除草劑藥害
蔣永正
行政院農委會農業藥物毒物試驗所
摘 要
雜草在農業栽培系統中，對作物生產構成了顯著而具延續性的限制，目前在世界大部份地區之作物田，除草劑已成為雜草防治的主要手段。50 %的除草劑主要是影響光合作用光反應之電子傳遞，其餘則分別為抑制胺基酸的生合成、干擾細胞正常分裂或具有植物生長素活性等；而近10年所發展之除草劑，其作用位置集中在細胞內重要的酵素分子上，如ALS, ACCase和PPG-oxidase等。除草劑在進入植物細胞轉移到作用位置的過程中，發生一連串的代謝作用，改變了藥劑原來的化學結構與生物活性，構成植物對藥劑產生容忍性的基礎。植物對這些外來化合物的代謝包括三個歷程，首先會經過氧化、還原或水解作用，將原本親脂性化合物變為兩性化合物（amphophilic），同時降低其在細胞內移動性；其次經過共軛作用（conjugation）再轉變為親水性，移動性也因而更加被限制，甚至成為不可移動；第三個步驟則再經過二次共軛作用，形成的不溶聚合物，與細胞壁成份相結合，或被區隔在液胞中，使原本對細胞有毒的物質變成不可移動且無毒性的形式。因此除草劑對雜草防治的有效程度，和實際到達細胞內作用位置上之濃度是否足以抑制正常的生理作用進行，且形成了一個不可恢復的傷害結果有關；此即構成除草劑對不同植物種間之選擇性差異，得以應用於實際田間之雜草防除，且不會導致作物發生藥害的基礎；因此已推薦之除草劑在施用過程中若有發生藥害的情形，對目標作物而言：主要是施用方法的誤差，如重覆曝藥或敏感生育期施藥，導致累積在作用位置上之藥劑無法經由代謝過程完全解毒；對非目標作物而言，則為施用時藥劑的飄散（drifting），或隨灌溉水到達非目標作物區內，導致敏感作物的生長抑制，或在土壤中之殘效過長，影響後作植株之正常生育。與藥害發生有關之因子包括；農藥本身性質（主成份及添加劑），和噴施系統（噴施壓力與藥液粒子大小等），受到環境因子的影響，使作物接觸到超過其自身可解毒之藥量範圍，而於敏感作物或敏感生育期的表現的異常現象。
關鍵詞：除草劑、藥害、非目標作物。
Herbicides
are extremely useful tools for managing weeds in modern day agriculture. Prior
to phytotoxic action, herbicide must be absorbed by
the plant and reach its site of action, usually located within an organelle in
the plant cell. Once at the active site, the herbicide may accumulate to
concentrations that will elicit a phytotoxic respones. Several factors can influence the final herbicide
concentration at sites of action, including root or shoot absorption,
translocation from sites of absorption to sites of action, or metabolism of the
herbicide to nonphytotoxic compounds. The
physicochemical characteristics of the herbicide molecule include- ing lipophilicity and acidity,
the plant cell membranes, and the electrochemical potentical
in the plant cell control herbicide absorption and accumulation. Most
herbicides move across plant membranes via nonfacilitated
diffusion because the membrane’s lipid bilayer is
permeable to neutral, lipophilic xenobiotics.
Passive absorption of lipophilic, ionic herbicides or
weak acids can be mediated by an ion- trapping mechanism where the less lipophilic, anionic form accumulates in alkaline
compartments of the plant cell. Herbicides also may accumulate in plant cells
by conversion to nonphytotoxic metabolites, binding
to cellular constituents, or parti- tioning into lipids. Many herbicides can be used
selectively in crops because they are rapidly metabolized to inatctive products before the crop plant is damaged. Weeds
which lack this metabolic capability remain susceptible. Herbicides registered
for specific crops generally have a large safety factor in terms of crop
tolerance to compensate for possible user misapplication and adverse
environmental situations. However, it is impossible to anticipate and predict
every conceivable misuse pattern, unusual environmental and edaphic
conditions, variations in tolerance of cultivars, and
combinations of herbicides with fertilizers and pesticides made by the user.
The major reasons for herbicide injury include: failure to follow label
directions, problems with application equipment, herbicide drift and
volatilization, herbicide residues, chemical interactions and
incompatibilities, and equipment contamination. Once injury occurs, there are
no true remedial measures that can be implemented to overcome or reverse the
damage. Prevention is the only sure cure.
Key
words: Herbicides, Herbicide injury, Nontarget crops