Aiming to test the existence of constraints in the ability of Pinus canariensis seedlings to withstand water stress under low irradiance, we carried out a greenhouse trial combining two highly contrasted light environments (90 and 5% of full sunlight) and two levels of water availability (42 and 10% in volume). Shaded seedlings displayed an extremely low dry weight, 2% of well-illuminated seedlings biomass at the end of the experiment (158 days). Light × water interactions were significant for most traits considered. However, the complementary allometric analysis showed that the effect of water availability under high irradiance could be explained through ontogenetic differences related to plant size. Shaded plants exhibited higher root biomass and reduced leaf area ratio in response to low water availability, and this was confirmed by allometric analysis. By contrast, plants under full sunlight showed only apparent plasticity; i.e. ratios for both watering treatments belonged to common allometric trajectories. Moreover, only the seedlings under low irradiance underwent a significant decrease in specific needle area due to a shortage in water availability. We, therefore, conclude that the effect of the imposed drought was more dramatic in the shaded environment.