Proline metabolism in cherry tomato exocarp in relation to temperature and solar radiation By M. A. ROSALES * , J. J. RÍOS, R. CASTELLANO, A. I. LÓPEZ-CARRIÓN, L. ROMERO and J. M. RUIZ Departamento de Fisiología Vegetal, Facultad de Ciencias, Universidad de Granada, 18071 Granada, Spain. (e-mail: rosales@ugr.es) (Accepted 23 May 2007) SUMMARY Given the importance of proline and its metabolism in resistance to abiotic stress, the objective of this work was to study the influence of temperature and solar radiation on proline metabolism in the exocarp fraction of cherry tomato fruits. Solanum lycopersicum cv. Naomi plants were grown in an experimental greenhouse.Three fruit samples were taken over the entire production period. The first sampling was at the beginning of harvest [85 d after transplanting (DAT)], the second at mid-harvest (160 DAT), and the third at the end of harvest (229 DAT).Temperature and solar radiation values peaked during the third sampling, which coincided with an increase in lipid peroxidation (lipoxygenase activity) and malondialdehyde content, indicating the presence of oxidative stress during this period. However, yield values did not vary during the production cycle of cherry tomatoes.An increase in proline content was found at 229 DAT,with two enzymes responsible for proline synthesis:  1 -pyrroline-5-carboxylate synthetase (P5CS) and ornithine--aminotransferase (OAT). In contrast to previous work, proline dehydrogenase (PDH), which degrades proline, also increased its activity during this period. Under these conditions, the accumulation of proline, and proline degradation by PDH, could be considered to be a mechanism for resistance to stress by eliminating O 2 and forming H + ions.This would limit any major formation of reactive oxygen species (ROS), and thus cell necrosis, reducing losses in productivity. T emperature and light are environmental variables that can severely limit the productivity and nutritional quality of a crop (Adams et al., 2001). One response mechanism to environmental stress in plants is the accumulation of osmoprotector solutes, such as proline (Hare et al., 1999; Ruiz et al., 2002; Rivero et al., 2004; Claussen, 2005). Proline is important in stress responses as it protects protein structures from denaturation, stabilises cell membranes by interactions with phospholipids, detoxifies hydroxyl radicals, and can also be used as a source of energy and N (Hare et al., 1999; Claussen, 2005). In higher plants, proline can be synthesised from glutamate or ornithine (Bryan, 1990). In the first biosynthetic step from glutamate, proline formation is catalysed by a bifunctional enzyme,  1 -pyrroline-5- carboxylate synthetase (P5CS). The principal enzyme in charge of proline synthesis from ornithine is ornithine-- aminotransferase (OAT; Delauney and Verma, 1993). Proline accumulation also depends on its rate of breakdown, which is catalysed by the mitochondrial enzyme, proline dehydrogenase (PDH; Hare et al., 1999). Increased proline accumulation in stressed plants may also be caused by protein breakdown (Becker and Fock, 1986), inhibition of protein synthesis (Dhindsa and Cleland, 1975), a decline in amide and amino-acid export to growth organs (Tully et al., 1979), or inhibition of leaf development (Davies and Van Volkenburg, 1983).Thus, proline accumulation in stress situations may be triggered by: (i) greater proline synthesis, prompted by increased P5CS and/or OAT activity (Sánchez et al., 2001; Ruiz et al., 2002; Rivero et al., 2004); (ii) reduced degradation, due to lower PDH activity (Ruiz et al., 2002; Rivero et al., 2004; Claussen, 2005); and/or (iii) degradation of proteins rich in proline and hydroxyproline, a reduction in protein synthesis, or a decrease in proline use for protein synthesis (Chaitanya et al., 2001; Claussen, 2005). The effect of high temperature on proline accumulation in some plant species has been studied. In cotton, the accumulation of soluble proteins and proline in leaves is considered to be an important component of heat tolerance (Ashraf et al., 1994). Similarly, barley plants subjected to high temperature (42ºC) registered a sharp rise in proline accumulation (Georgieva et al., 2003). Recently, Rivero et al. (2004) demonstrated that, in tomatoes under heat stress (35ºC), the proline content of leaves increased significantly with respect to the optimum growth temperature (25ºC). This was due to an induction of the main enzymes involved in proline synthesis (P5CS and OAT), as well as to an inhibition of the enzyme responsible for degrading proline (PDH), thereby causing a overall stimulation of the stress-resistance machinery of the plant. However, no studies exist concerning proline metabolism in fruits exposed to abiotic stresses. Higher temperatures and solar radiation within a greenhouse subject cherry tomato fruit to oxidative stress, due to the generation of reactive oxygen species (ROS; Adams et al., 2001; Rosales et al., 2006). The result is greater lipid peroxidation, measured as malondialdehyde (MDA) content and lipoxygenase *Author for correspondence. Journal of Horticultural Science & Biotechnology (2007) 82 (5) 739–744