Fusarium oxysporum f. sp cubense inoculation and hypoxia alter peroxidase and phenylalanine ammonia lyase activities in nodal roots of banana cultivars (Musa sp.) differing in their susceptibility to Fusarium wilt

The impact of O-2 deficiency on the interaction of banana roots and the Fus arium wilt pathogen, Fusarium oxysporum f. sp. cubense (Foc), was determine d on the basis of changes in the activities of enzymes involved in phenol m etabolism (phenylalanine ammonia lyase, PAL, and peroxidase, PER). The root systems of banana cultivars differing in their known field resistance to F usarium wilt were either inoculated with Foc or not and either continuously aerated, continuously exposed to hypoxia, or exposed to hypoxia for 48 h a nd then reaerated. Hypoxia stimulated PAL and PER activities in nodal roots of bananas and Foc inoculation stimulated PER activity. Hypoxia, in the presence of Foc, affe cted PAL activity among cultivars in a manner consistent with their observe d field resistance to Fusarium wilt. Foc inoculation increased PER activity in all cultivars over and above that caused by hypoxia alone. Foc inoculat ion, followed by hypoxia, affected the cultivars differently, with the corr esponding changes in PER activity seemingly correlated with their resistanc e to Fusarium wilt. The resistant cultivar Goldfinger has an inherently higher capability of st imulating the production of PAL and PER enzymes than cv. Williams (normally resistant to Fusarium wilt but succumbs when waterlogged) or cv. Gros Mich el (susceptible). While Williams could respond to hypoxia by increasing PAL and PER activities more than Gros Michel, the elicited level of activity m ay not be enough to prevent disease development under waterlogged condition s. While hypoxic treatment might be a way to stimulate the root's defence m echanism, it has to be at a level, duration and timing that would enhance P ER and PAL activities without causing irreparable damage to root functions. Postanoxic injury, for example, could impair host resistance to wilt. The differences in rapidity and degree of increase in PAL and PER activitie s under Foc inoculation, followed by hypoxia, appear to be associated with resistance to Fusarium wilt. The breakdown in resistance of Williams (a Cav endish cultivar) observed during waterlogging, needs further investigation, especially if PAL is synthesised in the more aerated parts of the root, aw ay from the stele where the defence mechanism needs to operate.