Photosynthetic and Physiological Characteristics of the Evergreen Ligustrum japonicum and the Deciduous Cornus officinalis

The evergreen, broad-leaved, Ligustrum japonicum and the deciduous Cornus officinalis are distributed in the central and southern parts of Korea and are widely used as landscape plants. In this study, we observed changes in the physiological characteristics of the two species across seasons by observing the changes in leaf photosynthesis; chlorophyll fluorescence; chlorophyll, carotenoid, and carbohydrate contents, osmolality and total ions. With a low soil moisture content due to low rainfall in June, the deciduous C. officinalis showed a 0.3 mmol m⁻² s⁻¹ stomatal conductance and open stomata allowing for photosynthesis. However, the evergreen L. japonicum showed a 0 mmol m⁻² s⁻¹ stomatal conductance and almost completely closed stomata, indicating a high water use efficiency. Cornus officinalis exhibited a higher leaf net photosynthetic rate (P_N), stomatal conductance (gs), transpiration (E), and carbon fixation efficiency (CE) than L. japonicum in all months except for November at which time its leaves fell from senescence. In winter, L. japonicum showed a dramatic decrease in its photosynthetic rate and F_v/F_m ratio under conditions of lower than average air temperature and soil moisture; furthermore, it showed a high ratio of the photochemical quantum yield Y(NO) and almost no photosynthetic activity despite it having leaves. Ligustrum japonicum appears to be tolerant to water stress and low temperature by a maintenance of osmosis through the increase and accumulation of soluble carbohydrates in winter. In conclusion, the two species showed different physiological characteristics under a water stress environment.