PS 15-194 - Comparative study of leaf structural and functional signs of Quercus robur L. growing in ravine forests in the southern part of the Ukrainian Steppe

Abstract: The resistance of forest ecosystems to extreme environmental conditions depends mostly on the sensitivity of the main species. Quercus robur L. is one of the main forest-forming and most valuable species in Ukraine, in particular of ravine forests in the southern part of the Ukrainian Steppe, where they play the important environmental, water- and soil-protective role. Climate aridization, an increase in the frequency and severity of droughts exacerbate the problem of protecting ravine forests in the steppe zone. To determine the traits ensuring oak tolerance to steppe arid conditions we investigated the leaf micromorphology and ulrastructure of mesophyll cells in trees from the Steppe zone (Dnipropetrovsk Region), growing on the gentle slope of the ravines (southern border of the community area) and solitary trees growing on the tops of ravines (southern border of the Forest-Steppe ecoregion). The methods of light, laser confocal, transmission and scanning electron microscopy were used.

It was found that leaves of a solitary tree on the ravine top differed from those of community area with the xeromorphic traits: a decreased size, an increased density of trichomes, and thickening of the periclinal walls of the adaxial epidermal cells, which obviously contribute to protecting leaves from direct sunlight and optimizing the water balance. Silicon fluorescence was detected in cell walls of the adaxial and abaxial epidermis, trichomes and stomatal guard cells, that suggests its contribution to the regulation of transpiration and light flux absorption, wherein the silicon content increased in leaves of a single tree. The presence of well developed granum system is a characteristic feature of chloroplasts in all investigated objects. Giant grana up to 2 μm containing almost 50 thylakoids were found. Rare starch grains are characteristic of chloroplasts in oaks from the community, with its almost complete absence in the leaves of solitary trees growing away from the community on the top of a ravine. The low starch content of the latter is consistent with the data of Bessonova and Chonkhova (2022) on a low starch with a simultaneous increase in the soluble sugars concentration in the steppe oak leaves (Dnipropetrovsk region) under drought compared to more abundant water supply. The obtained results indicate micromorphological and ultrastructural reactions of mature oak leaves to aridization, including an increase in the partial volume of thylakoids, an expansion of the vacuolar system, a decrease in the size of organelles which may be due to water and carbon deficiency.