Abstract
Ephemerals, widely distributed in the Gobi desert, have developed significant characteristics to sustain high photosynthetic efficiency under high light (HL) conditions. Since the light reaction is the basis for photosynthetic conversion of solar energy to chemical energy, the photosynthetic performances in thylakoid membrane of the spring ephemerals in response to HL were studied. Three plant species, namely two C3 spring ephemeral species of Cruciferae: Arabidopsis pumila (A. pumila) and Sisymbrium altissimum (S. altissimum), and the model plant Arabidopsis thaliana (A. thaliana) were chosen for the study. The ephemeral A. pumila, which is genetically close to A. thaliana and ecologically in the same habitat as S. altissimum, was used to avoid complications arising from the superficial differences resulted from comparing plants from two extremely contrasting ecological groups. The findings manifested that the ephemerals showed significantly enhanced activities of photosystem (PS) II under HL conditions, while the activities of PSII in A. thaliana were markedly decreased under the same conditions. Detailed analyses of the electron transport processes revealed that the increased plastoquinone pool oxidization, together with the enhanced PSI activities, ensured a lowered excitation pressure to PSII of both ephemerals, and thus facilitated the photosynthetic control to avoid photodamage to PSII. The analysis of the reaction centers of the PSs, both in terms of D1 protein turnover kinetics and the long-term adaptation, revealed that the unusually stable PSs structure provided the basis for the ephemerals to carry out high photosynthetic performances. It is proposed that the characteristic photosynthetic performances of ephemerals were resulted from effects of the long-term adaptation to the harsh environments.
Highlights
Ephemerals are widely distributed in the Gobi desert and play important roles in maintaining and restoring the desert ecosystems (Wang et al, 2003)
To eliminate the effects of the altered optical properties on the evaluation of relative electron transport rate through PSII (rETR) value in different samples, the lightsaturation index saturation index of photosynthetic capacity (Ek), which is independent of leaf absorptivity, was used to evaluate the rETR, and it was determined from the intercept point of α- slope and Pmax: Ek = Pmax/α-slope, where Pmax is the light-saturated rate of photosynthesis, and α-slope was calculated from the linear rise of the photosynthesis rate versus irradiance (Blache et al, 2011)
In order to reveal the mechanisms of ephemerals’ high photosynthetic efficiencies under strong light conditions, we investigated the characteristic responses to high irradiance in the three C3 species, a spring ephemeral A. pumila with close phylogenetic relationship to the reference plant A. thaliana and a second spring ephemeral S. altissimum widely spread in the Gobi district and a typical habitant in drought regions with high irradiances (Allen and Knight, 1984)
Summary
Ephemerals are widely distributed in the Gobi desert and play important roles in maintaining and restoring the desert ecosystems (Wang et al, 2003). To survive the harsh environmental conditions such as extremely high irradiance and high temperature, ephemerals in the desert areas have evolved many distinctive adaptation mechanisms. Ephemerals are able to accomplish their life cycle quickly under strong light conditions (Ehleringer et al, 1979; Yuan et al, 2009). Ephemerals possess high photosynthetic activity under strong light conditions because of their extremely high photosynthetic light saturation point that enables their photosynthetic apparatus to sustain efficient photosynthesis without suffering from any photodamage even under full sunlight (Ehleringer, 1983; Yuan et al, 2009). The electron transport and photosynthetic CO2 uptake in ephemerals are not saturated at light intensities under 2000 μmol photons m−2 s−1 (Ehleringer, 1983; Tu et al, 2012). In order to get a better understanding of ephemerals’ photosynthesis, it should be known which components of the photosynthetic primary reactions are modified
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