The global population is expected to reach 10.4 billion by 2023, of which 281.6 million people experienced food insecurity, and this will continue to be the case as the population continues to grow. To address this challenge, increasing crop productivity in stressful environments is one approach (Singh et al., 2023). Among the various abiotic stresses, drought is an important factor affecting crop productivity and is a serious problem due to climatic changes and the unpredictable and irregular distribution of rainfall patterns (Zeng et al., 2023). Therefore, it is crucial to develop drought-tolerant varieties and efficient crop management strategies to maintain productivity.

Globally, next to rice (Oryza sativa L.) and wheat (Triticum aestivum L.), maize (Zea mays L.) is the predominant crop grown for food, feed, and fuel. Globally, 50% of the maize cultivation areas are under rainfed conditions (Biradar et al., 2009). In the USA, 92% of the maize-growing region is rainfed (Kimm et al., 2020), and a similar situation has prevailed in China, sub-Saharan Africa, and India (Hu et al., 2023; Erenstein et al., 2022). Maize is more sensitive to drought stress than wheat (Kim and Lee, 2023). Drought stress during the vegetative, pollination, and grain-filling stages of maize reduced the grain yield by 25%, 50%, and 21%, respectively, suggesting that the reproductive phases are more sensitive than other growth stages (Sah et al., 2020). When drought stress occurs two weeks before anthesis, the yield decreases by 3%–4% per day (Nielsen, 2002). Yield decline depends on the intensity and duration of drought stress and crop stage. Based on weather and historical crop production data, rainfed maize grain yield in arid and semiarid regions is predicted to decline by 22% by 2050 (Schlenker and Lobell, 2010). In general, maize is more suitable for irrigated than rainfed ecosystems, as evidenced by the magnitude of the yield decrease due to differential rainfall patterns in the rainfed ecosystem (Qiao et al., 2021). Therefore, it is critical to develop a crop management strategy using an alternative approach to sustain the yield potential of rainfed maize.

Drought stress has diverse effects on crop growth, physiology, and grain yields. The major effects of drought stress are associated with the loss of photosynthetic activity, increased abscisic acid biosynthesis, improved osmotic adjustment, and improved antioxidant defense systems in maize (Sheoran et al., 2022; Gong et al., 2014). A reduction in photosynthesis may be associated with reduced stomatal conductance, which prevents water loss owing to reduced tissue turgor pressure and water potential (Singh and Reddy, 2011). Stomatal regulation is related to transpiration rate rather than photosynthetic rate because of high stomatal conductance, and photosynthetic rate levels off; however, the transpiration continues to rise linearly under mild drought stress (Condon et al., 2002; Djanaguiraman et al., 2024). However, the reduced photosynthetic rate is primarily due to stomatal limitation, which is evident from the decreased intercellular to atmospheric CO₂ levels (C_i/C_a ratio) under moderate drought stress (Flexas et al., 2002). A decrease in the photosynthetic rate is associated with reduced photochemistry and photosynthetic enzyme activity under severe drought stress (Aranjuelo et al., 2011; Edmeades et al., 1999).

Drought stress during the flowering stage results in a reduction in the number of grains per cob and lower grain yield in maize (Edmeades et al., 1999). Under severe drought stress, a reduced seed-set percentage is associated with loss of pollen and pistil functions (Boyer and Westgate, 2004), resulting in complete seed abortion (Li et al., 2023). Drought also increases the production of reactive oxygen species (ROS), such as superoxide radicals, hydrogen peroxide, and singlet oxygen, and excessive ROS production causes oxidative damage to cell organelles, especially chloroplasts, impairing the normal function of chloroplasts at the cellular level (Dietz and Pfannschmidt, 2011; Foyer and Noctor, 2009). The ability of the cell to scavenge the generated ROS depends on the activity of antioxidant enzymes, such as superoxide dismutase, catalase, and peroxidase (Faize et al., 2011; Yang et al., 2021). Foliar application of chemicals possessing antioxidant properties is an alternative approach for minimizing the effects of ROS under drought stress.

Recently, the use of nanoparticles with antioxidant properties to alleviate drought has increased (Djanaguiraman et al., 2024; Li et al., 2024). The use of nanomaterials to mitigate oxidative damage is more effective because of their exceptionally increased surface area (Omran and Baek, 2021; Xu et al., 2023). However, the antioxidant properties of nanomaterials differ depending on their chemical configuration, surface coating, charge, size, and crystallinity (Shah et al., 2017). The most commonly used metallic nanomaterials to alleviate oxidative damage are silver (Khan et al., 2020), ceria (Djanaguiraman et al., 2024), copper (Noman et al., 2020), copper oxide (Xiong et al., 2021), hematite (Al-Amri et al., 2020), selenium (Djanaguiraman et al., 2018), silica (Wang et al., 2021), silicon dioxide (Banerjee et al., 2021), titanium dioxide (Hu et al., 2020), and zinc oxide (Rai-Kalal and Jajoo, 2021). Overall, nano-zinc and nano-selenium possess antioxidant properties and can be used to mitigate drought-induced oxidative stress in plants. However, to our knowledge, the antioxidant properties of zinc selenide quantum dots (ZnSe QDs) under normal and drought stress conditions in plants have not yet been validated.

