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 plan ts 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 ho rmone-free medium, in the presence or absence of 100 mu mol/L putresci ne. The aim was to investigate histologically possible, and explant ty pe 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 c ell walls, both occurring in IAA and IAA+BA treatments. Effects of put rescine on specific cytological events, induced by IAA or BA in both m odel systems, were from day 4 to the end of culture. Putrescine strong ly counteracted the nuclear fragmentation, cell hypertrophy and xyloge nesis specifically induced by IAA during callus growth. Furthermore, t he polyamine strongly enhanced BA-induced formation of vegetative meri stemoids. However, the extent of putrescine effects declined when the explants were treated with the two hormones in combination, or culture d under hormone-free conditions.