Mitochondria isolated from etiolated shoots of blight-susceptible and blight-resistant corn plants were subjected, in various respiratory states, to the pathotoxin released by Helminthosporium maydis (race T). The addition of the pathotoxin to susceptible mitochondria caused respiratory rate and oxidative phosphorylation changes. The addition of pathotoxin to susceptible mitochondria suspended in a potassium chloride reaction medium induced an immediate and irreversible swelling regardless of the respiratory state of the mitochondria. This membrane swelling correlates with the observed respiratory and coupling effects of the pathotoxin. In all instances, mitochondria isolated from blightresistant corn failed to exhibit any of the above responses to the pathotoxin.

Light and electron microscopic studies of corn plants (Zea mays I.) exposed to Pb in hydroponic solution showed that the roots generally accumulated a surface Pb precipitate and slowly accumulated Pb crystals in the cell walls. The root surface precipitate formed without the apparent influence of any cell organelles. In contrast, Pb taken up by roots was concentrated in dictyosome vesicles. Dictyosome vesicles containing cell wall material fused with one another to encase the Pb deposit. This encased deposit which was surrounded by a membrane migrated toward the outside of the cell where the membrane surrounding the deposit fused with the plasmalemma. The material surrounding the deposit then fused with the cell wall. The result of this process was a concentration of Pb deposits in the cell wall outside the plasmalemma. Similar deposits were observed in stems and leaves suggesting that Pb was transported and deposited in a similar manner.Many researchers have shown that plants will accumulate Pb either from soil, stem, or foliar application (1,3,8,9,13,14,16,25). However, the reports conflict as to the amounts accumulated and the amounts that can be translocated. Little has been done to localize the Pb accumulated within an organ, although Hammett (5-7) indicated that much of the Pb was associated with cell walls and nuclei, and Suchodoller (24) found that it was concentrated in the root cortex.It was the purpose of this research to study the localization of accumulated Pb and characterize the method of accumulation as specifically as possible. MATEIUALS AND METHODSCorn (Zea mays L. Wf9 X M14) was grown by a standard hydroponic procedure (19) in a greenhouse or was grown on paper toweling in a moist chamber.

Weed infestations severely reduce the yield grown corn with such yield reductions generally being attributed to competition. Noncompetitive (allelopathic) mechanisms have to date, however, received little consideration in agronomic situations. In the greenhouse, interference (competition + allelopathy) of the growth of corn (Zea mays L. Wf9 ✕ M14) by giant foxtail (Setaria faberii Herrm.) was determined in mixed culture treatments. When corn was seeded into pots with 6‐week‐old giant foxtail, corn height, fresh weight, and dry weight were reduced by as much as 90% when compared to comparable plants grown in monoculture. Competitive and allelopathic mechanisms were separated through the use of a stairstep apparatus, in which a nutrient solution passed through the rhizosphere of giant foxtail, into the rhizosphere of corn and was subsequently recycled through the system. The stairstep apparatus was used to determine the allelopathic interactions between corn and giant foxtail seedlings, mature giant foxtail, whole dead giant foxtail plants, and mascerated dead giant foxtail leaf and root material. Mature giant foxtail inhibited the growth of corn approximately 35% through an allelopathic mechanism. Elimination of competition through the use of stairsteps apparatus implicates a possible allelopathic mechanism in the interference of corn by giant foxtail that involves the exudation and leaching of phytotoxins from the roots of giant foxtail. Phytotoxins leached from dead giant foxtail reduced corn growth by as much as 50%. The relationships of allelopathy to competition, crop rotation, herbicidal activity, and physiological processes are discussed.

The uptake of cadmium by corn roots (Zea malls L.) and shoots from hydroponic solutions was determined over time intervals from 3-12 days. This uptake from solutions containing 1-40 mg cadmium/liter of solution was correlated with leaf chlorophyll content, dry matter production, and tissue concentration of zinc and iron. Cadmium concentration in roots and shoots increased with increased time of treatment. Roots of plants treated for 12 days contained about twice the cadmium found in shoots (approximately 200-1,100/ag cadmium/g dry weight in roots and approximately 100-450 pg cadmium/g dry weight in leaves). The dry weight of both roots and shoots, and of leaf chlorophyll concentration, decreased with increased cadmium tissue concentrations. The zinc concentration decreased, but the iron concentration increased, in leaves and roots as the cadmium concentration increased. These data resulted in a linear relationship between the increase of tissue cadmium and the increasing iron/zinc ratio. While the chlorosis of cadmium-treated leaves appeared to be comparable to that resulting from iron deficiency, the data presented indicate that an iron deficiency is not the cause of leaf chlorosis in cadmium-treated plants. A more important factor in cadmium toxicity may be the apparent correlation between an increase in tissue cadmium concentrations and the corresponding increase in the iron/zinc ratio. Additional Index Words: heavy metal pollution, chlorosis, ion uptake.

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