How to recognize ozone symptoms
The following general guidelines can be given to identify ozone symptoms in plants (adapted from the Manual on Methods and Criteria for Harmonized Sampling, Assessment, Monitoring and Analysis of the Effects of Air Pollution on Forests Part X).
Symptom identification for broadleaf species
Visible ozone-like symptoms can be identified and distinguished from symptoms caused by other biotic/abiotic factors by following the general guide lines as described below:
· Symptoms are more severe on mid-aged and older leaves than on younger leaves. Older leaves are the first to develop symptoms (age effect).
· Shaded portions of the leaves (i.e. if two leaves overlap) usually do not show any injury (shade effect).
· Visible ozone injury normally does not go through the leaf-tissue. Visible symptoms are most likely confined to the upper leaf surface, typically expressed as tiny purple-red, yellow or black spots (described as stipple) or sometimes as a general even discoloration, reddening or bronzing.
· Both, stippling and even discoloration, only occur between the veins (interveinal) and do not affect the veins.
· Injured leaves appear to senesce faster and drop sooner.
Symptom identification for conifer species
Visible ozone and visible ozone-like symptoms for conifer species can be assessed by following the general guide lines as described below:
· Visible ozone and visible ozone-like symptoms for conifer species are expressed at the upper parts of the crown, on the upper side of the branches and needles.
· Chlorotic mottling is the most common symptom described for conifer needles; it is the result of chronic exposure to ozone and can be described as yellow or light green areas of similar size without sharp borders between green and yellow zones. However, not all needles in a fascicle may be uniformly affected.
· Chlorotic mottling frequently appears only in needles older than 1 year (second-year needles and older). That is, the observed symptom seems to increase with increasing needle age (age effect).
· Chlorotic mottling is more distinct on light-exposed needle areas in comparison to shaded ones.
· It is easier to observe the mottling, if several needles are held close to each other, forming a “plane” of needles.
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