with different drought intensities and recovery from drought
stress. The findings showed NP aggregates formation in
plant tissues, pores of large size in roots and stomata in
leaves were closed rapidly. There was an increase in total
chlorophyll and carotenoid content of leaves in the test
plants. The plants showed changes in antioxidant enzymes,
cell injury, osmolyte, metabolite profile and the membrane
stability indices. The study proved that application of NPs
directly in soil was suitable for post-drought recovery of
barley plants (Ghorbanpour et al. 2020).
Counteract membrane damage
The effects of engineered nanoparticles such as Ag, Co, Ni
(metals) and CeO 2 , Fe 3 O 4 , SnO 2 , TiO 2 (metal oxides) on
seedlings of basil (Ocimum basilicum L.) grown in mix of
20% sandy soil and 80% peat were investigated by Antisari
et al. (2018). The results indicated that the test metal-NPs
were accumulated in the roots and the selected NPs, i.e., Ag,
Co, CeO 2 and Ni, were translocated from the root to shoot,
leaves and then to edible part of the test plant. The relative
short exposure accumulated Ca in roots that counteracted the
membrane damage by nanoparticles (Antisari et al. 2018).
Induced root formation
The study carried out by Ahmad et al. (2020) on ZnO and
CuO-ENPs application on in vitro formation of root,
antioxidant (non-enzymatic) activities and steviol glycosides
(SGs) in Candyleaf (Stevia rebaudiana) regenerants. The
results of NP treatment showed that the percentage of rooting and SGs—rebaudioside A and stevioside—were
increased. The phytochemical studies (flavonoid content,
phenolic content) and 2,2-diphenyl-1-picryl hydrazyl
(DPPH)-free radical scavenging activity were high in
regenerants. The biochemical and morpho-physiological
responses of candyleaf were proved to elicit defense
against test ENPs (Ahmad et al. 2020).
Increased in vitro regeneration
The study by Zia et al. (2020) analyzed the effect of silver
nanoparticles (AgNPs) on in vitro regeneration of carnation
cultivars cv. Noblessa, cv. Antigua and cv. Mariposa. The
number of shoots/explant of cv. Noblesse and cv. Antigua
and cv. Mariposa showed the highest regeneration rate. The
study concluded that test nanoparticles were effective for
increasing in vitro shoot multiplication and regeneration of
plants (Zia et al. 2020).
Improved yield and nutritional quality
The effects of nCeO 2 and nCuO on yield and nutritional
quality of cucumber by foliar application was studied by
Wang et al. (2020a, b) in three week-old cucumber seedlings
grown in soil. The test plants were evaluated for parameters
such as Ce, Cu and other nutritional elements, stomatal
conductance (Gs), transpiration rate (E), net photosynthesis
rate (Pn), yield, fruit size, weight and firmness. The results
showed increase in the fresh weight and reduced Zn content
in test plant fruits treated with nanoparticles (Hong et al.
2016). Another study by Wang et al. (2020a, b) on Chinese
scallion (Allium fistulosum) plants from soil amended with
CuO particles [nano (nCuO), bulk (bCuO) and CuSO 4 ]
grown in greenhouse conditions. These plants were used to
evaluate the allicin content, nutrient element and enzymatic
antioxidants. The test plants showed enhanced nutrient and
allicin contents in scallion by nCuO treatment and suggested
the use of nanofertilizer for onion crop (Wang et al.
2020a, b).
Further, the bulbs of Allium cepa were assessed for
mitotic index (MI) and chromosomal aberrations (CAs) after
treatment with TiO 2 and ZnO-NPs, and their mixtures (1:1)
by Fadoju et al. (2020). The results of recovery test in treated
bulbs showed transient CAs induced by both NPs and the
frequency of aberrations was high. The finding proved the
potential of tested NPs to induce mutation in somatic cells of
Fig. 2 Physiological responses
to engineered nanoparticles
(ENPs) by test plants
Plant Physiological Responses to Engineered Nanoparticles
89
stress. The findings showed NP aggregates formation in
plant tissues, pores of large size in roots and stomata in
leaves were closed rapidly. There was an increase in total
chlorophyll and carotenoid content of leaves in the test
plants. The plants showed changes in antioxidant enzymes,
cell injury, osmolyte, metabolite profile and the membrane
stability indices. The study proved that application of NPs
directly in soil was suitable for post-drought recovery of
barley plants (Ghorbanpour et al. 2020).
