Water shortage and poor water quality are critical challenges to gardening and landscaping in many regions of the world. Population growth and increased urbanization have increased competition for fresh water among agriculture, industry, and municipal water users (Lea-Cox and Ross, 2001). Therefore, water conservation and the improvement of irrigation efficiency are important in landscape water management (Nicolas et al., 2008; Niu et al., 2006). With watering restrictions, the effect of drought stress is exacerbated on plant establishment and survival, and selection of drought-tolerant plants becomes increasingly important for the development of sustainable landscapes.
Roses (Rosa ×hybrid L.) are some of the most common garden plants in the world, and garden roses are one of the most popular and widely cultivated flowering plants. Moreover, some are valued for having ornamental fruit, and they can be used as hedges, screens, and groundcovers. Their cultivation, however, presents several challenges to gardeners because of their limited resistance to diseases, temperature extremes, drought, and salt stresses. Garden rose cultivars are well adapted to temperate climates, but there is little science-based knowledge about its tolerance to different climatic and soil conditions throughout the world. The Texas AgriLife Extension Service has established a brand designating a group of rose cultivars as “Earth-Kind®,” and these roses provide the consumers with garden plants that require minimum fertilizer, water, and pesticides while growing in gardens or landscapes (Aggie Horticulture, 2012).
Knowing the responses of rose plants to environmental stresses is becoming an increasing concern in arid and semiarid regions. Niu and Rodriguez (2009) reported that Rosa ×fortuniana had greater shoot growth and leaf area under cyclic drought stress and was considered to be more tolerant to drought than the other three cultivars (R. ×hybrid ‘Dr. Huey’, Rosa multiflora, and Rosa odorata). Reduction of leaf area by drought is caused by reduced cell expansion and cell division, leaf rolling, and death of apical parts of leaves and whole leaves (Blum, 1996). Plants with a larger leaf area have a higher ratio of transpiration to evaporation, which results in higher WUE (Kingeman et al., 2005). In a study of five low-maintenance rose cultivars under water stress conditions, ‘Pink Meidiland’ had the smallest leaf surface area, whereas ‘Ferdy’ had the largest root-to-shoot ratio and lowest leaf area ratio, which may contribute to larger leaf water reserve and moisture uptake for better adaptation to drought conditions (Henderson et al., 1991).
In a study by Niu and Rodriguez (2009), Rosa ×odorata, which was considered to be the least drought-tolerant cultivar, had lower leaf Pn, E, and gS than the other three cultivars investigated under SMC between 10% and 20%. Similar results were found with oleander (Nerium oleander L.) wherein drought-tolerant clones under water deficit conditions had greater gas exchange rates than those less tolerant to drought (Niu et al., 2008). In a study on two miniature roses (‘Poulhappy Charming Parade’ and ‘Poulbian Bianca Parade’), gS and Pn in drought-treated plants were restored to rates comparable to well-irrigated plants after re-watering, although E, gS, and Pn were reduced under drought-stressed conditions (Williams et al., 1999). They also concluded that drought-stressed miniature roses produced more dry matter per volume of water consumed compared with well-irrigated plants, which might result in improvement of plant drought tolerance by using water more efficiently. Egilla et al. (2005) found that relative water content, E, and gS were decreased under drought-stressed conditions in Chinese hibiscus (Hibiscus rosa-sinensis L.). In another study on ponderosa pine (Pinus ponderosa Dougl.) and big sagebrush (Artemisia tridentata Nutt.), plants grown under drought had greater WUE than those under well-irrigated conditions (DeLucia and Heckathorn, 1989).
Knowing how garden roses respond to stress will provide useful information that breeders can use in developing adaptable rose cultivars and lead to a better understanding of why certain cultivars withstand unfavorable environmental conditions better than others. Earth-Kind® is a special designation given to select rose cultivars by the Texas AgriLife Extension Service through the Earth-Kind landscaping program. It is based on the results of extensive research and field trials and is awarded only to those roses demonstrating superior pest tolerance combined with outstanding landscape performance (Aggie Horticulture, 2012). Our objectives were to investigate the relative drought tolerance in three Earth-Kind® garden roses (‘Old Blush’, ‘RADrazz’, and ‘Belinda’s Dream’) and one (‘Marie Pavie’) observed to withstand heat and drought stresses in southern landscapes. We evaluated the response of growth, water relations, and gas exchange rates of these rose cultivars to drought stress. We postulated that greenhouse studies could provide insight about physiology of plants under drought-stressed conditions, which is a basis for species selection in dry landscapes.
Aggie Horticulture 2012 Texas AgriLife Extension Service. Earth-Kind Roses. 31 May 2012. <http://aggie-horticulture.tamu.edu/earthkind/roses/about.html>
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