Herbicide resistance is the genetic capacity of a weed population to survive an herbicide treatment that, under normal use conditions, would effectively control the resistant weed population. Weeds have been evolving in conventional crop cultivars worldwide from selection pressure placed on them from repeated use of herbicides. In this chapter, we intend to explain the biochemical and molecular basis of herbicide resistance in weeds. On the other hand, herbicide resistance can be a useful tool so that weed scientists can use as important approach to control and manage weeds. There are several strategies for the production of HR crops by genetic engineering and the methods used in this process will be discussed in this chapter.

Herbicide tolerance is commonly associated with reduced absorption and translocation of the herbicide; we hypothesized that the mechanism of dicamba tolerance in wild tomato (Solanum lycopersicum) accessions is due to these. The absorption and translocation of dicamba were investigated at a drift rate of 2.8 g ae ha-1, in three predetermined dicamba-tolerant wild accessions (TOM199, TOM198, and TOM300), and compared with two dicamba-susceptible (DS) commercial tomato cultivars [Money Maker (MM) and Better Boy (BB)]. Dicamba was quantified in three different parts of the tomato plant: two upper leaves, two lower leaves, and the roots, at 1, 3, and 7 days after treatment (DAT). Both MM and BB absorbed more dicamba then all the three DT accessions. The overall translocation pattern of dicamba was similar between DS cultivars and DT accessions, thus suggesting that tolerance to dicamba in wild accessions may not be associated with reduced translocation, but instead with reduced uptake of the herbicide. Additionally, reduced dicamba absorption in DT accessions may be attributed to their leaf characteristics, such as the presence of narrower leaves (3.42 leaf length/width ratio) and higher trichome density (20 no. mm-2) in DT accessions, than compared to DS cultivars (1.92 leaf length/width ratio, and 8 no. mm-2 trichome density).

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