Nucleolar dry mass and area changes in onion epidermal cells following wounding and inoculation.

Abstract

Nucleolar dry mass and area means of inner epidermal cells in the equatorial region of white onion bulbs were compared with those 15 to 20 mm from the wound 48 hr later. Both nucleolar traits increased during the 48 hr as follows: dry mass of two nucleoli per nucleus, 83%; and, the area of two nucleoli, 60%. After the bulb was inoculated with a weakly pathogenic isolate of Aspergillus niger, means for nucleoli in the adjacent 4 cm increased 62% for nucleolar dry mass and 40% for nucleolar area. Because nucleoli in host cells respond positively to both wounding and inoculation, the effect of aggressive fungal pathogens on these cells and their organelles should be compared with that from cells collected from the inoculation site at the time of wounding. Nucleolar shape changes following wounding or inoculation suggests activation of the nucleolar organizer region to form bumb-bell type nucleoli which can cause interpretational problems when numbers and sizes of nucleoli per nucleus are being studied.