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In this work we examined the differences between two related fir species ( Abies pinsapo Boiss. and A. alba Mill . ) with respect to their ability to manage light as a key resource, considering both their C fixation capacity and the “loss” of water through stomata in response to different light regimes. Thus, we carried out three experiments where we measured instantaneous gas exchange rates (net photosynthesis, stomatal conductance to water) under different light conditions. In the first experiment, individuals from both species were subjected to a moderate but constant level of irradiance. In the second one, every individual received a certain light dose through several periods of time characterized by different levels of constant irradiance. Finally, in the third experiment, dark periods of increasing duration were imposed to plants at steady-state of their gas exchange response, under the same level of irradiance employed in the first experiment. We used the time record of the data collected to calculate the total amount of C fixed and the water “lost” in each situation. Both species differed in their intrinsic ability for C fixation and avoidance of water loss, but showed a similar response with respect to the ratio between C fixed and water lost under different circumstances, what suggests that regulation of gas exchange in both species tends to keep this ratio constant under varying light conditions, playing light a critical role as an environmental signal for maximizing water use efficiency. [ABSTRACT FROM AUTHOR]