Partial flooding enhances aeration in adventitious roots of black willow (Salix nigra) cuttings
Introduction
In wetlands, soil flooding initiates a chain of reactions leading to reduced soil oxidation–reduction potential (Eh). Flood-tolerant plants use several strategies to cope with low soil Eh conditions. Aerenchyma development has been considered as a mechanism critical to plant's ability to cope with anaerobiosis. This gas transport system allows plants to transport the atmospheric O2 to the underground organs to maintain aerobic respiration and to oxidize various reducing compounds in the rhizosphere (Pezeshki, 2001). In some species, poor aeration increases aerenchyma formation and hence porosity (Burdick, 1989; Laan et al., 1989; Armstrong et al., 1994). Aerenchyma forms in roots either lysigenously by cell separation and collapse or schizogenously by cell separation without collapse (Armstrong et al., 1991).
Rhizosphere oxygenation by radial oxygen loss (ROL) from roots is also of great importance for wetland plants to overcome anaerobic conditions. High correlation was found between ROL and soil Eh intensity. It was reported that soil redox potential of –250 mV resulted in enhancement of ROL as much as 3-fold in Taxodium distichum compared with those under well-aerated condition (Kludze et al., 1994b). The radial diffusion of O2 can immobilize or detoxify potential soil toxins including acetic and butyric acids produced by microbial metabolism and soil-reducing compounds (Armstrong, 1979; Mckee et al., 1988; Kludze et al., 1994b). ROL from some plants has been shown to increase soil Eh, which enables those plants to survive in an otherwise anoxic condition (Tessnow and Baynes, 1978). It also supports aerobic nitrifying or nitrogen fixing bacteria in the rhizosphere (Hoffmann, 1990; Ueckert et al., 1990). The potential for nitrification in the rhizosphere is a major consideration underlying the current use of wetlands for the purification in both natural and artificial effluents (Reddy et al., 1989; Armstrong et al., 1994). In addition, ROL from roots to rhizosphere inhibits methanogenesis, promotes methane (CH4) oxidations and thus reduces potential efflux of CH4 from the plants. Wetlands contribute 40–50% of total emission of CH4 to the atmosphere, which accounts for 7–9% of global warming (Armstrong and Armstrong, 2001).
Numerous studies have been conducted to investigate the effects of flooding on root aerenchyma formation and ROL for plants developed from seeds. However, little is known about these processes in adventitious roots that are developed on cuttings of woody species. Jackson and Attwood (1996) evaluated aerenchyma formation of Salix viminalis cuttings waterlogged for 4 weeks and noted that aerenchyma in the upper roots (100 mm) was enhanced by flooding. But it appears that no data are available regarding aeration of the root system under partially flooded conditions. This experiment was designed to quantify the effects of continuous flooding (CF) and partial flooding (PF) on root aerenchyma formation and ROL in black willow (S. nigra) cuttings under laboratory condition. Black willow is commonly found in floodplains and bottomland hardwood forests of the southeastern United States (Mitsch and Gosselink, 1993). Cuttings of this species are extensively used as planting material for soil stabilization, erosion control, and habitat rehabilitation along highly eroded streambanks (Schaff et al., 2003). Black willow is subjected to dynamic hydrologic conditions in riparian systems. Depending on the slope and depth to base flow, it may be exposed to continuously flooded or partially flooded conditions. Herein we use the expression “partially flooded” to refer to sites where the water table elevation is above much of the root zone. We expected that black willow would be under strong selective pressure to adjust morphologically and physiologically to both continuously flooded and partially flooded conditions in the riparian systems. We hypothesized that: (1) CF would enhance both root porosity (POR) and ROL; (2) roots located in the flooded zone of the PF treatment would have similar aerenchyma formation to those under continuously flooded condition, while those in drained zone should have similar porosity to those in well-aerated condition (i.e., under partially flooded condition, roots located in drained zone would be likely to have less aerenchyma tissue developed than those grown in flooded zone). PF would also stimulate ROL from roots (both zones combined); (3) due to these morphological responses, minimal disruption of photosynthetic and growth parameters would be shown under CF and PF, which would be indicated by no reduction in net photosynthesis (), stomatal conductance (gs), chlorophyll content (Chl), height growth, and biomass; and (4) no change would be detected in chlorophyll fluorescence parameters such as dark fluorescence yield (F0), efficiency of excitation capture of open photosystem II (PS II; ) in dark-adapted leaves and yield of energy conversion (Y), which suggests the resilience of PS II to flooding.
Section snippets
Plant materials
Black willow (Salix nigra Marshall) cuttings were collected from a localized population on the Loosahatchie River in western Tennessee, USA on June 1, 2004. Each cutting was 0.5 cm in diameter at the base and 25 cm in length. All existing branches were removed from each cutting to conform to common planting practices.
Experimental procedures
Cuttings were planted on June 2 in a laboratory. Pots 25 cm deep and 5 cm in diameter were constructed of PVC pipe and filled with two parts sand and one part soil (v/v) up to 20 cm.
Soil measurements
Soils in all groups were aerated on day 0 before the initiation of treatment. Soil Eh in C remained above +493 mV for the duration of the experiment, indicating oxic condition in this treatment. But soil was reduced in CF after the treatment initiation and the level of soil Eh in this treatment remained in the mildly reduced range (−4 to −104 mV) for the duration of the study. In PF treatment, soil was oxic in the upper drained portion (above +455 mV) but mildly reduced in the lower flooded zone
Discussion
Results from this study indicated that S. nigra, like many other wetland species (Smirnoff and Crawford, 1983), is capable of constitutively developing extensive aerenchyma tissue even in drained roots (Fig. 2). In some species, root aerenchyma is further promoted by hypoxia (Justin and Armstrong, 1987; Jackson and Attwood, 1996; Chen et al., 2002; Colmer, 2003) to enhance internal transfer of atmospheric or photosynthetic O2 between the plant parts above the water and the flooded tissues and
Acknowledgements
The authors gratefully acknowledge the following colleagues for their assistance with data collection during the course of conducting this experiment: Don Baud, Janice Roberts, and Wei Wang. This work was supported in part by a grant from The University of Memphis Faculty Research Grant Fund. This support does not necessarily imply endorsement by the University of research conclusions. Additional funding was provided from USDA-ARS National Sedimentation Laboratory, Cooperative Agreement No.
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