Experiments with young plants of seven cultivars and species in the Gramineae (Lolium perenne L. cv. S 23, Lolitim multiflorum Lam. cv. S 22, Lolntm muUifiorum Lam. cv. Tetrone, Triticum aestivum L. cv. Janus, Avena sativa L. cv. Peniarth, Hordeum vulgare L. cv. Zephyr, Avena fatua L.), showed that those grown at a high nitrogen level were more susceptible lo paraquat than those grown at a low level. Similarly, high levels of phosphorus increased the susceptibility of plant5 of A. saliva to this herbicide. However, potassium, at the levels used, had no efFect on paraquat phytotoxicity. Two further experiments, wiih Agropyron repens (L.) Beauv., indicated that nitrogen did not influence the phytotoxicity of paraquat; neither did it encourage the recovery of foliage, nor the production of rhizomes following spraying, despite a direct efTeci of nitrogen on the growth and development of the unsprayed plants. A mechanical defoliation treatment was les.s effective than paraquat in controlling the growth of this weed. Swards of!,, perenne grown under a low nitrogen regime were slightly more susceptible lo paraquat and showed less recovery than those grown under the higher nitrogen conditions. Tlie results are discussed In the context of the physiology of herbicides and of agricultural practices.
Summary. Necrosis at the stem‐end of potato tubers following the application of diquat to the foliage is rare and has occurred mainly in seasons of low rainfall. Damage in the field has been associated with tuber residues of diquat of about ten times the normal level and the extent of damage has shown a linear relationship with the concentration of chemical in the tubers. Normally an increase in the dose of diquat applied causes only a slight increase in amount of chemical in the tubers, but under conditions where damage occurred, tuber residues increased markedly with increase in application rate.
The factors influencing diquat residues in potato tubers were examined in pot experiments. Highest residues were found when plants were subjected to drought and to a period of darkness following spraying at a time when senescence of the foliage had commenced and the tubers were still increasing in fresh weight. Further experiments at a similar stage of growth showed that high residues of diquat caused stem‐end rot only when plants were subjected both to a soil moisture stress sufficient to induce wilting before spraying and to reduced light intensity and a high atmospheric humidity after spraying. It is suggested that these conditions promote the rapid downward movement of a high concentration of diquat in the xylem.
In a preliminary experiment with a range of chemical desiccants, only sodium arsenite gave appreciable tuber damage when applied to the foliage.
R^sum^: Zusammenfassung Es wird der EinfluU von Slickstolf auf die Struktur und Pliysiologie von Biaitern besprochen; die Ergebnisse werden mit friJheren Ergebnissen verglichen, bei denen sich gezeigt hiilte, daO mit Paraquat gespritzte ganze Pflanzen wcnigcr empfindlich waren, wenn sie bei niedrigem Stickstoffniveau gewachsen waren.
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