Leaf chlorophyll fluorescence and reflectance of oakleaf lettuce exposed to metal and metal(oid) oxide nanoparticles

BACKGROUND

Most nanoparticles (NPs) have a significant impact on the structure and function of the plant photosynthetic apparatus. However, their spectrum of action varies significantly, from beneficial stimulation to toxicity, depending on the type of NPs, the concentration used and plant genotypic diversity. Photosynthetic performance can be assessed through chlorophyll a fluorescence (ChlF) measurements. These data allow to indirectly obtain detailed information about primary light reactions, thylakoid electron transport reactions, dark enzymatic stroma reactions, slow regulatory processes, processes at the pigment level. It makes possible, together with leaf reflectance performance, to evaluate photosynthesis sensitivity to stress stimuli.

RESULTS

We investigated effects of different metal and metal(oid) oxide nanoparticles on photosynthesis of oakleaf lettuce seedlings by monitoring the chlorophyll a fluorescence light radiation and reflectance from the leaves. Observations of ChlF parameters and changes in leaf morphology were carried out for 9 days in two-day intervals. Spectrophotometric studies were performed at 9th day. Suspensions of NPs with the following concentrations were used: 6% TiO2, SiO2; 3% CeO2, SnO2, Fe2O3; 0.004% (40 ppm) Ag; 0.002% (20 ppm) Au. Nanoparticles were applied directly on the leaves which caused small symptoms of chlorosis, necrosis and leaf veins deformation, but the plants fully recovered to the initial morphological state at 9th day. Leaf reflectance analysis showed an increase in FRI for SiO2-NPs and CeO2-NPs treatments and ARI2 for Fe2O3, however, WBI and PRI coefficients for the latter nanoparticle were lower than in control. Chlorophyll a fluorescence parameters have changed due to NPs treatment. Fe2O3-NPs caused an increase in Fv/F0, PIABS, ET0/RC, DI0/RC, ABS/RC in different time points in comparison to control, also Ag, Au and SnO2 treatment caused an increase in Fv/F0, PIABS or ET0/RC, respectively. On the other hand, TiO2-NPs caused a decrease in Fv/Fm and Fv/F0 parameters, but an increase in DI0/RC value was observed. SnO2-NPs decreased PIABS, but increased ET0/RC than compared to control. Nanoparticles affected the shape of the O-J-I-P curve in slight manner, however, further analyses showed unfavourable changes within the PSII antenna, manifested by a slowdown in the transport of electrons between the Chl molecules of the light-harvesting complex II and the active center of PSII due to NPs application.

CONCLUSION

Changes in ChlF parameters and leaf reflectance values clearly proved the significant influence of NPs on the functioning of the photosynthetic apparatus, especially right after NPs application. The nature of these changes was strictly depended on the type of nanoparticles and sometimes underwent very significant changes over time. The greatest changes in ChlF parameters were caused by Fe2O3 nanoparticles, followed by TiO2-NPs. After slight response of O-J-I-P curves to treatment of the plants with NPs the course of the light phase of photosynthesis stabilized and at 9th day were comparable to the control curve.

Tissue specific disruption of photosynthetic electron transport rate in pigeonpea (Cajanus cajan L.) under elevated temperature

Nutritional rich pigeonpea (Cajanus cajan [L.]), a perennial shrub member of family Fabaceae is the sixth most important grain legume of the world. Continuous rise of temperature and current global climate scenario limits plant growth and performance but photosynthetic machineries are adversely affected. The aim of this study was the analysis of tissue specific photosynthetic pigments and photosynthetic electron transport rate under elevated temperature. Two different stages of both leaf (young and mature), and pod (young and mature) were chosen, and photosynthetic pigment and J-I-P tests were measured. Leaves and pods were detached and incubated in normal temperature (25°C) for 24 h in two different condition one light irradiance (100 µmol m^-2 s^-1) and another darkness as control, and treated with high temperature 45°C for 24 h and repeated previous measurements were taken. Tissue specific photosynthetic pigments variation were observed; J-I-P parameters clearly revealed that elevated temperature has greater influence on photosystem II (PSII) electron transport than photosystem I (PSI), and significant changes were observed in pods than leaves. Young tissues were adversely affected by elevated temperature. In addition, the J-I-P tests and energy pipeline model indicated that PSI electron transport rate of leaves and pods appeared to be more thermo-tolerance than those in PSII. Only a minor drop in pigments pool and photosynthetic performance was observed after 24 h of darkness. O-J-I-P transients can be used as a sensitive, nondestructive method for measuring heat stress damage and a special tool for investigating action sites of high temperature stress. Findings of this study will contribute to basic understanding of photosynthetic performance, and to screen potential thermo-tolerant genotypes of pigeonpea to sustain in either current scenario of climate change or/and erratic future climatic conditions.

IMPACT OF BLEACHING STRESS ON THE FUNCTION OF THE OXYGEN EVOLVING COMPLEX OF ZOOXANTHELLAE FROM SCLERACTINIAN CORALS(1)

Global climate change is leading to the rise of ocean temperatures and is triggering mass coral bleaching events on reefs around the world. The expulsion of the symbiotic dinoflagellate algae is believed to occur as a result of damage to the photosynthetic apparatus of these symbionts, although the specific site of initial impact is yet to be conclusively resolved. Here, the sensitivity of the oxygen evolving complex (OEC) to bleaching stress was studied as well as its natural variation between seasons. The artificial electron donor, diphenyl carbazide (DPC), was added to cultured, freshly isolated and expelled (bleaching treatments only) zooxanthellae suspensions. Chl a fluorescence and oxygen production measurements showed that upon addition of DPC, no restoration of diminished photochemical efficiency occurred under control or bleaching conditions. This result was consistent between 12 h and 5 d bleaching treatments on Pocilloporadamicornis, indicating that the OEC is not the primary site of damage, and that zooxanthellae expulsion from the host is a nonselective process with respect to the functioning of the OEC. Further experiments measuring fast induction curves (FICs) revealed that in both summer and winter, the temperature when OEC function was lost occurred between 7°C and 14°C above the sea surface temperature. FIC and oxygen production measurements of P. damicornis during exposure to bleaching stress demonstrated that the thermotolerance of the OEC increased above the temperature of the bleaching treatment over a 4 h period. This finding indicates that the OEC has the capacity to acclimate between seasons and remains functional at temperatures well above bleaching thresholds.