The main biotic constraints of sweetpotato in the tropics are sweetpotato weevil (Woolfe, 1992; Stathers et al., 2005; Shonga et al., 2013; Ehisianya et al., 2013 ), alternaria blight, sweetpotato virus disease (SPVD) (Ames et al., 1996; Mwanga et al., 2002; Miano, 2008;
McGregor et al., 2009), and root-knot nematodes (Meloidogyne spp) mostly found in the temperate zones (Mwanga et al., 2002; Grüneberg et al., 2009). Moreover, SPVD is a combination of sweetpotato chlorotic stunt virus (SPCSV) and sweetpotato feathery mosaic virus (SPFMV), (Clark, 1988; Miano, 2008; McGregor et al., 2009). In summary, there are
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about 20 viruses or virus like diseases, about 35 bacterial and fungal diseases, about 20 nematodes, and about 20 insect pests that affect sweetpotato (Martin and Jones, 1986;
Ames et al., 1996; Ndunguru and Kapinga, 2007).
Moisture stress due to drought is becoming a major abiotic constraint to crop production worsened by climate change (Amede et al., 2004; Claessens et al., 2012; Nakashima and Yamaguchi-Shinozaki, 2013). Soil moisture availability determines the external water status at the boundaries of the plant (soil and air) and in the internal plant water status within the tissue of the plants. Moisture stress begins when the readily available soil water in the root zone is exhausted (Kramer and Boyer, 1995). Drought stress reduces photosynthesis, and translocation of assimilates thus effectively reducing the yield (Anselmo et al., 1998; Anjum et al., 2011). However breeding drought tolerant varieties may ensure high yield production under conditions of limited water availability (Sorrells et al., 2000). Mechanisms such as drought escape, drought tolerance, drought avoidance, and drought recovery enable the crop to tolerate drought and produce some yield (Ekanayake, 1990; Sorrells et al., 2000).
Drought tolerant sweetpotato varieties produce higher quantity and quality of yields in absence of sufficient rains and irrigation compared to other varieties.
1.2.1 Moisture stress
Drought may occur in the early, middle, or end of the cropping season, which triggers multiple and mixed responses during the stress and recovery time (Mundree et al., 2002).
This phenomenon complicates studies of the genes responsible for drought tolerance.
Moreover, interactions of the genotypes with environmental factors such as soil chemistry, soil texture and weather complicate drought tolerance studies (Sorrells et al., 2000; Jaleel et al., 2009). Thus, drought evaluation experiments may require use of greenhouse or drip irrigated rain-out shielded fields where water can be precisely controlled (Laurie et al., 2013).
Additionally, field trials should be large enough and replicated to eliminate micro environmental errors (Amede et al., 2004). Also, screening for drought in sweetpotato is further complicated if drought tolerance is measured using yield components (e.g. number of storage roots M-2, number of storage roots plant-1, root weight plant-1, dry matter instead of the yield ha-1, because each component is sensitive to drought at different times and in different modes (Makihara et al., 1999).
Root growth characteristics play important roles in drought stress tolerance in plants (Ahmad et al., 2009). Deep rooted varieties that continue to grow by tapping water from deeper layers are bred for upland areas with deep soils and unreliable rainfall. Likewise, in lowland areas with unreliable rainfall, genotypes with roots with the capacity to penetrate the
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hardpan layer of compacted soil at about 20-25 cm below the soil surface are bred (Champoux et al., 1995; Ray et al., 1996).
The desired traits for drought tolerance in crops include its ability to; 1) reduce water loss to the soil and to air, 2) maintain cell turgor pressure for a long time, 3) survive loss of cell turgor pressure, 4) protect cells against oxidative damage caused by continued absorption of radiation when the stomata is closed thus preventing photosynthesis and, 4) to recover after water is available (Ahmad et al., 2009; Anjum et al., 2011)
1.2.2 Plant response to drought stress
Crops under water stress undergo three stages of dehydration; alarm, resistance and exhaustion (Amede et al., 2004). The stage of alarm occurs under mild drought stress where assimilation and transpiration are similar to normal well watered plants and plant soil water uptake meets evapo-transpiration requirements (Mundree et al., 2002). The resistance stage occurs when the photosynthetic capacity of the plant is reduced below the maximum potential level due to drought stress. At this stage, the plants develop adjustment mechanisms and regulatory metabolic processes to cope with the water stress (Anjum et al., 2011). Finally, the exhaustion stage occurs when the plants strive to survive and delay death due to prolonged drought stress. Once the crop reaches this stage, its recovery after rain or irrigation depends on genotype and duration of the water stress (Sinclair and Ludlow, 1986).
