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Rhododendrons (Rhododendron) are ornamental plants that exhibit poor thermotolerance. Salicylic acid (SA) and Ca regulate the physiological and biochemical mechanisms in plants adapted to adverse environmental conditions. This study investigated the role of SA and CaCl2 in managing heat tolerance of Rhododendron ‘Fen Zhen Zhu’. Plants of the triennial Rhododendron ‘Fen Zhen Zhu’ were pretreated with SA and CaCl2, alone and combined. Following this pretreatment, the plants were subjected to 38/30 8C (day/night) incubation for 6 days, and then allowed to recover for 20 days under 25/17 8C (day/night) in a chamber. Changes in morphology were observed and recorded. Data were collected on plant chlorophyll content, malondialdehyde (MDA) content, H2O2 level, antioxidant enzyme activity, and total soluble protein content. The results revealed that the plant growth was considerably affected by heat stress, the leaves became brown and withered, and the plant defoliated. Under heat stress, chlorophyll content and total soluble protein levels decreased. Peroxidase (POD) activity and superoxide dismutase (SOD) also decreased, whereas the H2O2 and MDA content increased. Individual or combined application of SA and CaCl2 had a positive effect on plant growth, chlorophyll content, total soluble protein levels, and enzymatic antioxidant activity under heat stress. In general, the effect of the combined application of SA and CaCl2 was superior to individual application. In addition, treatment with high CaCl2 concentrations effectively alleviated the decrease in chlorophyll content. However, at low SA and CaCl2 concentrations, SOD and POD activity and total soluble protein accumulation increased whereas MDA and H2O2 levels decreased. These results suggest that SA and CaCl2 may interact to alleviate heat stress. Global climate change may not only increase CO2 concentration, but also sustain elevated temperatures (Intergovernmental Panel on Climate Change, 2013; Lavania et al., 2015). As a result, the distribution pattern of plant communities will be negatively affected and the scenario where the vegetation in different landscapes is drastically affected over a period of time might become a reality (Grover et al., 2013). Although some ornamental plants have the capacity to evolve and cope with temperature fluctuations, this capacity might not keep pace with global warming, reduced ornamental value, and economic losses (Tao et al., 2015). Heat stress not only affects the phenotype of a plant, causing leaf etiolation and wilting, but it alters the anatomy, physiology, and photosynthesis capability of plants as well, even at the genetic level (Chen et al., 2014; Mishkind et al., 2009; Perez et al., 2009). Heat stress induces overproduction of reactive oxygen species (ROS) in plant tissues, thus seriously affecting the balance between the generation and degradation of ROS. Furthermore, plants respond quickly to various abiotic stresses by regulating complex physiological, biochemical, and molecular mechanisms for acclimation and adaptation to stress (Chandel et al., 2013; Chen et al., 2014). Antioxidants act as ROS scavengers to mitigate oxidative injury and to maintain metabolic functions; these include antioxidant enzymes and nonenzymatic low molecular weight antioxidant molecules (Semida and Rady, 2014a, 2014b). Heat tolerance can be improved by genetic selection as well as with the use of exogenous regulators, which aid the adaptation of physiological response in plants. SA and Ca are recognized as signal molecules known for their roles in plant adaptation to changing environments (Janda and Ruellan, 2015; Laanemets et al., 2013). SA, a small phenolic compound, plays a significant role in regulating physiological processes in plants involved in response to various abiotic stress such as drought (Nazar et al., 2015), cold (Sibozaa et al., 2014), heat (Boatwright and Karolina, 2013), intense light (Zhao et al., 2011), and salinity (Agami, 2013; Manaa et al., 2014; Rady and Mohamed, 2015). Application of SA enhances the photosynthetic rate and maintains the stability of the cell membrane through regulating enzymatic activity (Ghanta et al., 2014), which eventually alleviated oxidative stress (Wasti et al., 2012), but the underlying mechanism remains unclear (Wang et al., 2014). This effect of SA on oxidative stress response was observed in a variety of plants: Solanum lycopersicum (Manaa et al., 2014; Wasti et al., 2012), Arabidopsis thaliana (Borsani et al., 2001), Vigna radiata (Singh et al., 2014), and Triticum aestivum (Kumar et al., 2015; Li et al., 2013; Zhao et al., 2011). In different T. aestivum cultivars, variations in the expression levels of heat stress–associated proteins have been documented (Kumar et al., 2012). In contrast, there are limited studies on heat tolerance of flowers, especially the heat tolerance of Rhododendron. Several studies have shown that Ca is of pivotal importance for various regulatory mechanisms in plants as a secondary messenger, controlling a variety of cellular processes such as stomatal movements (Hubbard et al., 2012; Laanemets et al., 2013), antioxidant enzyme activity, and lipid peroxidation under multiple stresses (Khan et al., Received for publication 2 Feb. 2016. Accepted for publication 6 May 2016. We thank the International Science and Technology Cooperation Foundation (A213021501), the National Natural Science Foundation of China (k305021110), and the Shaanxi Natural Science Foundation (2012JQ3008). Corresponding author. E-mail: bingbing2003915@163.com. J. AMER. SOC. HORT. SCI. 141(4):363–372. 