Induced micronutrient toxicity inPetunia hybrida∗

Plant micronutnent toxicity symptoms of petunia (Petunia hybrida) cv. Ultra Crimson Star were experimentally induced and characterized. Starting at the 3–4 true leaf stage, plants grown in peat‐lite mix in 11 cm pots were constantly fed for 5 weeks with fertilizer solutions containing 0.25, 0.5, 1, 2, 3, 4, 5, and 6 mM plus the control concentration of boron, copper, iron, manganese, molybdenum and zinc at pH 6.5. The control solution had 20 μM B, 0.5 μM Cu, 10 μM Fe, 10 μM Mn, 0.5 μM Mo, and 4 μM Zn. Foliar toxicity symptoms were produced when the nutrient solutions contained 0.5 mM (5.4 ppm) B, 0.5 mM (32 ppm) Cu, 0.5 mM (28 ppm) Mn, 0.25 mM (24 ppm) Mo, and 0.25 mM (16 ppm) Zn. Iron in the range of concentrations tested did not cause foliar toxicity. Reduction in dry matter yield was evident when 2 mM (22 ppm) B, 1 mM (64 ppm) Cu, 6 mM (335 ppm) Fe, 0.5 mM (28 ppm) Mn, 0.25 mM (24 ppm) Mo, and 0.5 mM (33 ppm) Zn were used in the fertilizer solution. Leaf chlorophyll contents decreased as Cu and Mn concentrations increased. Leaf chlorophyll contents increased as Fe in the fertilizer solution was elevated. The threshold tissue micronutrient levels that caused toxicity symptoms were also determined.