Abstract
Hygrophila difformis, a heterophyllous amphibious plant, develops serrated or dissected leaves when grown in terrestrial or submerged conditions, respectively. In this study, we tested whether submerged leaves and ethylene-induced leaves of the heterophyllous, amphibious plant H. difformis have improved photosynthetic ability under submerged conditions. Also, we investigated how this amphibious plant photosynthesizes underwater and whether a HCO3− transport system is present. We have analysed leaf morphology, measured underwater photosynthetic rates and HCO3− affinity in H. difformis to determine if there are differences in acclimation ability dependent on growth conditions: terrestrial, submerged, terrestrial treated with ethylene and submerged treated with an ethylene inhibitor. Moreover, we measured time courses for changes in leaf anatomical characteristics and underwater photosynthesis in terrestrial leaves after submersion. Compared with the leaves of terrestrially grown plants, leaf thickness of submerged plants was significantly thinner. The stomatal density on the abaxial surface of submerged leaves was also reduced, and submerged plants had a significantly higher O2 evolution rate. When the leaves of terrestrially grown plants were treated with ethylene, their leaf morphology and underwater photosynthesis increased to levels comparable to those of submerged leaves. Underwater photosynthesis of terrestrial leaves was significantly higher by 5 days after submersion. In contrast, leaf morphology did not change after submergence. Submerged leaves and submerged terrestrial leaves were able to use bicarbonate but submerged terrestrial leaves had an intermediate ability to use HCO3− that was between terrestrial leaves and submerged leaves. Ethoxyzolamide, an inhibitor of intracellular carbonic anhydrase, significantly inhibited underwater photosynthesis in submerged leaves. This amphibious plant acclimates to the submerged condition by changing leaf morphology and inducing a HCO3− utilizing system, two processes that are regulated by ethylene.
Highlights
Higher land plants do not grow under submerged conditions and aquatic plants, such as Egeria densa, cannot grow in terrestrial growth conditions
H. difformis acclimates to a submerged environment in the long term by developing leaves with a characteristic morphology that are capable of photosynthesis optimized for the submerged environment
Results from the pH drift experiment and measurement of underwater photosynthesis at different pH values showed that submerged leaves and submerged terrestrial leaves were able to use bicarbonate but submerged terrestrial leaves had an intermediate ability to use HCO3− that was between terrestrial leaves and submerged leaves (Fig. 6A and C)
Summary
Higher land plants do not grow under submerged conditions and aquatic plants, such as Egeria densa, cannot grow in terrestrial growth conditions. On. Horiguchi et al – Terrestrial and submerged leaves in the amphibious plant. Hygrophila difformis, a heterophyllous amphibious plant, develops serrated and dissected leaves when grown in terrestrial and submerged conditions, respectively. Dissected leaves develop when H. difformis is treated with ethylene under terrestrial growth conditions (Li et al 2017). This morphological change is regulated by ethylene, it remains unknown whether ethylene induces the leaves to functionally acclimate with the underwater condition. Total inorganic carbon, the substrate for photosynthesis, exists as CO2 in the terrestrial condition but dissolution of carbon in water provides CO2 and bicarbonate (HCO3−) and carbonate (HCO32−) ions. The growth rates of aquatic plants are different depending on the DIC constituents in the environment (Hussner et al 2016; Dülger and Hussner 2017)
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