**1. Introduction**

Herbicides can penetrate plants through their aerial structures (leaves and stems), subterraneous (root, rhizome, stolon, and tuber), and young structures such as radicles and caulicles. The main route of penetration of the herbicides in the plant is a function of a series of intrinsic and extrinsic (environmental) factors. Absorption of herbicides by roots or leaves is influenced by the availability of the products at the sites of absorption and environmental factors (temperature, light, relative humidity, and soil moisture), which also influences the translocation of these to the site of action [1].

Among the biochemical and physiological mechanisms, the change in the absorption, translocation, or metabolism of resistant weed biotypes has been reported on several species for different herbicides. These resistance mechanisms have been studied over the last years, allowing the development and improvement of analytical techniques to diagnose this type of resistance [2]. However, an up-to-date, organized description and standardization of research procedures and methodology on the use of radioisotopes for detection of resistant weeds, through different mechanisms of absorption, translocation, and metabolism in comparison with susceptible weeds are lacking in the literature.

Radioisotopes are used on several research areas, such as for the metabolism of drugs and pesticides, environmental studies to determine biological routes and mass balance studies for organic compounds, and the ones that are most frequently used are tritium and 14C. The method for using radiolabeled herbicides may be quantitative or qualitative, allowing associating the resistance to the reduced absorption and/or translocation, and/or to the accelerated metabolism in several weed species [3]. Therefore, it is important to understand concepts and measurement units of the main analytical techniques that use labeled molecules with 14C to study the biochemical and physiological resistance mechanisms to herbicides, as well as for studies that evaluate the destination of these molecules on the environment. Understanding these mechanisms is fundamental for management alternatives to be planned or to improve the effectiveness of the product [4].

Considering the above, the objective of this chapter was to conduct a description of the research procedures and the methodology related for detection of resistant weeds using 14C-herbicide absorption, translocation, and metabolism compared with susceptible weeds.