Counteract membrane damage
The effects of engineered nanoparticles such as Ag, Co, Ni
(metals) and CeO 2 , Fe 3 O 4 , SnO 2 , TiO 2 (metal oxides) on
seedlings of basil (Ocimum basilicum L.) grown in mix of
20% sandy soil and 80% peat were investigated by Antisari
et al. (2018). The results indicated that the test metal-NPs
were accumulated in the roots and the selected NPs, i.e., Ag,
Co, CeO 2 and Ni, were translocated from the root to shoot,
leaves and then to edible part of the test plant. The relative
short exposure accumulated Ca in roots that counteracted the
membrane damage by nanoparticles (Antisari et al. 2018).
Induced root formation
The study carried out by Ahmad et al. (2020) on ZnO and
CuO-ENPs application on in vitro formation of root,
antioxidant (non-enzymatic) activities and steviol glycosides
(SGs) in Candyleaf (Stevia rebaudiana) regenerants. The
results of NP treatment showed that the percentage of rooting and SGs—rebaudioside A and stevioside—were
increased. The phytochemical studies (flavonoid content,
phenolic content) and 2,2-diphenyl-1-picryl hydrazyl
(DPPH)-free radical scavenging activity were high in
regenerants. The biochemical and morpho-physiological
responses of candyleaf were proved to elicit defense
against test ENPs (Ahmad et al. 2020).
Increased in vitro regeneration
The study by Zia et al. (2020) analyzed the effect of silver
nanoparticles (AgNPs) on in vitro regeneration of carnation
cultivars cv. Noblessa, cv. Antigua and cv. Mariposa. The
number of shoots/explant of cv. Noblesse and cv. Antigua
and cv. Mariposa showed the highest regeneration rate. The
study concluded that test nanoparticles were effective for
increasing in vitro shoot multiplication and regeneration of
plants (Zia et al. 2020).
Improved yield and nutritional quality
The effects of nCeO 2 and nCuO on yield and nutritional
quality of cucumber by foliar application was studied by
Wang et al. (2020a, b) in three week-old cucumber seedlings
grown in soil. The test plants were evaluated for parameters
such as Ce, Cu and other nutritional elements, stomatal
conductance (Gs), transpiration rate (E), net photosynthesis
rate (Pn), yield, fruit size, weight and firmness. The results
showed increase in the fresh weight and reduced Zn content
in test plant fruits treated with nanoparticles (Hong et al.
2016). Another study by Wang et al. (2020a, b) on Chinese
scallion (Allium fistulosum) plants from soil amended with
CuO particles [nano (nCuO), bulk (bCuO) and CuSO 4 ]
grown in greenhouse conditions. These plants were used to
evaluate the allicin content, nutrient element and enzymatic
antioxidants. The test plants showed enhanced nutrient and
allicin contents in scallion by nCuO treatment and suggested
the use of nanofertilizer for onion crop (Wang et al.
2020a, b).
Further, the bulbs of Allium cepa were assessed for
mitotic index (MI) and chromosomal aberrations (CAs) after
treatment with TiO 2 and ZnO-NPs, and their mixtures (1:1)
by Fadoju et al. (2020). The results of recovery test in treated
bulbs showed transient CAs induced by both NPs and the
frequency of aberrations was high. The finding proved the
potential of tested NPs to induce mutation in somatic cells of
Fig. 2 Physiological responses
to engineered nanoparticles
(ENPs) by test plants
Plant Physiological Responses to Engineered Nanoparticles
89