Induced Micronutrient Toxicity in ‘Touchdown’ Kentucky Bluegrass

Abstract

The growth of turfgrass may be affected by excess levels of micronutrients. The objectives of this study were to determine changes in growth responses, leaf pigmentation, and chloroplast structure of Kentucky bluegrass (Poa pratensis L. cv. Touchdown) as affected by elevated levels of micronutrients. Plants were established in peat‐lite mix in 11‐cm pots using a standard nutrient solution and were thereafter constantly fertilized with nutrient solutions supplemented with individual treatments of 0.5, 1, 2, 4, 6, 8, or 12 mM of boron (B), chloride (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), or zinc (Zn) for 7 wk. The control solution had (in μM) 20 B, 0.5 Cu, 40 Fe, 10 Mn, 0.5 Mo, and 4 Zn. The lowest micronutrient concentrations that showed visual toxicity symptoms were 0.5 mM B, 12 mM Cl, 1 mM Cu, 4 mM Fe, 0.5 mM Mo, or 4 mM Zn. A reduction in dry matter yield was evident when the nutrient solution contained 2 mM B, 4 mM Cu, 2 mM Mo, or 4 mM Zn. Elevated levels of Cl, Fe, or Mn did not alter the biomass yield. Chlorophyll‐a (Chl‐a), chlorophyll‐b (Chl‐b), and carotenoid contents decreased as B concentration increased. Chlorophyll (Chl) content, particularly that of Chl‐b, increased with elevated levels of Fe. The altered Chl‐b levels induced by Fe correlated with increased levels of light harvesting chlorophyll (LHC) apoproteins and granai membrane accumulations. The loss of Chl in the B and Mo treatments correlated with decreased stroma membrane level and smaller chloroplast size, respectively. A linear relationship between the tissue and nutrient‐solution concentration existed for each micronutrient, suggesting a passive uptake by the plants. Findings of this study may be used in identifying altered growth and physiological characteristics of Kentucky bluegrass caused by supraoptimal levels of micronutrients.