Some aspects of carbon relations in the barley – Helminthosporium teres complex. I. The effects of infection upon carboxylation in vivo and in vitro

Abstract

First leaves of seedlings of the barley varieties Parkland (susceptible) and C.I. 5791 (resistant) were inoculated with the net blotch organism Helminthosporium teres. In Parkland, over the 7-day infection cycle (inoculation to sporulation), (1) ribulosediphosphate (RuDP) carboxylase activity, chlorophyll levels, and carbon fixation in the light all fell to ca. 50% of control levels and (2) the activities of phosphoenolpyruvate (PEP) carboxylase and the malic enzyme, and carbon fixation in the dark (which was 0.5% that in the light in healthy tissue), rose 200–400%, peaking just before sporulation of the pathogen. Quantitative analytical studies of the early products of 14CO2 fixation in the light indicated that carbon entered metabolism predominantly via the Calvin cycle in healthy and infected plants. However, β-carboxylation, which in all samples was higher in the light than in the dark, showed a significant increase in susceptible tissues upon H. teres infection. In C.I. 5791 (hypersensitive reaction), the metabolic changes were similar to those in Parkland, with the magnitude of the metabolic response being related to the extent of fungal development; the effect persisted until the growth of the pathogen was inhibited. The changes induced in carbon fixation in situ in response to infection could be reproduced to a limited extent by physical damage, but the response to pathogenesis, especially in susceptible tissues, was probably more specific. The possible significance of this response is discussed.