PHYSIOLOGICAL-CHANGES INDUCED IN BIRCH SEEDLINGS BY SUBLETHAL APPLICATIONS OF GLYPHOSATE

Sublethal (0-2.1 kg/ha) effects of glyphosate were evaluated on white birch (Betula papyrifera Marsh.) seedlings grown outdoors and under controlled-environment conditions. Leaf fluorescence, photosynthetic pigments, ethylene, and changes in the shikimic acid pathway were used as indicators of these effects. Leaf fluorescence steadily increased in treated birch seedlings for 9 days, indicating decreased photosynthetic efficiency. After this period, a gradual decline occurred, suggesting recovery of the photosystems. Chlorophyll a and b levels decreased within 4 days of application, whereas carotenoid content increased. Treated seedlings exhibited increased ethylene production compared with control seedlings. Shikimic acid levels increased 10-fold within 24 h of application, even at 1% of the field rate, and were still elevated in the spring of the following year. Gallic acid, another compound on the shikimic acid pathway, also increased in treated seedlings. Of the several parameters measured, glyphosate had the greatest effect on shikimic acid and differences occurred long before visual injury was observed. Shikimic acid level showed responses to low concentrations of glyphosate that produced no visible symptoms.