Seasonal production of lenticel tissues was compared between Norway spruce trees (Picea abies (L.) Karst.) from a mountain site (1200 m), where they are autochthonous, and seven allochthonous lowland sites (250–600 m).
The periodic changes of lenticel structure were grouped into four stages, based on the degree of their opening: phase 1 - winter dormancy; phase 2 - beginning of meristem activity in spring; phase 3 - production of non-suberised filling tissue in early summer, which causes the disruption of the closing layer formed in the previous growing season; and phase 4 - differentiation of a new closing layer in late summer. Structural changes in lenticels of P. abies may be interpreted as a long-term reaction to climatic conditions, balancing transpiration and respiration. During the most active period of wood production, lenticels were found in their most permeable phase, phase 3. The production of a new closing layer takes place when summer temperatures reach maximum values, and when demand for effective regulation of transpiration is high. During phase 4 transpiration is successfully controlled because differentiating cells of the new closing layer are already suberised, although not in their final rounded shape, and therefore have small intercellular spaces. High annual variability in stratification of lenticel tissues, such as the proportion between closing layer and filling tissue, wall thickening and size of intercellular spaces, also indicates possible long-term regulation mechanisms for transpiration.