Hence, we hypothesized that the exogenous application of nanomaterials (ZnSe QDs) possessing antioxidant properties is an effective and convenient approach to mitigate drought-induced oxidative stress in maize. The aim of this study was to quantify the (i) toxicity potential of ZnSe QDs and (ii) drought mitigation potential of ZnSe QDs by assessing the transpiration rate, photosynthetic rate, oxidant production, antioxidant enzyme activity and seed yield of maize under limited soil moisture levels.

The major findings of this study are: (i) the synthesized ZnSe QDs were quasi-spherical with 10 nm, confirmed through XRD, SEM, and TEM, (ii) the exposure of earthworm and azolla to ZnSe QDs up to 20 mg L⁻¹ did not cause any toxic effects, but, above this concentration, harmful effects occurred, (iii) foliar application of ZnSe @ 20 mg L⁻¹ to maize plants grown under progressive soil drying restricted the transpiration rate at higher soil moisture level through partial stomatal closure compared to water spray, and (iv) application of ZnSe @ 20 mg L⁻¹ to rainfed maize reduced leaf oxidative damage and increased photosynthetic rate and seed set percentage, resulting in higher seed yield.

The XRD pattern of the ZnSe QDs showed three major diffraction peaks at 2θ values of 27.2, 45.3, and 53.6, corresponding to the (111), (220), and (311) diffraction planes, respectively, which matched the cubic zinc-blende structure (Indirajith et al., 2014; JCPDS card: 80-0021). The broad diffraction peaks of the (111), (220), and (311) planes indicate the nanocrystalline size of ZnSe ( Figure 1A ). Based on XRD data, it is concluded that (i) capping with cysteine has not changed the nature, phase and crystallinity of ZnSe, (ii) ZnSe had an average diameter of ~4 nm, indicating strong confinement, (iii) in addition to ZnSe peaks there was no additional peaks associated with the formation of ZnO or Se phase.

The SEM image showed that the ZnSe QDs were spherical and aggregated with an average particle size of 10 nm–20 nm ( Figure 1B ). The agglomeration of the particles is due to their high crystallinity (Gupta et al., 2016). The formation of semi-spherical particles may be due to the strong binding affinity of n-acetyl cysteine to the ZnSe surface, leading to the reduction of surface energies at all surfaces, resulting in the formation of ZnSe QDs (Bain et al., 2015). Similarly, the TEM image indicated that the particle was spherical, and the particle size was between 5 nm and 6 nm ( Figure 1C ), which agrees with the XRD data.

The HRTEM image shows that the ZnSe QDs had a good crystalline nature, as evidenced by the SAED patterns. The ZnSe QDs showed a quasi-ring-like diffraction pattern with several spots, which explains the crystalline nature of ZnSe at the nanometer scale ( Figure 1D ) (Sridevi et al., 2022). The interplanar spacing of ZnSe was 0.32 nm and 0.21 nm, corresponding to the (111) and (220) plane, respectively, of the cubic structure of the ZnSe phase, which was in close agreement with XRD data ( Figure 1E ) (Verma et al., 2015).

The FTIR spectra of the ZnSe QDs showed a broad peak at 3,257 cm⁻¹, which was assigned to the –OH stretching mode ( Figure 1G ) (Mahapatra et al., 2014). The peaks at 1,581 cm⁻¹ and 1,373 cm⁻¹ were associated with the C=O stretching vibration mode of ZnSe QDs (Sridevi et al., 2021), which indicated that the formed product had low water content and an acetyl group as a capping agent in the ZnSe QDs. The peaks at 664 cm⁻¹ and 731 cm⁻¹ were associated with ZnSe (Gupta et al., 2022). The absence of an intense symmetric stretching band at 2,548 cm⁻¹ in ZnSe, which is a characteristic feature of pure n-acetyl cysteine, indicates that many deprotonated n-acetyl cysteine molecules capped the surface of ZnSe.

The survival of earthworms up to 14 d in natural soil spiked with ZnSe QDs showed no mortality up to 500 mg L⁻¹, and the growth rate (mg day⁻¹) started to decrease from 15 mg L⁻¹ of ZnSe QDs. Similar results were observed by McShane et al. (2012) and Vidane et al. (2023), who reported that exposing earthworms to rutile TiO₂ at up to 10,000 mg kg⁻¹ did not result in mortality. In general, earthworms avoid digging holes for food in polluted soils (Jovana et al., 2014) and use the stored glycogen and proteins in the body (Ye et al., 2016). The decrease in body weight at higher concentrations of ZnSe (>25 mg L⁻¹) might be due to starvation ( Figure 2A ; LD₅₀). The decrease in the growth rate of azolla at higher concentrations (>25 mg L⁻¹; Figure 2B ) of ZnSe could be associated with a reduced photosynthetic rate (Oukarroum et al., 2013; Zarate-Cruz et al., 2016). A similar observation was made in Arabidopsis challenged with 100 mg L⁻¹ ZnO (Landa et al., 2012).

