Seasonal variability of vertical patterns in chlorophyll-a fluorescence in the coastal waters off Kimbiji, Tanzania

A study on the vertical pattern of chlorophyll-a (Chl-a) fluorescence was undertaken in the Mafia Channel offKimbiji, Tanzania. Data was collected during the Southeast Monsoon (SEM) and Northeast Monsoon (NEM) seasons. There was higher Chl-a concentration of 0.1 to 1.1 mgm-3 in the surface layer off Kimbiji to about 50 m depth due to the presence of mixed layer depth (MLD) which allowed water mixing in the layer. A deep Chl-a maximum was recorded at around 40 m depth during the NEM and between 40 and 70 m in the SEM. Surface water between longitude 39.9°E and 40.2°E had low Chl-a from the surface to about 50 m depth due to poor nutrient input. The NEM had an insignificantly higher Chl-a value than the SEM (p > 0.05) which differed from other studies in which Chl-a was higher during the SEM than the NEM, than, the Chl-a concentration was higher at the surface during the SEM than during the NEM. Satellite data showed higher Chl-a in the SEM than NEM, localized along the Mafia Channel. During the SEM season the wind pushes higher Chl-a water from the Mafia Channel towards the north and leads to a higher concentration at Kimbiji.

Coastal submesoscale processes and their effect on phytoplankton distribution in the southeastern Bay of Biscay

Submesoscale processes have a determinant role in the dynamics of oceans by transporting momentum, heat, mass, and particles. Furthermore, they can define niches where different phytoplankton species flourish and accumulate not only by nutrient provisioning but also by modifying the water column structure or active gathering through advection. In coastal areas, however, submesoscale oceanic processes act together with coastal ones, and their effect on phytoplankton distribution is not straightforward. The present study brings the relevance of hydrodynamic variables, such as vorticity, into consideration in the study of phytoplankton distribution, via the analysis of in situ and remote multidisciplinary data. In situ data were obtained during the ETOILE oceanographic cruise, which surveyed the Capbreton Canyon area in the southeastern part of the Bay of Biscay in early August 2017. The main objective of this cruise was to describe the link between the occurrence and distribution of phytoplankton spectral groups and mesoscale to submesoscale ocean processes. In situ discrete hydrographic measurements and multi-spectral chlorophyll a (chl a) fluorescence profiles were obtained in selected stations, while temperature, conductivity, and in vivo chl a fluorescence were also continuously recorded at the surface. On top of these data, remote sensing data available for this area, such as high-frequency radar and satellite data, were also processed and analysed. From the joint analysis of these observations, we discuss the relative importance and effects of several environmental factors on phytoplankton spectral group distribution above and below the pycnocline and at the deep chlorophyll maximum (DCM) by performing a set of generalized additive models (GAMs). Overall, salinity is the most important parameter modulating not only total chl a but also the contribution of the two dominant spectral groups of phytoplankton, brown and green algae groups. However, at the DCM, among the measured variables, vorticity is the main modulating environmental factor for phytoplankton distribution and explains 19.30 % of the variance. Since the observed distribution of chl a within the DCM cannot be statistically explained without the vorticity, this research sheds light on the impact of the dynamic variables in the distribution of spectral groups at high spatial resolution.

Effects of Bacterial Leaf Blight on Photosynthesis and Chlorophyll a Fluorescence in Susceptible and Resistant Rice Cultivars

Bacterial leaf blight (BLB) caused by Xanthomonas oryzae pv. oryzae is a serious disease affecting rice. Understanding of the effects of this disease on photosynthesis and chlorophyll (Chl) a fluorescence is important in rice management. We studied the effect of BLB on enzyme activities, photosynthetic rate (Pn) and Chl a fluorescence transient in susceptible Neiwuyou 8015 and resistant Shenzhou 98 rice cultivars. BLB had a negative effect on rice net photosynthesis (Pn) and stomatal conductance (Gs). Superoxide dismutase (SOD), polyphenol oxidase (PPO), phenylalanine ammonia lyase (PAL) activities, malondialdehyde (MDA) contents were increased while Chl content was decreased, indicating that rice photosynthetic functions were damaged by BLB. The effect of BLB on photosynthesis was greater in susceptible rice than in resistant rice. A significant difference of Chl a fluorescence transient curves was observed between BLB treatments and healthy ones. Parameters measured in transient rice Chl a fluorescence showed the photosynthetic reaction center was inactive after BLB. These findings will help in evaluating rice resistance and may be useful for rice disease management. © 2021 Friends Science Publishers

