Ecological significance of C(4)-like photosynthesis in the submersed aquatic angiosperm Hydrilla verticillata.

Abstract

Hydrilla verticillata (L.F.) Royle exhibits facultative C$\sb4$-like photosynthesis. A laboratory and field physiological study in north-central Florida was designed to investigate the spatial and seasonal patterns in photosynthetic physiology and environmental quality, the contribution of C$\sb4$-like photosynthesis to carbon gain, and the relationship between photosynthetic physiology and plant density in Hydrilla. Field observations demonstrated steep horizontal gradients in water chemistry across a Hydrilla population. Distinct mat and edge micro-habitats were identified. The mat habitat exhibited higher plant biomass, greater midday depletion of CO$\sb2$ and HCO$\sb3\sp-$ and supersaturation of dissolved O$\sb2$. The edge habitat exhibited lower biomass, no supersaturation of dissolved O$\sb2$ or depletion of CO$\sb2$ and HCO$\sb3\sp-$. Hydrilla C$\sb4$-like phenotypes were only observed in the mat. A reciprocal transplant experiment demonstrated that C$\sb4$-like mat plants exhibited 66% greater carbon gain than C$\sb3$ edge plants when measured in the mat habitat. Mat and edge plants performed equally well in the edge habitat. The mat habitat reduced carbon gain of both mat and edge plants over 80%. Mat plants exhibited a 60% greater photosynthetic nitrogen-use efficiency (PNUE) compared to edge plants when measured in the mat habitat. Laboratory experiments demonstrated that induction of C$\sb4$-like gas exchange characteristics in Hydrilla was associated with increased phosphoenolpyruvate carboxylase activity. Increased carbon gain of C$\sb4$-like plants resulted from reduction in both photorespiration and dark respiration. Induction of C$\sb4$-like photosynthesis was associated with high plant density and reduced CO$\sb2$ concentration, and increased pH and dissolved O$\sb2$ concentration. These data demonstrated that facultative C$\sb4$-like photosynthesis enhanced carbon gain under sub-optimal growth conditions. The evolution of C$\sb4$-like photosynthesis in Hydrilla was potentially selected by stressful conditions in the mat habitat. Reduced quantum yield of mat plants compared to edge plants explained the facultative nature of C$\sb4$-like photosynthesis in Hydrilla. C$\sb4$-like photosynthesis enabled Hydrilla to maintain carbon gain during CO$\sb2$ depletion and partially explained its competitive success in freshwater systems in north-central Florida.