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Mature leaves are the primary source of sugars, which give rise to many secondary metabolites required for plant survival under adverse conditions. In order to study the interaction of field-grown cork oak (Quercus suber L.) with the environment, we investigated the seasonal variation of minerals and organic metabolites in the leaves, using inductively coupled plasma atomic emission spectrometry, elemental analysis and nuclear magnetic resonance spectrometry. Statistical analysis showed that the data strongly correlated with seasonal climate and were divided in three groups corresponding to: (1) spring-early summer, (2) summer and (3) autumn-winter. The concentration of N, P, K and leaf ash content were highest in spring (recently formed leaves), reached the minimum during the hot and dry summer and increased slightly during the rainy period of autumn-winter. Conversely, Na, Mg and Ca concentrations were lowest in spring-early summer and increased during summer and autumn-winter, the Ca concentration increasing five-fold. Two cyclitol derivatives, quinic acid and quercitol were the major organic metabolites of the leaves. Their concentration along the season followed opposite trends. While quinic acid predominated during spring-early summer, when it contributed 12% to the leaf osmotic potential, quercitol was predominant during autumn-winter, when its contribution to leaf osmotic potential was about 10%. This different preponderance of the two compounds is expressed by the quercitol/quinic acid ratio, which can be as low as 0.2 in early summer and as high as 9 in winter. Sucrose and glucose concentrations also increased during autumn-winter. Evidence for the quercitol protective role in plants during stress is discussed, and on the basis of structural similarity, it is suggested that quinic acid could have an identical importance, with a protective role against heat and high irradiance. It is concluded that the marked changes in Q. suber leaf composition throughout the year could have important implications in the plant capacity to endure climatic stress.