**2. Employment of nanotechnology for the improvement of photosynthetic activity and plant productivity**

Photosynthesis is the most fundamental and vital physiological process in plant kingdom. It converts the light form of energy into chemical form in chloroplasts using chlorophyll and CO<sup>2</sup> and H2 O as raw materials, and stores in the bonds of sugar molecules. This form of energy is later used as the energy currency to regulate various processes. In green plants, chloroplasts are the site of synthesis for chemical energy, that is, carbon-based fuels. With the help of light energy, the captured atmospheric CO<sup>2</sup> is converted into different forms of sugars [53]. Photosynthetic apparatus utilized less than 10% of the sunlight [54], and there are possibilities to improve the solar energy conversion efficiency in photosynthetic organisms. The improvement in photosynthetic efficiency requires broadening the range of solar light absorption [55] particularly in the near-infrared spectra which are able to penetrate deeper into living organisms. With unique properties and higher stability, the nanomaterials can form chloroplast-based photocatalytic complexes having an enhanced and improved functional property under *ex vivo* and *in vivo* conditions [11]. It is clear that neither all the absorbed photons are involved in electron flow under intense light conditions nor chloroplast captures maximum solar energy under non-saturating light [56, 57]. The SWNTs have discrete optical and electronic properties and a broad range of absorption spectra (ultraviolet, visible and near-infrared). The enhancement of light reaction after the insertion of SWNTs in chloroplasts isolated from commercially available baby spinach leaves (*Spinacia oleracea* L.) has been observed [11]. Chloroplast does not have a broad range of absorption spectra and it cannot absorb spectra outside its absorption ranges of spectra. The boosted photosynthetic reactions might be attributed to electronic bandgap of semiconducting the SWNTs which converts the absorbed solar light into photosynthetic excitons [58]. Depending on their inherent light interaction capabilities, nanoparticles (NPs) interfere and alter the photosynthetic efficiency, photochemical fluorescence and quantum yield in plants. Keeping up with the importance of process, the researchers attempted either to mimic the process of photosynthesis artificially or to improve the existing efficiency in planta using nanotechnology-based inventions. The plants have been augmented to harvest more light energy by delivering carbon nanotubes into chloroplast. These carbon nanotubes serve as artificial antennae allowing chloroplast to capture wavelengths of light outside the normal range, that is, ultraviolet, green and near-infrared [11, 16]. Various reports are available on the enhancement of photosynthetic activity in plants through *in vivo* or *ex vivo* approaches. In subsequent text, a few cases will be highlighted to show the relevant progresses made by a nano-technologist for the improvement of agronomic attribute.

Plant photosystems include reaction centres (RCs) and the antenna chlorophylls; they are held in the membrane by weak intermolecular interactions. The antenna chlorophyll absorbs photons and transfers to the RCs and then electrons are transferred to the next electron acceptor. Naturally, photosynthetic machinery absorbs light within certain wavelength intervals. It has been reported that if nanoparticles conjugate with these RCs and antenna chlorophyll, there is an exciton enhancement effect [59]. Nanoparticle conjugate with light-harvesting complex absorbs a wider range of wavelength interval. Nanomaterials conjugated with a photosynthetic system strongly increase the rate of production of excited electrons due to the plasmon (metal nanoparticle having an oscillating free electron) enhancement effect. This excited electron can be used for photocurrents or chemical reactions. The association of metal nanoparticle with photosynthetic system has been reported to enhance the efficiency of photosystem. The incorporation of metal nanoparticles with light-absorbing chlorophyll molecules enhances the photon field which is referred as plasmon enhancement effect. Thus, the production of exited electrons has been reported to increase due to plasmon resonance and electronhole separation [60]. In support of this, experimental proofs were generated for the increased rate of the formation of ATP molecules. With hybrid structure, the rate of formation of the excited electron was reported to enhance as compared to photosystem alone [61]. Artificial structures composed of a photosynthetic system and various metal nanoparticles also display strong enhancements of photosynthetic efficiency, and this cause the parallel increases in light absorption by chlorophylls and energy transfer from chlorophylls to nanoparticles [60, 62, 63].

