Responses of CO2 concentrating mechanisms to variable CO2 in different leaf regions of Vallisneria natans and V. spinulosa

Abstract Vallisneria are submerged macrophytes characterized by linear or banded leaves, and the capacity for both bicarbonate (HCO 3 − ) use and crassulacean acid metabolism (CAM). A notable feature of Vallisneria leaves is the presence of a maturity gradient from the tip to the base. However, detailed studies on the variation of CO 2 -concentrating mechanisms (CCMs) within Vallisneria leaves from the tip to the base remain scarce. In this study, two common Vallisneria plant, Vallisneria natans and V. spinulosa , were selected to investigate how the tip and the base of the leaves respond to variable CO 2 . The pH-drift data showed that the tip and the base of both V. natans and V. spinulosa could use HCO 3 − under high CO 2 (HC) and low CO 2 (LC) treatments. Notably, the capacity for HCO 3 − use was significantly higher in the leaf tip compared to the base in both species, regardless of the [CO 2 ]. Moreover, the mode of HCO 3 − utilization was identical between the tip and the base in both V. natans and V. spinulosa and was independent of [CO 2 ]. Titratable acidity results indicated that both the tip and the base of V. natans and V. spinulosa performed CAM metabolism under LC treatment. Compared to the base, the diurnal acidity change was significantly higher at the tip under LC in both V. natans and V. spinulosa . The chlorophyll contents of V. natans and V. spinulosa in the tip region under HC treatments were significantly higher than those in the base under LC treatments. These findings reveal the spatial heterogeneity of CCMs within Vallisneria leaves, enhancing our understanding of how these species acclimate to CO 2 -fluctuating freshwater.