Structure and ultrastructure of Pinus halepensis primary needles

This study using light, transmission and scanning electron microscopy, describes the structure of Pinus halepensis primary needles collected from seedlings 22 and 24 weeks after emergence. P. halepensis is a circum-mediterranean pine of high importance for ecosystem maintenance and reforestation, in which primary needles are formed three to four years before being replaced by secondary needles. The needle surface shows spine-like structures and deeply sunken stomata, covered by wax, which however does not occlude the epistomatal chamber. Cross sections of the rhomboidal-shaped leaves show a uniseriate epidermis composed by living cells with a thin cuticle. Hypodermal cells do not occur under the epidermal layer. The plicate mesophyll, with large intercellular spaces and resin ducts, is also described. The vascular bundles are surrounded by transfusion tissue, bounded by a uniseriate endodermis. The endodermal cells have unmodified thin walls which lack Casparian strips, but plasmodesmata between adjacent endodermal cells occasionally occur. Moreover, plasmodesmata between mesophyll and endodermal, and endodermal and transfusion parenchyma cells interconnect these tissues. Transfusion tracheids are numerous with abundant bordered pits with a torus. The xylem has the usual structure, and the mature sieve elements have very thick fibrous walls. The fine structure of primary needles of the P, halepensis population in North Eubea (Greece) indicates that the typical morpho-anatomic characteristics of the pine leaf, which are generally correlated with resistance to aridity and are described for secondary needles, are not present. Rather, the features of non-vascular tissues may enable a high photosynthesis and are coherent with an (indefinite) growth model for the juvenile phase of P. halepensis.