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Remote sensing methods enable detection of solar-induced chlorophyll a fluorescence. However, to unleash the full potential of this signal, intensive cross-disciplinary work is required to harmonize biophysical and ecophysiological studies. For decades, the dynamic nature of chlorophyll a fluorescence (ChlaF) has provided insight into the biophysics and ecophysiology of the light reactions of photosynthesis from the subcellular to leaf scales. Recent advances in remote sensing methods enable detection of ChlaF induced by sunlight across a range of larger scales, from using instruments mounted on towers above plant canopies to Earth-orbiting satellites. This signal is referred to as solar-induced fluorescence (SIF) and its application promises to overcome spatial constraints on studies of photosynthesis, opening new research directions and opportunities in ecology, ecophysiology, biogeochemistry, agriculture and forestry. However, to unleash the full potential of SIF, intensive cross-disciplinary work is required to harmonize these new advances with the rich history of biophysical and ecophysiological studies of ChlaF, fostering the development of next-generation plant physiological and Earth-system models. Here, we introduce the scale-dependent link between SIF and photosynthesis, with an emphasis on seven remaining scientific challenges, and present a roadmap to facilitate future collaborative research towards new applications of SIF.

Highlights • Modest brownification did not affect primary production, but increased bacterial production. • Concentration of inorganic nitrogen was the primary driver for the phytoplankton development. • Brownification benefitted picophytoplankton. Climate change is projected to cause brownification of some coastal seas due to increased runoff of terrestrially derived organic matter. We carried out a mesocosm experiment (15 d) to test the effect of this on the planktonic ecosystem expecting reduced primary production and shifts in the phytoplankton community composition. The experiment was set up in 2.2 m3 mesocosm bags using four treatments, each with three replicates: control (Contr) without any manipulation, organic carbon additive HuminFeed (Hum; 2 mg L−1), inorganic nutrients (Nutr; 5.7 μM NH4 and 0.65 μM PO4), and combined Nutr and Hum (Nutr + Hum) additions. Measured variables included organic and inorganic nutrient pools, chlorophyll a (Chla), primary and bacterial production and particle counts by flow cytometry. The bags with added inorganic nutrients developed a phytoplankton bloom that depleted inorganic N at day 6, followed by a rapid decline in Chla. Brownification did not reduce primary production at the tested concentration. Bacterial production was lowest in the Contr, but similar in the three treatments receiving additions likely due to increased carbon available for heterotrophic bacteria. Picoeukaryotes clearly benefited by brownification after inorganic N depletion, which could be due to more effective nutrient recycling, nutrient affinity, light absorption, or alternatively lower grazing pressure. In conclusion, brownification shifted the phytoplankton community composition towards smaller species with potential effects on carbon fluxes, such as sinking rates and export to the sea floor.

The effects of increased photosynthetically active radiation (PAR) and ultraviolet radiation (UVR) on species diversity, biomass and photosynthetic activity were studied in fast ice algal communities. The experimental set-up consisted of nine 1.44 m(2) squares with three treatments: untreated with natural snow cover (UNT), snow-free (PAR + UVR) and snow-free ice covered with a UV screen (PAR). The total algal biomass, dominated by diatoms and dinoflagellates, increased in all treatments during the experiment. However, the smaller biomass growth in the top 10-cm layer of the PAR + UVR treatment compared with the PAR treatment indicated the negative effect of UVR. Scrippsiella complex (mainly Scrippsiella hangoei, Biecheleria baltica and Gymnodinium corollarium) showed UV sensitivity in the top 5-cm layer, whereas Heterocapsa arctica ssp. frigida and green algae showed sensitivity to both PAR and UVR. The photosynthetic activity was highest in the top 5-cm layer of the PAR treatment, where the biomass of the pennate diatom Nitzschia frigida increased, indicating the UV sensitivity of this species. This study shows that UVR is one of the controlling factors of algal communities in Baltic Sea ice, and that increased availability of PAR together with UVR exclusion can cause changes in algal biomass, photosynthetic activity and community composition.

