Variation for polyphenol oxidase activity in stems of Medicago species

Polyphenol oxidase (PPO) activity was detected in stems of both glandular-h aired and,glabrous wild Medicago species. Two PPO isoforms appeared in each Medicago species, the heavier isoform ranged from 120 to 180 kDa and the l ighter one from 65 to 75 kDa. The substrate affinity for 4-methyl catechol was quite low (K-m around 0.5 mM) in all the species examined. The PPO spec ific activity present in the stem epidermis of insect-resistant Medicago sp p. was remarkably higher than that measured in the same tissue of susceptib le alfalfa, thus suggesting a possible role of PPO in resistance against bi otic stresses. ((C) Inra/Elsevier, Paris.).