Genetic Studies of Physiological and Morphological Traits Associated With Drought Tolerance in Cassava Genotypes
ABSTRACT
This study was undertaken to identify key traits that are closely related to cassava yield under drought stress and also identify stable high yielding cassava genotypes under varying environments. A survey was conducted using in semi-structured questionnaire in three districts in the Northern Region involving 120 farmers to identify farmers’ perception on drought in cassava cultivation, production constraints, mitigation strategies and preferences for improved cassava genotypes. To identify ideal genotypes that meet farmers’ preferences,
150 cassava genotypes from local and exotic sources were assembled and assessed for diversity using morphological traits and simple sequence repeat markers. Subsequently 20 genotypes were selected and evaluated under irrigation and no irrigation to assess genetic variability in abscisic acid content, carbon isotope ratio, stomatal conductance, leaf temperature and root yield. Stability of genotypes for physiological and yield traits were also assessed using the additive main effect and multiplicative interaction (AMMI) and GGE biplot analyses. The survey indicated lack of credit as the most important constraint facing cassava cultivation in the region. Drought was the second most important constraint and the intensity was observed to be increasing. Majority of the farmers also preferred early maturing cassava varieties that are high yielding with good plant type and marketability. Factorial analysis of the morphological traits grouped the genotypes based on their origin with few exceptions. Principal component analysis further identified plant height, branching habit, distance between leaf scars, colour of end branches, root yield, harvest index and number of roots per plant as the traits contributing most of the variability in the groups. High level of heterozygosity was revealed by the simple sequence repeat markers which grouped the genotypes into seven distinct clusters irrespective of sources. Genetic variability was established for abscisic acid content which was higher under stress than irrigation. ABA content was negatively correlated with root yield, harvest index and above ground biomass
yield meaning it can be used as indirect selection criteria against unproductive genotypes. Narrow genetic variation was observed for carbon isotope ratio which was higher under irrigation than no irrigation. Carbon isotope ratio was positively correlated with above ground biomass yield but not root yield. Stomatal conductance and leaf temperature were significantly different among genotypes and environments but genotype x environment interaction was not significant. Broad sense heritability estimates were high for most of the traits except stomatal conductance, above ground biomass yield, root number and stem diameter. AMMI analysis of plant height, severity of cassava mosaic disease, root yield, root length/girth ratio, above ground biomass yield and harvest index indicated stronger effect of environment than genotypes for all traits except CMD. The study established for the first time relationship between ABA content and cassava root yield on the field. Extension of roots to lower soil depts (L/G ratio) was also found to be detrimental to storage root yield. It was also found from this study that carbon isotope ratio influence above ground biomass and not storage root yield under stress conditions. Based on AMMI selections and the GGE biplot analysis, three genotypes, MM96/1751, UCC2001/449 and 00/0203 were identified as high yielding and stable across environments. These can be used as donor parents in improving local farmer-preferred varieties. Six genotypes (UCC2001/449, 96/1708, MM96/1751, 00/0203, 96/409 and I91934) had significantly higher root yield than the best local farmer preferred variety. These can be tested on-farm for official release to farmers for cultivation.
CHAPTER ONE
GENERAL INTRODUCTION
Cassava (Manihot esculenta Crantz) is the third most important source of calories in the tropics after rice and maize with millions of people depending on it in Africa, Asia and Latin America (FAO, 2014). It can be cultivated under marginal ecologies characterised by poor, erratic rainfall and extended periods of drought where most crops will fail (Hillocks, 2002; El-Sharkawy, 2004). The production areas are mostly confined to developing tropical and sub-tropical countries with over 40% of world production occurring in Sub-Saharan Africa (El-Sharkawy, 2006). It is grown by resource-poor farmers, mainly women, often on marginal lands for food security and income generation (Okogbenin et al., 2003). It also has the potential to produce starch for industrial purposes as well as feed for livestock production at a relatively cheaper cost than maize (Nweke et al., 1994). Cassava is grown in four out of the five major agroecological zones of Ghana covering nine out of the ten regions of Ghana (SRID, 2009). Apart from serving as the main source of carbohydrates to meet the dietary requirements of most rural dwellers, it contributes immensely to Ghana’s Agricultural Gross Domestic Product (AGDP) (Ofori, 2005). According to FAO (2014), the crop earned the country over US$ 1.51 million, which was second to only yam (US$ 1.69 million) in foreign exchange. The crop is mainly consumed as ampesi (boiled storage roots) or in the form of fufu (pounded boiled cassava) with the remainder being processed into gari or kokonte, dried products for storage. In the northern parts of Ghana, it is mainly processed into dried chips for storage by resource poor farmers who use the flour prepared from it for preparing tuo zaafi, a local staple.
