more glyphosate than the roots. In both plant types, glyphosate was localized in the meristems and young, Maize (Zea mays L. var. Bonnie) transformed with a actively growing leaves. Specific glyphosate activity gene encoding a 5-enolpyruvylshikimate 3-phosphate (the amount of glyphosate per unit dry weight of tissue) synthase with altered sensitivity showed over 100-fold in the major sinks of the transgenic declined towards greater resistance to the herbicide glyphosate (N- the end of the treatment period but remained relatively [phosphonomethyl]glycine) in comparison with its non- constant in the parental control. In conclusion, enhantransformed progenitor (parental control) at the third- cing glyphosate resistance by genetic transformation leaf stage. Studies with [14C]-glyphosate at a dosage influenced the absorption, translocation and distribulethal to the parental control, but sublethal to the tion of this herbicide in whole plants. transgenic, revealed that a maximum of 45‐65% of the applied dose was absorbed, with greater absorption Key words: Zea mays, glyphosate (N-[phosphonomethyl]occurring in transgenic plants. Translocation of glycine), transgenic, absorption, translocation, source‐ glyphosate was closely related to its absorption (r sink. value 0.956) with approximately 15% more of the applied dose being mobilized in transgenic plants than the parental controls. Analysis of electronic autoradi- Introduction ograms along the treated leaf lamina found discrete internal regions of glyphosate accumulation closely Glyphosate is a foliar applied, broad spectrum, postassociated with the site of application. These regions emergence herbicide capable of controlling annual and contained lower amounts of glyphosate towards the perennial grasses and dicotyledonous weeds (Franz et al., basipetal end of the lamina. Glyphosate present in the 1997). This herbicide also has favourable environmental treated leaf lamina was almost completely transloc- features including rapid soil inactivation and degradation ated in transgenic plants, while in the parental controls to harmless products, and low toxicity to non-plant life. more remained and the leaf became necrotic. In both A major limitation for glyphosate usage has been its nontypes of maize there was a small accumulation of selectivity. However, there is now the possibility of this herbicide in the tip region of the leaf which was not limitation being overcome in many situations by the mobilized. Younger shoot tissues and roots were major introduction of crop plants genetically engineered for sinks for translocated glyphosate accumulating enhanced resistance to this herbicide ( Kishore et al., approximately 25‐40% of the applied dose depending 1992). upon treatment. In the parental control, equal amounts The mode of action of glyphosate is by inhibition of of glyphosate were found distributed between young 5-enolpyruvyl shikimate 3-phosphate synthase ( EPSP shoot tissues and roots; while in transgenic plants, the synthase), a key enzyme of the shikimate pathway