White to tan small spots on the leaves of the gourd Turk’s Turban in the following photographs are due to ozone. They were taken on 6 August 2004.
The interveinal necrotic (brown) tissue in the watermelon leaf below is the result of damage from exposure to high concentration of ozone.
Exposure to ozone caused the small, white spots on the pumpkin leaves in the following photographs. Sometimes part of a leaf blade is not injured due to stomates being closed.
Injury may occur primarily in one age group of leaves, which are the leaves that were most photosynthetically active when ozone concentration in the air was high, and thus these leaves had fully open stomates. Injury due to exposure to high ozone often appears suddenly and throughout a planting, in contrast with symptoms due to disease which develop slowly over time and often exhibit a pattern indicative of spread in affected planting. Sometimes when sections of leaves are bent over such that the lower surface is exposed directly to sunlight they become bleached.
In early August 2008 when ozone reached concentrations high enough to cause injury, plants generally were wilting under the hot and dry conditions then, consequently stomates were closed limiting ozone influx into leaves and thus very little damage was observed.
