Leaf blade anatomy of the rare Siberian flora species Mertensia sibirica (L.) G. Don fil. (Boraginaceae)

The authors present the findings of a leaf blade anatomy study for the rare relict Siberian flora species Mertensia sibirica (L.) G. Don fil. (Boraginaceae). They collected samples for the study from natural habitats in Chita Region (Chikoy Range) and then planted them in the introduction area of the Siberian Botanic Garden (Tomsk) located in the southern taiga subzone of Western Siberia. The parameters of the photosynthetic and stomatal complex of M. sibirica were studied for the first time. It was found out that the rosette and cauline leaves of the species under study are hypostomatous, with an anomocytic stomatal complex. The epidermis is single-layer. On average, the adaxial epidermis has larger cells vs. abaxial epidermis. The leaf mesophyll is 242.90–369.90 µm thick, dorsiventral. The adaxial side of the leaf comprises glandular trichomes surrounded with pronounced rosettes of cells in the base part. The cauline leaf significantly differs from the rosette leaf in finer cells of its adaxial and abaxial epidermis (and, consequently, their larger number per 1 mm2), while the adaxial epidermal cells are thicker, and in a larger number of stomata in the abaxial epidermis. The palisade mesophyll in the cauline leaf is more developed vs. the rosette leaf, while the cells are longer and the palisade/spongy mesophyll ratio is higher. The rosette leaves have a more developed system of vascular tissues vs. cauline ones, as they play the main role in providing plants with water and nutrients. The contribution of the cauline leaf palisade mesophyll to the photosynthetic potential of M. sibirica is higher vs. that of the rosette leaf (the ratio between palisade and spongy mesophyll is 0.45 vs. 0.36, respectively), which characterizes the cauline leaf as more heliophytic. The stomatal complex and mesophyll parameters under study are primarily characterized by low variance. As for dermal tissue parameters, medium variance is typical of the thickness and size of the abaxial and adaxial epidermal cells. Coefficients of variation for the cells of the upper mesophyll layer (CV=31.2–41.6%) and the number of stomata on the lower epidermis of the rosette leaf (CV=21.5%) demonstrate medium and high variance. A very high coefficient of variation (116.2–174.0) is registered for the adaxial epidermis parameter characterizing the density of trichomes per 1 mm2. The study results were used to develop an optimal M. sibirica cultivation regime under conditions of introduction in the southern taiga subzone of Western Siberia.