Although the tropical forest understorey represents an extremely light-limited habitat, we investigated the nature of nutrient limitation of photosynthesis and electron transport on 160 seedlings of the tropical tree species Alseis blackiana, with a nitrogen, phosphorus and potassium factorial experiment now in its twelfth year. The study was conducted in a lowland tropical forest at the Barro Colorado Island Nature Monument in Panama.  Photosynthetic light response curves were measured simultaneously with chlorophyll fluorescence to determine maximum photosynthetic rates, stomatal conductance, and electron transport rate, and light availability was calculated as percent transmittance using hemispherical photographs above each study seedling. We used multiple regression to account for variability in light availability when comparing the responses of photosynthetic parameters to nutrient addition.

Results/Conclusions
When light availability was taken into account, multiple regression indicated that electron transport rate showed a marginally significant increase with nitrogen addition. Maximum photosynthetic rate increased with nitrogen and potassium addition and showed a marginally significant nitrogen-potassium interaction. Stomatal conductance increased and intrinsic water use efficiency (photosynthesis per unit stomatal conductance) decreased strongly with potassium addition. These results show that even in a tropical forest where nitrogen is relatively abundant, nitrogen is a non-substitutable resource for light harvesting and carbon assimilation. The effects of potassium on stomatal conductance and photosynthesis suggest that potassium addition improves the ability of stomates to respond to short-duration high-intensity light pulses, which provide a major proportion of understorey plant carbon gain. Overall, our results highlight the importance of plant nutrition in a habitat often considered to be limited only by light.