Inhibition of Dark CO(2) Fixation and Photosynthesis in Leaf Discs of Corn Susceptible to the Host-specific Toxin Produced by Helminthosporium maydis, Race T.

The host-specific toxin produced by Helminthosporium maydis, race T, causes 50% inhibition of dark fixation of (14)CO(2) by leaf discs of susceptible (Texas male sterile) corn when it is diluted to approximately 1/10,000 of the volume of the original fungus culture filtrate. Dilutions of 1/10 or less are required for equivalent inhibition of discs prepared from resistant (N) corn. Root growth and photosynthesis were considerably less sensitive (dilution values 1/3000 and 1/1200, respectively), as was leakage of (14)C induced by toxin from preloaded discs. Based on literature values for dilutions causing ion leakage or inhibition of mitochondrial oxidation, toxin dilutions several orders of magnitude greater bring about inhibition of dark CO(2) fixation. Preincubation of discs in light increased sensitivity of dark fixation to toxin and an effect of light on symptom development was shown. Phosphoenolypruvate carboxylase activity in extracts of roots or leaves was not affected by toxin nor was the enzyme level altered in excised leaves treated with toxin. Inhibition of dark fixation of CO(2) provides a bioassaay for race T toxin which is both reliable and rapid.

Metabolism of Separated Leaf Cells: III. Effects of Calcium and Ammonium on Product Distribution During Photosynthesis with Cotton Cells.

Separated mesophyll cells from cotton (Gossypium hirsutum var. Stoneville 1613 Glandless) were isolated with pectinase and mechanical agitation. The separated cells had rates of light-dependent CO(2) fixation between 50 to 100 mumoles CO(2) per mg chlorophyll per hour. The presence of Ca(2+) in the incubation medium did not significantly affect the type of photosynthetic products formed, but 2 mm Ca(2+) did cause a 50% decrease in the appearance of photosynthetic products in the incubation medium. The movement of all types of products (sugars, organic, and amino acids) out of the cells was reduced similarly by the Ca(2+). Light had no affect on the movement of products out of the cells, whereas 1 mm ethylenediaminetetra-acetate greatly increased the movement. The addition of 1.6 mm NH(4)Cl to the cell suspensions caused a large increase in the amount of fixed (14)C appearing in the amino acid fraction and a decrease in the sugar fraction. These metabolic changes in the cells were reflected in the movement of products out of the cells so that the incubation medium also contained a larger amount of label in amino acids and a smaller amount in sucrose. Although the cell plasma membrane restricted the movement of soluble products, it did not discriminate significantly between the types of products moved.