The pigment antenna constraining the light reactions: photosynthetic energy redistribution indicated by leaf absorbance changes

<p>The solar energy absorbed by the vegetation light-harvesting antenna complexes supplies the photosynthetic light reactions with a highly efficient transfer of quantum energy. The absorbed energy is efficiently transferred from one molecule to another, until being used by the reaction centres for the further carbon reactions. The energy transfer to the reaction centres is hereby highly regulated by the variable aggregation of pigments in the antenna complexes, allowing for quick and slower adjustments according to the incoming solar radiance. To control and protect the pigment antenna and the reaction centres from a potentially harmful solar radiance excess, these regulated photoprotective mechanisms are activated at different time scales at the antenna level, allowing vegetation to adapt to changing light conditions. The understanding of these energy regulative processes from optical measurements is essential in order to monitor plants' adaptation strategies to stressful environments and changing climates from remote sensing data.</p><p>Using high-spectral resolution leaf spectroscopy in a controlled laboratory set-up, we have observed detailed and significant absorbance shifts controlled by the pigment antennas themselves. Simultaneous measurements of both upward and downward spectrally-resolved leaf radiance (Lup(λ), Ldw(λ), W m<sup>-2</sup> sr<sup>-1</sup> nm<sup>-1</sup>) allowed us to observe the specific absorbance changes at leaf level, including changes in chlorophyll (Chl) a fluorescence emission (Fup(λ), Fdw(λ), W m<sup>-2</sup> sr<sup>-1</sup> nm<sup>-1</sup>). Interestingly, these changes due to shifts in energy redistribution were: 1) observed in the PAR region and even far beyond 700 nm, and 2) indicated a prominent role of both Carotenoid and Chl a molecules in the creation of alternative energy sinks, i.e. constraining the energy transfer to the reaction centres. Hereby, a significant redistribution of photosynthetic light energy was observed in the 500-800 nm range, highlighting this spectral region to be of potential interest for remote sensing. We further revealed that these energy redistributions do not necessary occur in parallel with Chl a fluorescence changes, illustrating the importance of different energy redistribution mechanisms constraining the photosynthetic light reactions. To conclude, a good quantitative understanding of all mechanisms of energy regulation in plants based on VIS-NIR wavelengths is essential 1) to be able to understand these trends using remote sensing data, 2) to better model the adaptations of vegetation to changing climate and environmental conditions, and 3) potentially better predict future trends in dynamic global vegetation models.</p>

Amplification of abiotic stress tolerance potential in rice seedlings with a low dose of UV-B seed priming

UV-B radiation is a major abiotic stress factor that adversely affects the growth and productivity of crop plants including rice (Oryza sativa L.). However, on the other hand, lower doses of UV-B radiation applied to seeds can have a priming effect on plants emerging from it. In this study, seeds of O. sativa var. kanchana were primed with UV-B radiation (6 kJ m–2) and were further subjected to NaCl, polyethylene glycol 6000 (PEG) and UV-B stress. The effects of UV-B priming in imparting NaCl, PEG and UV-B stress tolerance to rice seedlings were analysed through various photosynthetic features and antioxidative mechanisms. PSI and PSII activity levels as well as chl a fluorescence were found to be significantly higher in the UV-B primed and unstressed seedlings. When stress (NaCl, PEG and high UV-B) was imposed, increased PSI and PSII activity levels, chl a fluorescence and metabolite accumulation (proline, total phenolics and sugar) as well as nonenzymatic (ascorbate and glutathione) and enzymatic (superoxide dismutase, catalase, ascorbate peroxidase) antioxidants were recorded in UV-B primed and NaCl-stressed plants followed by UV-B primed and UV-B–stressed plants, and primed and PEG-stressed, compared with unprimed and stressed conditions. The results indicate that UV-B priming in rice seedlings effectively enhances the NaCl stress tolerance potential in rice to a greater extent than UV-B and PEG stress tolerance potential. The cost-effectiveness of UV-B seed priming is predominantly clear from the differing tolerance responses of rice seedlings exposed to different stress conditions.