Screening of Maize (Zea mays L.) Genotypes for Adaptation on Contrasted Acid Soils in the Humid Forest Zone of Cameroon

Soil acidity is a major limiting factor for maize productivity in the humid forest zone of Cameroon. A yield loss of up to 60% has been reported in acid tolerant population. The objective of this study was to determine the level of tolerance of some selected inbred lines under Aluminum and Manganese toxicities on chosen experimental site soils and to classify them into specific pools. The experiment was laid out in a Split-plot design and genotypes were completely randomized within the blocks. Three replications were used. An assessment of 52 inbred lines of maize which consisted of 25 IRAD Cameroon lines, 3 IITA lines and 24 lines from CIMMYT-Colombia was then carried out in a contrasted acidic soil with Al (Nkoemvone site) and Mn (Nkolbisson site) toxicities Original Research Article Petmi et al.; JEAI, 14(6): 1-15, 2016; Article no.JEAI.29333 2 based on six agro-morphological parameters. Soil correction was made up by the application of 2 t ha -1 of dolomite lime. The soil analysis revealed that, at Nkolbisson plot there was Mn toxicity of 90. 6 (ug/g), while at Nkoemvone Al toxicity was more presented with 2.32 (cmol (+) kg). At Nkolbisson, 5.76% of the genotypes proved to be efficient, 9.43% were tolerant, 75% were susceptible and 15.09% were negative control. At Nkoemvone, 7.54% of inbred lines expressed themselves as efficient, 3.77% were tolerant, 75.47% were susceptible, 13.20% were negative control and 5.66% of the genotypes presented floral abnormalities called “mentle”. ATP S5 30Y-1 and CML 535 distinguished themselves as ubiquitous and CML 304 was the most susceptible variety to both agro-ecologies. The dendrogram obtained by the non-hierarchical classification analysis of endogamous genotypes showed three groups of maize genotypes at Nkolbisson and four groups at Nkoemvone. The best genotypes at Nkolbisson were: ATP-14 (4.08 t/ha), Camlnb1 17 F (3.93 t/ha), ATP S9 30Y-1 (3.86 t/ha), CML 437 (2.72 t/ha) and CML 535 (2.54 t/ha) and at Nkoemvone were: Clgp1 17 (3.9 t/ha), CML 322 (2.24 t/ha), CML 479 (2.24 t/ha) and CML 533 (2.05 t/ha). Diffusion of these acid-tolerant genotypes offers a sustainable strategy to increase yield productivity of acids soils in the humid forest zones.


