MORPHOGENESIS IN CULTURED THIN-LAYERS AND PITH EXPLANTS OF TOBACCO .1. EFFECT OF PUTRESCINE ON CELL-SIZE, XYLOGENESIS AND MERISTEMOID ORGANIZATION

Superficial thin layers and pith explants from vegetative tobacco plants were cultured on a caulogenic medium (1 mu mol/L IAA + 10 mu mol/L BA), or with IAA alone (1 mu mol/L), BA alone (10 mu mol/L) or on a hormone-free medium, in the presence or absence of 100 mu mol/L putrescine. The aim was to investigate histologically possible, and explant type independent, putrescine effects on cytological events specifically induced by the hormone treatments during caulogenesis and callogenesis in vitro. In both types of explants the earliest cytological events (48 h) observed were fragmented or multilobed nuclei and newly formed cell walls, both occurring in IAA and IAA+BA treatments. Effects of putrescine on specific cytological events, induced by IAA or BA in both model systems, were from day 4 to the end of culture. Putrescine strongly counteracted the nuclear fragmentation, cell hypertrophy and xylogenesis specifically induced by IAA during callus growth. Furthermore, the polyamine strongly enhanced BA-induced formation of vegetative meristemoids. However, the extent of putrescine effects declined when the explants were treated with the two hormones in combination, or cultured under hormone-free conditions.