Drought is an increasing problem in many parts of the world (IPCC, 2007). Australia is the driest inhabited continent with the vast majority of the land receiving less than 600 mm of rain each year (Bowman, 2000) and declining as a result of climate change. Although Australia's population is mostly concentrated in major urban zones along the higher rainfall seaboards, even these areas are affected by drought (McWilliams, 1986). Similarly, the rapidly growing western United States is being affected by drought and climate change (Barnett et al., 2004). In both Australia and the western United States, up to half of municipal water supplies are applied to urban landscapes (AWRA, 2000); thus, areas such as irrigated urban parks, gardens, and sports fields are increasingly targeted for water conservation.
Low water-requiring or “water-wise” urban landscaping is an increasingly important policy tool in arid/semiarid and water-limited areas in Australia and the western United States (Hurd et al., 2006, Kjelgren et al., 2000). Water-wise landscaping can maintain acceptable appearance during drought when designed with more drought-tolerant plants than turf and when irrigation is zoned by plant water needs (Mee et al., 2003). Appropriate species selection for water-wise landscaping can be improved with knowledge of drought tolerance and water stress response mechanisms among candidate species (Kjelgren et al., 2000). Drought and water stress response mechanisms have been well studied in woody forest (Crombie et al., 1988) and annual crop species (Subbaro et al., 1995). This general knowledge provides the physiological framework (Maseda and Fernandez, 2006) for understanding drought and water stress responses when combined with appropriate measures of performance for specific urban landscape species (e.g., He and Joyce, 2007).
Herbaceous perennial wildflowers dominate arid regions of Australia (Johnston and Joyce, 2006) and the western United States (Mee et al., 2003), and thus are a rich source of drought-tolerant plants for water-wise landscaping. Characterizing herbaceous wildflower water deficit stress response mechanisms can inform selection of drought-tolerant herbaceous perennial species appropriate for urban landscapes (Chapman and Auge, 1994). Herbaceous perennials that avoid drought through wilting, dormancy, or dieback are less acceptable than species that tolerate drought by maintaining intact foliage (Zollinger et al., 2006). Some drought tolerance responses are better suited to urban landscapes. Deep-rooted, drought-avoiding species that become stressed and lose visual quality in shallow urban soils would be less suitable than species that withstand drought through stomatal closure and dehydration tolerance (Stewart et al., 2004).
Integrating the overall drought responses and performance measures in the context of duration and severity of urban drought is an important consideration on a species or even a cultivar level (Maseda and Fernandez, 2006). The isohydric:anisohydrogic model provides a potentially useful conceptual framework for understanding different drought response strategies (Schultz, 2003) in assessing performance and enhancing herbaceous species selection for low water-requiring landscapes. Anisohydric species tolerate greater soil water depletion by maintaining open stomata and generating more negative internal water potential while scavenging soil water under water stress, particularly if deep-rooted (West et al., 2007). However, during prolonged drought, anisohydric species risk cavitation damage and are slow to recover and can outcompete and suppress adjacent species (West et al., 2008). Isohydric species maintain less negative internal water potentials through partial stomatal closure during drought to avoid cavitation (West et al., 2007) but at the cost of reduced photosynthesis (West et al., 2008). Isohydric species may be a more conservative and hence appropriate choice for low-water landscapes with variable soil and water conditions.
Australia has an abundance of seemingly drought-tolerant native wildflower species in its vast interior arid zones. Many of these species have desirable ornamental qualities and apparent drought tolerance and are thus candidates for water-wise landscaping in Australia and overseas (Johnston and Joyce, 2006). However, little is known about the water stress response mechanisms and performance of these species. We investigated water deficit stress in three herbaceous Australian native ornamental species, two originating from dry regions and one common to southeast Asia rainforests, in terms of isohydric/anisohydric behavior under isolated and competitive growing environments.
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