Dynamics of regulated YNPQ and non-regulated YNO energy dissipation in sunflower leaves exposed to sinusoidal lights.

Better understanding of photosynthetic efficiency under fluctuating light requires a specific approach to characterize the dynamics of energy dissipation in photosystem II. In this study, we characterized the interaction between the regulated YNPQ and non-regulated YNO energy dissipation in outdoor- and indoor-grown sunflower leaves exposed to repetitive cycles of sinusoidal lights of five amplitudes (200, 400, 600, 800, 1000 µmol m-2 s-1) and periods (20, 40, 60, 90, 120 s). The different light cycles induced various patterns of ChlF emission, from which were calculated the complementary quantum yields of photochemical energy conversion YII, light-regulated YNPQ, and non-regulated YNO non-photochemical energy dissipation. During the light cycles, YNO varied in complex but small patterns relative to those of YNPQ, whose variations were mostly mirrored by changes in YII. The YNO patterns could be decomposed by fast Fourier transform into a main (MH) and several upper harmonics (UH). Concerning YNPQ dynamics, they were described by sinusoidal regressions with two components, one constant during the light cycles but increasing with the average light intensity (YNPQc), and one variable (YNPQv). Formation and relaxation of YNPQv followed the intensity of the sinusoidal lights, with lags ranging from 5 to 13 s. These lags decreased with the amplitude of the incident light, and were shorter by 37% in outdoor than indoor leaves. YNPQv and UHs responses to the growth conditions, amplitudes, and the periods of the sinusoidal light were closely correlated (r = 0.939), whereas MH and YNPQc varied similarly (r = 0.803). The analysis of ChlF induced by sinusoidal lights may be a useful tool to better understand the dynamics of energy dissipation in PSII under fluctuating lights.

How much biomass do plant communities pack per unit volume?

Aboveground production in terrestrial plant communities is commonly expressed in amount of carbon, or biomass, per unit surface. Alternatively, expressing production per unit volume allows the comparison of communities by their fundamental capacities in packing carbon. In this work we reanalyzed published data from more than 900 plant communities across nine ecosystems to show that standing dry biomass per unit volume (biomass packing) consistently averages around 1 kg/m(3) and rarely exceeds 5 kg/m(3) across ecosystem types. Furthermore, we examined how empirical relationships between aboveground production and plant species richness are modified when standing biomass is expressed per unit volume rather than surface. We propose that biomass packing emphasizes species coexistence mechanisms and may be an indicator of resource use efficiency in plant communities.