INTRODUCTION
Acid soils cover approximately 3950 million ha, which is about 30% of the total ice free land area on the earth [1,2]. In the tropics, more than 8 million ha of acid soils are planted with maize, and 17% of tropical Africa is covered by acid soil [1]. In Cameroon, acid soil covers up to 75% of the soil, and this is mainly in the humid forest zones (HFZ). Acid soils in these zones are characterized by low pH, deficiency in Ca, Mg, P, K and Mo contents and toxic levels of Al and Mn [3]. These characteristics limit the fertility of the soil and inhibit root development, thus leading to low water and nutrient uptake and low maize yields [4]. [5] reported the average yield of maize production ranged from 0.8 -1 t/ha. However, this production is not sufficient to meet the demand of the population. In 2008, the quantity of maize flour imported was estimated to 24,815 tons [6] and this continue to increase until to date. Moreover, over the past decades, farmers' interest in maize production has increased, and maize has become a cash crop like coffee and cocoa, and is now an important source of income [7,8]. There is an increasing demand for the crop because of its use as feed in animal production and in the brewing industry. The annual demand of maize for human consumption and animal feed in Cameroon was estimated to 870,000 and 320,000 tons, respectively [9]. Maize production in Cameroon has been increasing steadily from an estimated 966,000 metric tons in 2004 [10] to 1,647 036 tons in 2013 [11]. These increases have mainly been due to increases in area harvested (832,400 ha) rather than yield increase per unit area (0.8 -1t/ha). The HFZ which covers an area of 21.7 million hectares, by virtue of its bimodal rainfall pattern holds promise for increasing the output for maize if the main fertility constraint of low soil pH can be solved. Soil amendment with lime, phosphorus and organic matter has been suggested to bring unproductive acid soil under acceptable agricultural production. However, such solutions are temporary and expensive for the resource-poor farmers [12]. To correct acid soil in one ha area, 2 to 4 tons of dolomitic lime are required and should be applied 2-3 years for better plant growth [13]. Significant genetic variation for tolerance to soil acidity has been reported. Early studies demonstrated qualitative inheritance [14]. Quantitative inheritance to Al resistance was later demonstrated [15,2,16]. Considerable progress has been made in breeding maize for acid soil tolerance through recurrent selection [3,17]. Developing acid-tolerant maize genotypes is an effective and sustainable way of alleviating the impact of Al toxicity in maize production areas. Studies have shown that Al-tolerant maize genotypes outperformed the adapted local and susceptible genotypes by 13% and 61%, respectively [18]. These results suggest that growing Al-tolerant maize genotypes will ensure a high sustained maize productivity. Five maize open pollinated varieties (OPVs) from Cameroon were reported to have some level of tolerance to soil acidity [19]. Three of these (ATP-SR-Y, ATP-S4 SYN Y and ATP SYN I-W) have been found to give 13% increase in grain yield over local varieties in the humid forest area of Cameroon [19]. However, most of the materials used exhibited significant additive genetic variance x environment interaction, suggesting that the materials had specific adaptation [15]. Only ATP SR Y out of these three acid tolerant population is the open-pollinated variety released and commercialized in Cameroon [20]. However, the impacts of climate change Associates to acid soils with aluminum and manganese toxicities increase yield losses in maize [21]. Yield losses of 60% have been reported in this acid tolerant population (ATP SR Y) [19]. There is a need of reducing yield loss due to soil acidity especially Al toxicity and increasing yield productivity per unit area. Thus it is necessary to develop other varieties adapted in many areas that will consider these constraints in order to provide farmers with maize cultivars which offer ecological, economical and a permanent solution, contributing to sustainable crop production in acid soils [22]. To identify and improve varieties that would perform well under acid soil, selections would have to be based on performance across a range of environments. This would lead to germplasm with broader adaptation. One way to obtain such germplasm is by introgression of exotic germplasm to locally adapted cultivars. The objective of this study is to evaluate the genetic potential of introduced inbred lines and identify high yielding and perform one under acid soil conditions and enables the breeder to choose appropriate combinaison for hybrid production or cultivar development programs.

Study Area
Two experimental sites were used for this study and were situated in a humid forest area of Cameroon, with bimodal rain fall. The first site is located at Nkolbisson (Yaounde) and the second at Nkoemvone (Ebolowa) and they are both separated by a distance of 180 km (Fig. 1.).
At Nkolbisson (11°36'E; 3°44'N), the mean annual temperature is 23.5°C, with an annual mean rain fall of 1560 mm, the vegetation is caducifoliated semi-deciduous forest. The soil has a sandy-clay texture with a strong hydromorphic tendency and with Manganese toxicity of (90.6 c mol (+) kg), a saturation rate of 39.02%, a pH H20 of 5.12, an efficient Cation exchange capacity (ECEC) of 4.20 C mol (+) kg and a C/N ratio of 13.86 c mol (+) kg.

Fig. 1. Geographical location of study sites
At Ebolowa (2°40'N 12°24'E), the site was located at the heart of humid forest zone at 615 m altitude above sea level and with a semi-deciduous vegetation. Its climate is of the Guinean type with an average rain fall of 1875 mm/year and a mean temperature of 24°C. The soil has a clay texture with Aluminium toxicity (2,326 c mol (+) kg), a saturation rate of 32.74%, pH H20 of 4.33, an ECEC of 3.5 C mol (+) kg and a C/N ratio of 9.28 c mol (+) kg (Table 1).

Germplasm
The germplasm used was made up of 52 maize inbred lines genotypes consisted of 25 IRAD Cameroon lines, 3 IITA (Nigeria) lines and 24 lines from CIMMYT-Colombia. The characteristics of these maize inbred lines genotypes are presented in Table 2.

Seed Sowing and Fertilizer Application
On each experimental site, land was ploughed and divided into two major parts with an alley of 2m. On one part of the field, the acidity of the soil was corrected with the incorporation of 2t/ha of dolomite. As soon as the rain started during the first rainy season, maize inbred lines were sown on rows of 4 m long with 9 hills. The distance between two consecutive rows was 75 cm and 50 cm between two consecutive hills in a row. Two maize seeds were planted per hill with no thinning. Plant density at planting was approximately 53,333 plants/ha. Weeds and insects were chemically controlled.
Mineral fertilizers were applied twice [22]: the basal application was done 15 days after sowing and was composed of a bag of 100 kg NPK 14-24-14 + 5(S) + 3.5 (MgO) with a bag of 50 kg/ha of urea. The second application was done 32 days later, at a dose of 50 kg/ha of urea.