Plants manifest different types of responses to moisture due to water stress (Jaleel et al., 2009; Anjum et al., 2011; Nakashima and Yamaguchi-Shinozaki, 2013). During rapid water stress, plants tend to orientate leaves parallel to incident light. Cell growth and expansion is diverted to restitution of physiological integrity, and stomatal adjustment (Arve et al., 2011).
Moreover, under long term water stress, crops tend to reduce leaf area while increasing root density modifying root leaf ratio (Ahmad et al., 2009). Usually, root depth and density are used to determine the level of drought tolerance of genotypes. Also, crops under water stress alter plant cell solute concentration to reduce the osmotic cell potential while maintaining cell turgor pressure (Mundree et al., 2002; Arve et al., 2011). This allows turgor related physiological processes such as stomata movement and cell growth to go on despite low water potential. Some plants modify soil plant water gradient through cell solute concentration, hence increasing the amount of water transpired and leading to yield increase (Mundree et al., 2002). Solute concentration in plant cells occurs through cellular water loss, breakdown of primary metabolites (protein, carbohydrates and fats), and translocation of assimilates (solutes) to the cell sinks that increases cell solute concentration (Kramer and Boyer, 1995; Mundree et al., 2002). Under moderate levels of drought stress, roots may
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absorb and accumulate inorganic macro and micro ions such as potassium, calcium, sodium, chloride, magnesium in cells, coupled with reduced cell growth, hence reducing osmotic potential (Munns, 1988 ). For example, tepary beans are more drought tolerant than common beans because they possess higher osmotic adjustment potential. Thus, transferring the osmotic adjustment gene from tepary beans to common beans can improve their drought tolerance (Parsons and Howe, 1984).
The rate of transpiration in crops depends on size of the transpiring area, the number and size of stomata, and the conductivity of the cuticle (Amede et al., 2004). Stomatal transpiration accounts for 90% of crop water loss (Anjum et al., 2011; Ahmad et al., 2009).
This is followed by water loss through the cuticles, which is influenced by its thickness, and presence or absence and type of wax (Monneveux and Belhassen, 1996). Additionally, leaf rolling, leaf colour or leaf reflectance influence transpiration (Arve et al., 2011). Early seedling establishment, early high vigour, rapid canopy development, and leaf area maintenance minimize evaporation while maximizing transpiration and are termed as drought tolerance mechanisms (Subbaro et al., 1995). Stomata closure is one of the first steps of defence against drought stress in many crops because it is a more rapid and flexible process relative to other mechanisms like root growth or reduction of leaf area. The effect of drought stress on stomata closure may be measured by the CO2 fixation rate. Thus, drought tolerant genotypes maintain higher CO2 fixation even at prolonged drought stress.
Crops under mild water stress tend to have increased root growth and root density but reduced shoot growth and evapo-transpiration (Ahmad et al., 2009). Stomatal closure is an effective survival strategy for intermittent stress, although it may not work for terminal stress (Amede et al., 2004).
1.2.3 Water use efficiency
Plants open stomata to admit CO2 for photosynthesis, and to transpire water (Mundree et al., 2002). Stomatal conductance is more strongly correlated to photosynthetic parameters than soil water status itself (Anjum et al., 2011). Leaves and stems may transpire through non- stomatal surfaces. Plants transpire to cool during high air temperatures and transport soil nutrients and chemicals synthesized in the roots to the leaves through the transpiration stream (Peuke et al., 2002). Crop production per unit transpiration is referred as water use efficiency (WUE). The ratio of photosynthesis (A) / transpiration (T) is referred as transpiration efficiency (Peng et al., 1998). Water productivity per unit of evapo-transpiration is the mass of crop production divided by the total mass of water transpired by the crop and lost from the soil by evaporation. Plants transpire several hundred times more water than is present in their tissues at any one time. Drought water stress reduces photosynthesis, total
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dry matter accumulation, and yield, depending on time, duration and severity of the drought stress (Boonjung and Fukai, 1996; Anjum et al., 2011).
