Characterization and slow growth in Vitro conservation of potato (solanum tuberosum L.) in Kenya
Most of the potato (Solanum tuberosum L.) germplasm in Kenya is undocumented and uncharacterized and therefore of little practical value and use to breeders. The extent of genetic erosion is little known and in vitro conservation is limited by lack of cost effective protocols. Therefore, studies were carried between 2006 and 2010 with the aim of characterizing potato and developing a cost effective starch-based in vitro conservation protocol for potato. The studies comprised a survey to assess the extent of genetic erosion and farmers' perceptions of genetic erosion in potato; morphological and molecular characterization; agronomic characterization; disease characterization; and evaluation of the potential of cassava starch as an alternative low cost gelling agent for potato in vitro conservation media at normal propagation temperature. Survey results showed that only nine of 29 varieties which were once widely grown in Kiambu West district were still being grown, while another 11 had been introduced. A genetic erosion of 31 % had therefore occurred in the study area; however, farmers were not bothered by loss of varieties. Morphological analysis of 48 cultivars showed that there was little variation among morphological traits and not all the cultivars could be uniquely identified based on morphological characters. The genetic diversity and relationships of 48 cuItivars examined using 22 simple sequence repeats markers showed that there is a narrow genetic base of the Kenyan potato cultivar collection. Highly significant positive correlations were found between total tuber yield and % ware yield (r=0.616), average tuber weight (r=0.665), days to maturity (r=0.367), total tuber number (0.712), plant height (r=0.637), seed yield (r=0.506), ware tuber number (r=0.846) and ware yield (r=0.914). The path coefficient analysis based on tuber yield as a dependent variable revealed that the number of tubers/plant and average tuber weight showed positive direct effects on tuber yield (0.789 and 0.716, respectively) while number of stems/rrr' had negative direct effects (-0.022) on tuber yield. Under the method of covering freshly harvested seed tubers with polythene sheets to break dormancy, short dormancy cultivars had a significantly higher number of sprouts per tuber, number of stems/plant, number of tubers/plant, harvest index, ware yield, seed yields and total tuber yields than the medium and long dormancy cultivars. On the basis of relative area under disease progress curve, 26 of 32 cultivars were delineated into distinct late blight resistance classes. Cultivars Kenya Karibu, Tana Kimande, Kihoro, Kenya Sifa and Kenya Karibu (white flowers) were rated as resistant to late blight while Kerrs' Pink, Ngure, Desiree, , ROmano and Pimpernel were rated as susceptible. Seed borne virus infections significantly reduced total yield, ware yield, seed yield, average tuber weight, and number of tubers per plant for all the varieties. Experiments to evaluate the potential of cassava starch as an alternative low cost gelling agent for potato in vitro conservation media at normal propagation temperature showed that plantlet survival and condition of plantlets after 18 months of conservation was similar when cassava starch (8%) + agar (0.25%) and agar (0.8%) alone were used as gelling agents for all the three varieties studied. Gelling agent costs were reduced by 16.6 and 24.4 % when cassava starch (8%) + agar (0.25%) and cassava starch (8%) alone were used as the gelling agents instead of agar alone. Media gelled with cassava starch (8%) alone had poor clarity and gel strength indicating its unsuitability for conservation. Use of table sugar as a source of carbon instead of laboratory grade sucrose resulted in a 96.4% reduction in carbon costs. It is concluded that there is potato genetic erosion in Kiambu West district, farmers' awareness of the importance of conserving potato varieties is low and SSR markers were more rapid and reliable in differentiating cultivars than morphological markers. None of the local cultivars evaluated were resistant to viruses, hence the need to broaden the genetic base of the Kenyan potato cultivar collection. Covering freshly harvested seed tubers with polythene sheets as a dormancy breaking method is unsuitable for medium and long dormancy cultivars and more effective methods of dormancy breaking are required. Use of cassava starch (8%) + agar (0.25%) and table sugar instead of agar and laboratory grade sucrose has the potential to reduce costs of in vitro potato conservation.