Galston and Baker have established that riboflavin is a photoreceptor in the destruction of auxin by visible light by in vivo and in vitro experiments. Carrol showed that riboflavin also reduced the activity of 2,4–dichlorophenoxyacetic acid (2,4–D). He believed that the reduction in activity occurred only in the presence of living plant material. The present studies using corn seedlings confirm the reduction in 2,4–D toxicity by riboflavin when the seedlings and substrate were illuminated. In addition, the inactivation of 2,4–D by riboflavin by light in vitro was also shown.

The weedy annual and perennial members of Euphorbia are widespread in the United States, occurring in every state. In at least nineteen states E. esula is considered serious enough to be declared noxious by law (PMA, 1946). In other states, to date, the members of this group occupy the position of a minor nuisance, to be controlled in the general clean-up of other weeds. E. corollata is noxious in Idaho and Wyoming; E. lucida in Idaho and South Dakota.

A high degree of control of annual weeds in cotton by the pre-emergence application of dinitro-secondary-butyl-phenol or its salts has led to appreciable commercial interest. Leonard, et al. showed that the above chemical was promising for controlling annual weeds in cotton, when applied to the soil as a pre-emergence herbicide, and there have been many reports on the method since that time. It was recognized that this treatment did not control perennial weeds such as nutgrass and Johnson grass, and that some additional treatment would be required to control these weeds. In January 1948, one of the authors had the opportunity to see sugarcane weeds in Puerto Rico controlled by the post-emergence application of fortified oil emulsions, and the procedure was included for study in the 1948 cotton tests. The primary interest of the authors was to find a spray that would control the perennial weeds and which, at the same time, would not be injurious to cotton. It was felt that the annual weeds could be controlled rather well by the pre-emergence treatment. The effect of fortified oils and oil emulsions and unfortified oils on cotton hypocotyls and on grass shoots showed that the fortifying chemicals (dinitro-secondary-butyl-phenol and pentachlorophenol) were quite injurious to cotton hypocotyls, but that some unfortified oils showed considerable promise for selectively weeding cotton.

The employment of selective herbicidal sprays to kill or inhibit one or more undesirable plant species growing in close association with desirable species has become a common practice in producing certain crop and pasture species. Many crop and weed plants which are equally inhibited if exposed to certain herbicides during the germination period may show marked differential response when the tops of the same plants are sprayed with the materials. For example, the common small grains and corn are readily inhibited by the herbicide 2,4–dichlorophenoxyacetic acid (2,4–D) if exposed to solutions of the herbicide at the germination stage. Notwithstanding, 2,4–D is widely used in spraying these crop plants at certain stages to control weeds without causing significant damage to the crops. The lack of serious damage to these species from applications of 2,4–D to their tops may be attributed to several factors. Firstly, the spray may be repelled and shed harmlessly to the soil by the crop species and retained by the weedy ones. Secondly, the spray may be retained about equally well by the foliage of all species, but absorption and translocation of the herbicide to susceptible tissues, such as the stem and root apical meristems, may be less efficient in the non-susceptible species than in the susceptible ones. Possible differential accumulation of herbicides at the site of toxic action by susceptible and tolerant plants also may be important. Thirdly, the differential response of plant species to certain herbicidal sprays may be attributable to an inherent physiological capacity of the non-susceptible plant to tolerate the herbicide even in intimate association with the cellular components. Fourthly, some herbicidal materials appear to be readily taken up by the normal absorbing organ, the roots, of plants, but are not absorbed efficiently by the aerial plant parts because of certain physio-chemical relationships and internal morphological barriers. Fifthly, the position of the stem growing point may be a factor influencing the differential response of plants to herbicidal sprays. The apical bud of the stem in most broadleaf plants is usually wet by foliage sprays, whereas the stem bud of grasses is protected by several enveloping leaves. Sixthly, some species may inactivate the herbicide more rapidly than others and hence are not highly responsive.