Contrasting phellem patterns associated with phellogen longevity in Quercus suber and Quercus rotundifolia (Fagaceae)

Abstract Key Message Maintaining a persistent phellogen (no rhytidome) may be associated with radial anticlinal cell division rates that increase with branch diameter, whereas the opposite trend is observed when the phellogen is instead replaced and a rhytidome develops. Abstract Quercus suber L. (cork oak) maintains its first phellogen (cork cambium) active throughout the life of the tree, unlike Quercus rotundifolia Lam. (round-leaved oak) and many other tree species, in which the first phellogen is eventually replaced and a rhytidome develops. Because periderm ontogeny provides a spatial record of past cell division activity, we developed a quantitative image analysis workflow to compare phellem cell patterning in young branches of both species as an anatomical proxy for radial anticlinal cell division frequency in the phellogen. Sample-level analysis of Q. suber and Q. rotundifolia branches showed that the relationship between branch diameter and the frequency of radial anticlinal cell divisions inferred from phellem row counts (Div/10c) differed markedly between species (species × diameter correlation): Div/10c increased with diameter in Q. suber but did not in Q. rotundifolia , with the clearest divergence in branches ≥ 7 mm diameter. Q. suber branches also showed higher phellem cell density in this size class. These findings represent early anatomical evidence for different phellem organisations in the two Quercus species, that may later result in their contrasting periderm developmental strategies.