Methods of Using Nanoparticles
4.3 Hydroponic Treatment
Hydroponics is a type of hydroculture involving mineral nutrient solutions to grow plants, in water, without soil. Though more suitable for semiaquatic plants, ter-restrial plants may also be grown with their roots immersed in the nutrient solution Fig. 4.3 Confocal microscopy images of BY-2 incubated with SWNT/FITC: a brightﬁeld image;
bfluorescence image; c DIC image under high magniﬁcation; d fluorescence image under high magniﬁcation; e overlay of C and D. Scale bars are 100 lm for (a) and (b) and 10 lm for (c–e).
Reprinted with permission from Liu et al. (2009). Copyright (2009) American Chemical Society
or in an inert medium, such as perlite or gravel. Hydroponics provides an oppor-tunity to better understand the relationship between nutritional status and plant growth, in addition to assessing the impact of biotic and abiotic factors on the plant development. Hydroponic systems are ideal for isolating factors affecting plant growth and to establish relationship of elements from hydro/pedosphere to bio-sphere. In soil–plant systems, the activity of a particular element inevitably depends on numerous other environmental factors than its own activity in the interfaces of roots, which in many cases may alter the properties and ultimately the actual signature effects of the elements (Cornelis et al.2012). Many a times, in the case of NMs, the materials may interact with the microorganisms, enzymatic factors, pH changes, etc. in the soil, which can neither be accurately monitored nor controlled.
In the case of a closed system like hydroponics, an accurate maintenance of essential parameters could be achieved, which is evident from the large number of works being carried out with this system to analyze various NM–plant interactions.
Stampoulis et al. (2009) elucidated the effects of MWCNTs, Ag, Cu, Si, and Zn oxide on biomass of zucchini using a batch hydroponic experimental system.
Germination was induced on moist paper, and 4-day-old seedlings were subse-quently transferred to amber vials with 7.5 mL of 25 % Hoagland solution.
Cultures were maintained at optimum conditions for 14 days and further transferred to 40-mL amber vials containing 39 mL of solution with either nanoparticles or corresponding bulk materials at 1000 mg/L.
Hydroponic cultures of onion (Allium cepa) bulbs were used to assess the effects of cobalt and zinc oxide NPs on the root elongation, root morphology, and cell morphology, as well as their adsorption potential (Ghodake et al.2011). Healthy and fresh onions were washed under running tap water, and scales were removed.
Onions were grown for three days without the NPs and when the length of the roots reached between 1.3 and 1.5 cm, the plants were transferred to fresh solutions of CoO and ZnO NPs. The physiological parameters were measured at different intervals over a span of three days, and average values were recorded. Signiﬁcant adsorption of CoO NPs into the root system was observed.
Feichtmeier et al. (2015) analyzed the reversibility and effects of citrate-coated Au NPs on barley by cultivating the seeds in Au NP-containing nutrient solution for two weeks with subsequent transfer of the seedlings to Au NP-free media. The stability of Au NPs in the cultivation media was also investigated over a period of two weeks. Barley (Hordeum vulgare) was cultivated for two weeks in a hydro-culture media on afloating layer consisting of low-density polyethylene granulate (LD-PE). The granulate is used to provide necessary anchorage to the plant roots and access to sufﬁcient nutrient supply from the medium. Au NP hydrosol (1, 3, 5, 8, and 10lg mL−1) was supplemented with 0.022 g of an MS basal medium. For initial germination of barley seeds, a room temperature, dark chamber incubation was arranged. After two days, the cultures were set in a chamber with a 16-h light and 8-h dark period cycle at 21.5 °C. Barley plants were harvested after 14-day exposure and further used for determination of fresh biomass and other parameters.
It is claimed that Ag NPs are safe and efﬁcient agents against disease causatives in agriculture. To understand the protein populations and sub-populations along
with the environmental Ag NPs stresses, Mirzajani et al. (2014) employed a pro-teomic approach on rice. The seeds were germinated in sterile water ﬁlled Petri dishes, for seven days. Post-germination, seedlings were transferred to rice-speciﬁc growth cultivation media and cultured in a phytotron under suitable growth con-ditions, with the renewal of hydroponic media everyﬁve days. Ten-day-old plants were treated with Ag NPs (18.34 nm) colloidal solution at concentrations of 0, 30, and 60 mg/mL for 20 days. Post-incubation plants were removed from the NP-augmented solution and thoroughly washed with water prior to analysis.
