PHYSICAL INJURY, RE-CRYSTALLIZATION OF WAX TUBES AND ARTIFACTS - IDENTIFYING SOME CAUSES OF STRUCTURAL ALTERATION TO SPRUCE NEEDLE WAX

The study presents investigations of epicuticular wax morphology on spruce needles (Picea abies (L.) Karst.) performed by ambient temperature scanning electron microscopy. To use alterations to epicuticular wax morphology as a successful tool in forest damage research, it is necessary to distinguish environmental influences and artefacts from common pollution effects. The present paper summarizes observations of alterations to epicuticular waxes independent of pollution effects, recorded in various field studies. These observations are supplemented by controlled experiments to investigate wax alterations owing to solvents and mechanical injury. Fissures in the antechamber wax were the result of needle shrinkage during drying. Wax alterations, where only the antechamber wax was affected by a continuous loss of the structural integrity of wax tubes, were classified as storage artefacts since they occurred exclusively on needles stored in air-tight glass vials and were unrelated to needle age, sampling site or pollution influence. Such wax alterations were never observed in needles stored in air-permeable paper bags. Mechanical injury also resulted in severe morphological alterations to epicuticular wax structures. The wax tubes on the surface as well as in the antechamber were squashed and flattened. On smoothed and squashed wax layers a significant regrowth of wax tubes was occasionally observed, unrelated to needle age. Though the tubular form of the re-crystallized waxes was similar to those originally present on the surface, the tube diameters were much wider. Since redeveloped wax tubes also occurred on dead needles which had been artificially injured, this process could be classified as re-crystallization analogous to wax re-crystallization out of solvent extracts. The re-crystallization took place within 48 h after the mechanical injury occurred. The implications of the reported wax alterations for a successful use of epicuticular wax morphology as a bioindicator are discussed.