Ecological Plasticity of the Photosynthetic Apparatus of Hibiscus syriacus L. under Pressure of High Temperature, Insolation, and Air Pollution

This paper describes the integrated effect of high summer temperature, intensive insolation, gas pollution, and dust in the air on the pigment content and net oxygen production (apparent photosynthesis) and dark respiration rates of common hibiscus (Hibiscus syriacus L.) leaves. The study included three observation sites: Tashkent Botanical Garden (Academy of Sciences, the Republic of Uzbekistan), the public garden in the central part of the city of Tashkent, and a mountain holiday camp. The research was carried out in 2017 during the period of active vegetation. The results of experiments showed a high adaptive potential of H. syriacus L., as well as its adaptiveness to stress environmental factors of the semiarid zone, provided a sufficient level of its irrigation. The ecological plasticity of the photosynthetic apparatus of hibiscus plays a key role in species adaptation to environmental conditions. It has been revealed that the growth of the plants of the Tashkent Botanical Garden under shading conditions leads to the formation of large, wide, thin, shade leaf blades, which can be considered as the manifestation of sciomorphosis. Heliomorphosis features of hibiscus leaves were identified in mountains, where sun leaves with significantly smaller, thickened, and compacted blades are formed under high insolation. The adaptive significance of these structural leaf modifications is to strengthen photosynthetic capacity for compensating the deficiency in sunlight (in the case of sciomorphosis), and, on the contrary, to provide mutual shading for photosynthetic elements as a measure of protection against the damaging effect of redundant solar radiation (in the case of heliomorphosis). This provides carbon dioxide assimilation and organic matter production for maintaining the constant energy balance for the plant under different environmental conditions. The study of the temperature correlation of dark respiration and net oxygen production rates has shown that these processes are more resistant to temperature injuries under more extreme environmental conditions. In addition, mature H. syriacus leaves have a higher resistance than young leaves; i.e., the plant adapts to possible temperature drops in the process of its ontogenesis. Taking into account the optimal measuring conditions, the net oxygen production rate of H. syriacus during the period of active vegetation is approximately at the same level under different growing conditions (0.20 ± 0.05 μmol of O₂/(dm² s)); this is considered a norm of reaction of the net production (apparent photosynthesis) rate of H. syriacus and determines the specific features of its photosynthetic apparatus.