2016. 363 2010). Thus, exogenous application of Ca has become an important research topic due to concerns about the effects of climate change on plants. Cytosolic Ca content increases under heat stress, which might alleviate heat injury and initiate cellular repair (Bamberg et al., 1998; Gong et al., 1998). The activities of antioxidant enzymes in plants pretreated with CaCl2 either increase or do not decrease to the same extent as in the control group (Tan et al., 2011). It has been shown that Ca treatment prevents solute leakage from the cytoplasm, stabilizing cell membrane surfaces, and regulating hormone metabolism in plants (Hirschi, 2004). However, the effect of Ca treatment on thermotolerance of Rhododendron is poorly studied. Individual exogenous application of SA and Ca in mitigating heat stress has been studied extensively (Manaa et al., 2014). Combination application of SA and Ca improved growth, photosynthetic pigment concentration, and mineral nutrition in S. lycopersicum during salt stress (Manaa et al., 2014). Studies on SA and Ca, individual or combined, for T. aestivum under salt stress revealed that the combined application proved more effective in reducing oxidative stress (AlWhaibi et al., 2012). This research suggests that SA and Ca interact to reduce stress. Other studies also support this conclusion (Guo et al., 2015; Xu et al., 2008). Rhododendron is one of the most popular ornamental plants worldwide, with both medicinal and edible value (Dampc and Luczkiewicz, 2013). At present, genetic variation (Li et al., 2015; Zhao et al., 2012), flowering-related gene manipulation (Cheon et al., 2012), selective breeding of some species (Shen et al., 2015), and the identification of the major chemical constituents in leaf were the main subjects in studies on Rhododendron (Lou et al., 2015). Influenced by genetic factors, this genus is widely distributed at high altitudes, in cold and moist areas. Thus, heat intolerance is a major constraint in Rhododendron cultivation and landscape applications. Rhododendron ·hybridum was obtained by hybridization over several generations: Rhododendron simsii, Rhododendron phoeniceum, Rhododendron indicum, and Rhododendron capitatum. Rhododendron ‘Fen Zhen Zhu’ is a compact plant, with pink flowers, and an early, long flowering period, from January to March. Despite its high ornamental value, it has poor thermal resistance. A limited number of studies have investigated heat resistance of Rhododendron, and the influence of SA and Ca pretreatment on heat tolerance in ornamental plants, including Rhododendron, has rarely been reported. Because of its poor thermal resistance and difficulty in growing it in gardens, commercial Rhododendron is currently cultivated in greenhouses, thus requiring more energy, manpower, and resources. For these reasons, the influence of SA and CaCl2 treatments, applied alone and in combination, on Rhododendron ‘Fen Zhen Zhu’ was assessed by observing changes in physiology and morphology of plants exposed to high temperatures. The results will provide theoretical basis and some practical recommendations for improving thermotolerance in Rhododendron, which can be used to develop management practices for its cultivation in garden and to reduce energy consumption and enhance cultivation in summer. In addition, the effect of SA and CaCl2 application in Rhododendron will provide basis for further studies on molecular mechanisms responsible for the interaction of the two compounds. Materials and Methods PLANT MATERIAL, GROWTH CONDITIONS, AND TREATMENTS. Rhododendron ‘Fen Zhen Zhu’, collected from a greenhouse in the city of Dandong, were grown in plastic pots (12 cm high, diameter 16 cm at the top, and 10 cm at the bottom) in a greenhouse in Yangling, Shaanxi province, China. The culture substrate was a 1:1 (v:v) mixture of peat and pine needle mulch. In July 2015, plants of similar stature, height 28–30 cm and crown 18–21 cm in diameter, were selected and transferred to a growth chamber [25/17 C (day/night), 80% relative humidity, photoperiod of 14 h light/10 h dark, and light irradiance of 150 mmol m s] for 1 week. A tray was placed under the pots to retain water. After 7 d, the leaves were sprayed daily until saturation with different concentrations of SA (0.5, 1.0, and 2.0 mM) and CaCl2 (3.5, 7.0, and 10.5 mM) solutions alo

Effects of Salicylic Acid and Calcium Chloride on Heat Tolerance in Rhododendron ‘Fen Zhen Zhu’