Introduction

Moisture stress is one of the major abiotic factors that affect potato production worldwide (Yuan et al., 2003). Vulnerability of potato to drought has been attributed mainly to the crop’s shallow root system and low capacity of recuperation after a period of water stress (Iwama and Yamaguchi, 2006). Potatoes have sparse and shallow root system (Kashyap and Panda, 2003; Onder et al., 2005) with a depth ranging from 0.5 to 1.0 m (Vos and Groenwold, 1989). About 85 % of the total root length is concentrated in the upper 0.3 m of soil. Due to this, potato extracts less of the available water from the soil compared to other crops (Weisz et al., 1994). Even short periods of water shortage can reduce tuber production and quality (Miller and Martin, 1987). The relative inability of potato to withstand drought limits its productive range to areas with adequate rainfall or reliable irrigation. Consequently, the ideal conditions for potato production include high and nearly constant soil matric potential, high soil oxygen diffusion rate, adequate incoming radiation and optimal soil nutrients (Yuan et al., 2003). It is estimated that the average global potato yield could be increased by at least 50 % if water supply to the crop could be optimised.

The negative impacts of moisture stress on potato production are likely to increase over the next decades due to climate change and the expansion of potato production in drought prone areas. Due to drought, it is estimated that potential potato yield will decrease by 18 to 32 % between 2040 and 2069 (Hijmans, 2003). Recently, drought affected Russia and led to losses of around 30% on industrial potato farms in the Central and the Volga Valley in 2010 (GAIN, 2010). Furthermore, crop models predict reduction in potato yields by about 30 % in Poland as a result of water deficit (http://www.climateadaptation.eu/poland/agriculture-and-horticulture/). Insufficient water supply may occur almost anywhere in the world; in arid tropical and sub-tropical zones where crop production is only possible with irrigation, short periods of drought often arise because of inappropriate irrigation techniques or shortage of rainfall. In the temperate zones, both short and long periods of drought may occur due to irregular rainfall particularly on soils with low water holding capacity. In warm tropical areas, the negative effect of water stress is exacerbated by high temperature. Even with good irrigation practices, water stress may occur because of high transpiration rates especially during mid-day when root system cannot completely meet the water requirements of the plant leading to increased water potential and consequent reduction in the rate of photosynthesis (Minhas and Sukumaran, 1988).

In most plants, moderate drought leads to a reduction in stem height, number of green leaves and leaf length (Deblonde and Ledent, 2001). Drought limits crop productivity by affecting photosynthetic processes at the canopy, leaf or chloroplast level, either directly, or by feedback inhibition if transport of photosynthates to sink organs is limited (Jones and Corlett, 1992). Plants grown under drought conditions tend to have lower stomatal conductance, thus helping to conserve water and maintain an adequate leaf water status; however, this reduces leaf internal carbon dioxide (CO₂) concentration and photosynthesis (Chaves et al., 2002). The effects of drought on plants depend on intensity, duration and rate of progression of imposed drought (Pinheiro and Chaves, 2011).  Moisture stress first causes stomatal closure thus reducing CO₂ uptake for photosynthesis; this leads to reduced plant growth and yield (Serraj et al., 2004; Mafakheri et al., 2010; ScienceDaily, 2008). Sugar concentration within the leaf tissue increases to increase the osmotic potential of the plant; this leads to feedback inhibition of photosynthesis (Basu et al., 1999). Drought also leads to increased accumulation of reactive oxygen species (ROS) in plants such as superoxide radical (O₂^-) and hydrogen peroxide (H₂O₂). Overproduction of ROS can disrupt normal plant metabolism through impaired enzyme activity due to oxidative damage, protein degradation, DNA and RNA damage and membrane lipid peroxidation, which can ultimately culminate in cell death (Finkel and Holbrook, 2000).

