Role of Ca in the metabolism of phenolic compounds in tobacco leaves (Nicotiana tabacum L.)

Given the essential role played by phenol metabolism in many resistance responses to different types of stress, the aim of the present work was to determine how different application rates of calcium may influence this metabolic process. Increased calcium in the nutrient solution in which tobacco plants were grown considerably reduced the foliar concentration of phenolic compounds. Calcium clearly exerted a positive influence on the activities of enzymes (phenylalanine ammonia-lyase, polyphenol oxidase and peroxidase) involved in the metabolism of the phenolics. High dosages of calcium (5 mM) promoted more oxidation than synthesis of these compounds, thus explaining the lower concentration of the phenolics. Abbreviations: BSA – bovine serum albumine; PAL – phenylalanine ammonia-lyase; POD – peroxidase; PPO – polyphenol oxidase Introduction The processes which have been most thoroughly studied and which most directly involve phenolics are ̈ Phenols, among the most widely distributed natural related to pest and disease resistance (Dubeler et al. products in the plant kingdom, carry strong physio1997; Ruiz et al. 1999a). In addition, the metabolism logical and ecological implications (Ruiz and Romero of phenolics has been associated with injuries (Smith 2001). The metabolism of phenolics involves a series 1982), with resistance to thermal stress (Christie et al. of enzymes that participate in the synthesis and the 1994), with tolerance against exposure to UVrays and oxidation of these compounds. The reaction catalysed ozone (Rasmussen et al. 1991) and finally to changes by phenylalanine ammonia-lyase (PAL, EC 4.3.1.5), in the nutritional state of nutrients such as boron ̈ the deamination of L-phenylalanine to produce trans(Cakmak and Romheld 1997; Ruiz et al. 1998, cinnamate, is commonly regarded as a key step in the 1999a), nitrogen (Wojtaszek et al. 1993) and calcium ̃ ́ biosynthesis of phenolics and is affected by a number (Castaneda and Perez 1996; Penel et al. 1999). 21 of factors (Hao et al. 1996). Calcium (Ca ), one of the essential nutrients for On the other hand, phenolics are degraded to diplants, plays a major role in the initiation of many quinones by peroxidases (POD, EC 1.11.1.7) and signal transduction processes in higher plant cells, principally polyphenol oxidases (PPO, EC 1.14.18.1) including bud formation, polar growth, gas-exchange ̈ ̈ (Soderhall 1995; Thypyapong et al. 1995). Many regulation, secretion, movements and lightand horstudies have demonstrated that both enzymes increase mone-regulated growth and development (Hepler and in response to biotic and abiotic stress (Kwak et al. Wayne 1985). In addition, this nutrient actively in1996; Ruiz et al. 1998, 1999a). fluences one of the processes most vital to plant 173 Plant Growth Regulation 41: 173–177, 2003.