Light (20-450 μmol photons m(-2) s(-1)), temperature (3-11 °C) and inorganic nutrient composition (nutrient replete and N, P and Si limitation) were manipulated to study their combined influence on growth, stoichiometry (C:N:P:Chl a) and primary production of the cold water diatom Chaetoceros wighamii. During exponential growth, the maximum growth rate (~0.8 d(-1)) was observed at high temperature and light; at 3 °C the growth rate was ~30% lower under similar light conditions. The interaction effect of light and temperature were clearly visible from growth and cellular stoichiometry. The average C:N:P molar ratio was 80:13:1 during exponential growth, but the range, due to different light acclimation, was widest at the lowest temperature, reaching very low C:P (~50) and N:P ratios (~8) at low light and temperature. The C:Chl a ratio had also a wider range at the lowest temperature during exponential growth, ranging 16-48 (weight ratio) at 3 °C compared with 17-33 at 11 °C. During exponential growth, there was no clear trend in the Chl a normalized, initial slope (α*) of the photosynthesis-irradiance (PE) curve, but the maximum photosynthetic production (P(m)) was highest for cultures acclimated to the highest light and temperature. During the stationary growth phase, the stoichiometric relationship depended on the limiting nutrient, but with generally increasing C:N:P ratio. The average photosynthetic quotient (PQ) during exponential growth was 1.26 but decreased to

A new approach for modelling plant growth using the software AMAPpara is presented. This software takes into consideration knowledge about plant architecture which has been accumulated at the Plant Modelling Unit of CIRAD for several years, and introduces physiological concepts in order to simulate the dynamic functioning of trees. The plant is considered as a serial connection of vegetative organs which conduct water from the roots to the leaves. Another simple description of the plant as a network of parallel pipes is also presented which allows an analytical formulation of growth to be written. This recurring formula is used for very simple architectures and is useful to understand the role of each organ in water transport and assimilate production. Growth simulations are presented which show the influence of modifications in architecture on plant development. (Resume d'auteur)

Change in plants as bicarbonate rises Freshwater plants can be broadly divided into two major categories according to their photosynthetic traits: Some use carbon dioxide as their carbon source, whereas others use bicarbonate. Iversen et al. found that the relative concentrations of these two inorganic carbon forms in water determine the functional composition of plant communities across freshwater ecosystems (see the Perspective by Marcé and Obrador). They created global maps revealing that community composition is structured by catchment geology and not climate (in contrast to the terrestrial realm, where the trait composition is structured by temperature and rainfall). Anthropogenic influences from land-use change are causing large-scale increases in bicarbonate concentrations in freshwater catchments and are thus leading to wholesale changes in the composition of their aquatic plant communities. Unlike in land plants, photosynthesis in many aquatic plants relies on bicarbonate in addition to carbon dioxide (CO2) to compensate for the low diffusivity and potential depletion of CO2 in water. Concentrations of bicarbonate and CO2 vary greatly with catchment geology. In this study, we investigate whether there is a link between these concentrations and the frequency of freshwater plants possessing the bicarbonate use trait. We show, globally, that the frequency of plant species with this trait increases with bicarbonate concentration. Regionally, however, the frequency of bicarbonate use is reduced at sites where the CO2 concentration is substantially above the air equilibrium, consistent with this trait being an adaptation to carbon limitation. Future anthropogenic changes of bicarbonate and CO2 concentrations may alter the species compositions of freshwater plant communities. One sentence summary: The widespread photosynthetic trait of freshwater plants, bicarbonate use, has a global biogeography controlled by catchment characteristics

The photosynthetic rate of (Willd.), (Hedw.) and (Sw.) grown in plastic containers was monitored with infrared gas analyser in open air under natural weather conditions. It proved that the photosynthetic rate of wet moss cushions was satisfactorily predicted by temperature and light intensity. In dry moss cushions this kind of model gave too high an estimate for photosynthetic rate. Water requirements of each moss species were found to be moderate, and water content of moss cushions limited photosynthetic rate only under serious water deficiency.Pleurosium schreberiHylocomnium splendensDicranum undulatum

The aim of the present paper was to study the annual production of (Brid.) Mitt., (Hedw.) B.S.G and Sw. as a function of light available for photosynthesis. The productivity of the above moss species is studied using the harvested quadrats method in Norway spruce () stands of the site type representing different stand density classes (basal area from 0 to 34 m2/ha) in Southern Finland.Pleurozium schreberiHylocomnium splendensDicranum polysetumPicea abiesMyrtillus Pleurozium schreberiDicranum polysetumHylocomnium splendens

Light intensity inside the canopy varies considerably both in space and time. A new apparatus was developed which is disturbed as little as possible by the above-mentioned variation. The construction is based on the linear relationships between light intensity (measured using silicon diodes) and photosynthesis. This procedure permits linear operations (summing and integration) to be carried out on the output of the diodes without any loss of accuracy. There are five diodes in each assimilation chamber. A model, in which the independent variables include ligth, measured with the present equipment, and temperature, fits the photosynthetic rates well even inside the canopy. The PDF includes a summary in Finnish.

Data were collected over 3 yr during the growing season in a 20-yr-old stand at the Forestry Field Station of the University of Helsinki, Finland, to study the relations of photosynthesis and transpiration to environmental factors (irradiance, temperature, water content of soil and air) and metabolic processes.