HERBICIDE MODE OF ACTIONS AND SYMPTOMS OF PLANT INJURY BY HERBICIDES: PLANT GROWTH REGULATOR HERBICIDES (SYNTHETIC AUXINS)

The growth regulator herbicides are synthetic chemicals acting as auxin-type phytohormones. Application of growth regulator herbicides promotes the disbalance in phytohormone levels in the sensitive plants resulting in metabolic defects and disruption of normal plant growth. The majority of growth regulator herbicides are relatively fast uptaken by plant leaves (some of them also by roots) and translocated (especially by phloem) into the tissues showing high growth activity. This is causing significant systemic and long-term efficacy on perennial weed species. The main symptoms of phytotoxicity are growth and reproduction anomalies. Dicotyledonous plants show typical leaf and stem twisting, leaf deformations, elongations, and/or abnormal growth, production of calluses or tumours, and root dwarfing. Consequently, meristem chloroses occur together with willing and necroses. Treated sensitive trees can show bark cracking. In case of maize and cereals, following injury symptoms can be found: leaf twisting, stem bending, brittle stem, tiller number reduction, anomalies during earing, anomaly growth of aerial roots (maize). Herbicides of this group are predominantly used for control of dicotyledonous weed species in cereal stands. The most commonly used active ingredients are MCPA, MCPP, dicamba, fluroxypyr, aminopyralid, and clopyralid. Some growth regulator herbicides can be used also in dicotyledonous crop stands. In oilseed rape stands, there can be used clopyralid, picloram, aminopyralid, and quinmerac; in sugar beet stands clopyralid or quinmerac, in field pea MCPB, and in poppy stands fluroxypyr. Beside this, herbicides of this group are used also in some other crops and vegetables, minor under our conditions. The active ingredient triclopyr is used as an arboricide to control weedy trees and shrubs and weeds in pastures and meadows, in forest management, and on non-agriculture land.