot ratio and root length shows that the differences in the maturity group of the genotypes does not affect these traits.
Table 6 shows the mean performance of the genotypes for some agronomic traits.There was no significant difference (P>0.05)among the genotypes for all the traits measured.No significant difference was observed among the genotypes for the agronomic traits measured at seedling stage.This shows that the genotype has similar pattern of root and shoot development.However, 2009 EVDT a drought tolerant variety gave a better response in terms of root development response followed by Sammaz 29.This is an indication that Sammaz 29 can be improved to be drought tolerant.
Correlation among some agronomic traits of the genotypes is presented in Table 7.Most of the correlation among the traits showed no significant relationship (P>0.05).There was positive and highly significant (P<0.01)correlation in the correlation among leaf temperature with fresh root weight (0.64), fresh shoot weight (0.65), and dry shoot weight (0.64); and with dry root weight (0.50) and shoot length (0.64) at P<0.05.This shows that a change in leaf temperature will result in a change in these traits.Also root traits were found to have positive effect on yield.Results obtained are in accordance with those reported by Khan et al. (2002), Dhanda et al. (2004), Awan et al. (2007) and Rauf et al. (2007).As root-shoot ratio was negatively correlated with some traits so selection for low root-shoot ratio will decreases the performance of other important seedling traits (Khan et al., 2010).Similar results have also been reported earlier by Echarte and Tollenaar (2006) and Ojo et al. (2006).
A high correlation was observed between plant and ear height.A high correlation between plant height and ear height has been reported by Hallauer and Miranda (1995), Nato and Miranda (2001), and Salami (2002).The close relation among these traits will cause them to respond similarly during improvement.

Conclusion
Drought, which is a rising threat of the world, can be adapted to with genotypes with efficient root system.Improvement in root and physiological traits of maize genotypes can lead to improvement in level of tolerance to drought.The genotypes used in this study shows a good response to drought and no variability was observed between the genotypes for the root and physiological traits observed.This is an indication that maturity period did not influence the response of maize to these traits as such maize has similar response pattern to root and physiological traits.Leaf and canopy temperature has relationship with root weight, shoot weight and leaf number.This shows root characteristics can be improved by increasing the leaf temperature.The existing correlation between leaf temperature and root system is an important factor which breeders should consider in production of these genotypes.
The lack of significant difference in the physiological traits of the genotypes which included a drought tolerant genotype (2009 EVDT) shows that these genotypes can also be improved to become drought tolerance genotypes.Also irrespective of maturity group of the maize, their root and physiological responses are the same.

Figure 1 .
Figure 1.Meteorological data of the experimental area in 2014.

Table 1 .
List of genotypes used for the study.

Table 2 .
Mean performance for some agronomic traits of different maturity group of maize.

Table 3 .
Pairwise comparison for some agronomic traits of different maturity group of maize.

Table 4 .
Mean performance for some physiological traits of different maturity groups of maize.

Table 5 .
Mean squares for some root and shoot traits of different maturity group of maize in screen house.

Table 6 .
Mean performance for root and shoot traits of different maturity group of maize in screen house.
NS = Not significant.

Table 7 .
Correlation matrix of some physiological, root and agronomic traits of maize.