Studies in the Principles of Phytotoxicity

Abstract

It has previously been shown that the concentration of 3:5-dinitro-ocresol required in the external medium to halve (a) the growth rate of the mould Trichoderma viride or (b) the respiration rate of yeast increased same 75–100 times as the pH changed from 4 to 7. In the present investigation it has been found that when solutions or suspensions of 3:5-dinitro-o-cresol are applied as sprays to seedlings of Brassica alba the concentration needed to kill half the plants changes by a factor of less than 2 over the same pH range. On the other hand, when Lemma minor is grown in nutrient solution containing the toxicant, the concentration required to halve the rate of frond multiplication increases sharply as the pH is raised from 5.4 to 6.8. Moreover, when leaf disks of B. alba are vacuum infiltrated with buffered solutions of 3:5-dinitro-o-cresol the concentration needed to halve the respiration rate rises 50-fold between pH 4 and 8. It is considered that the essential condition which determines the similarity of the result for L. minor, the leaf disks, and micro-organisms is that a relatively large volume of solution containing the toxicant is in direct contact with the tissues. In contrast, under the conditions of spraying the volume of spray droplets relative to that of plant tissue is small. Thus the pH effect ie masked since the buffering capacity of the cells exceeds that of the droplets, with the result that 3:5-dinitro-o-cresol largely acts upon the cells at a pH determined by the tissues, a pH which may bear little relation to that of the original solution. Supporting evidence is produced that the pH factor operates in the same way for other weak acids, such as the chlorophenoxyacetic acids.