265
NPs Upregulated Photosynthesis and Secondary Metabolism
TiO 2 NPs enhanced the vegetative growth of spinach plants by increasing their
photosynthetic efficiency and photosynthetic rate. TiO 2 NPs were found to increase
the chlorophyll content, ribulose bisphosphate carboxylase/oxygenase activity, and
photosynthetic rate of plants (Zheng et al. 2005). TiO 2 NPs reportedly increased
the absorption of light in the red and blue region of spinach. Enhancement in the
excitation energy absorbed by light harvesting complex (LHC) II was observed,
which further increased the energy transferred from photosystem PS I to PS II (Su
et al. 2007). TiO 2 NPs also increased the LHC II content in the thylakoid membranes of spinach plants. NPs entered the spinach chloroplasts and interacted with
PS II to promote the transfer of energy from chlorophyll b and carotenoids to chlorophyll a. Hence, the process involving the conversion of light energy to electron
energy namely, electron transport, water photolysis, and oxygen evolution were
also accelerated (Zheng et al. 2007). Later, TiO 2 NPs were also reported to improve
ATP synthesis and transport across the membrane. Enhanced chloroplast coupling
and increased activity of the ATP synthase enzyme were responsible for enhanced
growth of plants on NP exposure. The noncyclic photophosphorylation activity of
chloroplasts was higher than cyclic photophosphorylation activity on NP exposure.
Simultaneously, easier entry of Ca
2+
and Cl
-
was also favored into the oxygenevolving complex during NP exposure (Hong et al. 2005a; Lei et al. 2007). TiO 2
NPs have also been reported to promote electron transport by accelerating the primary charge separation of PS II and P680
+
(Su et al. 2008a). This was further supported by the fact that TiO 2 NPs also enhanced the activity of
ribulose-1,5-bisphosphate carboxylase/oxygenase (RUBISCO) enzyme. They
increased the protein expression and activity of RUBISCO enzyme (Gao et al.
2006; Linglan et al. 2008). Hence, these factors were collectively responsible for
the better growth of spinach on TiO 2 NP exposure.
Nitrogen is the main component required for the normal structure and function
of chlorophyll. TiO 2 NP exposure was found to increase the nitrogen assimilation of
spinach plants. These NPs accelerated the direct reduction of NO
3–
to NH
4+
.
Subsequently, there was enhanced transformation of NH
4+
into organic nitrogen like
proteins and chlorophyll. Simultaneously, the activities of important nitrogenassimilating enzymes were also enhanced on NP exposure (Yang et al. 2006). Ag
NPs induced the blockage of ethylene receptors and enhanced indole-3-acetic acid
efflux from plant roots to induce phytostimulatory effects on A. thaliana and poplar
plants (Wang et al. 2013b). Graphene ribbons improved the vegetative growth and
metabolism of wheat by improving their secondary metabolism and nitrogen
sequestration. Graphene ribbon exposure was found to enhance the accumulation of
carbohydrates, amino acids, and fatty acids. Ribbons also improved the integrity
and permeability of cell membranes (Hu and Zhou 2014). TiO 2 NPs were also
reported to increase the photosynthesis in tomato leaves under mild heat stress conditions. TiO 2 NPs induced increase in the transpiration rate and regulated photosystem II (PS II) energy dissipation. The nonregulated PS II energy dissipation was
10 Phytoresponse to Nanoparticle Exposure
NPs Upregulated Photosynthesis and Secondary Metabolism
TiO 2 NPs enhanced the vegetative growth of spinach plants by increasing their
photosynthetic efficiency and photosynthetic rate. TiO 2 NPs were found to increase
the chlorophyll content, ribulose bisphosphate carboxylase/oxygenase activity, and
photosynthetic rate of plants (Zheng et al. 2005). TiO 2 NPs reportedly increased
the absorption of light in the red and blue region of spinach. Enhancement in the
excitation energy absorbed by light harvesting complex (LHC) II was observed,
which further increased the energy transferred from photosystem PS I to PS II (Su
et al. 2007). TiO 2 NPs also increased the LHC II content in the thylakoid membranes of spinach plants. NPs entered the spinach chloroplasts and interacted with
PS II to promote the transfer of energy from chlorophyll b and carotenoids to chlorophyll a. Hence, the process involving the conversion of light energy to electron
energy namely, electron transport, water photolysis, and oxygen evolution were
also accelerated (Zheng et al. 2007). Later, TiO 2 NPs were also reported to improve
ATP synthesis and transport across the membrane. Enhanced chloroplast coupling
and increased activity of the ATP synthase enzyme were responsible for enhanced
growth of plants on NP exposure. The noncyclic photophosphorylation activity of
chloroplasts was higher than cyclic photophosphorylation activity on NP exposure.
Simultaneously, easier entry of Ca
2+
and Cl
-
was also favored into the oxygenevolving complex during NP exposure (Hong et al. 2005a; Lei et al. 2007). TiO 2
NPs have also been reported to promote electron transport by accelerating the primary charge separation of PS II and P680
+
(Su et al. 2008a). This was further supported by the fact that TiO 2 NPs also enhanced the activity of
ribulose-1,5-bisphosphate carboxylase/oxygenase (RUBISCO) enzyme. They
increased the protein expression and activity of RUBISCO enzyme (Gao et al.
2006; Linglan et al. 2008). Hence, these factors were collectively responsible for
the better growth of spinach on TiO 2 NP exposure.
Nitrogen is the main component required for the normal structure and function
of chlorophyll. TiO 2 NP exposure was found to increase the nitrogen assimilation of
spinach plants. These NPs accelerated the direct reduction of NO
3–
to NH
4+
.
Subsequently, there was enhanced transformation of NH
4+
into organic nitrogen like
proteins and chlorophyll. Simultaneously, the activities of important nitrogenassimilating enzymes were also enhanced on NP exposure (Yang et al. 2006). Ag
NPs induced the blockage of ethylene receptors and enhanced indole-3-acetic acid
efflux from plant roots to induce phytostimulatory effects on A. thaliana and poplar
plants (Wang et al. 2013b). Graphene ribbons improved the vegetative growth and
metabolism of wheat by improving their secondary metabolism and nitrogen
sequestration. Graphene ribbon exposure was found to enhance the accumulation of
carbohydrates, amino acids, and fatty acids. Ribbons also improved the integrity
and permeability of cell membranes (Hu and Zhou 2014). TiO 2 NPs were also
reported to increase the photosynthesis in tomato leaves under mild heat stress conditions. TiO 2 NPs induced increase in the transpiration rate and regulated photosystem II (PS II) energy dissipation. The nonregulated PS II energy dissipation was
10 Phytoresponse to Nanoparticle Exposure