Source–sink relationship and photosynthesis in the horn-shaped gall and its host plant Copaifera langsdorffii Desf. (Fabaceae)

The horn-shaped gall of Copaifera langsdorffii is induced by an unidentified species of Diptera: Cecidomyiidae and stands out among the other gall morphotypes by its bizarre shape and high infestation level over the years. Its role as a sink of photoassimilates in comparison to the non-galled leaflets was assessed through the quantification of nitrogen, carbohydrates, pigment content and the apparent relative electron transport rate (ETR) during different developmental stages. The levels of nitrogen and starch decrease in galls as leaflets mature. Total soluble sugars and the water-soluble polysaccharide concentration were higher in galls, whereas, chlorophyll content and ETR values were higher in leaflets rather than in galls. However, the latter presented significantly higher concentrations of total carotenoids. The low nitrogen and chlorophyll contents are related to the low photosynthetic activity, indicating that the CO2 assimilation in galls is insufficient to fully supply their metabolism, reinforcing it as a sink of photoassimilates. The amount of sugars allocated to the gall tissues corroborates this hypothesis, and is probably used as energy supply for both gall structure maintenance and herbivore nutrition. The reduced intercellular spaces on the horn-shaped gall structure imply little gas exchange and, thus, hypoxic conditions on the gall tissues. However, the incipient photosynthesis might be important to provide oxygen to the structure and avoid hypoxia, enabling gall metabolism. ► The galls on Copaifera langsdorffii are a sink of photoassimilates. ► The polysaccharides and soluble sugars contents were higher in galls. ► Nitrogen and chlorophyll contents are related to the low photosynthetic activity. ► Photosynthesis in gall tissues could provide oxygen to the structure.