Experimental Design
The trials were carried out in each experimental site on an area of 1326.8 m². The experiment setup was a split plot; where the main plot was the soil type made of the native acid soil with Aluminum or Manganese toxicity known as treatment "O" and the corrected acid soil known as treatment 'T'. The subplots were made of genotypes. The genotypes were arranged into a complete randomized block design with three replications.

Data Collection
The following phenotypic parameters were measured: plant height (HP) and ear insertion height (HIE), the ears weight (WE) and grain moisture content (MC) measured in the field during harvest. Grain yield (Y in kg/ha) of the genotypes were obtained using the formula below: Y: Grain yield in tons/ha. EFW: Ear field weight in kg SP: Shelling percentage estimated at 0.83 MC: Moisture content DMP: Dry mass percentage estimated at 85% when the relative moisture content is 15% -Percentage yield loss PYL due to the acidity of the soil was calculated following the formula: PYL: Percentage yield loss YC: Grain yield in corrected soil YA: Grain yield in acidic soil

Data Analysis
Data were subjected to the analysis of variance using the SAS 9.0 software package. The

Genotypes identification adapted in acidic soil with Al and Mn toxicity
At Nkolbisson, grain yield of inbred lines on acidic soil varied from 0.11 t/ha (CLA 154) to 4.11 t/ha (CLA 183), making a deviation of about 4 t/ha between the most productive and the least productive. 17 genotypes produced a yield greater than 2t/ha and were grouped into three classes [2][3][3][4][5]. Genotypes CLA 183 (4.11 t/ha), ATP-14 (4.08 t/ha), 9450 (4.07 t/ha) and CML 535 (4.07 t/ha) were revealed as the best varieties on the acidic soil at Nkolbisson.
Grain yield on corrected acid soil (control) varied from 0.37 t/ha (ATP S5 20Y-2) to 5.57 t/ha (CML 358), making a difference of about 5.2 t/ha. In the control, 22 genotypes produced a yield greater than 2 t/ha and are grouped into three classes [2-3[, [3][4][5]. Genotypes CML 358 (5.57 t/ha), ATP-14 (5.00 t/ha) and ATP-46 (4.42 t/ha) were revealed as the best varieties.   (Fig. 3). Equally, Clgp1 17 were the best strain 2, 88069, ATP S6 21Y-At Nkoemvone, four lines responding on both ils gave yield losses 14.70% and 0%. These lines were grouped as efficient and tolerant genotypes. They are Clgp1 17 (3.9 t/ha), CML 479 (2.24 t/ha), CML 533 (2.05 t/ha) and ATP S9 30Y-1 (1.04 t/ha). The yield and yield loss percentages rom the evaluated parents showed that 2 genotypes out of the 43 left were retained as tolerant on acid soil of Aluminum toxicity (CML 535 (31.84%) and CML 439 (16.79%)). 6 lines presented very regressive losses between -163.45% and quite considerable yields on control treatments: they were characterized as negative control genotypes on acid soils at Nkoemvone. They are: CML 322 400%), D 300-17 255.81%), ATP-49 163.43%). Besides, 40 parents which expressed yield less than 1 t/ha and percentage yield losses greater than 50 % were said to be susceptible genotypes to the soil A comparative study of screened the two sites revealed that ATP S5 30Y CML 535 were the best among all, both at Nkoemvone and at Nkolbisson. Hence they are considered as ubiquitous strains, while 15 genotypes perfectly expressed themselves susceptible on both experimental sites and were equally most susceptible of all tested genotypes.

Classification of genotypes into specific pools
The dendrogram obtained after analysis by grouping genotypes based on yield at Nkolbisson (Fig. 4.)

