Seasonal courses of chlorophyll a fluorescence, gas exchange parameters and water status, joined to environmental variables, were measured in four age classes of stone pine (Pinus pinea L.) natural regeneration, selected within a natural light gradient in order to cover the whole range of incident light detected within a stone pine regular stand throughout 2007 and 2008. Selection of light conditions was based a priori on the distance of seedlings to the parent trees, expressed by crown radius distances. One-year old seedlings were worse acclimated to the extreme drought conditions recorded in summer (especially in 2008), showing lower predawn and midday Fv/Fm values, slightly lower photosynthetic rates at midday, lower ΦPSII values, lower water potentials (Ψ pd and Ψ min ), and high mortality (90%). The oldest seedlings (class 4, 51-200 cm high), although they do not experience the lowest Ψ min in July 2008, were not able to maintain a positive carbon gain at midday, even showing the lowest photosynthetic rates. However, they rapidly recovered once the climatic conditions were softened in early autumn. During late autumn low Fv/Fm values were found especially in one-year old seedlings, indicating that maximal photochemical efficiency of PSII is sensitive to the autumn temperatures below cero recorded during both years. Thus, stone pine regeneration acclimates its physiology to the seasonal course of temperature and water availability, and this acclimation depended on age class. However, seedling physiology is only slightly affected by differences in light environment, probably related to the low variability recorded in GSF values (0.55 ± 0.01), due to the low density of the stand. We suggest that such low stand densities cannot assure one-year old seedlings' survival, but are sufficient for older seedlings.Key words: stone pine; natural regeneration; drought; cold; gas exchange; photochemical efficiency; water status. ResumenCambios estacionales en la actividad fisiológica del regenerado bajo un gradiente natural de luz en una masa natural de Pinus pinea Durante 2007 y 2008 se midió la evolución estacional de la fluorescencia a la clorofila, de los parámetros de intercambio gaseoso y del estado hídrico, junto con variables ambientales, en el regenerado de cuatro clases de edad de pino piñonero (Pinus pinea L.) en una masa regular de esta especie. Las plantas se eligieron dentro de un gradiente natural de luz, con el objeto de cubrir todo el rango de luz incidente en este tipo de masas. La selección de las condiciones de luz se realizó a priori basándose en la distancia de las plantas a los árboles adultos, expresada en radios de copa. Las plantas de un año se aclimataron peor a las condiciones de sequía extrema del verano (principalmente en 2008), mostrando valores menores de Fv/Fm al mediodía y al amanecer, de tasa fotosintética al mediodía, de ΦPSII, de potenciales hídricos (Ψ pd y Ψ min ) y una mayor mortalidad (90%). Las plantas de mayor edad (clase 4, 51-200 cm de altura), aunque no experimentaron ...