The host-specific toxin produced by Helminthosporium maydis, race T, causes 50% inhibition of dark fixation of "4CO2 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 "C induced by toxin from preloaded discs. ions induced by most of these toxins has centered attention on host membranes as a site of action (5,19,21,22). A major problem in the identification of a primary site for race T toxin is the considerable variation in toxin dosage and time of exposure that has been reported for each effect. In this laboratory, for example, 50% inhibition of susceptible corn root growth can be achieved with 1/3000 dilution of race T toxin, whereas dilutions of 1/ 10 to 1 / 100 apparently are required for rapid induction of ion leakage (6, 7) or inhibition of mitochondrial oxidation (9, 13, 16).At present, it is impossible to decide whether the variations arise from differences in toxin potency, the nature of the assay systems employed, or fundamental differences in the innate sensitivity of the processes under examination. It seems reasonable that the primary site would show the greatest sensitivity; that is, would require the least amount of toxin for a response.As a first approach to the problem, a quantitative bioassay for standardizing activity of toxin from various sources becomes essential. Inhibition of root growth has been used for other host specific toxins. Although usable for race T toxin, we have found this method time-consuming, tedious, and unreliable because of variations in seed germination and seedling vigor. During studies of the effect of T toxin on several physiological and biochemical processes of corn leaves, we observed significant reduction in dark fixation of CO2. In addition to its intrinsic interest in connection with a mode of action, we also have found the phenomenon to be useful as a reliable, sensitive, and rapid bioassay. MATERIALS AND METHODS

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 C02 fixation between 50 to 100 umoles COa per mg chlorophyll per hour. The presence of Ca'+ in the incubation medium did not significantly affect the type of photosynthetic products formed, but 2 mM Cae+ 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'+. Light had no affect on the movement of products out of the cells, whereas 1 mM ethylenediaminetetraacetate greatly increased the movement. The addition of 1.6 mM NH4Cl to the cell suspensions caused a large increase in the amount of fixed 14C 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 discrminate significantly between the types of products moved.Techniques have recently been developed in this laboratory (3, 5) and others (13) for the isolation of separated mesophyll cells by use of a crude pectinase preparation. These separated cells are capable of photosynthetic CO2 fixation for 20 or more hours with formation of products similar to those found in leaf tissue (5). The cells are capable of taking up amino acids and uracil and uridine from the incubation medium and incorporating them into proteins and RNA, respectively (3). The use of separated cells for the study of leaf metabolism offers at least two advantages. First, the cells can be handled similarly to unicellular algae, thus facilitating the removal of uniform aliquots. Second, and more importantly, all cells in

The integration of mathematics and science teaching and learning is gaining interest in many elementary and secondary classrooms. National organizations such as SSMA, NCTM, and AAAS are giving strong support for integrating of mathematics and science in the curriculum. If mathematics and science are to be successfully integrated, it would seem reasonable that mathematics and science courses for teachers also be integrated. This article describes (a) reasons for integrating methods courses, (b) the evolution of an integrated methods course in a small university, (c) topics covered in the course together with descriptions of student assignments and activities, (d) procedures used in student evaluation, and (e) student opinions of the integrated course.

Extracts, mainly from mesophyll cells, were obtained by grinding cells in a Waring Blendor; then extracts of parenchyma sheath cells were obtained by exhaustive grinding of the blender residue in a roller mill or mortar with sand. The specific activities of P-glycolate phosphatase, glycolate oxidase, catalase and reduced nicotinamide adenine dinucleotide- (NADH-) hydroxypyruvate reductase were fourfold higher in extracts of the parenchyma sheath cells than in the mesophyll cells from corn, sugarcane, and Atriplex rosea. P-Glycerate phosphatase was mainly located in the mesophyll cells. The total activity of glycolate oxidase in plants without CO2-photorespiration averaged about one-third that found in other plants on a wet-weight basis. Glycolate oxidase activity in Atriplex rosea, without CO2-photorespiration, was about the same as in Atriplex patula, with CO2-photorespiration. It is concluded that enzymes for glycolate metabolism are present in all leaves in substantial amounts and are located in both cell types, although a higher specific activity is in the parenchyma sheath cells. Thus it is proposed that photorespiration occurs in all plants, but that CO2 evolution from glycolate metabolism is not manifested in plants which have high levels of activity for the C4-dicarboxylic acid cycle of CO2 fixation.

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