Acclimation of photosystem II to high temperature in two Wedelia species from different geographical origins: implications for biological invasions upon global warming

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Abstract
More intense, more frequent, and longer heat waves are expected in the future due to global warming, which could have dramatic ecological impacts. However, few studies have involved invasive species. The aims of this study were to examine the effect of extreme heating (40/35 °C for 30 d) on the growth and photosynthesis of an alien invasive species Wedelia trilobata and its indigenous congener (Wedelia chinensis) in South China, and to determine the development of this invasive species and its potential adaptive mechanism. In comparison with W. chinensis, W. trilobata suffered less inhibition of the relative growth rate (RGR) and biomass production due to high temperature, which was consistent with the changes of photosystem II (PSII) activity and net photosynthetic rate (Pn). High temperature caused a partial inhibition of PSII, but the adverse effect was more severe in W. chinensis. Measurement of the minimum fluorescence (Fo) versus temperature curves showed that W. trilobata had a higher inflexion temperature of Fo (Ti), indicating greater thermostability of the photosynthetic apparatus. Moreover, comparisons of absorbed light energy partitioning revealed that W. trilobata increased xanthophyll-dependent thermal dissipation (ΦNPQ) under high temperature, while retaining the higher fraction of absorbed light allocated to photochemistry (ΦPSII) relative to W. chinensis. The results suggest that the invasive W. trilobata has a high thermostability of its photosynthetic apparatus and an effective regulating mechanism in energy partitioning of PSII complexes to minimize potential damage and to retain greater capability for carbon assimilation. These factors confer greater heat stress tolerance compared with the native species. Therefore, the invasive W. trilobata may become more aggressive with the increasingly extreme heat climates.

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