Effects of drought on potato crop

Drought may affect potato crop in a number of ways: 1) by reducing the amount of productive foliage thereby decreasing plant growth (Deblonde and Ledent, 2001), 2) by decreasing the rate of photosynthesis per unit of leaf area and 3) by shortening the growth cycle/vegetative period (Kumar et al., 2007). This result in reduced number (Eiasu et al., 2007) and size (Schafleitner et al., 2007) of tubers produced Potato yields and quality are influenced by the timing, duration and intensity of rainfall or irrigation (Ekanayakeand Midmore, 1989; Jeffery, 1995); it is possible to increase yields through well-scheduled irrigation programmes throughout the growing season (King and Stark, 1997). Sensitivity of potato to water stress varies with the developmental stage of the crop; plant emergence and tuberization are two critical periods when water stress most affects final tuber yield (Martínez and Moreno, 1992). Drought after planting may delay or even inhibit plant emergence while insufficient water supply between plant emergence and beginning of tuber bulking may lead to slow growth rate of the foliage, small leaves and small plants. Minhas and Bansal (1991) showed that tuber initiation is the most sensitive stage to water stress; drought during this period can reduce the number of tubers produced per plant (King and Stark, 1997).Moisture stress at stolon and tuber initiation not only restrains foliage and plant development but also limits the number of stolons formed leading to reduced tuber numbers and therefore a reduction in final yields (Deblonde and Ledent, 2001). Tuber size and quality are closely related to moisture supply during tuber bulking period and total yield of potatoes is most sensitive to water stress during this period. Water stress during tuber bulking stage leads to a reduction in the leaf expansion rate, inhibits the development of new leaves and encourages plant senescence resulting in decreased leaf area index (LAI) (Kumar and Minhas, 1999; Susnoschi and Shimsi, 1985).  Gandergander and Tanner (1976) showed that mild water stress of –3 to –5 bars greatly reduce leaf expansion in potatoes. For best tuber yields, a 120-150 day potato crop requires 20-27.5 inches (508-698.5 mm) of water. Moisture stress can reduce yields, produce misshapen tubers, negatively affect processing quality and increase common scab incidence (Mane et al., 2008). Tuber characteristics such as shape, dry matter and reducing sugars contents can be influenced by water stress during the vegetative period. Shape defects such as dumb-bell shaped, knobby or pointed end tubers can be caused by short periods of moisture stress during the tuber bulking stage (MacKerron and Jefferies, 1988). Misshapen tubers can also occur due to secondary growth which mainly occurs in dry soils when temperatures rise (Lugt, 1964). These hot and dry conditions may also result in poor cooking quality (glassiness) of the tubers, jelly end or translucent tuber ends. They also result in high content of reducing sugars in tubers which causes difficulties during processing. Tubers from water stressed plants often have higher contents of total sugars and dry matter than well-watered plants (Levy, 1983). Steckel and Gray (1979) found that the dry matter content of potato tubers grown under low soil moisture was much higher than in tubers from well-watered plants. However, Levy (1983) found that tubers from stressed plants of cultivar up-to-date and cultivar Troubadour had a lower dry matter content while cultivar Alpha had higher dry matter content than well-watered controls.

Plant response to water stress

Reduction in plant size and leaf area, early plant senescence, and prolonged stomata closure are plant responses to drought (Vos and Groenwold, 1989). Drought resistance in plants can be classified according to the mechanisms exhibited; these include drought escape, avoidance, tolerance and recovery. According to Levitt (1980), drought avoidance includes closure of stomata or possession of a large root system while drought tolerance includes capacity for osmotic adjustment or rapid resumption of photosynthetic activity. Drought tolerance is primarily attributed to maintenance cell turgor and it includes osmotic adjustment and cellular or tissue elasticity (Obidiegwu et al., 2015). Responses to drought stress can also be partitioned into (i) avoidance of tissue water deficits/dehydration, (ii) tolerance of tissue water deficits, and (iii) efficiency mechanisms (Turner, 1986; Jones, 2014). Avoidance of tissue water deficits can be achieved by means of “drought escape,” where plants grow only during periods of ample moisture and often involve rapid phenological development. The drought escape process is significant in arid regions where adapted annuals might combine short life spans with high rates of growth and gas exchange while utilizing the maximum moisture content in soil (Maroco et al., 2000). Avoiding tissue dehydration can also be achieved by enhanced water uptake as a result of increased root depth or altered rooting patterns (Jackson et al., 2000) or by reduced water loss due to stomatal closure or adjustments of the leaf energy balance through reduction in light absorption or modifications to heat and mass transfer in the leaf boundary layer (Larcher, 2000; Mitra, 2001; Jones, 2014). Drought tolerance has been associated with control of plant growth and carbon transfer under water stressed conditions (Tourneux et al., 2003), enhanced water use efficiency (Alva et al., 2012), and osmotic adjustment (Heuer and Nadler, 1998). Tolerance of tissue water deficits most commonly involves maintenance of turgor either through osmotic adjustment (OA) (Morgan, 1984) or as a result of rigid cell walls or decreased cell size (Wilson et al., 1980) even when the tissue water potential declines. In an agricultural context, farmers and breeders tend to define drought tolerant cultivars as those that maintain yield under drought conditions. Potential efficiency mechanisms for improvement of crop drought tolerance include improvements in the water use efficiency (WUE) and improvements in the efficiency with which assimilate is converted to harvestable yield (HI).

Drought tolerant cultivars employ multiple strategies to survive under water-limited conditions and to produce higher yields than sensitive cultivars. One such strategy is to obtain as much water as possible from the soil by forming a well-developed root system (Lahlou and Ledent, 2005). The second strategy is to retain more water in the plant. Relative water content (RWC) is one of the most reliable indicators for defining water retention in plants (Rampino et al., 2006; Sanchez-Rodriguez et al., 2010). Studies have shown that RWC decreases in response to drought stress (Bürling et al., 2013; Shaw et al., 2002). The third strategy of drought tolerant cultivars is to increase the capacity to defend against oxidative damage caused by drought stress. Oxidative damage is characterized by overproduction of ROS such as O₂^- and H₂O₂ resulting in lipid peroxidation and even cell death (Imlay, 2003; Ashraf, 2009; Ashraf, 2010). To counteract ROS, plants produce various types of antioxidants; activation of antioxidants is associated with the degree of drought tolerance of the plant species (Sunkar et al., 2006). Partial root zone drying has been shown to enhance antioxidant activity in potato tubers (Jovanovic et al., 2010).