Koo et al. (2015) explored the impact of surface-modiﬁed quantum dots on stability, uptake, and translocation in Arabidopsis, and subsequent transfer to pri-mary consumers, cabbage looper (Trichoplusia ni). Arabidopsis samples were exposed to CdSe/CdZnS QDs with three different surface coatings: poly(acrylic acid-ethylene glycol) (PAA-EG), polyethylenimine (PEI), and poly(maleic anhydride-alt-1-octadecene)- poly(ethylene glycol) (PMAO–PEG), which are anionic, cationic, and relatively neutral, respectively. Arabidopsis seeds were sown in 1/16 strength Hoagland solution fortiﬁed with 0.8 % agar in a seed holder hydroponic system (Kit 140 HD, Aquaponics, Belgium). Post-germination, Arabidopsis plants were transferred to 15-mL conical tubes for four weeks followed by a subsequent transfer to 1/16 strength Hoagland solution amended with either PAA-EG or PEI QDs at 10lg/mL for seven days before analysis.
Schwabe et al. (2015) investigated the uptake of cerium dioxide NPs by hydroponically grown wheat, pumpkin (Cucurbita pepo), and sunflower (Helianthus annuus var. Iregi) plants. Pre-sterilized seeds were germinated on water-irrigated paper and, after 6–8 days of soaking, were subsequently transferred to a hydroponic system with 1 L 20 % Hoagland solution and cultured for 26 days.
Plants were then exposed for 6 days to CeO2-NPs suspension at 100 mg L−1, dispersed by ultrasonication in 1 L of 20 % Hoagland medium (Fig.4.4). For stabilization of the nanoparticles in suspension, gum arabicum (GA) was supple-mented at a concentration of 60 mg L−1.
The determination of unique characteristics of NPs after their entry into the plant system is highly restricted with currently available technologies. Therefore, Dan et al. (2015) developed an enzymatic digestion and single-particle inductively coupled plasma-mass spectrometry (SP-ICP-MS) analysis, for simultaneous deter-mination of Au NP size, size distribution, particle concentration, and dissolved Au concentration in tomato plant tissues. For this purpose, pre-sterilized tomato (Solanum lycopersicum) seeds were initially germinated on deionized water-moistened ﬁlter paper for seven days. Seedlings of uniform features were transferred to 50 mL of quarter-strength Hoagland solution in polypropylene cen-trifuge tubes. Post-20 days of culture in the nutrient solution, plants were trans-ferred to fresh 50-mL centrifuge tubes containing only deionized water for a further two days. The water was subsequently replaced with solutions of different con-centrations of PVP-coated 40 nm Au NPs. The plants were exposed to the NPs solution for a period of 4 days before harvesting for enzyme extraction and SP-ICP-MS analysis.
Zhang et al. (2014) reported on the accumulation and elimination of CuO NPs and CdS/ZnS QDs in aquatic mesocosms with Schoenoplectus tabernaemontani cultivated in hydroponic mesocosms. S. tabernaemontani rhizomes were thor-oughly washed and acclimatized in 25 % strength Hoagland nutrient solution for four weeks. The plants were eventually transferred to 2-L vessels containing Hoagland nutrient solution spiked with CuO NPs and CdS QDs and cultured for 21 days prior to analysis.
Six-inch cuttings of poplar plants (Populus deltoides nigra, DN-34) were used to evaluate the vegetative uptake and subsequent translocation and transport of commercially available Au NPs into plant cells in a hydroponic culture system.
Plants were grown for 25 days in 1/2 strength Hoagland solution prior to NP exposure. Different concentrations of Au NPs (3 mL of 498± 50.5, 247 ± 94.5, and 263± 157 ng/mL) and Au(III) ions (5.0, 10.0, and 20.0 mg/L) were added to 200 mL of deionized water in 250-mL glass conical flask test reactors with a PTFE-faced septum sampling port. The reactors were covered with aluminum foil Fig. 4.4 Scheme of the experimental setup for hydroponic system. a Cotyledons were removed before harvest; b stem; c meristem incl. newest leaf not older than 7 d; 1–5 leaf count; *I–*VI marked points for chlorophyll measurements. Reproduced from Schwabe et al. (2015), with permission from Royal Society of Chemistry
and kept at 23± 1 °C under optimum culture conditions. Deionized water satu-rated with oxygen was injected into the reactors twice per day to compensate for the evapotranspiration loss (Zhai et al.2014). Further cases of hydroponic application of NMs to plants are presented in Table4.3.