Metabolism and distribution of grasses in tropical flooded savannas in Venezuela

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Abstract
Flooded savannas in south-west Venezuela consist of seasonal, marshy and flooded grassland communities, the distribution of which depends on soil level. Seasonal grasslands are dominated by tuft-forming, rhizomatous C4-grasses (exemplified byPaspalum chaffanjoniiin this study), while flooded grasslands are dominated by stoloniferous grasses which develop rooted, floating culms during the rainy season(Leersia hexandraandHymenachne amplexi-caulis).The interface between these two communities is a marshy grassland dominated by stoloniferous grass species which tolerate flooding to a depth of 10 to 25 cm(Panicum laxumandLeersia hexandra).All perennial grass species in flooded grasslands behave as typical C3 plants, while marshy grasslands are dominated byPanicum laxum,a species with reduced photo-respiration, low RuBP/PEP-carboxylase ratios, and low absolute RuBP-carboxylase activity compared to C3 grasses. It also has lower photosynthetic rates than the other grass species.Hymenachne amplexicaulisappears to be the least drought tolerant from the species selected, in accordance with its distribution in the wettest side of the flooding gradient studied. The other species showed marked reduction in relative water content and pronounced increase in leaf proline content in drought experiments lasting 6 to 30 days. Alcohol dehydrogenase increased markedly in response to anaerobiosis in the root environment in the tuft forming grasses, while the stoloniferous species with ascending culms were least affected by this treatment, probably as a result of better aeration inside the culms and also to the production of adventitious roots in upper nodes. Nitrate reductase activity increased as a result of anaerobiosis in the roots but not in the leaves, of all species exceptLeersia hexandra.No difference among the species was found in malate accumulation, or the activity of malic enzyme.