INSTYTUT DENDROLOGII

Wax layer and its morphological variability in selected Salix species

The objectives of this study were: (1) to recognize the morphological structure and variability of the epicuticular wax layer on leaves in Salix species; (2) to assess the usefulness of features of the leaf micromorphology in taxonomic studies; and (3) to determine if the wax layer structure is related to leaf position on the shoot and to the stage of leaf blade development.

Material for this study consisted of leaves of 36 Salix species and 4 hybrids, collected from individuals growing in Poland and abroad: Salix acutifolia, S. alba, S. alpina, S. amplexicaulis, S. arctica, S. armeno-rossica, S. aurita, S. caprea, S. cinerea, S. daphnoides, S. dasyclados, S. elaeagnos, S. eriocephala, S. fragilis, S. hastata, S. helvetica, S. herbacea, S. lapponum, S. myrsinifolia, S. myrtilloides, S. pantosericea, S. pentandra, S. phylicifolia, S. polaris, S. purpurea, S. repens, S. reticulata, S. retusa, S. schwerinii, S. silesiaca, S. starkeana, S. triandra, S. turanica, S. udensis, S. viminalis, S. xanthicola, S. alba × fragilis, S. amplexicaulis × xanthicola, S. pentandra × fragilis, S. purpurea × viminalis).

The investigations showed that:

1. The adaxial leaf surface in the analyzed Salix species is covered by a homogenous smooth wax layer.

2. The abaxial wax layer is either formed as a smooth layer, or covered by peculiar structures, for which the term "conicoid" was coined.

3. Conicoids are structures composed of apically convergent wax filaments. They may differ in size, and sometimes in wax filaments density.

4. Conicoids are formed during leaf blade development. A scheme of its formation is proposed.

5. There are differences between the analyzed species in the structure of the wax layer on the abaxial leaf surface. They enable grouping of the species into four categories: (1) with a smooth wax layer, (2) with large conicoids, (3) with medium-sized conicoids, and (4) with small conicoids.

6. The wax layer structure can be used as an additional identifying feature in willow species, but it does not depend on their subgeneric grouping. The relation between the wax layer structure and the division of the genus Salix into sections is generally poor.

7. The structure of the wax layer depends on leaf position on the shoot. On the oldest leaves, it is poorly developed. On the successive leaves, conicoids have a structure and size typical of the given species. On the young leaves, which are not fully developed yet, the process of wax formation is on-going, so its structure is not the same as in fully-developed leaves, typical of the given species.

8. No relationship was found between the presence of conicoids and epidermal cell size, cuticle thickness, and cuticle ultrastructure.