Abstract
In the present paper, the writer deals with the experimental evidence on the cause of so-called “vascular browning”, which is induced in the watermelon tissues infected by Fusarium oxysporum f. niveum.
Microchemical observations on the injured tissues showed that, in the early stage of the disease development, the discoloration was limited to xylem parenchyma, wherein remarkable increase of phenolic substance and abnormal activation of phenoloxidase were noticeable, and was severely on progress in advance of the invading hyphae into cells, that the browning of cells seemed not to play an important role for a chemical barrier to invading hyphae, since the growth of mycelium in the browned cells was also recognized in the later stages of the disease development.
Salicylic acid and another phenolic substance were isolated from watermelon roots as a vascular browning precursor. And the qualitative and quantitative changes of the precursors in the tissues during the infection process were assayed by using spectrophotometer and paper electrophoresis (Figs. 4 and 5).
Fungal pectic enzyme, being functional in producing cell maceration, and fusaric acid that is toxic to the cell, might be indirectly responsible for the browning in vivo.
Effects of salicylic acid and the related compounds on the growth of the causal fungus were investigated. From the results, it was shown that the antibiotic action of salicylic acid was dependent on the COOH radical rather than the OH.
