Our foods contain trace amounts of a wide range of heavy metals: some of these have a biochemical function, others are contaminants. Trace metals can contaminate foods through agricultural technology, industrial pollution, geological sources and food processing. Some results from recent Canadian monitoring surveys are given with particular emphasis on data from lead and cadmium analyses. Consideration is given to the criteria that can be used to determine the priorities in the design of a monitoring program for trace metals in foods.

The in oitro fungistatic activity of some twenty-four metal cations has been determined against Alternaria tenuis and Botrytis fubae. The metal salts, mainly nitrates, were tested in aqueous solution without added spore germination stimulant. The logarithm of the metal ion concentration at the E D 50 value has been found to conform to the exponential relationship with electronegativity proposed by Danielli & Davies (1951). These results are discussed in relation to the site of action of metal cations on the fungal cell. I N T R O D U C T I O NThe concept of a general relation between metal ion toxicity and some physical or chemical property of the metal has held interest since the beginning of this century. Mathews (1904) considered the toxicity of metal ions to eggs of the fish Fundulus heteroclitus to be related to the ease with which the ions gained or lost electric charges and hence to the electrode potential of the metal. Later work by Jones (1939, 1940) has shown that for a given degree of toxic action on the stickleback, Gasterosteus aculeatus, and the planarian, Polycelis nigra, the logarithm of the toxic concentration of some eighteen metal ions is a linear function of the standard electrode potential.

SUMMARYThe electrokinetic behaviour of intact conidia and cell walls of Neurospora crassa was studied using a micro-electrophoresis technique. By chemical and enzyme treatments it has been established that amino, carboxyl and phosphate groups are integral components of the spore surface; acid phosphate groups, however, were not found on the surface of washed cell walls. The fungicide dodine acetate reduced the negative charge on conidia to zero and, with increasing concentration, gave a positive charge to the spores: at lower fungicide concentrations the negative charge on the surface of cell walls and stabilized protoplasts was also neutralized. These results are consistent with an ionic reaction between the dodine cation and the carboxyl and phosphate groups of the cell. There was no evidence that the toxic reaction between dodine acetate and N. crassa conidia is located on the spore surface-the spores were completely killed before there was a perceptible reduction in electrophoretic mobility.

SUMMARYUptake of the dodine cation from acetate solutions by conidia of Alternaria tenuis and Neurospora crassa was characterized by a rapid rate of sorption, a ‘Langmuir type’ adsorption isotherm, and independence of temperature: all of which suggests an ionic bonding mechanism. Metal cations competed with dodine for the anionic binding sites of the cell—regarded as carboxyl and phosphate groups—and dodine uptake also decreased as ionization of the carboxyl group was suppressed.Cell walls of A. tenuis had a greater capacity to bind dodine than did those of N. crassa. Binding at the cell wall may detoxify some of the large amount of dodine that must be accumulated by the spores to achieve toxicity. The dodine retained by N. crassa cell walls could not be exchanged or desorbed by washing and is probably bound covalently rather than by weaker ionic bonds. At sub‐lethal concentrations there was no evidence that dodine disorganized cell wall structure.Disruption of spores which had been incubated with 14C‐labelled dodine showed the fungicide to be associated with intra‐cytoplasmic organelles. It is suggested that dodine reacts with the protoplast membrane so as to alter its permeability and allow more dodine to penetrate into the cytoplasm where it may destroy intracellular membrane structure.

Using B4Cu a quantitative study has been made of the factors influencing the uptake of copper, from buffered solutions of pH 5.6, by conidia of Neurospora crassa. The