Insights into carotenoid dynamics in non-foliar photosynthetic tissues of avocado

Leaves are the main photosynthetically active tissues in most plants. However, stems and fruits are also important for the overall carbon balance of the plant because of their contribution to fixation of the CO2 released by respiration. Photosynthesis could not be possible without a complete set of photoprotection mechanisms, which include the ubiquitous violaxanthin (V) cycle and the taxonomically restricted lutein epoxide (Lx) cycle. In this work, we characterise carotenoid stoichiometry in photosynthetic stems and fruits of avocado in comparison with that of leaves and specifically whether Lx is present in these tissues and also whether it is involved in a light-driven cycle. Avocado was selected as model species to study whether both cycles were functional in non-foliar photosynthetic structures (stems and fruits). An unusual pigment composition was observed in avocado fruit, with a high content of cis-V and cis-Lx, suggesting a different photosynthetic function. In stems, both xanthophylls de-epoxidated upon illumination, but only V recovered in the dark, indicating the existence of a possible 'truncated' Lx cycle. Lx in fruits was de-epoxidated only when its pool was higher than a threshold of 30 mmol mol-1 chlorophyll, indicating a high non-photoconvertible pool of Lx. We conclude that, at least in stems, the dynamic regulation of photosynthetic activity could also depend on the Lx cycle.