Morphological and physiological responses of Arundo donax and Phragmites australis to waterlogging stress
Introduction
Arundo donax L. (giant reed) and Phragmites australis (Cav.) Trin. Ex Steudel (common reed) are tall perennial, rhizomatous grasses (Poaceae). The erect culms of A. donax are 6 to 10 m tall, while those of P. australis are 2–6 m. Arundo resembles an outsized P. australis. Both are highly productive, successful, cosmopolitan species (Eller et al., 2017; Pompeiano et al., 2019). Phragmites has high genetic diversity and phenotypic plasticity and spreads rapidly through vegetative and sexual reproduction (Lambertini et al., 2008; Kettenring et al., 2015). Arundo has low genetic diversity, the seeds are rarely fertile and reproduction is vegetative (Saltonstall et al., 2010). The lineage of P. australis that is native to sub-Saharan Africa is “haplotype Med” (Canavan et al., 2018), while A. donax is an invasive weed, originally from India (Nackley et al., 2014). Both are robust and form monospecific stands in a wide range of freshwater and brackish wetlands in temperate and tropical regions (Pompeiano et al., 2019).
Both Arundo and Phragmites provide many ecosystem services. The stems of both species are used as building material. The leaves are used as fodder for livestock, as roof-thatching material and for weaving mats (Dudley, 2000). Both are used for erosion control as they possess extensive root systems and help maintain wetlands. The woody, hollow stems of A. donax are used to manufacture flutes, organ pipes and woodwind instruments. Arundo is also a leading candidate crop for the production of energy, fuels and chemicals due to its high biomass production (Rossa et al., 1998; Fabbrini et al., 2019). Phragmites is used in constructed wetlands for waste water treatment, while in disturbed wetlands, it facilitates hydrological succession and wetland restoration (Eller et al., 2017; Canavan et al., 2018). Populations of Arundo and Phragmites expand rapidly and adversely affect water resources, biodiversity and ecosystem integrity. Both reeds have a competitive advantage over other species through efficient lateral expansion and clonal reproduction (Canavan et al., 2018).
In this study, A. donax and P. australis were chosen as the experimental species because both are invasive, share similar habitats, form dense monospecific stands and compromise the integrity of ecosystems. When these species co-occur, there is competitive interaction for space and resources. We compared morphological and physiological responses of A. donax and P. australis, grown either singly or together in pots, to various soil moisture regimes. The species that is more adaptable to soil moisture will have a competitive advantage for survival and expansion in wetlands. Morphometric, gas exchange, chlorophyll fluorescence and soil characteristics were used to measure adaptation to soil moisture. The waterlogging tolerance coefficient (WTC) was used to measure differences between the species.
Section snippets
Plant material and experimental design
Young culms (ca. 6 cm) of P. australis and A. donax were collected from the Palmiet Nature Reserve, Durban, South Africa (29° 82’ S, 30° 95’ E) and planted in potting trays containing loamy soil. Plants were maintained in an air-conditioned glasshouse set at 25°C (day), 20°C (night), relative humidity 70% and maximal photosynthetic photon flux density (PPFD) of 820 µmol m−2 s−1. After four weeks, plants with an average height of 10–15 cm were transferred to 20 L plastic pots containing a loamy
Soil redox potential (Eh)
Soil Eh values were pooled to determine whether treatments were effective in creating a broad range of conditions from oxidized to reduced (Fig. 1). Mean soil Eh in the WD treatments was between +58 to +83 mV, indicating moderately reduced conditions. Waterlogging significantly reduced soil Eh compared to the WD treatment. Mean soil Eh ranged from +5 to +15 mV in the ½F treatments and from -80 to -83 mV in the F treatments. Soil Eh was negatively correlated with soil moisture (r = -0.92, P <
Discussion
The negative relationship between soil Eh and soil moisture, in plants grown separately and together, indicated that Eh decreased significantly as soil moisture increased. Mean soil Eh decreased with increase in soil moisture (+83 to -83 mV), suggesting that soils were moderately reduced and hypoxic in the waterlogged treatments (Patrick and DeLaune, 1977). The Eh data indicated that the waterlogged treatments were sufficiently reduced for the formation of products of anaerobic metabolism such
Conclusions
This study demonstrated that the responses of Arundo donax and Phragmites australis, grown separately or together, to waterlogging stress were similar. These responses included increases in culm density, shoot elongation, aerenchyma development, biomass accumulation and production of above-ground adventitious roots. Under prolonged waterlogging, the capacity of the plant for ventilation of below-ground organs is exceeded, resulting in hypoxia and reduced growth. These adaptations to
Credit author statement
LT performed the experiments and obtained the data with supervision from KKN and GN.
All authors contributed equally to the analyses of the data and the writing of the manuscript.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This research was sponsored by the National Research Foundation (grant number 93560 to G Naidoo) and the University of KwaZulu-Natal.
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