Barsha Bhushan Swain

Thiamethoxam is a widely used neonicotinoid known for its effect on soil microbial community. Synechocystis sp. PCC 6803 is a soil cyanobacterium and contributes organic carbon to soil microbial community through active photosynthesis, aiding synergistic soil microbial activity. However, not many studies have been made to decipher the effect of thiamethoxam on the photosynthetic electron transport of soil microalgae and cyanobacteria. We measured the effect of thiamethoxam (0–225 μM) on the growth, physiology, and photochemical performance of Synechocystis sp. PCC 6803 during 24 hr of exposure with an aim to explore its effect on photoelectron transport. Thiamethoxam caused a decrease in the growth and pigment content of Synechocystis, with more of an effect on chlorophyll a and carotenoids than on phycobiliproteins. A concentration-triggered increase in lipid peroxidation and photosynthetic pigment fluorescence was also observed. The OJIP fluorescence transient showed a continuous rise of the O-J phase with thiamethoxam concentration. The performance-indicating parameters φP0 and φE0 decreased, whereas the stress-indicating parameters VJ, M0, and φD0 increased with thiamethoxam treatment. The reaction center functions (Abs/RC, TR0/RC, and ET0/RC) indicated the increase of inactive reaction center with thiamethoxam treatment. The data agree with those obtained by atrazine treatment and indicated that the insecticide imposed a Photosystem II (PS II) herbicide–like effect on the acceptor side of PS II of Synechocystis. The inhibition of PS I reduction and decreased PS I functions support our conclusion. Our observation will enable researchers to evaluate the on-site effects of thiamethoxam on crop plants.

Leaf ontogeny of tropical evergreen tree species lasts several months with changes in size, shape, colouration and internal tissue distribution of leaves. Leaf initiation in Saraca asoca generally occurs once in a year during February–April, followed by very limited leafing thereafter. We measured the rate of photosynthesis, chlorophyll a fluorescence, energy quenching and PSII functions during the leaf ontogeny process. Observations were taken up to 35 days after opening of lamina (DAOL). Significant increase in the synthesis and accumulation of photosynthetic pigments but negative net photosynthesis was noticed during initial days of the ontogeny. The leaf moved from heterotrophy to autotrophy with gradual improvement of PSII functions. The ratio of intercellular CO2 (Ci) and ambient CO2 (Ca) showed significant change at ≥11 DAOL. Increase in the age of the leaf (between 5 and 28 DAOL) caused decrease in O-J rise and corresponding increase in J-I and I-P rise as well as of fluorescence maximum (FM) of the OJIP curve. The improvement of the electron transport components of the donor side of PSII was seen with increase in the functional oxygen evolving complex. The functional improvements of the donor and acceptor side of PSII during leaf ontogeny are discussed.