Effect of mineral composition and medium pH on Scots pine tolerance to toxic effect of zinc ions

Abstract

Effect of mineral composition and pH of nutrient medium on the growth of Scots pine (Pinus sylvestris L.) seedlings and their tolerance to toxic effect of zinc ions was investigated by modifying the standard Knop-Hoagland medium (pH 6.6) (M1). The improved medium (M2) was more acid (pH 4.5) and notable for 2.7-fold lower osmolarity, with equimolar ratio between ammonia and nitrate nitrogen, assimilable form of iron (9 μM Fe2+), and the presence of 0.02 μM Co2+ and 2 mM Cl−. Owing to balanced mineral composition and low pH, M2 enhanced seedling growth under normal conditions and reduced the toxic effect of 100 μM ZnSO4. Rapid development of the root system (main root, the number and total length of lateral roots) on M2 was maintained even under stress conditions (100 μM Zn2+); as a result, the ratio between the biomass of aboveground organs and the biomass of the root system remained on the level of 2.5, whereas on M1, it increased to 3.9. At the same time, on M2 we observed accelerated production of the needles with elevated content of photosynthetic pigments therein. Investigation of the operation of individual components of antioxidant system (superoxide dismutase, catalase, ascorbate peroxidase, and guaiacol peroxidases) and peculiarities of proline accumulation showed that Scots pine seedlings grown on M2 experienced less oxidative stress than the seedlings grown on M1. The probable physiological foundation of the observed phenomena is discussed.