genotypes into specific
The dendrogram obtained after analysis by grouping genotypes based on yield at Nkolbisson (Fig. 4.) showed that, it was made up of three of 31 averagely oup II had 3 tolerant genotypes and group III had 18 susceptible genotypes. The distance between group I and II and that between group II and III was 77.42 and 86.66 respectively. Each group encloses parents contains varieties that were efficient (CML 535, CML 486, CML 437) and tolerant 1, ATP S5 31Y-2, Camlnbg1 17 (F). Observations within studied parameters in each group revealed that group II appears to be the best with good ears, Y (4.33 t/ha), HP (1.59 m), HIE (0.73 m) and GR (64.81%) higher than group I and III. The dendrogram obtained after analysis by grouping genotypes based on yield at Nkoemvone showed that, it was made up of four large groups (Fig. 5.). Group I was made up of 17 averagely tolerant genotypes; group II had 8 tolerant genotypes. Group III had five averagely susceptible parents and group IV had 21 susceptible genotypes. The distance between groups I and II was 20.19, that between groups II and III is 23.60 and that between groups III and IV is 40.38. Hence, each group encloses parents that look alike phenotypically.
In group I, some efficient varieties (CML 479, ATP S9 30Y-1) and tolerant (CML 435) were obtained after screening, Group II was the best with good ears (1.6), Y (1.68 t/ha), HP (1.28 m), HIE (0.57 m), GR (40.27%) and these data were higher than those in groups I, III and IV.    On corrected acid soil at Nkolbisson, quick lime had a significant effect on genotypes from germination to maturity. This result is similar to the one with the results obtained by [29] for where Calcium improvement compensates the acidification produced by biological activity, increasing the lowering of the soil pH and therefore favoring the assimilation of soil nutrients.
On corrected acid soil at Nkoemvone, dolomitic lime equally had a significant effect on the evaluated genotypes in the course of maturity comparatively to the results of [23]. In addition, low yield and high yield losses obtained in this corrected plot are due to repercussions of stems borer and hedgehog.These results confirm those conducted by [30]. This author had proved on cereals pathology study in Cameroon that yields losses due to these crops pests vary from 15 to 50% compared to total annual production.
A similar study of screened genotypes on the two sites showed that ATP S5 30Y-1 and CML 535 manifested as the best in terms of yield. These genotypes tolerance ability, retained on contrasting soil are considered as ubiquitous lines.  [23,31]. In addition, the number of susceptible parents (75.47%) obtained on acid soil at Nkoemvone was higher than that obtained on acid soil at Nkolbisson (69.81%). Therefore, the toxicity due to Aluminum was the greatest limitation to the growth and development of plant under acidic soils [26,32].

Classification of genotypes into specific pools
At Nkolbisson, three groups of strains were obtained (group I, II and III). Genotypes ATP 14, ATP 43, ATP S9 30Y-1, ATP S5 31Y-2 and Camlng1 17 (F) of group I initially defined as tolerant from origin confirmed to actually be tolerant. [33] Identified Clgp1 17 and CML 357 as tolerant to acid soils and similarly the genotypes Clgp1 17 and CLA 18 were found to be as tolerant by [33]. Furthermore, 9450 which had been detected as tolerant by [33,23] figures in group II.
At Nkoemvone, four groups of inbred lines were obtained (group I, II, III and IV). These results showed that group I was made up of 17 averagely tolerant endogamous varieties. Group III distinguished itself with 5 averagely susceptible genotypes while group IV presented 21 very susceptible lines. Group II proved to be the best. The averagely tolerant variety CML 357 (group I) and the tolerant varieties Clgp1 17 (group II) were shown to be tolerant by [22]. Averagely tolerant varieties CLA 18 and 9450 (group I) were shown to be tolerant by [33]. These results were similar to those of [34] and [14], who showed that, maize cultivars present a great variability of soil acidity tolerant genes, meaning that; tolerance of maize in acidic soil is controlled by major genes [21].

CONCLUSION
The evaluation of 52 inbred lines under Aluminum and Manganese toxicities soils showed a great variability on the level of tolerance of genotypes to the different types of soil acidity studied. Some numbers of efficient, tolerant and susceptible lines were identified. Also, common groups of progeny for acidity tolerance were known: 3 groups at Nkolbisson and 4 groups at Nkoemvone. The best progeny of Nkolbisson were: ATP-14 (4.08 t/ha), Camlnb1 17 F (3.93 t/ha), ATP S9 30Y-1 (3.86 t/ha), CML 437 (2.72 t/ha) and CML 535 (2.54 t/ha). The best genotypes at Nkoemvone were: Clgp1 17 (3.9 t/ha), CML 322 (2.24 t/ha), CML 479 (2.24 t/ha) and CML 533 (2.05 t/ha). The results of this study showed that maize cultivation on acid soils could lead to grain yield reduction of 60% or more in tropical environments. Grain yield loss due to soil acidity could be minimized by the development of hybrids from crosses between locally adapted inbred lines and those introduced from CIMMYT Colombia and soil program. Farmers of Center, South and East regions of Cameroon would benefit if they adopt hybrids developed with these inbred lines identified to be efficient and tolerant genotypes.