| Abstract: | The biological effects of phytotoxic compounds will be dependentupon uptake, the amounts reaching the site of toxic action,and the toxicity at cell level. When the materials are appliedas sprays to growing plants absorption through the roots, retentionby and penetration into the shoot, transport and localized accumulationare factors which may determine differences in response eitherbetween compounds or between species. The precise assessmentof relative toxicity must therefore involve studies of the effectson whole plants and at cell level. For such assessments it is essential to determine in the firstplace the change in the biological effects brought about bya wide range of dosage. The percentage inhibition of germinationbears a sigmoid relationship to the amount of toxicant in theexternal medium, while the same relationship holds between percentagemortality and the concentration of the spray solution. Accuratecomparisons of relative toxicity can only be obtained when thevariation in reaponse is measured at several dosages and thedata treated by the methods of probit analysis. Using such techniques in germination or spraying experimentsit has been demonstrated that the relative toxicities of chlorinatedphenoxycetic acids, alkyl phenylcarbamates, dinitro-alkylphenols,pentachlorophenol, thioacetic acid, and formamide are greatlydependent on the species. In fact, the order of toxicity maybe reversed from one species to another, while between compoundsthe results of germination tests may or may not be comparablewith those obtained by spray applications. The physical characteristics of the spray solution will in partdetermine the degrees of retention and penetration. For compoundswith a low solubility in water the addition of a hygroscopicsubstance may increase the percentage kill. According to thespecies, spray solutions of a low surface activity may be moreor less toxic than those with a high activity, while the relativeeffects of oil emulsions and aqueous sprays vary between species. For compounds which are freely translocated, the methods ofgrowth analysis are of value for assessing the toxic effects,especially of non-lethal dosages. Since the effects on the growthof the component parts of the plant may be widely divergent,conclusions based on a single criterion are likely to be erroneous.Where movement of the compound is restricted, such as with hydrocarbons,an assessment of toxicity can be obtained by measuring the degreeof localized damage following on the application to the leavesof individual droplets of varying size. Lemna minor has the twin advantages that the experimental conditionscan be controlled and that in some respects its response tophytotoxic compounds is akin to that of unicellular organisms.Since with some compounds at any one dose the depression inthe growth rate remains constant with time (e.g. nitrophenols),while for others the depressant effect is cumulative (e.g. dichloro-phenoxyaceticacid, cupric salts), the nature of the growth response mustfirst be established before comparisons between compounds canbe made. For studies of relative toxicity at cell level two methods haveken employed. The external concentrations in the agar mediumrequired to halve the growth rate of Trichoderma viride havebeen determined, or the dosages required to bring about a standardeffect on the respiration of yeast have been estimated. It is concluded that only by using a range of species and anumber of techniques can relative toxicity be established withprecision. |