Translocation is less affected by water stress than the photosynthesis and transpiration (Boyer, 1976). Thus, late sowing for example in a month after the onset of rains exposes the ability of a genotype to translocate reserves to the sink during the onset of the terminal drought (Ludlow and Muchow, 1990). In most crops, the assimilate reserves originate from pre-anthesis to post-anthesis events, depending on the amount of reserves in the stem, therefore the availability of these reserves is dependent on the timing of water stress (Amede et al., 2004).
1.2.4 Drought tolerance mechanisms
Plants respond to drought through various mechanisms (Mundree et al., 2002; Nakashima and Yamaguchi-Shinozaki, 2013). Some of the drought resistant mechanisms are;
synchrony of fast growth stages to water supply, early growth vigour, stomatal regulation, developmental plasticity, increased root depth and density, leaf area maintenance, synchrony of storage root filling to water supply, mobilization of assimilates from source to sink, and osmotic adjustment of roots and shoots (Mundree et al., 2002; Amede et al., 2004;
Nakashima and Yamaguchi-Shinozaki, 2013). Traits that indicate drought tolerance are;
biomass, growth vigour, root depth and density, storage roots per vine, storage root weight, storage root yield, early maturity, degree of translocation, production efficiency, and drought sensitivity index, plant water potential, osmotic potential, relative water content (RWC), and relative growth rate (RGR) (Amede et al., 2004; Painawadee et al., 2009; Anjum et al., 2011;
Arve et al., 2011; Ciarmiello et al., 2011).
Plants cope with drought through: 1) drought avoidance, by development of deep roots that penetrate and explore deeper and greater soil volume, 2) drought escape where short duration crops complete their lifespan before soil moisture from fairly reliable rainfall vanishes, and 3) drought tolerance where plants adapt to slow onset of drought by modifying their chemical constitution to retain as much water as possible through osmotic adjustment to protect themselves from irreversible damage during stress (Ludlow and Muchow, 1990;
Mitra, 2001; Mundree et al., 2002; Bennett, 2003).
Drought avoidance mechanism occurs where primary and secondary leaves continue expanding at decreasing soil water potential (SWP). This mechanism is detected by determining both the leaf area at moderate drought stress and leaf water potential (LWP) at 50% transpiration reduction (Amede et al., 2004). Drought escape is where the crop will
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cope with low soil water through earliness of development, plasticity of growth, and efficient water uptake by increased rooting depth and density and increased hydraulic conductivity (Mundree et al., 2002). Drought tolerance arises through osmotic regulation that maintains cell turgor measured by determining SWP and LWP at decreasing relative water content (RWC) (Amede et al., 2004; Yang and Miao, 2010; Anjum et al., 2011). Also, drought tolerance arises through the plant having low critical soil water potential (SWP) for non- recovery upon re-watering measured by determining RWC and LWP below which plants will not recover (Amede et al., 2004).
Other mechanisms include efficient conservation of tissue water by leaf rolling and shrinkage, reduction of cuticle water loss through wax deposits, regulation of stomata aperture and osmotic adjustment (Anjum et al., 2011). Plants may also cope with incident light intensity by producing new growth at a time of the year with moderate light intensity, under shading by plants within a canopy, or leaves on individual plants. Plants also control light absorption through reduction of leaf reflectance using leaf hairs, waxes and leaf orientation adaptation. At the cellular level, water stress is reduced by absorption of excitation energy by chlorophyll, as well as dissipation of excitation energy as heat through the xanthophyll cycle. Also, plants reduce water stress through active oxygen formation and free radical scavenging defence mechanism through enzymatic and non-enzymatic antioxidants (Amede et al., 2004). The excess free radicals cause drought induced photo- oxidative damage referred to as leaf chlorosis or necrosis when the crop is exposed to high light intensity coupled with drought stress and or mineral deficiency (Mundree et al., 2002;
Anjum et al., 2011).
1.3 Breeding for drought tolerance