Photosynthetic consequences of phenotypic plasticity in response to submergence: Rumex palustris as a case study.

Survival and growth of terrestrial plants is negatively affected by complete submergence. This is mainly the result of hampered gas exchange between plants and their environment, since gas diffusion is severely reduced in water compared with air, resulting in O2 deficits which limit aerobic respiration. The continuation of photosynthesis could probably alleviate submergence-stress in terrestrial plants, but its potential under water will be limited as the availability of CO2 is hampered. Several submerged terrestrial plant species, however, express plastic responses of the shoot which may reduce gas diffusion resistance and enhance benefits from underwater photosynthesis. In particular, the plasticity of the flooding-tolerant terrestrial species Rumex palustris turned out to be remarkable, making it a model species suitable for the study of these responses. During submergence, the morphology and anatomy of newly developed leaves changed: 'aquatic' leaves were thinner and had thinner cuticles. As a consequence, internal O2 concentrations and underwater CO2 assimilation rates were higher at the prevailing low CO2 concentrations in water. Compared with heterophyllous amphibious plant species, underwater photosynthesis rates of terrestrial plants may be very limited, but the effects of underwater photosynthesis on underwater survival are impressive. A combination of recently published data allowed quantification of the magnitude of the acclimation response in this species. Gas diffusion resistance in terrestrial leaves underwater was about 15,000 times higher than in air. Strikingly, acclimation to submergence reduced this factor to 400, indicating that acclimated leaves of R. palustris had an approximately 40 times lower gas diffusion resistance than non-acclimated ones.