Appropriate control tissues for host-pathogen studies in onion.

Abstract

Inner epidermal tissue of onion bulb-scale was studied by means of quantitative interference microscopy at the time of inoculation (with Aspergillus niger) and again after 48 hours of incubation. Nuclear area (NA), dry mass per unit area (DM/A) and nuclear dry mass (NDM) of epidermis cells 15-20mm ahead of mycelium were not statistically different from cells initially removed from the inoculation site. NDM of cells immediately ahead of advancing mycelium (0-5mm from the inoculation site) averaged 42% lower than that of cells obtained at the time of inoculation; NA was 46% lower; whereas, DM/A was 8% higher. We conclude that either sample of inner epidermal tissue, one removed from the inoculation site at the start of an experiment or one from 15-20mm away from the inoculation site 48 hours after the start of the experiment, is an appropriate control for determining the degree of host response to a pathogen within 0-5mm of the mycelium. The alteration and killing of host cells ahead of advancing mycelium suggested secretion of toxic metabolites by the fungus.