Abstract

The impact of O₂ deficiency on the interaction of banana roots and the Fusarium wilt pathogen, Fusarium oxysporum f. sp. cubense (Foc), was determined on the basis of changes in the activities of enzymes involved in phenol metabolism (phenylalanine ammonia lyase, PAL, and peroxidase, PER). The root systems of banana cultivars differing in their known field resistance to Fusarium wilt were either inoculated with Foc or not and either continuously aerated, continuously exposed to hypoxia, or exposed to hypoxia for 48 h and 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, affected PAL activity among cultivars in a manner consistent with their observed field resistance to Fusarium wilt. Foc inoculation increased PER activity in all cultivars over and above that caused by hypoxia alone. Foc inoculation, followed by hypoxia, affected the cultivars differently, with the corresponding changes in PER activity seemingly correlated with their resistance to Fusarium wilt.

The resistant cultivar Goldfinger has an inherently higher capability of stimulating the production of PAL and PER enzymes than cv. Williams (normally resistant to Fusarium wilt but succumbs when waterlogged) or cv. Gros Michel (susceptible). While Williams could respond to hypoxia by increasing PAL and PER activities more than Gros Michel, the elicited level of activity may not be enough to prevent disease development under waterlogged conditions. While hypoxic treatment might be a way to stimulate the root’s defence mechanism, it has to be at a level, duration and timing that would enhance PER 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 activities under Foc inoculation, followed by hypoxia, appear to be associated with resistance to Fusarium wilt. The breakdown in resistance of Williams (a Cavendish cultivar) observed during waterlogging, needs further investigation, especially if PAL is synthesised in the more aerated parts of the root, away from the stele where the defence mechanism needs to operate.