Histological analysis of methyl jasmonate-induced gummosis in petiole of culinary rhubarb (Rheum rhabarbarum L.)

The application of methyl jasmonate (JA-Me, 0.5%, w/w) as a lanolin paste to the middle of intact petiole (leaf stalk) of growing culinary rhubarb (Rheum rhabarbarum L.) induced gum formation in the entire petiole below and above the treatment. Histological analysis of petioles stained with safranin-fast green observed in light and fluorescence microscopes revealed different stages of gum duct formation. The fust reaction to the JA-Me treatment was observed on the 2nd day after the JA-Me application, it being manifested by the dissolving of middle lamella in parenchymal cells. The accumulation of liquid gum in intercellular spaces of parenchymal tissue followed by the dissolution of cell walls and protoplasts leading to gum duct formation was also observed on the 5th day. Specialized secretory cells were not recognized. Some of parenchyma cells contained dense gum deposits. Gum formation in rhubarb petioles found in parenchymal tissue on the perimeter of the petioles adjacent to bark while their middle part remained intact. The general structure of the bark in rhubarb plants treated with or without JA-Me was almost similar. The more intensive safranin-staining of the cell walls in the vascular bundles was observed on the 5th and the 7th days after the JA-Me treatment. On the 7th day after the JA-Me application, gum accumulation caused rupturing of the petiole periderm followed by gum extrusion. Processes of gum exudation are discussed based on histological analyses.