Drought is one of the critical environmental stresses that affect the establishment, survival, growth, and performance of shrubs and trees in urban and suburban landscape environments (Cregg, 2004; Fernández et al., 2006). Watering restrictions, which are becoming more common in many parts of the world, exacerbate the effect of drought stress on the establishment and survival of these plants. One strategy to improve the landscape may be the selection of drought-tolerant plants.
Plants express various responses to drought stress and develop a wide range of mechanisms from morphological to physiological aspects. For example, the smaller, thicker leaves with thicker cuticles, more cuticular wax, and a higher specific leaf mass in Mexican redbud [Cercis canadensis L. var. mexicana (Rose) M. Hopk.] contribute to water-conserving capability and survival in arid and semiarid regions compared with eastern redbud (C. Canadensis; Tipton and White, 1995). When plants are under drought-stressed conditions, relative water content, water potential, and turgor of cells decrease and the concentration of ions and other solutes in the cells increase, thereby decreasing the osmotic adjustment (Tezara et al., 1999). However, in a study by Björkman et al. (1980), there was no osmotic adjustment in oleander (Nerium oleander L.) plants under drought-stressed conditions.
Photosynthesis is the primary process for plant biomass production and is one of the physiological processes most sensitive to environmental stresses (Hsiao and Acevedo, 1974; Huang, 2004). Consequently, the ability to maintain a reasonable rate of photosynthesis under stressful conditions can be a good indicator of a plant's adaptability. A drought-resistant cultivar Koroneiki of olive (Olea europea L.) performed better and was able to maintain higher leaf photosynthetic rate under high air vapor pressure deficit compared with other green olive cultivars (Hagidimitriou and Pontikis, 2005).
Chlorophyll fluorescence has been routinely used to detect and quantify the tolerance of plants to various stresses (Percival and Sheriffs, 2002; Willits and Peet, 2001). However, the effect of drought stress on chlorophyll fluorescence values is contradictory, possibly because of differences in the severity of drought stress and species. For example, chlorophyll fluorescence values of field-grown papaya (Carica papaya L.; Marler and Mickelbart, 1998), bigtooth maple (Acer grandidentatum Nutt.; Bsoul et al., 2006), and a number of greenhouse-grown herbaceous plants (Starman and Lombardini, 2006) were not influenced by drought stress. However, F0 of banana (Musa L.) leaves increased and Fm decreased under drought stress (Thomas and Turner, 2001).
Variations exist in characteristics of drought tolerance among cultivars (Zwack and Graves, 1998), seed sources (Cregg and Zhang, 2001), and provenance (Bsoul et al., 2006; St. Hilaire and Graves, 2001). These variations in response to stress among genotypes offer opportunities for selecting plants with optimal characteristics for high rates of photosynthesis and productivity. For example, by characterizing the gas exchange in response to temperature among genotypes and provenance, heat tolerant genotypes and cultivars have been selected for holly (Ilex L.; Ranney and Ruter, 1997), rhododendron (Rhododendron hyperythrum Hayata; Ranney et al., 1995), and raspberry (Rubus idaeus L.; Stafne et al., 2000). It is generally considered that genotypes originating from xeric environments can tolerate drought stress better than those from mesic environments. St. Hilaire and Graves (2001) concluded that maple (Acer L.) taxa from more xeric provenances are more likely to perform better in arid environments than trees from mesic environments. In addition, seedlings of Pinus sylvestris L. from drier central Asian seed sources survived drought stress longer than seedlings from more mesic European and coastal seed sources (Cregg and Zhang, 2001). However, Cregg (1994) found that seedling survival of Pinus ponderosa Dougl. ex Laws. to drought stress was poorly correlated to climate indices of the seed sources.
Oleander, native to southern Asia and the Mediterranean region (Dirr, 1998), is a fast-growing, tough, versatile evergreen shrub that can reach up to 6 m tall but is usually trimmed at 2 to 3 m to form a round shrub. It grows well in warm subtropical regions and is commonly used as an ornamental plant in landscapes, parks, and along roadsides. About 400 cultivars have been named, representing a wide variety of flower colors (Mackay et al., 2005). Early work illustrated, although no information on cultivar was given, that leaf gas exchange of oleander decreased as soil moisture content decreased (Björkman et al., 1980). However, no relationship was found between leaf water potential and leaf stomatal conductance (g S) for oleander (Gollan et al., 1985).
Two representative commercial cultivars, Hardy Pink and Hardy Red, and two breeding lines, EP1 and EP2, were selected for this study. EP1 and EP2 were selected because these plants were still alive despite not being irrigated for ≈8 years in a semiarid climate. Thus, this study was undertaken to compare the growth, photosynthetic characteristics, and leaf water relations of two breeding lines with two commercial cultivars in response to drought stress.
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