Preferential Allocation of Currently Assimilated Carbon Induced by the Source Leaf Position on Young Cork Oaks (Quercus suber L.) in Late Spring

The whole-plant preferential allocation patterns of recently assimilated carbon by the source leaves of six-year-old cork oaks (Quercus suber L.) were assessed 7 days after a 14CO2 pulse-labelling in late spring (end of May). The 14CO2 assimilation was separately induced on attached leaves on branches located at the top-down 30% of the crown height, in the middle 40% and at the bottom-up 30% of the crown height of twelve plants. Our results showed that the top source leaves retained the highest amount (64%) of their own current produced carbohydrates compared to either lower (49%) or middle (42%) source leaves. The top source leaves preferentially export current carbohydrates to their most proximal sinks, namely, other leaves or their branches. However, lower source leaves exported the highest amount of current carbon, about 37%, preferentially to the root system. Roots displayed the greatest sink strength for the available current carbohydrates, due to their largest biomass (between 69% and 75% of the whole plant biomass), when other strong sinks, such as the annual leaves, were fully expanded. Taken together, our data revealed that carbon supply by leaves and delivery to roots are critical for maintaining root growth in cork oak under Mediterranean seasonal drought conditions.