Effects of magnesium deficiency on needle ultrastructure and growth of Scots pine seedlings

Abstract

Effects of magnesium (Mg) deficiency on 3-year-old Scots pine (Pinussylvestris L.) seedlings were studied during a field experiment extending over two growth periods. Seedlings were grown in quartz sand in 7.5-L pots watered with nutrient solutions in which the Mg content was reduced to 30 and 0% of the control level (15 mg/L in the first and 3 mg/L in the second growth period). During the first growing period the Mg content of needles at the 0% Mg level was significantly lower than at the 30% level, and in the second growth period a decline was clear at both deficiency levels. Swelling of phloem cells was observed in samples taken after 4 weeks' exposure, and the frequency of swollen phloem cells increased towards autumn and through the second growth period. At the end of the first growing period a decrease in the number of thylakoids per granum and an increase in the number of plastoglobuli in chloroplasts of mesophyll cells were detected at both deficiency levels. During the second growing period the tips of needles from the previous year in the 0% Mg level group became brown, and these needles were shed in autumn from some of the seedlings. The chloroplasts in these needles were rounded and the thylakoids were abnormally organized. Structural symptoms caused by Mg deficiency were observable before visual changes and before a clear decrease in Mg content had occurred, thus showing the value of structural observations in early diagnosis of this stress.