Photosynthesis and photoprotection responses to water stress in the wild-extinct plant Lysimachia minoricensis

Lysimachia minoricensis is an endemic species of the Balearic Islands that has become extinct in the wild, but persists in botanical gardens. Attempts of re-introducing the species into its natural habitat, which consisted in temporary dry streams, have failed. Low genetic variability has been reported for the garden individuals, suggesting that a reduced potential to adapt to environmental changes could be among the reasons for its extinction. In the present study, we particularly test whether photosynthesis and photoprotection responses of this species to water stress could help explaining the lack of success of this species in its natural habitat.Plants of L. minoricensis were grown in pots in a growth chamber. Soil water depletion was imposed over 20 days by stopping irrigation. Early stomata closure was observed in response to soil water depletion while leaves desiccated progressively. Although net photosynthesis was low in irrigated plants, due to a remarkably low mesophyll conductance to CO2, substantial photosynthetic activity was kept at severe drought, where leaf relative water content was as low as 50%, suggesting that L. minoricensis is a very drought-tolerant species. In parallel with decreased photosynthesis, thermal dissipation of the excess light and photorespiration progressively increased. The former was linearly related to increased de-epoxidation of the xanthophylls cycle. Photoprotection was effective, as pre-dawn maximum photochemical efficiency was maintained higher than 0.75 through the entire experiment. Moreover, photosynthetic capacity was largely (80%) recovered only 24 h after re-watering. These results show that stomatal regulation, photosynthetic metabolism and photoprotection in L. minoricensis are well adapted to water stress, suggesting that additional factors may be responsible for its status as a wild-extinct plant.