**2. Employment of nanotechnology for the improvement of** 

100 Photosynthesis - From Its Evolution to Future Improvements in Photosynthetic Efficiency Using Nanomaterials

Photosynthesis is the most fundamental and vital physiological process in plant kingdom. It converts the light form of energy into chemical form in chloroplasts using chlorophyll and

energy is later used as the energy currency to regulate various processes. In green plants, chloroplasts are the site of synthesis for chemical energy, that is, carbon-based fuels. With

sugars [53]. Photosynthetic apparatus utilized less than 10% of the sunlight [54], and there are possibilities to improve the solar energy conversion efficiency in photosynthetic organisms. The improvement in photosynthetic efficiency requires broadening the range of solar light absorption [55] particularly in the near-infrared spectra which are able to penetrate deeper into living organisms. With unique properties and higher stability, the nanomaterials can form chloroplast-based photocatalytic complexes having an enhanced and improved functional property under *ex vivo* and *in vivo* conditions [11]. It is clear that neither all the absorbed photons are involved in electron flow under intense light conditions nor chloroplast captures maximum solar energy under non-saturating light [56, 57]. The SWNTs have discrete optical and electronic properties and a broad range of absorption spectra (ultraviolet, visible and near-infrared). The enhancement of light reaction after the insertion of SWNTs in chloroplasts isolated from commercially available baby spinach leaves (*Spinacia oleracea* L.) has been observed [11]. Chloroplast does not have a broad range of absorption spectra and it cannot absorb spectra outside its absorption ranges of spectra. The boosted photosynthetic reactions might be attributed to electronic bandgap of semiconducting the SWNTs which converts the absorbed solar light into photosynthetic excitons [58]. Depending on their inherent light interaction capabilities, nanoparticles (NPs) interfere and alter the photosynthetic efficiency, photochemical fluorescence and quantum yield in plants. Keeping up with the importance of process, the researchers attempted either to mimic the process of photosynthesis artificially or to improve the existing efficiency in planta using nanotechnology-based inventions. The plants have been augmented to harvest more light energy by delivering carbon nanotubes into chloroplast. These carbon nanotubes serve as artificial antennae allowing chloroplast to capture wavelengths of light outside the normal range, that is, ultraviolet, green and near-infrared [11, 16]. Various reports are available on the enhancement of photosynthetic activity in plants through *in vivo* or *ex vivo* approaches. In subsequent text, a few cases will be highlighted to show the relevant progresses made by a nano-technologist for the

Plant photosystems include reaction centres (RCs) and the antenna chlorophylls; they are held in the membrane by weak intermolecular interactions. The antenna chlorophyll absorbs photons and transfers to the RCs and then electrons are transferred to the next electron acceptor. Naturally, photosynthetic machinery absorbs light within certain wavelength intervals. It has been reported that if nanoparticles conjugate with these RCs and antenna chlorophyll, there is an exciton enhancement effect [59]. Nanoparticle conjugate with light-harvesting complex absorbs a wider range of wavelength interval. Nanomaterials conjugated with a

O as raw materials, and stores in the bonds of sugar molecules. This form of

is converted into different forms of

**photosynthetic activity and plant productivity**

the help of light energy, the captured atmospheric CO<sup>2</sup>

improvement of agronomic attribute.

CO<sup>2</sup>

and H2

Artificially, the quantum dots (artificial antennae absorbing light efficiently in a wide range of photon energies from solar spectrum) conjugated with a reaction centre complex of *Rhodobactor sphaeroides* purified from natural light-harvesting complexes showed an efficient transfer of excitation energy to reaction centre. The efficient energy transfer from QDs to the bacterial RC clearly offers an opportunity of the utilization of nanocrystals to enhance the photosynthetic biological functioning [59]. A silver nanowire conjugated with light-harvesting complex from the dinoflagellate *Amphidinium carterae* showed strong enhancement in fluorescence intensity of protein-bound chlorophyll molecules [64]. The increase with silver nanowire conjugate was recorded up to an average of 10-fold increase in chlorophyll fluorescence [65], and this indicates a higher rate of generation of excitations in the chlorophylls [66].

