Rhythmic growth rings of wood and their relationship with the foliage in oak seedlings grown in a favourable environment

Abstract

The anatomy of wood was studied in Quercus robur L. and Quercus suber L. seedlings exhibiting 3-8 units of extension, each with a tier of photosynthesizing leaves in their upper parts, generated as the result of rhythmic shoot growth under favourable conditions. At all the axis levels examined (i.e. the base of each of the different units of extension, four other equidistant levels within the first unit of extension and the upper part of the taproot), the wood displayed rings when treated with Wiesner reagents. This indicated cinnamaldehyde groups present in lignins. No rings appeared when the Maule reaction was used for specific detection of syringyl subunits in lignins. A trend towards a periodical arrangement of xylem parenchyma bands was also found when sections were treated with I2/KI. The number of rings coincided with the number of leaf tiers above the level of measurement, and did not vary inside the first unit of extension. Thus, the rings are called rhythmic growth rings. In sections of the first and the second units of extension, and in the taproot, the area and width of a given rhythmic growth ring were highly correlated with the total area of leaves present above the level of measurement at the presumed time of growth ring formation. Moreover, stem diameter at the base of the units of extension was highly correlated with the leaf area above. These results indicate that differentiation of xylem, particularly its lignification, varies rhythmically in oak seedlings. They imply that wood production is linked to the photosynthesizing and/or transpiring area of the plant. Thus, during a growth cycle of Q. robur and Q. suber seedlings, there appears to be integration of the primary metabolic activities with the laying down of rhythmic growth rings.