Studies in the Principles of Phytotoxicity: V. INTERRELATIONSHIPS BETWEEN SPECIFIC DIFFERENCES IN SPRAY RETENTION AND SELECTIVE TOXICITY

In a detailed study of the factors which control spray retentionby the shoot, five species with markedly different morphologicalcharacteristics have been examined (Helianthus annuus, Brassicaatha, Linum usitatisrnnwn, Hordeum vulgare, and Pisum sativum).The amount of spray retained, expressed as volume per unit weightof shoot or unit leaf area, is dependent on the stage of developmentof each species, the volume of spray applied, and the surfacetension of the spray solution. The interrelationships betweenthese factors are complex for all the second-order interactionsproved to be highly significant. Under one set of condi tionsthere may be a tenfold difference in retention between two specieswhile under another set the corresponding difference may beas little as twofold. Between a spray output of 14 and 143 ml./m.³ the level of retentionby P. sativum and H. vulgare is linearly related to output butfor H. annuus and B. alba the relationship is curvilinear. Whenthe surface tension is reduced below 40 dynes/cm. retentionby P. sativum and H. vulgare is increased, but for B. alba andH. annuus retention is decreased if the output exceeds 50 ml./m.For a given output, a reduction in the mean droplet size augmentsretention by H. vulgare but not by H. annuus. Differences in retention either between species or between stagesof develop ment are linked with the ratio of leaf area to shootweight, the nature of the leaf surfaces, the angle of incidenceat which the droplets strike the leaf, and localized accumulation.Retention is maximal when the leaf-area ratio is high, the surfacescompletely wetted, and the laminae set in a horizontal plane:once this maximum value is reached any additional spray receivedruns off the shoot. The relationship between retention and the concentration ofthe spray solution required to bring about some standard biologicalresponse has been examined for two groups of compounds—thosewhich are freely translocated (triethanolarnine salts of 2:4-dichloro-,2:4:5-trichloro-, and z-methyl-4-chloro-phenoxyacetic acids)and those which are not (arnrnonium 2:4-dinitro-6-sec-butylphenate).For the first group the equi-effective concentration is inverselyand linearly related to retention since the total amount receivedis of primary importance in determining the inhibition of growth,though the position of the droplets must be taken into account.Placing micro-drops containing the growth regulator on the cotyledonsof L. usitatimmwn inhibits the growth of the shoot less thana spray application of the same amount over the whole shoot:for H. annuus placing the growth regulator on either the firstor second pair of leaves or on the first internode induces moreinhibition of growth than application to the cotyledons. With ammonium dinitro-sec-butylphenate the relationship betweenretention and concentration is also inverse but approximatelylogarithmic for L. usitatisrimum. For some other species thechanges in retention consequent upon a reduction in the surfacetension are not matched by comparable changes in the equi-effectiveconcentration. For B. alba and H. annuus though lowering thesurface tension may decrease retention it may also Himiniahthe concentration because the fall in retention is more thanoffset by a greater penetration of the compound into the shoot. It is concluded that differences in spray retention betweenspecies can play a major role in determining selective actionand the conditions under which this factor may operate mosteffectively are discussed.