NANOTECHNOLOGY: Effect of nanomaterials on plant growth

plant growth

The effect of nanoparticles on plant growth varies greatly with the type of nano-particle, concentration used and the plant species being studied. Further different nano-particles affect different growth processes of plants. The nanomaterial can enter the plant by binding to carrier proteins, through aquaporin, ion channels, endocytosis, by creating new pores or by binding to the organic chemical in the environmental media (Rico et al. 2011). Interaction of nanoparticles with edible plants has been recently reviewed by Rico et al. 2011. Confocal fluorescence image studies have revealed the capacity of single walled carbon nanotubes to traverse across both the plant cell wall and cell membrane (Liu et al. 2009). Compared to plant cell walls and membranes, the penetration of nanoparticles into seeds is expected to be difficult due to the significantly thick seed coat covering the whole seed. Khodakovskaya et al. (2009) demonstrated that CNTs could effectively penetrate seed coat, thereby influencing the seed germination and plant growth. Exposure of seeds to CNT s (40 ^g/ml in MS medium) enhanced tomato seed germination and growth rate. The presence of CNT s inside the seeds was also confirmed by Raman spectrum and transmission electron microscope. The CNTs create pores in the cell wall thus enhancing water uptake thereby promoting germination. The seeds treated with CNTs showed higher moisture content as compared to control seeds (Srinivasan and Saraswathi 2010). The positive effect of multi-walled carbon nanotubes (MWCNTs) on seed germination and root growth of crops like radish (Raphanus sativus), rape (Brassica napus), rye grass (Lolium perenne), lettuce (Lactuca sativa), corn (Zea mays) and cucumber (Cucumis sativus) (Lin and Xing 2007) are reported. Rice seeds treated with SWCNTs and MWCNTs also showed improved germination (Stamphoulis et al. 2009). Insurance market

Treatment of soybean (Glycine max) with a mixture of nanoscale SiO2 and TiO2 at low concentration resulted in improved germination and plant growth. The seedlings showed enhanced uptake of water and fertilizers and high nitrate reductase activity and better antioxidant system (Lu et al. 2002). Application of Nano-TiO2 (2.5 to 40 g/Kg soil) could promote growth of spinach by enhancing photosynthesis and nitrogen metabolism (Hong et al. 2005 a,b). The improvement in spinach growth under N-deficient conditions was related with N2 fixation by nano-anatase TiO2. Nano-anatase TiO2 on exposure to sunlight could chemisorb N2 directly or reduce N2 to NH3 in the spinach leaves, transforming into organic nitrogen and improving the growth of the spinach (Yang et al. 2007). Treatment with nano-anatase TiO2 promoted photosynthesis and growth of spinach under visible and ultraviolet illumination and accelerated electron transfer, photophosphorylation of chloroplast (Chl), water photolysis and oxygen evolution (Lie et al .2007). Xuming et al. (2008) further observed that, nano-anatase treatment resulted in enhancement of Rubisco mRNA amounts, the protein levels, and activity of Rubisco, thereby leading to the improvement of Rubisco carboxylation and high rate of photosynthetic carbon reaction. It could also promote antioxidant status of spinach chloroplast under UV-B radiation by removing ROS and activating SOD, CAT, GPX and APX and (Zheng et al. 2008). Similarly, nano-SiO2 showed a corresponding positive effect on growth of Changbai larch (Larix olgensis) with increasing concentration up to 500mg/L (Lin et al. 2004).

