The influence of convective flow on rhizome length in Typha domingensis over a water depth gradient

We present direct evidence for an ecological role of convective flow in the emergent macrophyte Typha domingensis Pers. A pond experiment was conducted to examine the influence of convective flow on rhizome length at water depths ranging from 5 to 65 cm. The leaves of half of the plants were pierced while the remaining plants were used as a control. Leaf piercing had the effect of preventing both internal pressurisation and convective flow. The plants responded to increasing water depth by altering internal morphology to permit greater rates of aeration. The lacunal cross-sectional area of the rhizome and leaf was greater in plants grown in deeper water. This coincided with a decrease in the internal resistance to both diffusion and bulk flow in the leaves and rhizomes with increasing water depth. However, there was no significant difference between the pierced and intact plants; both showed similar trends in internal resistance and lacunal cross-sectional area. This suggests the plants were not responding to oxygen supply per se but some other factor associated with water depth. Since the internal resistance in blind rhizomes was too high to allow any convective flow (mean resistance 1.1x10(9) Pa s m(-3)), diffusion must supply oxygen to the rhizome terminus. The mean rhizome length of intact plants was significantly longer at each depth compared with the rhizome lengths of pierced plants (P<0.05). At 5 cm the rhizome length was 32.1 and 25.8 cm for intact and pierced plants, respectively; at 65 cm the rhizome lengths were 40.3 and 28.2 cm, respectively. Due to the absence of convection in treatment plants, the diffusive pathlength includes the length of the portion of the culm below the water surface as well as the length of the rhizome. Hence, the total diffusive resistances of the pierced plants increased with water depth whereas there was no significant change in the diffusive resistances of control plants. It was concluded that convective flow plays an important role in increasing the capacity for rhizome extension by decreasing the diffusion pathlength especially in deep water. (C) 1998 Elsevier Science B.V. All rights reserved.