Linking Carbon Source-Sink Imbalance and Leaf Traits to Species’ Ecosystem Productivity in the Tropics

Tropical ecosystems are the largest biotic carbon sink in the world. It has been long hypothesised that the increase in atmospheric CO2 concentration will enhance primary productivity around the globe by facilitating carbon diffusion from the surrounding air to the carboxylation site in leaves. However, experimental data and theoretical background have suggested that ecosystem productivity is not only limited by environmental factors driving photosynthesis, but by plant's carbon growing needs. In other words, carbon is not “pushed” from the atmosphere to the leaf, but “pulled” by growth and metabolic processes, thus representing plants' carbon demand. When carbon supply by leaves exceeds carbon demand, an imbalance is reached, producing a cascade of signals that end up affecting photosynthesis, hence ecosystems productivity. There has been a lack of empirical studies on the effect of sugar imbalance on photosynthesis; we present here the first such study for tropical leaves.

We examine the effect of sugar imbalance over photosynthesis in cut branches of three tropical species found in Ankasa National Park, Ghana: D. staudtii, O. gore and U. guinensis. We measured photosynthesis before and 15 min after applying 3 treatments to 6 leaves each: control, scrubbing the cuticle and applying water, and scrubbing the cuticle and applying a sucrose solution at 0.7 M.

The sucrose addition trial decreased photosynthesis by almost 60% from its initial values in O. gore and U. guinensis whilst D. staudtii showed little to no differences to the control treatment after application. These finding are consisting with the source-sink imbalance theory, thus affecting the on-going increase in ecosystem productivity when plant carbon supply becomes satisfied.

We explore the significance of venation patterns in these three species, as a possible mechanism to explain the differences within observed results. Veins do not only supply water to the leaf (xylem), but acts as a piping system to translocate sugar from leaves to the rest of the plant (phloem). An increase in small veins and a better distributed network should enhance carbohydrates translocation, hence controlling photoassimilates levels in the mesophyll. Also, we hypothesised about the role of different phloem uploading mechanisms on translocation efficiency. This kind of experiments opens the window to new ecophysiological research on the effects of higher CO2 concentration and their trade-off with tropical ecosystem productivity.