Partial Reversal by Cytokinin and (2‐Chloroethyl)‐Trimethylammonium Chloride of Near‐Ultraviolet Inhibited Growth and Morphogenesis in Callus Cultures

Callus cultures of Haplopappus gracilis, Nicotiana tabacum and Allium cepa var. proliferum were in varying degrees inhibited by blue to near‐UV light obtained from fluorescent tubes. The inhibition was considerably reduced for Haplopappus cultures by 6‐(3‐methyl‐2‐buten‐1‐ylamino)‐purine (2 iP) and (2‐chloro‐ethyl)‐trimethylammonium chloride (CCC) in combination. Even separately these compounds stimulated growth in blue but not in white light. A high concentration of 2 iP reduced the inhibiting effects of near‐UV on tobacco tissue cultures, and a synergism was observed between 2 iP and CCC in respect to shoot formation in blue light. Allium callus was not significantly affected by CCC. It was also observed that the concentration of indol‐3‐acetic acid (IAA) was more important for the growth of tobacco cultures in blue than in white light. It is believed that the light‐inhibition of growth is partly due to a photoinactivation of IAA and that 2 iP and CCC might be active through processes controlling the levels of IAA and other growth hormones.