Lipoxygenase activity and proline accumulation in leaves and roots of olive trees in response to drought stress Adriano Sofo a, *, Bartolomeo Dichio a , Cristos Xiloyannis a and Andrea Masia b a Dipartimento di Produzione Vegetale, Universita ` degli Studi della Basilicata, Campus Macchia Romana, 85100, Potenza, Italy b Universita ` degli Studi di Bologna – Dipartimento di Colture Arboree – viale Fanin 46, 40127, Bologna, Italy *Corresponding author, e-mail: adriano.sofo@libero.it Received 1 June 2003; revised 20 September 2003 The olive tree (Olea europaea L.) is commonly grown in the Mediterranean basin and is able to resist severe and prolonged drought. Levels of proline (PRO) and malondialdehyde (MDA), and the lipoxygenase (LOX) activity were determined in 2-year-old olive plants (cv. ‘Coratina’) grown in environ- mental conditions characterized by high temperatures and high photosynthetic photon flux density levels and gradually subjected to a controlled water deficit for 20 days. Before and during the experimental period, leaf and root samples were collected and analysed for PRO and MDA. The levels of PRO increased in parallel with the severity of drought stress in both leaves and roots. Significant increases of LOX activity and MDA content were also observed during the progressive increment of drought stress in both leaf and root tissues. Measurements of transpiration and photosynthetic rate, stoma- tal conductance and substomatal CO 2 concentration were carried out during the experiment. The accumulation of PRO indicates a possible role of PRO in drought tolerance. The increases of MDA content and LOX activity show that the water deficit is associated with lipid peroxidation mechanisms. Introduction The maintenance of plant water potential during water deficit is essential for continued growth and can be achieved by osmotic adjustment mechanisms resulting from the accumulation of compatible solutes (such as proline, glycine betaine, organic acids, sugars as mannitol and sucrose) in the cytoplasm (Ingram and Bartels 1996, Hare et al. 1998). Proline (PRO) acts as a compatible osmolyte since it can accumulate to high concentrations without damaging cellular macromol- ecules. Proline can also serve as a nitrogen carbon source in the cell (Chiang and Dandekar 1995, Verbruggen et al. 1996). Moreover, PRO has a protective action which prevents membrane damage and protein denaturation during severe drought stress (Ain-Lhout et al. 2001), and replenishes the NADP 1 supply in altered redox potentials (Hare and Cress 1997, Hare et al. 1999). It has also been proposed that PRO can act as an electron acceptor, avoiding damage of photosystems due to their photoinhibition by activated oxygen species (Hare et al. 1998). LOX (EC 1.13.11.12) isoenzymes are nearly ubiqui- tous in the plant kingdom and are involved in many physiological processes such as flowering (Ye et al. 2000), seed germination (Suzuki and Matsukura 1997), pigment bleaching (Pastore et al. 2000), formation of flavour and aroma in plant products (Williams et al. 2000) and plant growth and development (Hildebrand et al. 1991). Isoenzymes of LOX differing in their sub- strate specificity and pH optimum, are located in the cytosol (Siedow 1991), microsomes (Feussner and Kindl 1994), plasma membrane (Macri et al. 1994) and oil bodies (Rodrı´guez-Rosales et al. 1998). LOXs catalyse the dioxygenation of polyunsaturated fatty acids con- taining a cis, cis-1,4-pentadiene backbone, producing hydroperoxy fatty acids, which are highly reactive com- pounds that are toxic to cells. These fatty acids are PHYSIOLOGIA PLANTARUM 121: 58–65. 2004 Copyright # Physiologia Plantarum 2004 Printed in Denmark – all rights reserved Abbreviations – CP, control plants; LOX, lipoxygenase; LWP, leaf water potential; MDA, malondialdehyde; MR, medium roots; PPFD, photosynthetic photon flux density; PRO, proline; SP, stressed plants; TR, thin roots. 58 Physiol. Plant. 121, 2004