Intra-seasonal trends in phloem traits in Pinus spp. from drought-prone environments

Abstract

Abstract Recent studies on the seasonal dynamics of secondary tissue formation in Mediterranean trees have shown that xylogenesis depends on species and site conditions, but many questions still remain open. On the other side of the cambium, even less information is available about phloem structure and timing of its formation. We analysed intra-annual phloem variation in width and cell traits in the conducting, non-collapsed phloem (CPH) of Pinus pinea and Pinus halepensis at Mediterranean sites in southern Italy and Spain. In all investigated trees, it was possible to differentiate among the non-conducting, collapsed phloem (NCPH), and the CPH. CPH showed no evident annual growth layers; no differences in radial dimensions of early- and late phloem sieve cells, and no cyclic patterns of axial parenchyma distribution. Since it was not possible to study the seasonality of the phloem growth, we analysed the entire CPH. CPH width showed seasonal fluctuations and was generally the widest during the maximum cambial activity and narrowest during summer and winter. The radial size of newly formed sieve cells varied in relation to seasonal dynamics of cambial activity and fluctuations in local weather conditions. The number of axial parenchyma cells in CPH increased during the summer. The observed intra-annual variations in CPH width and structure seemed to be correlated with seasonal weather conditions in order to ensure a sufficient amount of conducting phloem tissue for translocation of photosynthates and signalling molecules to the actively growing tissues along the stem of a tree growing in the harsh Mediterranean conditions.