Rose is one of the most common garden plants in the world. Due to its long history of cultivation and popularity, vast information exists on breeding, disease/pest management, culture and propagation, rootstock selection, and postharvest handling (Robert et al., 2003). However, little is known about plant responses to environmental stresses, such as drought stress, which is becoming an increasing concern in arid and semiarid regions. Therefore, in addition to enhancing flower color, size, and fragrance, improving resistance to environmental stresses will be one of the goals in future breeding and biotech programs (Pemberton, 2003).
Plants have developed various mechanisms to tolerate drought stress such as altered root to shoot ratio, smaller and fewer leaves, and altered stomatal function (Blum, 1996). Photosynthesis is the primary process for plant biomass production and is one of the most sensitive physiological processes to environmental stresses (Hsiao and Acevedo, 1974; Huang, 2004). Consequently, the ability to maintain a reasonable rate of photosynthesis under stress conditions can be a good indicator of a plant's adaptability. Hagidimitriou and Pontikis (2005) found that ‘Koroneiki’, a more drought-tolerant olive (Olea europea) cultivar, performed better and was able to maintain greater leaf photosynthetic rates under high air vapor pressure deficit compared with the other green olive cultivars in the research. In oleander (Nerium oleander), drought-tolerant clones developed more new shoots and had greater shoot growth during a cyclic drought stress period, while those less tolerant to drought had fewer new shoots and less shoot growth (Niu et al., 2008).
Most garden roses are produced by grafting using the T-budding technique (Pemberton, 2003). Different rootstocks are recommended in various areas in the world in accordance with climatic and soil conditions. For example, R. multiflora is used in the south-central United States, Canada, and Japan, whereas, ‘Dr. Huey’ is used in the western United States (Pemberton, 2003). Rosa ×fortuniana is used in areas with year-round temperate climate (Morrell, 1983). In the United States, R. ×fortuniana is mainly used in Florida, southeastern, and southwestern regions (Martin, 2008). Rosa odorata is one of the most popular rose rootstocks for greenhouse-cut roses (R. ×hybrida), but it is also valued for garden roses (Cabrera, 2002; Singh and Chitkara, 1982, 1987). To our knowledge, the only studies on drought tolerance for garden roses were conducted by Henderson et al. (1991) and Henderson-Cole and Davies (1993), who observed cultivar differences in drought tolerance. For greenhouse cut roses, Chimonidou-Pavlidou (1996, 2004) found that drought stress reduced cut flower yield, flower quality, and the growth of flowering shoots. For potted miniature roses (R. ×hybrida), drought stress during production lowered the postharvest quality (Williams et al., 1999, 2000). Different mechanisms in miniature rose cultivars in tolerating drought stress such as osmotic adjustment and stomata closure were reported (Riseman et al., 2001).
The performance of a grafted rose plant under drought stress depends on the drought tolerance of the scions and the rootstocks and their compatibility (interaction). No research has compared the drought tolerance of various rose rootstocks. Therefore, selection for drought tolerance in roses may begin with drought-tolerant rose rootstocks. In grapevines (Vitis vinifera), drought-tolerant rootstocks were used to improve the performance of the grafted plants under drought conditions (Iacono and Peterlunger, 2000). In citrus (Citrus spp.), it is well known that the inherent differences among rootstocks influence growth, yield, and fruit quality, making the selection of a rootstock an important consideration for a citrus orchard (Castle et al., 1993). Leaf gas exchange of grapevine is affected by rootstock, although this effect is scion-specific (Iacono et al., 1998). Niu and Rodriguez (2008) characterized the growth and ion uptake responses of four rose rootstocks, ‘Dr. Huey’ R. ×fortuniana, R. multiflora, and R. odorata, to various salinity levels dominated by chloride or sulfate. However, their response to drought stress remains unknown. Therefore, the objective of this study was to compare the growth, water relations, and gas exchange rates of the same four rose rootstocks in response to drought stress.
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