Lemnaceae are small freshwater plants with extraordinary high growth rates. We aimed to test whether this correlates with a more efficient photosynthesis, the primary energy source for growth. To this end, we compared photosynthesis properties of the duckweed Lemna minor and the terrestrial model plant Arabidopsis thaliana. Chlorophyll fluorescence analyses revealed high similarity in principle photosynthesis characteristics; however, Lemna exhibited a more effective light energy transfer into photochemistry and more stable photosynthesis parameters especially under high light intensities. Western immunoblot analyses of representative photosynthesis proteins suggested potential post-translational modifications in Lemna proteins that are possibly connected to this. Phospho-threonine phosphorylation patterns of thylakoid membrane proteins displayed a few differences between the two species. However, phosphorylation-dependent processes in Lemna such as photosystem II antenna association and the recovery from high-light-induced photoinhibition were not different from responses known from terrestrial plants. We thus hypothesize that molecular differences in Lemna photosynthesis proteins are associated with yet unidentified mechanisms that improve photosynthesis and growth efficiencies. We also developed a high-magnification video imaging approach for Lemna multiplication which is useful to assess the impact of external factors on Lemna photosynthesis and growth.

Photosynthesis needs to run efficiently under permanently changing illumination. To achieve this, highly dynamic acclimation processes optimize photosynthetic performance under a variety of rapidly changing light conditions. Such acclimation responses are acting by a complex interplay of reversible molecular changes in the photosynthetic antenna or photosystem assemblies which dissipate excess energy and balance uneven excitation between the two photosystems. This includes a number of non-photochemical quenching processes including state transitions and photosystem II remodeling. In the laboratory such processes are typically studied by selective illumination set-ups. Two set-ups known to be effective in a highly similar manner are (i) light quality shifts (inducing a preferential excitation of one photosystem over the other) or (ii) dark-light shifts (inducing a general off-on switch of the light harvesting machinery). Both set-ups result in similar effects on the plastoquinone redox state, but their equivalence in induction of photosynthetic acclimation responses remained still open. Here, we present a comparative study in which dark-light and light-quality shifts were applied to samples of the same growth batches of plants. Both illumination set-ups caused comparable effects on the phosphorylation of LHCII complexes and, hence, on the performance of state transitions, but generated different effects on the degree of state transitions and the formation of PSII super-complexes. The two light set-ups, thus, are not fully equivalent in their physiological effectiveness potentially leading to different conclusions in mechanistic models of photosynthetic acclimation. Studies on the regulation of photosynthetic light acclimation, therefore, requires to regard the respective illumination test set-up as a critical parameter that needs to be considered in the discussion of mechanistic and regulatory aspects in this subject.

Lemnaceae are aquatic freshwater plants with extraordinary high growth rates. We have studied selected physiological and molecular photosynthesis properties of the duckweed Lemna minor and compared these to the terrestrial model species Arabidopsis thaliana. Lemna and Arabidopsis plants grown under identical light intensities displayed similar photosynthesis characteristics, however, Lemna exhibited slighty better quenching efficiencies pointing to improved light utilization in the duckweed. Western-immuno-blot analyses of representative photosynthesis proteins suggest various post-translational modifications in Lemna that might be associated to this. Phospho-threonine phosphorylation patterns of thylakoid membranes uncovered differences between the two species. Testing the photosystem II antenna association of Lemna minor in dark and light by 77K chlorophyll fluorescence emission experiments, however, revealed a typical association as reported in terrestrial plants. High light stress experiments causing photoinhibition and subsequent recovery from it were not substantially different in Lemna when compared to Arabidopsis. We hypothesize that the molecular differences in Lemna photosynthesis proteins are associated with evolutionary adaptations to the aquatic life style and ultimately with the high growth rates. We also developed a video imaging approach for Lemna multiplication at high magnification that will be useful to assess the impact of external factors on Lemna photosynthesis and growth.