Cassava cultivation is severely affected by several biotic and abiotic stresses that impact negatively on production, consumption and marketability (Bull et al., 2011). Low yields in cassava have been attributed to the use of late bulking varieties, disease and pest susceptibility and low yielding potential of many varieties (Nweke, 1996). Yields are far below par in developing countries, particularly in the tropical regions (El-Sharkawy, 2007). Crop yields per unit area have remained low (<12 t/ha) (MoFA, 2012). Whereas progress has been made in developing disease-resistant varieties leading to the release of several varieties in Ghana (Manu-Aduening et al., 2005), little progress has been made in developing varieties for drought prone areas. Most of the released cassava varieties in Ghana targeted high yield potential in the forest and transition zones of Ghana which have bimodal rainfall pattern, totally different from the monomodal rainfall pattern of the Guinea Savannah ecologies. The presence of genotype by environment interaction in yield and yield components in cassava (Egesi et al., 2007a; Aina et al., 2009) means that the performance of these varieties cannot be predicted in the savannah zones. Additionally, these released varieties are often rejected due to their inability to adapt into the farmers’ cultivation systems and food (Manu-Aduening et al., 2005; Acheampong et al., 2013). Involving farmers in the development of crop varieties has resulted in faster rates of adoption and dissemination of improved varieties
(Kapinga et al., 1997; Odendo et al., 2002; Manu-tAald.,uening e 2014).
The cultivation of cassava has been under rain fed conditions making its performance climate dependent. However, the advent of climate has made the rainfall pattern more unpredictable thereby affecting crop productivity in the decades to come in drought-prone areas (Saxena and John, 2002; Turyagyenda et al., 2013I)n. creased temperatures are associated with increased rates of evapotranspiration resulting in decline in the yield potential of crops (FAO, 2011). Yield losses due to water deficits however, vary depending on timing, intensity and
duration of the deficit, coupled with other location-specific environmental stress factors such as high irradiance and temperature (Serraj et al., 2005; Farre and Faci, 2009). About 82 to 96% yield decline has been reported in cassava under severe moisture stress (Aina et al., 2007a). Drought affects crops through chlorophyll degradation and inhibition of photosynthetic capacity, leading to low yields (Epron and Dreyer, 1993; van der Mescht et
al., 1999; This et al., 2000; Jiang and Huang,i2001; L et al., 2004). A genotype that has the
ability to maintain its chlorophyll content under water deficit conditions is believed to be drought resistant or tolerant. The adaptation of plants under drought conditions is a complex phenomenon that depends on plants’ attributes and their interactions with environmental factors (Parry et al, 2005; Tardieu, 2005).
Past research activities have focused on evaluating introduced cassava genotypes for high yields, low HCN content and excellent cooking qualities and, subsequently, breeding for improved pest and disease resistance (Nweke et al., 1994) in humid ecologies, without much emphasis on resistance to abiotic stresses such as drought. Ability to identify drought-tolerant and resistant varieties is of paramount importance for the maximization of productivity potential in these areas of low rainfall and unpredictable conditions (Okogbenin et al., 2003). It has been suggested that the main way to ensure food security in the presence of drought is to increase the development and deployment of drought-tolerant crop varieties (EPA, 2003). Therefore, zone-specific research is necessary to identify cassava varieties that can meet the expectations and needs of farmers and consumers in the Guinea Savannah Zones of Ghana.
Cassava has been perceived to be a hardy crop with ability to withstand moderate moisture stress making research to improve its water use efficiency limited (Turyagyenda et al., 2010). Breeding crops for drought prone areas requires the identification and the establishment of
the genetic basis of key traits as well as their interactions with the environment to assess their stability (Rebetzke et al., 2002; Tardieu, 2005). Such traits should be positively and closely associated with yield to bring about the desired genetic improvement. Parry et al. (2005) proposed that directed breeding strategies must focus on key traits that are important to performance under drought stress. For instance, traits such as carbon isotope discrimination, which measures the extent to which photosynthesis is maintained under limited moisture, and stable chlorophyll content have been used in wheat and groundnut improvement under drought conditions (Condon et al., 2004; Arunyanark et al., 2008). Though an association has been found between carbon isotope discrimination and yield in cereals (Monneveux et al., 2005), the mechanism of carbon isotope discrimination and its potential in determining transpiration efficiency has been poorly exploited in root and tuber crops (Monneveux et al., 2013). Most crops have specific phases in their phenology that can be targeted for improvement against drought, it has not been easy to do so for cassava due to differential partitioning of dry matter into the above ground biomass and roots (Alves, 2002; El- Sharkawy, 2004). It has been difficult to separate productive traits from survival traits which in most cases are negatively correlated yield under drought but only useful for perennial crops (Mitra, 2001; Clary et al., 2004). The cassava plant is naturally a perennial shrub and can grow indefinitely alternating periods of vegetative growth and root bulking (Alves, 2002). It undergoes various physiological, morphological and adaptive mechanisms such as abscisic acid accumulation to induce leaf senescence, partial closure of stomata as well as root elongation to deeper soil depths (El-Sharkawy et al., 1992; Santisopasri et al., 2001; El- Sharkawy, 2007) to combat periods of moisture deficit. Identification of relevant traits closely associated with root yield, their genetic basis as well as stability across environments will provide opportunity for selection for high-yielding drought tolerant cassava varieties.
1.1 Objective of research
The main objective of this work was to determine the genetic basis of the physiological traits associated with cassava productivity under drought stress conditions for adaptation in the guinea savannah ecology of Ghana. The specific objectives were to:
- Identify farmers perception of drought, coping strategies, cropping systems and varietal preferences in cassava cultivation,
- Determine genetic variability among cassava genotypes in physiological traits associated with drought tolerance in cassava,
- Estimate broad sense heritability and stability of morpho-physiological traits associated with yield in different growing environments, and
- Determine the efficiency of abscisic acid and carbon isotope discrimination in identifying drought tolerant genotypes as well as the effect of drought stress on root bulking in
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