Seasonal dynamics of histological parameters of the needles of Scots pine (Pinus sylvestris L.) growing in conditions of excess ammonia

Abstract

Ammonia is one of the most important aerial pollutants which might affect morphology of conifers. The purpose of our study was to assess morphological and histological parameters of the needles of Scots pine (Pinus sylvestris L.) growing under elevated concentration (up to 17.5 μg m–3) ammonia (near JSC Achema, Lithuania) during the vegetation season (June–August, 2009). Current-year needles from the young age stand with higher concentration of air ammonia (17.5 μg m–3) were wider (p < 0.05; 1.11 times) when compared to the needles of Kačerginė site (3.9 μg m–3). The investigation of the seasonal dynamics of the needle morphology (length, width, thickness) and the thickness of the needle tissues in the cross-section (epidermis, hypodermis, mesophyll and central cylinder tissues – transfusion parenchyma and sclerenchyma) in most other cases did not reveal statistically significant differences between the trees growing in ammonia-polluted and relatively clean sites. It was true for both middle-aged and young age stands. Under the elevated concentration of air-ammonia, only some tendencies were observed for thicker needles (up to 1.05 times), thicker adaxial mesophyll (up to 1.10 times) and adaxial transfusion parenchyma (up to 1.06 times) of the needles. For middle-aged stands significant (p < 0.01) needle age differences were observed according to the abaxial and adaxial mesophyll thickness (up to 1.21 times thicker in 1-year-old needles), the same was true for the needles of the young age stand (up to 1.25 times). At the end of August, the average length of 1-year-old needles was 76 mm, width 1.7 mm and thickness 0.76 mm. For some histological parameters statistically significant (p < 0.05) seasonal differences were observed: the thickness of adaxial transfusion parenchyma was decreasing and the sclerenchyma width was increasing within the June–August period.