Zinc nano and zinc ethylenediaminetetraacetic acid (EDTA) mediated water deficit stress alleviation in pearl millet (Pennisetum glaucum (L.) R. Br.): Photosystem II electron transport and pigment dynamics

Water stress adversely affects the photosynthetic apparatus and pigment composition in plants, leading to reduced yields and compromised plant health. Zinc (Zn) foliar spray in nano form presents a potential solution to ameliorate water deficit stress. We attempt here a detailed dissection of electron transport in Photosystem II (PSII) through studies on chlorophyll a fast fluorescence kinetics and non-photochemical quenching (NPQ) and pigment dynamics in response to water stress. We also investigated the possible changes in these processes and the regulation of it by Zn Nano and Zn Ethylenediaminetetraacetic acid (EDTA) foliar sprays that may lead to amelioration of stress. Our results indicated that water stress created a "traffic jam" like situation in the electron transport system of Photosystem II, leading to decreased photosynthetic efficiency. Treatments with water deficit stress + Zn Nano (particle size < 90 nm) spray with Zn concentration at 20 mg L⁻¹ and water deficit stress + Zn EDTA spray (solid material size ∼ 100 µm) with Zn concentration at 240 mg L⁻¹ effectively ameliorated water deficit stress by its action on flux ratio parameters viz., quantum yield for electron transport (φ_E0), probability of electron transport beyond Q_A (ψ₀) and quantum yield of electron transport from Q_A^⁻ to PS1 end electron acceptors (ϕ_R0) and also the specific fluxes and phenomenological fluxes. These treatments positively influenced chlorophyll content, and xanthophyll components, including violaxanthin, antheraxanthin, and zeaxanthin, and reduced NPQ and the de expoxidation state. Higher concentrations of Zn Nano foliar spray (water stress + Zn Nano spray Zn @ 30 mg L⁻¹) did not ameliorate water deficit stress as effectively as the lower concentrations, although this higher concentration was not in any way toxic. This lack of stress amelioration at higher concentrations of Zn Nano spray may be due to physiological limitations of elemental zinc action within the plant. Our findings suggest that Zn foliar sprays in Nano and EDTA form at optimum concentrations can significantly improve plant resilience to water stress.