Is Mitigation of Drought Stress By Zinc Oxide Nanoparticles Driven By a Nano-Specific Mechanism or Alleviation of Micronutrient Deficiency?

J. W. Deakin1, A. Cartwright2, M. Potter2, D. Sparks3, J. Hortin3, A. J. Anderson2, D. W. Britt2, J. E. McLean3, A. R. Jacobson1
1Department of Plants, Soils, and Climate, Utah State University, 2Department of Biological Engineering, Utah State University, 3Utah Water Research Laboratory, Utah State University
Background
  • Nanoparticles (NPs) are particles less than 100 nm in any dimension.
  • NPs can affect crops differently than bulk fertilizers, through mechanisms such as direct uptake by plants.
  • ZnO NPs, when used as soil amendments, are reported to improve plant health and/or drought tolerance.
  • Many of the reported studies were conducted in Zn-deficient soil / growth media, leaving it unclear if the benefits from ZnO NPs are caused by a nanoparticle-specific benefit or simply the mitigation of a micronutrient deficiency.
  • Zn deficiency has been shown to decrease drought tolerance, potentially explaining the benefits reported when ZnO NPs are applied.
  • Pseudomonas chlororaphis isolate O6 (PcO6) is a beneficial, root-colonizing microbe that induces water stress tolerance in plants (Cho et al. 2008).
Literature Reviewed
Figure 1
Figure 1: Summary of the findings of 12 studies considering the effects of ZnO NPs on plant health, both in the presence and absence of drought. While not an exhaustive literature review, each paper that was identified on the topic is included above. Note that the majority of papers that do not include a Zn control (a comparison to currently available Zn fertilizers) report that ZnO NPs benefit the plants, while the majority of papers that include Zn controls conclude that the NPs have neutral or mixed effects.
Methods
  • Wheat seeds (v. Juniper) were inoculated with a probiotic, Pseudomonas chlororaphis isolate O6 (PcO6).
  • Inoculated seeds were planted in sand amended with ZnO NPs providing 0, 2.5, 5, 10, or 20 mg Zn/kg sand.
  • Modified, half-strength Hoagland’s solution was added at 0, 7, and 14 days to provide all essential plant nutrients, including Zn. 
  • Grown under white LED lightsImage 1Image 1: The pots for this experiment on the growth shelves. This photo was taken when the first seedlings emerged.
  • Water restricted at day 14 for water-stressed plants. This water-stress continued for 8 days 
  • Plants were watered daily to return to target water content: 
    • Non-stressed water content: 0.144 g/g
    • Water-stressed water content: 0.024 g/g
  • Drought stress was quantified by tissue dry mass and quantum yield of PSII (ΦPSII). ΦPSII measurements were made using a Licor 6800 Portable Photosynthesis System.
Preliminary Experiments
Wheat seedlings were grown in sand sufficient in all nutrients, including Zn. After establishment, severe drought was induced for 8 days. Image 1
Image 2: Photos of wheat seedlings grown in a Zn-sufficient medium in either the presence or absence of ZnO NPs. These images were taken after 8 days of drought. Insets show Luria-Bertani medium that was dabbed with roots from the pots at the conclusion of the 8 day drought period. The orange color indicates successful inoculation with Pseudomonas chlororaphis isolate O6 (PcO6), a beneficial root-colonizing bacterium.
 
 
Figure 2
Figure 2: Drought significantly reduced the quantum yield of PSII (ΦPSII), a measurement of photosynthetic efficiency. The addition of ZnO NPs as a sand amendment did not mitigate this reduction in photosynthetic efficiency.
 
Figure 3
Figure 3: The effects of ZnO NPs used as a sand amendment on the tissue dry mass production of wheat seedlings experiencing drought stress and grown in a Zn-sufficient medium. Drought reduced tissue production but, under these conditions, the addition of ZnO NPs did not significantly mitigate reductions in leaf dry mass caused by drought. Reductions of leaf water content caused by drought (not shown) were also not mitigated by the addition of ZnO NPs.
Image 2
Image 3: Scanning electron microscope (SEM) image of wheat roots grown in the presence of ZnO NPs. After 22 days of growth, no ZnO NPs could be located on the root surface.
 
 
Conclusions
 
  • ZnO NPs did not mitigate drought symptoms, as measured by tissue dry weight and ΦPSII, in wheat seedlings grown in a sand growth matrix with sufficient nutrients for wheat growth.
  • When evaluating the efficacy of ZnO NPs (or other novel agronomic amendments), experiments need to be designed in systems that represent currently available production techniques (i.e. nutrient-sufficient media).
  • Before adopting a new technology, crop advisors and growers need to review the methods used in the studies that tested the products to ensure that the study applies to their growing conditions and includes a fair comparison with current management practices.
Acknowledgements
This work was supported by the USDA National Institute of Food and Agriculture, AFRI project 2016-08771 and NSF CBET 1705874. The Utah Agricultural Experiment Station also provided funding through Projects 1280 and UTA-01341. USU Core Microscopy Facility and SEM instrumentation support was from NSF CMMI 1337932. Funds from the Utah Agricultural Experiment Station, USU Plants, Soils, and Climate Department, and the USU Office of Research were used to purchase the Li-6800. 

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