Abstract
Bacterial population and histological changes in leaf tissues of Citrus natsudaidai after inoculation of Xanthomonas citri by rubber-block press or infiltration method were examined.
When inoculated by rubber-block press method, bacteria invaded intercellular spaces near substomatal chamber, but did not increase significantly until disease developed at cell levels. Staining with phenolic thionin and orange-G combination, elucidated that enlargement of nucleolus and disappearance of chloroplasts occurred in two or three layers of spongy parenchyma cells near stomata 4-5 days, 3-4 days, and 2-3 days after incubation at 20C, 23C, and 25C, respectively. Bacterial number increased for 1 or 2 days, then remained constant or decreased, and finally increased remarkably when diseased cells increased in number. The interval between these two phases of bacterial increase reflected the process of cellular damage. There was no noticeable difference in time interval between two phases of bacterial increase in leaf tissues among several host plants, including hosts of various resistance.
When injected into spongy parenchyma, bacteria started to multiply soon after inoculation, especially when nutrients were added. Diseased cells, which were similar to those observed after rubber-block press inoculation, were found in one or two cell layers in spongy parenchyma 4 days after incubation at 20C, and extended to 4 to 5 layers 4 or 5 days later.
Water-soaked spots appeared under the surface of the leaf when diseased cells increased to about 10 layers and hypertrophied. Cell division in affected tissues was not observed. There were numerous bacteria in the intercellular spaces and peripheral spaces of the diseased tissues.
These results suggested that causal bacteria affect the adjacent parenchyma cells and disease them after a temperature-dependent incubation period, and that diseased cells support bacterial multiplication. These bacteria ooze out and affect surrounding host cells. Repetition of these process is considered to lead to symptom formation.
