Effects of high concentrations of calcium salts in the substrate and its pH on the growth of selected rhododendron cultivars

Abstract

For proper growth and development, rhododendrons need acidic soils, whereas calcium carbonate (CaCO₃) in the substrate markedly limits their growth. In this study, we analysed the reactions of rhododendrons to high concentrations of calcium salts and pH in the substrate. We used 4-month-old seedlings of Rhododendron 'Cunningham's White' and 1.5-year-old seedlings and rooted cuttings of R. 'Cunningham's White' and R. 'Catawbiense Grandiflorum'. Their reactions depended mostly on calcium salt type added to the substrate (sulphate or carbonate). An increase in concentrations of phenolic compounds was detected mostly in roots of the plants grown in a substrate with a high calcium carbonate content. Addition of calcium salts to the substrate caused a significant rise in total nonstructural carbohydrates in leaves and roots of the studied plants. As compared to the control, an increase in substrate pH in the variant with calcium carbonate limited the activity of acid phosphatase, while lowering of substrate pH in the variant with calcium sulphate, significantly increased its activity. Along with the rise in substrate pH, a remarkable increase was observed in the activity of nonspecific dehydrogenase (DHA) in the substrate with CaCO₃, as compared to the control. Unfavourable soil conditions (high calcium content and alkaline pH) caused a decrease in assimilation of minerals by the studied plants (mostly phosphorus and manganese). Our results show that the major factor limiting rhododendron growth is an increase in substrate pH, rather than an increase in the concentration of calcium ions.