Axial variability of anatomical structure and the scaling relationships in Scots pine (Pinus sylvestris L.) needles of contrasting origins

Abstract

Intraspecific variability of anatomical leaf traits is usually determined on the basis of median sections and interpreted as adaptive. We hypothesized that anatomical traits may relate to the position within the leaf, reflecting functional coordination e.g., between transport and gas exchange capacities and between supporting tissue and the supported leaf weight. Anatomical variation may thus be partly controlled by leaf size. We examined variability of transverse anatomical traits and stomata at different positions along the leaf axis using Scots pine (Pinus sylvestris L.) needles collected from three geographical locations with contrasting climate (Poland, Southern and Northern Sweden) and differing in length.Transverse xylem and phloem area, and the number of tracheids at a given cross-section scaled positively with length of the distal needle part and the number of distal stomata. Similarly, the number of vascular sclerenchyma fibers in a cross-section scaled with the length and volume of the distal needle part. Allometric slopes were less steep in the longer, southern-sites needles. Additionally, our study revealed an increased sclerification of epidermis towards the needle apex, the occurrence of widest tracheids in central or subapical regions, a reduced number of resin ducts in basal and apical parts, as well as overall basipetal decreases in duct diameter, the number of stomatal files and stomatal density.These results show that allometry accounts for significant amount of intraspecific variability in xylem, phloem and fibers, and that environmental adaptation involves also modification of the within-needle allometric relationships. The allometric component of variability, together with the nonlinear distribution of certain traits along needle length, should be considered when designing comparative studies of foliar phenotypes and scaling up from transverse sections to entire needles.