Photosynthetic Inhibitors

Life on Earth is dependent on sunlight. In the process known as photosynthesis, plants, algae and certain bacterias are capable of using this source of energy to drive the synthesis of organic compounds. The oxygenic photosynthesis results in the release of molecular oxygen and the removal of carbon dioxide from the atmosphere that is used to synthesize carbohydrates. This process, which involves a complex series of electron transfer reactions, provides the energy and reduced carbon required for the survival of virtually all life on our planet, as well as the molecular oxygen necessary for the survival of oxygen consuming organisms (Nelson, 2011; Nelson & Yocum, 2006; Rutherford & Faller, 2001). In modern agriculture, farmers continuously face a battle to achieve products in high yields and better quality to feed an ever increasing world population (Stetter & Lieb, 2000). The optimization of agriculture techniques demands, along with other requirements, the application of crop protection agents to control a variety of diseases and pests, among which are weeds. Weeds compete with crops for nutrients, water, and physical space, may harbor insect and disease pests, and are thus capable of greatly undermining both crop quality and yield. In view of the problems caused by weed species, their control is highly desirable. Among the methods to control weeds, the use of herbicides or weed killers has become the most reliable and least expensive tool for weed control in places where highly mechanized agriculture is practiced. Since the introduction of 2,4-dichlorophenoxyacetic acid (2,4-D) in 1946, several classes of herbicides were developed that are effective for broad-spectrum of weed control (Böger et al, 2002; Cobb, 1992; Ware, 2000). It is well know that various compounds can interfere with photosynthetic electron transport. This fact has been explored by agrochemical companies to develop an assortment of herbicides to control weeds. Some representative members of commercial photosynthetic inhibitors are diuron (1), atrazine (2), paraquat (3) e diquat (4) (Figure 1). The photosynthetic inhibitors can be divided into two distinct groups, the inhibitors of photosystem II exemplified by diuron (1) and atrazine (2) and the inhibitors of photosystem I such as paraquat (3) and diquat (4). It is worth to mention that compounds that inhibit photosystem II account for 30% of the sales in the herbicide market (Draber, 1992). Although there are a variety of herbicides that can control a broad spectrum of weeds, there is still a necessity for the development of new active ingredients. Several reasons can be mentioned to support this statement. Herbicides should have a favorable combination of properties, such as high specific activity, low application rates, crop tolerance, and low mammalian toxicity. Increasing public concern for environmental pollution derived from