Reduced transpiration rates by ZnSe QDs spray ( Figures 3B ; 4) indicated a restricted but continuous water flow that coordinates with stomatal movement to prevent desiccation (Sinclair, 2018). The decreased daytime hourly transpiration rate in ZnSe QDs and combined Zn and Se spray was due to decreased stomatal pore size ( Figures 5B, D, F ), indicating a positive relationship between the stomatal pore size and transpiration rate (Comstock, 2002). An early partial stomatal closure trait induced by ZnSe at high FTSW might have been linked to effective water use (Djanaguiraman et al., 2024). Increased water use under drought stress by nanoparticle foliar spray was observed in maize (Veroneze-Júnior et al., 2020), barley (Hordeum sativum distichum; Rajput et al., 2018), and sorghum [Sorghum bicolor L. (Moench); Djanaguiraman et al., 2024] and decreased transpiration rates resulted in increased WUE ( Figures 6A, B ). A similar observation was recorded in drought-stressed wheat sprayed with iron-oxide nanoparticles (Manzoor et al., 2023).

The importance of maintenaning tissue turgidity has been widely documented. In the present study, among the foliar sprays, the application of ZnSe QDs increased Ψw and RWC compared to water spray ( Figures 7A, B ), indicating that ZnSe QDs can alleviate drought stress by maintaining a higher tissue water content by increasing Ψw (Robbins and Dinneny, 2015). Foliar application of ZnSe and combined zinc and selenium under rainfed conditions decreased hydrogen peroxide content and malondialdehyde contents ( Figures 10A, B ). In contrast, the activities of the CAT and POX enzymes increased. Based on this observation, it could be concluded that ZnSe and Se acted as antioxidants by increasing the antioxidant enzymes (Tendenedzai et al., 2013) and/or due to their inherent property (Kuršvietienė et al., 2020).

The reduced F_o value in the ZnSe or Se treatments represents an efficient transfer of excitation energy from the antenna to the reaction centers, resulting in lower ROS production and more photochemistry (Genty et al., 1989). Furthermore, as shown by an increase in the F_v/F_m ratio, the enhanced photosynthetic rate induced by ZnSe or selenium foliar spray under drought stress could reduce the photooxidative damage of PSII by efficiently scavenging the ROS generated in the chloroplasts during the light reaction to zinc (Cakmak, 2000) and Se (Djanaguiraman et al., 2010).

Plants sprayed with ZnSe QDs or Se showed increased photosynthetic rate and decreased stomatal conductance and transpiration rate ( Figures 9A–C ). In the present study, the inverse relationship between stomatal conductance and photosynthetic rate was due to differences in leaf water potential (O'Toole and Cruz, 1980). The loss of the photosynthetic rate in water-sprayed plants may be associated with increased premature leaf senescence under drought stress caused by higher ROS levels (Djanaguiraman et al., 2010; Honda et al., 2021). In contrast, plants sprayed with ZnSe or Se showed decreased ROS content and increased activity of antioxidant enzymes ( Figures 10A–D ), thereby, delaying premature leaf senescence might be delayed (Djanaguiraman et al., 2010).

In maize, drought stress during pollination reduces grain yield by 4% per day and can be up to 8%, depending on the intensity of drought stress (Nielsen, 2002). In the present study, drought stress during the reproductive stage reduced seed number cob⁻¹ and grain yield ( Table 1 ; Boyer and Westgate, 2004). However, foliar application of ZnSe or Se increased grain yield by improving seed cob⁻¹, suggesting that reproductive success was improved by ZnSe or Se under drought stress (Tedeschini et al., 2015).

The synthesized ZnSe QDs were nanocrystalline and had a size of 4 nm to 10 nm in size. ZnSe QDs at concentrations >25 mg L⁻¹ were toxic to earthworms and azolla. Here, we demonstrate for the first time that ZnSe QDs increase drought tolerance by restricting the transpiration rate through partial stomatal closure. The increased photosynthetic rate in rainfed maize sprayed with ZnSe QDs is associated with delayed leaf senescence caused by reduced oxidative damage. The increase in grain yield was due to increased reproductive success, as evidenced by the increased seed cob⁻¹. Thus, foliar application of ZnSe at a rate of 20 mg L⁻¹ to rainfed maize increased grain yield by increasing photosynthesis and reproductive traits. Overall, it is evident that foliar application of ZnSe QDs to drought-stressed maize plants activates the antioxidant defense system, resulting in the alleviation of drought-induced oxidative damage in leaves. However, additional research is needed to understand the long-term effects of ZnSe QDs on soil, aquatic, and terrestrial organisms before their large-scale adoption.