Different CO2 acclimation strategies in juvenile and mature leaves of Ottelia alismoides
The freshwater macrophyte, Ottelia alismoides, is a bicarbonate user performing C4 photosynthesis in the light, and crassulacean acid metabolism (CAM) when acclimated to low CO2. The regulation of the three mechanisms by CO2 concentration was studied in juvenile and mature leaves. For mature leaves, the ratios of phosphoenolpyruvate carboxylase (PEPC) to ribulose-bisphosphate carboxylase/oxygenase (Rubisco) are in the range of that of C4 plants regardless of CO2 concentration (1.5–2.5 at low CO2, 1.8–3.4 at high CO2). In contrast, results for juvenile leaves suggest that C4 is facultative and only present under low CO2. pH-drift experiments showed that both juvenile and mature leaves can use bicarbonate irrespective of CO2 concentration, but mature leaves have a significantly greater carbon-extracting ability than juvenile leaves at low CO2. At high CO2, neither juvenile nor mature leaves perform CAM as indicated by lack of diurnal acid fluctuation. However, CAM was present at low CO2, though the fluctuation of titratable acidity in juvenile leaves (15–17 µequiv g−1 FW) was slightly but significantly lower than in mature leaves (19–25 µequiv g−1 FW), implying that the capacity to perform CAM increases as leaves mature. The increased CAM activity is associated with elevated PEPC activity and large diel changes in starch content. These results show that in O. alismoides, carbon-dioxide concentrating mechanisms are more effective in mature compared to juvenile leaves, and C4 is facultative in juvenile leaves but constitutive in mature leaves.
KeywordsBicarbonate use C4 metabolism Carbon dioxide-concentrating mechanism (CCM) Crassulacean acid metabolism (CAM) Freshwater macrophyte Leaf maturity
Crassulacean acid metabolism
Carbon dioxide-concentrating mechanism
Concentration of total inorganic carbon
Pyruvate phosphate dikinase
Ribulose 1,5-bisphosphate carboxylase-oxygenase
This research was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB31000000), the Chinese Academy of Sciences President’s International Fellowship Initiative to SCM and BG (2015VBA023, 2016VBA006) and the National Key Research and Development Program of China (2016YFA0601000). We thank the two reviewers for their helpfull comments.
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