Seasonal Cambial Anatomy and Development of Xylem inDalbergia sissooGrowing Under the Influence of Combined Air Pollutants

Seasonal behavior of vascular cambium and development of xylem were compared between the trees of Dalbergia sissoo growing under the influence of combined air pollutants (affected) and in relatively unpolluted area (normal). The duration of radial growth lasted for seven months in normal trees and five months in affected ones. In the normal trees, cambial cells began to divide in April with the sprouting of new leaves and ceased in November after reaching peak in August. The cambial cell division in affected trees started in June with the arrival of rains, reached peak in August and then ceased in November. Cell divisions in the cambium and development of xylem were suspended from December to March and December to May in normal and affected trees, respectively. Comparison between the length of fusiform cambial cells in normal and affected trees also showed significant variation. Xylem is diffuse porous with indistinct growth rings but it can be delineated by the presence of fine band of terminal parenchyma cells in trees of both localities. However, considerable variations occurred in the frequency and dimensions of xylem elements. Relatively more number of vessels with less lumen diameter was the characteristic feature of the xylem of trees growing under pollution stress. Similarly annual average length of xylem fibers was relatively less in the trees of pollution stress. Annual increment of xylem for two successive years was also found less in affected trees. Periodicity of cambium and xylem development is discussed in relation to the local climatic factors and phenology of the trees.