While dynamic regulation of photosynthesis in fluctuating light is increasingly recognized as an important driver of carbon uptake, acclimation to realistic patterns of light fluctuations is still largely unexplored. We subjected Arabidopsis thaliana (L.) plants to light fluctuations with distinct amplitudes and average irradiance. Using gas-exchange and thermal imaging, we examined how light fluctuations affected leaf structure, photosynthetic capacity, and water relations. We found a wider amplitude of fluctuations produced a stronger acclimation response. Large reductions of leaf mass per area under fluctuating light framed our interpretation of changes in pigment content and photosynthetic capacity, in that photosynthetic investment increased markedly on a mass basis, but only a little on an area basis. Moreover, leaf transpiration rose sharply, being nearly four times under fluctuating light. Our findings indicate that leaves growing under fluctuating light, although thinner, maintained their photosynthetic capacity; suggesting their photosynthesis may be more cost-efficient than those under steady light, but overall may incur increased maintenance costs. This is especially relevant for plant performance globally, because naturally fluctuating light created conflicting acclimation cues for photosynthesis and transpiration that may hinder progress towards ensuring food security under climate-related extremes of water stress.

Leaf fall in the autumn opens the forest canopy, allowing more solar radiation to be transmitted to the forest floor. Those understorey species that remain physiologically active may benefit from the sunlight received by assimilating additional carbon while conditions remain favourable. We monitored water and leaf pigment content, as well as photosynthetic capacity in six understorey species growing in three adjacent stands differing in their canopy tree species. Leaf fall, transmitted light and microclimate were followed in each stand. We found that leaf fall started earlier in the Betula pendula, than in the Quercus robur stand, and light transmission changed accordingly. Concurrently, understorey leaf senescence was generally earlier in the birch than in the oak stand, itself earlier than in the evergreen Picea abies stand. Neither atmospheric CO 2, humidity, nor temperature differed between stands. A change in light quality and/or increase in quantity following leaf fall drove the difference in the timing of senescence in the understorey. Species with later senescence were more able to use the increased light after leaf fall. Together these findings help provides a mechanistic foundation to predict how ecosystem functioning and carbon balance will be impacted by phenological shifts in response to global changes.