Carbohydrate Metabolism in Higher Plant Tissues Infected With Obligate Parasites.

Several reports (7, 8, 22) have described apparently fundamental changes in the rates and nature of respiratory pathways during infection of higher planit tissues by obligate parasites, particularly rust fungi. Although disease can cause increases of 400 % in rates with a marked lowering of C6/C1 ratios, there is considerable uncertainty about the carbohydrate changes which accompany the respiratory alteration. Allen (1) first reported increases in sucrose and hexoses in wheat infected by powdery mildew. Gerwitz and Durbin (10) have described increases in both reducing and non-reducing carbohydrates in wheat infected by rust fungi while otbers (5,15, 16) have reported marked decreases in both types of compounds. Shaw and Samborski (22) found increases at 3 and 7 days after infection of wheat with rust fungi, but at 5 and 10 days the diseased tissue contained less sucrose and hexose than healthy controls. Inman (13) has pointed out that the amount of infection and the stage of parasite development are two factors which are important in such studies. With heavy infection of the bean rust fungus, the levels of reducing and non-reducing carbohydrates increased by 50 and 300 %, respectively, just prior to sporulation. Subsequently, and within 72 hours, the levels fell to 50 % of control tissue. With progressively fewer infections per unit of leaf area the changes were neither as great nor as abrupt in time. In cases of light infection the concentrations of soluble carbohydrate did not fall below the levels in control plants. It is of interest that, despite the consistently low C6/C1 ratios in conjunction with high rates of respiration, only the usual hexoses and sucrose have been positively identified in detectable amounts in diseased tissue.