Aspects of drought tolerance that are important and should be considered in potato breeding programme are: 1) the effect of short periods of moisture stress on productivity and tuber quality, 2) survival and recovery of the plants after water stress and 3) water use efficiency. Regarding survival and recovery after water stress, two factors are important: 1) limited loss of soil cover and 2) recovery of expansion growth and of the photosynthetic rate. There are indications that potato is able to regain its photosynthetic rate after drought stress. Chapman and Loomis (1953) found that after “permanently wilted” potato plants were watered, the rate of photosynthesis recovered fully after three days. Bansal and Nagarajan (1987) studied recovery of leaf growth after a period of water stress in eight potato cultivars. They reported that some potato cultivars showed minimum growth reduction under stress and had rapid recovery on re-watering with final increase in the leaf length exceeding that of the unstressed controls. In the second group, moisture stress caused moderate reduction in growth and on recovery; the increase in leaf length was comparable to that of controls. The third group of cultivars was characterised by huge reduction in growth and on re-watering, the final leaf length was less than in the controls.

Indicators of drought stress or drought tolerance

Further, direct or indirect indications of drought tolerance can be obtained by measuring photosynthetic rate, water retention of excised leaves, depth and extension of the root system, anatomical structure of the leaf, yield under dry growing conditions in the field, the extent of wilting and recovery after severe drought stress, water use efficiency and transpiration rate (Minhas et al., 2003).

Breeding for drought tolerance in potatoes

A number of approaches have been used to alleviate the problem of drought; nevertheless, plant breeding seems to be the most effective and economical one. There exists genotypic variability among potatoes with respect to drought tolerance with some varieties performing better under drought conditions. These drought-tolerant potato cultivars can produce reasonable yields where grain crops fail especially when drought coincides with flowering and seed set in grains (Iwama and Yamaguchi, 2006). The genetic variability that exists within S. tuberosum and its relatives can be exploited to improve drought tolerance (Levy, 1983; Jefferies and MacKerron, 1987). Although modern S. tuberosum are highly susceptible to drought stress, several landraces as well as wild species of potato are adapted to harsh and water-scarce conditions (Vasquez-Robinet et al., 2008; Hijmans and Spooner, 2001). However, there is limited use of native potatoes in conventional breeding programs due to linkage drag and the fact that some of the traits contributing to drought tolerance in native and wild potatoes are associated with low yield (Cabello et al., 2012). Desirable drought phenotypic traits must be genetically associated with high yield under stress, be highly heritable, genetically variable, easy to measure, stable within the measurement period, and must not be associated with a yield penalty under unstressed conditions (Okogbenin et al., 2013). Because potato yields depend on the timing of water stress within the growing period (Spitters and Schapendonk, 1990) and upon climatic and soil conditions (Tourneux et al., 2003), then it is necessary to consider these factors before making recommendations of optimal phenotypes for any specific environments.

Drought tolerance is a genetically complex polygenic trait with multiple pathways implicated. Therefore, effective crop improvement for drought tolerance will require pyramiding of many disparate characters, with different combinations being appropriate for different growing environments (Obidiegwu et al., 2015). However, it is difficult to pyramid drought tolerance related genes in highly heterozygous tetraploid potato cultivars while considering other important economic traits; the situation is aggravated by linkage drag and distortion in segregation between inter-specific hybrids. Hopefully these issues may be circumvented by use of biotechnological approaches.

Most of the quantitative trait loci (QTL) mapping studies in potato have been performed on diploid populations because the efficiency of association mapping is much higher in diploid than in polyploid species. Target traits included leaf senescence (Malosetti et al., 2006), tuberization (Fernandez-Del-Carmen, 2007) and tuber shape, eye depth and flesh colour (Sliwka et al., 2008). Recently, QTL have also been identified in tetraploid populations for traits such as plant height, maturity, crop emergence, tuber size, and tuber quality traits (Bradshaw et al., 2008; Hoop et al., 2010). However, insight into genetics and genes underlying QTL that are related to drought tolerance is still limited in potato (Anithakumari et al.,  2012).

Conclusions

This review discusses traits that have already been identified and are useful in selecting for drought tolerance in potatoes. Drought tolerance or resistance is a complex polygenic trait with multiple pathways implicated.  Breeding for drought tolerance using conventional methods has challenges especially in tetraploid potatoes due to lower efficiency of association mapping in polyploids than in diploids.

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