Fluoride-induced symptoms have been described in many reviews (Weinstein and McCune 1970; Weinstein and McCune, 1971; Thomas and Alther 1966; Brandt and Heck 1977; Treshow and Pack 1970; Guderian et al. 1969; Hindawi 1970; Thomas 1961). The basis for the following description is Weinstein and McCune (1971) plus our own experience in the field in several countries.
Gaseous fluoride enters the leaf through the stomata (=pores) then it dissolves in the water permeating the cell walls. The natural flow of water in a leaf is towards the sites of greatest evaporation, which are the margins and tip. Carried by the water, the fluoride concentrates in the margins and tip, so it is these areas that generally are the first to show visible injury. Clearly, this concentration mechanism is one reason why fluoride can be so toxic to plants but there is an important corollary: Most of the leaf may have very little fluoride present and may function normally in terms of assimilation.
Generally, leaves are most sensitive when they are young and still expanding. Once fully developed, they may be many times more resistant. Therefore symptoms are more often seen in young, expanding leaves. Where fumigation is periodic, symptoms may reflect this as only those leaves that are at the sensitive stage of development when the fumigation occurs will develop injury. The rate at which symptoms appear depends on the weather. There can be a considerable lag between the time of exposure to the fluoride and the development of the symptoms.
Exposure to a high concentration causes necrosis of part or even the whole of the leaf. The term necrosis comes form the Greek nekros meaning a dead body. The tissues die.
The initial stages vary with species and both the speed of development of the symptoms and their appearance depend on the weather. In most monocotyledonous (narrow-leaved species including grasses and lilies) plants, the initial symptom is the development of chlorosis (= yellowing) at the tips and margins of elongating leaves.
Although necrosis is the symptom most frequently referred to in texts, often being called tip-burn, other symptoms are at least as common or, in some areas, more common.
(Davison A, Weinstein L. The effects of fluorides on plants. The Air Pollution Group, Department of Agricultural and Environmental Science, University of Newcastle upon Tyne, UK.).
You must log in to post a comment.