Effects of potassium deficiency and simulated acid rain, alone and in combination, on the ultrastructure of Scots pine needles

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Abstract
The effects of potassium deficiency and artifical acid precipitation, alone and in combination, on Scots pine (Pinus sylvestris L.) needles were studied using transmission electron microscopy. The seedling material was grown in quartz sand culture and watered with nutrient solution containing 45, 20, 10, or 5 mg of potassium/L; the concentration of other nutrients was constant and equally available for all seedlings. A portion of the seedlings from each group received consecutive acidified water spraying of pH 4.5, 4.0, 3.5, and 3.0, each for 4 days with a 3-day resting period between applications. The potassium deficiency caused an increase in the proportional vacuolar space and severe vesiculation of the tonoplast. Irregularly shaped lipid structures increased in number in the cytoplasm, and lipid bodies also appeared in the vacuoles and occasionally in the chloroplasts. The symptoms related to potassium deficiency were more severe in the transfusion parenchyma cells than in the mesophyll. The lowest level of potassium produced almost complete disorganization of the cellular structures in the transfusion parenchyma tissue, but severe changes were also seen in the mesophyll. The simulated acid rain treatment caused the formation of protrusions in the chloroplasts and an increase in irregularly shaped lipid structures in the cytoplasm throughout the mesophyll tissue, but no clear symptoms were observed inside the bundle sheath. In general, the changes caused by acid rain in the chloroplasts were slight and did not cause serious disorganization of these organelles. When the seedlings were exposed to combination treatment, the typical symptoms of both exposures were observable. The results suggest that the stresses caused by potassium deficiency and short-term foliar acid rain treatment can be distinguished in the needle ultrastructure.