| Abstract |
[Truncated abstract] A common response of plants to flooding is the formation of an adventitious root system. These stem-borne roots can stay suspended in the water column (aquatic roots), or grow down into the sediment. Previous research has focused on features of sediment adventitious roots, such as aerenchyma and radial O2 loss that enhance root function in anaerobic soil. Unlike sediment adventitious roots, aquatic adventitious roots grow in aerobic floodwaters and have the potential to form photosynthetically active chloroplasts, and, therefore, have additiional O2 sources to that supplied via the aerenchyma. This thesis contains a series of experiments investigating full and partial submergence tolerance and aquatic root growth and physiology in two herbaceous perennials, Cotula coronopifolia L. and Meionectes brownii (Hook. f.). Both species tolerated 4 weeks of complete submergence in glasshouse experiments and grew an extensive aquatic adventitious root system from submerged stems when partially or completely submerged. Aquatic root systems were a major constituent of the total plant drymass (up to 26.3 ± 2 % in C. coronopifolia and 20.8 ± 1 % in M. brownii, in partially submerged plants) and contributed up to 90% of the total root drymass. Aquatic roots potentially confer some benefits to plant health and growth during flooding. An aquatic root pruning experiment conducted on both completely and partially submerged plants found, compared to controls with intact aquatic roots, reduced relative growth rates in both completely and partially submerged plants and reduced stem and leaf Chla in completely submerged plants when these roots were removed. |
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The effects on aquatic root growth of submergence waters of various light availabilities (mean daily PAR approximately 5 μmol m-2 s-1 to 600 μmol m-2 s-1) were also tested for partially-submerged plants. In both species roots at the lowest PAR contained less chlorophyll (2-fold less in M. brownii and 3-fold less in C. coronopifolia), but there was no difference in aquatic root drymass after four weeks regardless of light availability in the water column (shoots above-water illuminated); C. coronopifolia gained 0.06 ± 0.01 g DM d-1 of aquatic roots while M. brownii gained 0.14 ± 0.01 g DM d-1. The length and diameter of these aquatic roots also did not differ with different PAR in the water column. M. brownii was chosen as the focus species for investigations of O2 dynamics in aquatic roots. Like sediment roots, aquatic roots can source their O2 via diffusive movement from the shoot. In addition, due to their growth into illuminated floodwater, aquatic roots can also potentially produce endogenous O2 via photosynthesis and utilise dissolved O2 from the water column. The aquatic roots of M. brownii contain a complete photosynthetic pathway... |