Metal nanoparticles have the ability to influence the energy conversion efficiency in photosynthetic systems. The binding to Au and Ag nanoparticles with chlorophyll molecule results in a novel hybrid system, which could produce 10 times more excited electrons due to plasmon resonance and fast electron–hole separation [60]. Electron transfer from excited fluorophore to Au or Ag nanoparticles has been reported [65, 67–69]. The concentration-dependent effects of Au nanoparticles (5–20 nm) on PSII chlorophyll, a fluorescence quenching in soybean leaves, have been observed [70]. Falco et al. [70] observed a shift in fluorescence towards a higher wavelength in Au nanoparticle-treated soybean leaves. An enhanced PSII quantum efficiency was reported in Ag nanoparticle-treated Indian mustard [71].

Giraldo et al. [11] reported 49% increase in electron transfer rate under *ex vivo* conditions (in extracted chloroplast from baby spinach leaves) after treatment with SWNTs. SWNTs also enhanced the light reaction *in vivo* in leaves of *A. thaliana*. Similarly, carbon nanotubes in spinach thylakoid improved photo-electrochemical activity under illumination [72]. Noji et al. [73] reported that nanomesoporous silica compound (SBA) conjugated with photosystem II (PSII) maintained the high and stable oxygen-evolving ability of PSII in *T. vulcanus*. The applied TiO<sup>2</sup> nanoparticles caused the transfer of charges between light-harvesting complex II (LHCII) and TiO<sup>2</sup> NPs because of their photocatalytic properties [74] which induced reduction–oxidation reaction. Ze et al. [75] reported an increased expression of LHCII b and contents of LHCII in the thylakoid membrane of *A. thaliana* after the application of TiO<sup>2</sup> nanoparticles. It was found that TiO<sup>2</sup> NPs promote the light absorption by chloroplast and regulate the distribution of light energy from PSI to PSII by increasing LHCII content, which in turn accelerate the transformation from light energy to electronic energy, water photolysis and oxygen evolution.

Fe<sup>2</sup> O3

*Zea mays* L. with silica (SiO<sup>2</sup>

**3. Future prospects**

**Acknowledgements**

**Conflict of interest**

**Abbreviations**

The authors declare no conflict of interest.

Deff Diffusion coefficient

FITC Fluorescein isothiocyanate

 nanoparticle spray on *Glycine max* had positive effects on root elongation and photosynthesis rate. Also, the elongation of root and an increase in seed germination were observed in

of ZnO nanoparticulates showed the highest germination and seedling vigor index [94].

Nanotechnology has enormous potential to create novel and improved functional properties in photosynthetic organelles and organisms for the enhancement of solar energy harnessing. The upward translocation from root to leaf opens up greater opportunities for their use in various delivery applications. The SWNTs delivered by this spontaneous mechanism have the potential for increasing chloroplast carbon capture by promoting chloroplast solar energy harnessing and electron transport rates. It has been shown that when nanoparticles enter into plant cell, various metabolic changes occur that leads to an increase in biomass, fruit/grain yield, and so on; therefore, further mode and action can be elucidated to evaluate the possibility of their uses. The nanomaterials have the potential to be utilized for the transport of DNA and chemicals into plant cells [95, 96] which offers new opportunity to target specific gene manipulation and expression in the specific cells of the plant. With nanomaterial, the output of a crop can be increased while reducing the input through a better understanding of nanoparticle interaction with plants. The nanobionics approach to engineer plant function will lead to a new area of research at the interface of nanotechnology and plant biology.

CSIR-CSMCRI PRIS 024/2018. The authors thankfully acknowledge the financial supports provided by the Govt. of India in the form of different R&D Projects through Council of Scientific and Industrial Research (CSIR). KK is thankful to CSIR, New Delhi, for financial support in the form of Senior Research Fellow (SRF) and AcSIR for registration in Ph.D. program.

