In vitro storage of prolific potato cultivars increases labour, space and material costs due to frequent sub-culturing. A method that reduces frequent sub-culturing can lower maintenance costs of potato storage. A study was undertaken to assess in vitro growth of potato under either (1) complete dark exposure, (2) dark exposure for three days only and then to light or (3) light exposure throughout. Also, micro shoots were placed either (1) horizontally, (2) upright or (3) inverted on culture media. There were significant differences between treatments with respect to number of shoots (P=0.0099) and roots (P=0.0279), but not with respect to shoot length when placed horizontally. Micro shoots placed upright and under light condition were significantly (P=0.0092) longer than those in the dark. Similarly, those under light exposure had significantly (P=0.0168) higher number of roots than those under dark condition. The number of leaves under light condition was significantly (P=0.0318) higher than those under dark condition. For inverted plantlets, significant differences between treatments with respect to shoot length (P=0.0114), number of roots (P=0.0045) and number of leaves (P=0.0004) were obtained. There were significant differences (P=0.003) with respect to explant position. Placing micro shoots under dark condition and at an upright position severely reduced explant growth. Exposing micro shoots to dark condition for three days and then to light also reduced explant growth. This study concludes that exposing potato micro shoots to dark and then to light condition in an upright position can reduce sub-culturing frequency and enables explants to recover from chlorosis. This procedure is necessary for in vitro potato storage as it reduces maintenance costs due to low sub-culturing frequency.
Key words: Chlorosis, conservation, explants, in vitro, regeneration, Thandizo.
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