DOES ALTERED NITROGEN-METABOLISM AND H2O2 ACCUMULATION EXPLAIN THE VITRIFIED STATUS OF THE FULLY HABITUATED CALLUS OF BETA-VULGARIS (L)

The habituated callus is a vitrified tissue which has two main biochem ical characteristics both leading to production of toxic forms of oxyg en: first the blockage of the porphyrin pathway and a lack of H2O2 det oxifying enzymes (catalase and peroxidases); secondly a deviation of t he nitrogen metabolism induced by NH3 accumulation. Ammonia detoxifica tion is ensured by increased glutamate dehydrogenase activity and accu mulation of both proline and polyamines. A putative linkage between pr oline synthesis and the HMP pathway, as proposed for animal proliferat ing tissues (Phang 1985), might explain a high purine biosynthesis and cytokinin autonomy.