) nanoparticles treatment [93]. Maize with a treatment of 1500 ppm

http://dx.doi.org/10.5772/intechopen.76815

103

Plant Nanobionics and Its Applications for Developing Plants with Improved Photosynthetic…

Nadtochenko et al. [62] observed an enhanced electron transfer efficiency in isolated photosynthetic reaction centres using alumina nanoparticles. The bread wheat (*Triticum aestivum* L.) showed an increase in grain number, biomass, stomatal density, xylem-phloem size, epidermal cells and water uptake after seed priming with MWCNT [76]. TiO<sup>2</sup> nanoparticles have been reported to protect chloroplasts from aging during long illumination regimes, promote chlorophyll formation and stimulate Rubisco activity, which in turn results in increased photosynthesis or enhanced photosynthetic carbon assimilation [71, 77, 78]. With exogenous application of TiO<sup>2</sup> , Qi et al. [79] observed an improved net photosynthetic rate, water conductance and transpiration rate. Nano-anatase was reported to promote electron transport chain reaction, photoreduction activity of PSII, evolution of O<sup>2</sup> and photophosphorylation of chlorophyll under both visible and ultraviolet light [80]. A higher photosynthetic carbon reaction due to Rubisco carboxylation was observed as a result of nano-anataseinduced marker genes for Rubisco activase mRNA, enhanced protein levels and activities of Rubisco activase [81]. On the contrary, the exogenous application of TiO<sup>2</sup> -anatase NPs resulted in a reduced PSII quantum yield, photochemical quenching, electron transfer rate, chlorophyll fluorescence and higher non-photochemical quenching and water loss [82]. Nano-TiO<sup>2</sup> reported to improve water absorption, seed germination, plant growth, nitrogen metabolism and photosynthesis [63, 76, 83, 84]. TiO<sup>2</sup> NPs were reported to alleviate heat stress through regulating stomatal opening [79].

Nano-TiO<sup>2</sup> (rutile) influences the photochemical reaction in spinach chloroplasts [85, 86]. The spinach treated with 0.25% nano-TiO<sup>2</sup> showed improved up-hill reaction and oxygen evolution. The noncyclic photophosphorylation activity was found to be higher than cyclic photophosphorylation in chloroplasts. This increase in photosynthesis with nano-TiO<sup>2</sup> might be associated with the activation of a photochemical reaction in spinach chloroplasts [85, 86]. Similarly, an increase in dry weight, chlorophyll formation, the ribulose bisphosphate carboxylase/oxygenase activity and the photosynthetic rate was reported in aged spinach treated with 2.5% nano-TiO<sup>2</sup> rutile [83]. The nano-anatase TiO<sup>2</sup> improved light absorbance, conversion of light energy to electron energy and ultimately to chemical energy, and this promotes carbon dioxide (CO<sup>2</sup> ) assimilation. Treatment of nano-anatase TiO<sup>2</sup> improved Rubisco-carboxylase activity 2.67 times in spinach as compared to control, which consecutively activates Rubisco carboxylation and eventually the rate of photosynthesis increase [87]. Pradhan et al. [88] found that Mn-NPs induced an increase in the hill reaction rate in mung bean (*Vigna radiata*).

In the recent time, NMs are used as a vital tool for improving plant growth and productivity under adverse environmental conditions, that is, salt stress. The Si nanoparticles in the soil have been shown to alleviate salt stress, enhance seed germination, improve activities of antioxidative enzymes, photosynthetic rate and leaf water content [89, 90]. Increased leaf, pod dry weight and grain yield were recorded in soya bean using nano-iron oxide [91]. The β-cyclo dextrin-coated iron nanoparticles penetrate the biological membranes of maize and increase the chlorophyll pigments (up to 38%) as compared to control [92]. The spray of citrate-coated Fe<sup>2</sup> O3 nanoparticle spray on *Glycine max* had positive effects on root elongation and photosynthesis rate. Also, the elongation of root and an increase in seed germination were observed in *Zea mays* L. with silica (SiO<sup>2</sup> ) nanoparticles treatment [93]. Maize with a treatment of 1500 ppm of ZnO nanoparticulates showed the highest germination and seedling vigor index [94].
