PEROXIDASE, POLYPHENOLOXIDASE, AND SHIKIMATE DEHYDROGENASE ISOZYMES IN RELATION TO TISSUE-TYPE, MATURITY AND PATHOGEN INDUCTION OF WATERMELON SEEDLINGS

Isozymes thought to be involved in disease resistance and maturation w ere investigated in watermelon tissues [Citrullus lanatus (Thunb.) Mat sum and Nakai] by isoelectric focusing in polyacrylamide gels (IEF). P eroxidase (PER) (EC 1.11.1.7), polyphenoloxidase (PPO) (EC 1.10.3.2), and shikimate dehydrogenase (SKDH) (EC 1.1.1.25), displayed tissue-spe cific isozymes. Root and cotyledon tissue expressed the most PER isozy mes with seven each. All tissue examined contained three PER isozymes between pI 6.5 and 5.5. Spectrophotometric PER analysis showed that to tal PER activity increased as tissues matured. Root and stem tissue co ntained the highest PER specific activity, followed by cotyledon, firs t true leaf and second true leaf Additional PER isozymes (pI = 3.5, 4. 5 and 5.2) were detected as the stem, cotyledon, and leaf tissue aged. When seedlings were inoculated (induced) with an avirulent race of Fu sarium oxysporum Schlechtend:.Fr. f. sp. niveum (E.F.Sm.) W. C. Snyder and H. N. Hans, pI 3.5 and 4.5 PER isozymes were enhanced. After indu ction, PPO displayed various isozyme patterns and SKDH did not express isozyme polymorphisms. Similar PER isozymes (pI = 3.5 and 4.5) were d etected in maturing cotyledon and stem tissue and leaf tissue of root- induced plants, suggesting that PER isozymes enhanced by an avirulent pathogen are also enhanced by maturation.