Spatial distribution of phlorotannins and its relationship with photosynthetic UV tolerance and allocation of storage carbohydrates in blades of the kelp Lessonia spicata

Abstract

The action of an intercalary meristem in the fronds determines the growth patterns of the sub-Antarctic kelp Lessonia spicata, which result in gradients in physiological activity and tissue composition. In this study, the allocation of phenolic compounds (phlorotannins) was examined in relation with the longitudinal patterns of photosynthesis (as 14C fixation and chlorophyll fluorescence) and storage of carbohydrates (mannitol and laminaran) in the blades. Moreover, exposures to UV radiation were carried out in order to test the photoprotective role of phlorotannins along different blade regions. The content of soluble phlorotannins was higher in the basal regions than in the middle and apical parts, which was correlated with the longitudinal allocation of mannitol and light-independent 14C fixation, two indicators of growth activity. In contrast, photosynthetic 14C fixation and allocation of laminaran were inversely related to the levels of phlorotannins along the blade. The photosynthetic characteristics measured using Imaging-PAM fluorescence did not show clear intra-blade patterns. UV exposures resulted in a decline in chlorophyll fluorescence (F v/F m) after 24 and 48 h (10 and 20 % respectively); however, no differential effects in different regions of the fronds were observed. The content of soluble phlorotannins in response to UV radiation was mostly determined by the time of exposure and less by the blade region: After a 6-h exposure phlorotannins decreased around 40 %, while after 48 h UV radiation stimulated the synthesis of phlorotannins by 22–99 %. Overall, after a 12-h UV exposure an increasing tendency of phlorotannins allocated in the basal regions of the blade was demonstrated. This study reinforces the idea that phlorotannins are important not only as secondary metabolites, but also their allocation in the thallus of kelps is integrated within the morpho-functional characteristics related to photosynthesis and biomass formation.