Effects ofmeta-topolin derivatives and temporary immersion on hyperhydricity and in vitro shoot proliferation inPyrus communis

Although micropropagation in temporary immersion systems might increase plant growth and multiplication, it can also cause specific problems such as hyperhydricity and losses by contamination. A new commercial temporary immersion bioreactor, SETIS (TM), was used to micropropagate two Tunisian pear cultivars, 'Arbi' and 'Mahdia 6'. The latter cultivar was endogenously contaminated bySphingomonas. Hyperhydricity was inevitable when 5 mu M benzyladenine was applied. However, the symptoms could be reduced by lowering the immersion frequency to 3 times per day. Applying 5 mu Mmeta-Methoxy topolin riboside (MemTR) ormeta-Topolin riboside (mTR) completely inhibited hyperhydric shoot formation. Moreover, the addition of Plant Preservative Mixture was effective to controlSphingomonasand allowed the plants to proliferate. For both pear cultivars, the highest number of shoots per explant was induced by 5 mu M MemTR, whereas the highest leaf area was obtained with 5 mu MmTR. The longest shoots were obtained with 5 mu MmTR for 'Arbi' and 5 mu M MemTR for 'Mahdia 6'. Key message Methoxy topoline-riboside (MemTR) and meta-topoline-riboside (mTR) were used as alternatives to benzyladenine to prevent hyperhydricity during the micropropagation of pears in a new temporary immersion system. These cytokinins also increased the number of good quality shoots, characterized by large leaves and longer shoots.