Lin and Xing (Lin and Xing 2007) studied effects of five types of nanoparticles (multi-walled carbon nanotube, aluminum, alumina, zinc, and zinc oxide) on seed germination and root growth of six higher plant species (radish, rape, ryegrass, lettuce, corn, and cucumber) and reported significant inhibition of germination of ryegrass germination by nZnO (2000 mg/L) and of corn by nZnO and nAl2O3 (2000 mg/L). Inhibition on root growth varied greatly among nanoparticles and plants. Root elongation of the tested plant species practically terminated by 2000 mg/L of nano-Zn or nano-ZnO. Fifty percent inhibitory concentrations (IC50) of nano-Zn and nano-ZnO were estimated to be near 50 mg/L for radish, and about 20 mg/L for rape and ryegrass. In another study by Lee et al. (2010) the effect of three concentrations (400, 2000 and 4000 mg/L) of four metal oxide nano-particles, four metal oxide nanoparticles, aluminum oxide (nAl2O3), silicon dioxide (nSiO2), magnetite (nFe3O4), and zinc oxide (nZnO) on the development of Arabidopsis thaliana using three toxicity indicators (seed germination, root elongation and number of leaves). Among these, nZnO was most phytotoxic, which (all concentrations) significantly inhibited development (seed germination, root elongation and number of leaves), followed by nFe3O4, nSiO2, and nAl2O3, which was not toxic. Root elongation was significant improved with nAl2O3 (all tested concentrations) and nSiO2 (400 mg/L) whereas nSiO2 (2000 and 4000 mg/L) and nFe3O4 (all concentrations) showed significant inhibition of root elongation. The increasing concentrations (5, 10, and 20 ^g ml-1) of the cobalt and zinc oxide NPs severely inhibit root elongation of Allium cepa under hydroponic conditions due to massive adsorption into the root system (cobalt NPs) and accumulation in both the cellular and the chromosomal modules (Zn Nps) thus signifying their highly hazardous phytotoxic nature (Ghodake et al. 2011). Three desert plants, Parkinsonia florida (blue palo verde), Prosopis juliflora-velutina (velvet mesquite) and Salsola tragus (tumbleweed) responded differently to seed treatment with different concentrations of ZnO nanoparticles (0 to 4000 mg L -1). Although germination was not significantly affected (P < 0.05) in any of the three plant species, root length in velvet mesquite was reduced at all ZnO NP concentrations used whereas root elongation in blue palo verde was reduced by16% (at 4000 mg ZnO NPs L-1), and in Tumbleweed root size diminished by 14% and 16% (at 500 and 2000 mg ZnO NPs L-1 respectively. Further X-ray Absorption Spectroscopic (XAS) studies demonstrated the biotransformed of ZnO NPs on/within the root in all three plant species (Rosa et al. 2011).

Liu et al. (2010) attempted to study the effect of fullerence at cellular level in transgenic seedlings. The treatment with fullerence resulted in retarded root growth with shortened length and loss of root gravitropism. Fluorescence imaging revealed the abnormalities of root tips in hormone distribution, cell division, microtubule organization, and mitochondrial activity. Genotoxic effects of CeO2 NPs treatment in soybean plants have also been demonstrated (Lopez-Moreno et al. 2010). Use of synchrotron X-ray absorption spectroscopy revealed presence of CeO2 NPs in roots. Random amplified polymorphic DNA assay showed the appearance of four new bands at 2000 mg L-1 and three new bands at 4000 mg L-1 treatment of CeO2 NPs indicating the effect of NPs on DNA replication. Ma et al 2010 studied the effect of four rare earth oxide nanoparticles, nano-CeO2, nano-La2O3, nano-Gd2O3 and nano-Yb2O3, on seven plant species (radish, rape, tomato, lettuce, wheat, cabbage, cucumber) by means of root elongation experiments. Low concentrations of rare earth ions had a positive effect on plant growth. A suspension of 2000 mgL-1 nano-CeO2 had no effect on root elongation of six plants except lettuce, whereas similar concentration of all other tested nano-particles severely inhibited the root elongation. The wheat plants were inhibited on seed incubation whereas lettuce and rape were inhibited on both seed soaking and incubation process. The fifty percent inhibitory concentrations (IC50) for rape were about 40 mg L-1 of nano-La2O3, 20 mg L-1 of nano-Gd2O3, and 70 mg L-1 of nano-Yb2O3, respectively. Calabrese and Baldwin (2002) explained the dose dependent effect of rare earth ions on plant growth by “hormesis effect” which is meant by a low-dose stimulation and high-dose inhibition. Further, it has also been reported that as compared to functionalized nano-particles, non-functionalized nano-particles have more inhibitory effect on plant growth (Canas et al. 2008). Thus, the inhibitory effect of nano-particles can be reduced by their functionalization. Moreover, functionalization also increases the specificity of nano-particles.

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