Response of banana (Musa sp.) roots to oxygen deficiency and its implications for Fusarium wilt

To investigate whether waterlogging predisposes bananas to infection by the Fusarium wilt fungus, we explored the short term response of banana roots to O-2 deficiency, a major component of waterlogging. We measured the O-2 profile across roots with an O-2 microelectrode and discovered that the stele of a root bathed in aerated solution (20.6 kPa O-2) experiences quite low O-2 concentrations (1.3 to 2.6 kPa). Using a root pressure probe we showed that an external dissolved O-2 concentration of 4 kPa reduces net nutrient transfer into the vascular tissue in the stele within 1 or 2 hours, suggesting the development of O-2 deficiency within the stele, even though the epidermis and the cortex may receive adequate Oz for oxidative phosphorylation. Thus nutrient loading of the stele is one of the earliest root functions affected by Oz deficiency. Anoxia in the medium stopped root elongation within 30 min. Reaeration after 4 h of anoxia restored elongation rate to only 50% of continuously aerated roots. Anoxia for more than 6 h killed the root tip. Banana roots develop aerenchyma (large air spaces), especially when grown in potting mix and even in the absence of waterlogging. The mature roots of Cavendish cultivars (AAA) have 10 % porosity in aerated nutrient solution and up to 28 % aerenchyma when grown in potting mix. Roots of cv. Goldfinger (AAAB) have about 6 % porosity in aerated nutrient solution and up to 15 %; aerenchyma in potting mix. Even with aerenchyma, the growth and functioning of the roots is very sensitive to O-2 deficiency. We are developing inoculation techniques that avoid severing the root, to determine the significance of Oz deficiency in the infection and colonisation patterns of Fusarium oxysporum f. sp. cubense in roots of banana cultivars.