Time-Course and Antioxidant Inhibition of Ethylene Production by Victorin-Treated Oat leaves

Abstract

WHEELER, H., and E. ELBEL. 1979. Time-course and antioxidant inhibition of ethylene production by victorin-treated oat leaves. Phytopathology 69: 32-34. Although an increase in ethylene production is one of the most sensitive respiration, decreases in C6/C1 ratios, and other changes clearly linked to responses to victorin, no increase in this gas was detected during the first 30 metabolism. Antioxidants, of which a-tocopherol was the most effective, min after susceptible oat leaves were treated with this pathotoxin. These inhibited ethylene production by victorin-treated tissues and also results do not support the concept that ethylene triggers initial changes in suppressed pathologic effects of victorin on losses of electrolytes and permeability and electrochemical potentials that occur within 5 min after chlorophyll. Failure of antioxidants to suppress ethylene production in exposure to victorin. The results also emphasize the significance of a similar healthy control leaves lends support to evidence that the pathways leading 30-min lag in less sensitive responses to victorin such as increases in to ethylene evolution in diseased tissues differ from those in healthy plants. The ability of victorin, a pathotoxic product of Helmintho- diluted to contain 100 units/ml, was detoxified (19), and this sporium victoriae Meehan and Murphy, to induce in susceptible solution and distilled water served as controls. oat tissues all of the visible and physiologic symptoms of disease Detached leaves, which had taken up victorin or other test found in plants infected by this pathogen is well-established (4,7,13, solutions through basal cut ends, were placed in containers sealed 15). Initial time-course studies of responses to victorin indicated with septa, and ethylene production at various intervals was that changes in permeability, estimated by rates of loss of electro- determined by gas chromatography as in a previous study (14). In lytes, occur almost instantaneously, but increases in respiration and one series of tests, this procedure was varied by using Beyer and other changes directly linked to metabolism occur only after a lag Morgan's vacuum evacuation method (1) in an attempt to measure of about 30 min (18). These results led to the hypothesis that the both evolved ethylene and that contained in the tissue. initial effect of victorin is on the plasmalemma (4,18,20), which The three antioxidants used were sodium ascorbate, propyl perhaps contains specific receptor sites only in susceptible tissues gallate, and a-tocopherol, the latter prepared as a mixture in Tween (7). Despite extensive research, all attempts to provide direct 80 (10); they were serially diluted in 0.005 M KH 2PO4 to maintain a evidence of initial effects of victorin on the plasmalemma or for pH of about 4.5, similar to that of solutions containing victorin. specific receptors for this toxin have been negative (4,13,16). Thus Solutions then were assayed for toxicity by root growth tests and the site of action of victorin and the significance of the delay in for effects on ethylene production by oat leaves. Thresholds for metabolic responses to this toxin remain unknown, toxicity and increases in ethylene production for the three One factor that complicates interpretation of present data on the compounds fell in the range of 10-4 to 5)X 10-5 M. Therefore, these sequence of changes in victorin-treated tissues is the insensitivity of compounds were used at 10-5 M in combination with victorin. the metabolic responses that have been studied. Concentrations of Methods previously described were used to measure losses of victorin 10-100 times higher than those that cause marked effects electrolytes (18) and chlorophyll content (17). on permeability and root growth are required to produce detectable effects on respiration (9,18), C6 / C1 ratios (9), or enzyme activity (6). RESULTS However, an increase in ethylene production was reported to be at