Genetic studies towards elucidation of drought tolerance of potato

Drought is a major threat to agricultural production, which makes drought tolerance a prime target for breeding approaches towards crop improvement. Drought is a complex polygenic trait and poses a challenge for drought tolerance breeding. Improving crops for drought tolerance at least requires the knowledge of the physiological mechanisms of the contributing traits and their genetic control. Thus, identification of genetic variation for drought tolerance is the first step towards drought tolerance breeding. The effect of drought stress on potato tuber yield and quality is very significant as potato is considered sensitive to water shortage. To understand the genetic factors underlying drought tolerance in potato, we performed drought stress experiments under green house and field conditions with moderate drought and severe drought stress conditions, respectively. In the field, potato genotypes were exposed to severe drought stress for two consecutive years starting from tuber initiation, which progressed to severe drought stress. In addition, we examined potato cultivars for moderate drought tolerance under greenhouse conditions where water application was reduced 50-60% from optimum amount starting from stolon formation. Morphological and physiological trait data were collected that allowed precise monitoring of the drought response of potato. Phenotypic data collected under severe drought stress conditions which includes traits like shoot and root biomass (fresh and dry), yield and chlorophyll content  were used for QTL mapping while data collected under moderate drought stress conditions was used for genome wide association mapping. With QTL mapping, 60 QTLs were identified controlling those traits both under well-watered and drought stress conditions. In the drought tolerance evaluation of the potato cultivars under greenhouse conditions we identified significant marker trait associations for both above- and belowground traits. Many of the QTLs detected for drought tolerance traits were specific to either moderate or severe drought tolerance conditions. However, a few QTLs showed an overlap between these drought stress environments. This demonstrates the presence of common genomic regions controlling drought tolerance traits under moderate and severe drought stress conditions. In addition, from the two years of field drought stress experiments we selected a subset of genotypes that showed contrasting responses to drought stress. We used these genotypes to further examine the relationship between canopy development and tuber yield under severe drought stress conditions. Canopy development was measured for several time points and the data were used for curve fitting. From the curve-fit, parameters related to the different developmental phase of canopy were extracted. We observed that there is positive correlation between canopy parameters and tuber yield under drought stress conditions. The evaluation of potato for drought tolerance under field and greenhouse conditions has resulted in the identification of several QTLs that can be interesting to be used for enhancing drought tolerance in potato. Furthermore, the use of model derived parameters gave a better insight into the relationship between canopy development and tuber yield under water stress conditions.