GENETIC STUDIES TOWARDS ELUCIDATION OF DROUGHT TOLERANCE OF POTATO

Abstract

Potato (Solarium tuberosum L.) is a staple food with great economic value that ranks as the fourth most important food crop in the world. Globally potato is cultivated on 19 million hectare, being 8tn in terms of area under cultivation and with an estimated 325 million tons of annual production (Food and Agricultural Organization of the United Nations, 2012). Potato production provides food, employment and income as a cash crop (Scott et al. 2000). Potatoes have a high productivity per unit area with relatively little water consumption and take a short production time, thus being a candidate crop for food security. The cultivated potato S. tuberosum is autotetraploid (2n=4x=48). The domestication of potato dates back 6000 years in the central Andes, which is present-day southern Peru and northern Bolivia, when the native people started to select wild potato species for human use (Spooner et al. 2005). The modern cultivated potato (Solanum tuberosum) was domesticated from wild potato species of the Solanum brevicaule complex (Spooner et al. 2005). The genus Solanum has over 220 wild tuber bearing potato species and seven cultivated potato species (Hawkes and Jackson 1992). The variation in ploidy level is one of the most important features in potato taxonomy. The chromosome numbers in the wild species vary from diploid (2n=2x=24), triploid (2n=2x=36), tetraploid (2n=4x=48), pentaploid (2n=5x=60), to hexaploid (2n=6x=72), while in cultivated potatoes this ranges from diploid to pentaploid. The majority of the diploid species are self-incompatible while tetraploids are self-compatible allopolyploids with disomic inheritance (Hawkes 1990). Wild and cultivated potato genetic resources provide a variety of reproductive and genetic features associated with species differentiation and breeding applications. Cultivated potatoes can be classified as landraces or improved varieties. Landraces are native varieties still grown in South America today while improved varieties are grown around the world. Landrace potato cultivars are native to two areas in South America: the upland Andes from eastern Venezuela to northern Argentina and the lowlands of southcentral Chile (Ames and Spooner 2008). It was in the year 1557 that potato was first introduced to Europe (Rios et al. 2007). The origin of the "European" potato is disputed with two competing hypotheses, one suggesting its origin from the Andes while another one suggests it to originate from lowland Chile. For the last 60 years it was accepted that European potato could have an Andean origin but recent studies suggest the European potatoes most likely came from both Andean and Chilean landraces (Rios et al. 2007). By the 1700s, potato cultivation was widespread in Europe and its worldwide cultivation began soon after (Hawkes and Francisco-Ortega 1993). The Irish potato famine caused by potato late blight disease, Phytophtera infestans, caused widespread famine and migration in Europe beginning in 1845. Late blight remains one of the most serious potato diseases worldwide, yet the potato crop persisted as a staple food throughout Europe. Although there is no clear recored when potatoes was introduced to Africa, the first introduction of potato to Ethiopia was in 1858 by a German immigrant, Wilhelm Schimper (Kidane-Mariam 1980). However, the adoption of potato crop by the Ethiopian farmers occurred very gradually for several decades and its wider adoption occured only at the end of 19th century (Gebremedhin et al. 2001). As a non-cereal crop, potato is regarded as a secondary crop despite its potential as a food security crop. However, efforts are being made by different sectors including governmental research centerts and non governmental organization to increase the production potato in the suitable highland ares of the country. Potato production Potatoes are grown in about 125 countries with annual productions approaching to 325 million tons (FAO, 2012). Potatoes are consumed by more than a billion people worldwide on a daily basis. For a long period of time potatoes held a particular importance in temperate climates but this has changed in the last 20 years when the world potato production has undergone major changes. In the last few years, there has been a dramatic increase in potato production in the developing nations mainly due to an increase in productivity and area harvested (FAO 2013). At present, developing nations account for more than half of the global potato area and production (Haverkort and Struik 2015). Currently, the major potato producing countries are China, India, The Russia Federation, Ukraine and USA (FAO 2013). In Africa, Ethiopia ranks at the 11th place in potato production with an estimated annual production of 525 000 tons as of 2007 (FAO 2008). Ethiopia has the potential to be the highest potato producing country in Africa with widely available highland areas that best suit potato production. Potato can potentially be grown in 70% of arable land estimated to be lOMha (FAO 2008; Hirpa et al. 2010).However, the current potato production in Ethiopia occupies small (0.16Mha) part of the available arable land (Hirpa et al. 2010). In Ethiopia, there are four major potato production areas that include the central, eastern, northwestern and southern parts of the country (Hirpa et al. 2010) and Figure 1. Collectively these areas accounts for the country's 83% of potato farmers, where 40% are located in north western of the country (CSA 2008/2009).