In this experiment, the foliar application of silicon concentrations enhanced gas exchange parameters, pigment contents, antioxidant capacity, and mini-tuber yield of potato which are consistent with previous reports regarding the beneficial influences of Si supplementation on antioxidant activities, growth and yield of other crops including cucumber [5], barley [27], soybean [29, 30], banana [14] and tomato [8].
In this study, the increasing Pn and pigment content was apparent at concentrations above 1.6 mmol L− 1 of both Si sizes. These results were in agreement with studies on tomato, cucumber, and soybean which indicated the positive influence of Si supplement on the Pn and Chl content of leaves in hydroponic conditions [5, 8, 30]. Furthermore, our results indicated that DPPH radical scavenging was clearly increased by Si particles in the tuberization stage of potato. There might be numerous mechanisms involved in the influence of silicon on net photosynthesis however, an increase in antioxidant capacity as a result of Si induction may be the possible mechanism. Feng et al. (2010) [10] showed that Si could promote net photosynthesis, which is related to the individual role of Si in protecting photosynthesis apparatus from ROS damages. Furthermore, in similar previous reports it was proposed that silicon helps to improve the stability of cell plasma membranes [29], the integrity (thylakoids, grana lamellae) and function of chloroplast [8, 9, 10] and following that the electron transport chain in thylakoid membranes for the production of ATP and NADPH will be protected against ROS [9] by promoting antioxidant system for detoxifying reactive oxygen species [10, 30]. Our results also showed that total phenol content in potato leaves was enhanced significantly by Si at above 0.8 mmol L− 1 levels. These observations are in agreement with the findings of Gagoonani et al. (2011) [17], who applied 1.5 mM Si which improved phenolic components of leaves at the control and Al toxicity treatments in Brago officinalis L. seedling was grown in hydroponically medium. Moreover, Shetty et al. (2011) [18] reported that the application of 3.6 mM Si stimulated phenolic acids and flavonoids in rose (Rosa hybrida). They suggested that Si can promote the expansion of genes encoding enzymes and transcript levels in the phenylpropanoid pathways compared with untreated Si plants. Phenolic components can impress plant development via lignin and pigment biosynthesis or accumulation in the subepidermal layers of plant tissues [30] and the up-regulation biosynthesis of phenols in chloroplasts could enhance radiation intercept in leaves [31]. So, increasing phenol content induced by Si has influenced the structure, function, and protection system of potato leaves, especially in the chloroplast. Since increase of DPPH radical scavenging and total phenols content under the influence of silicon was simultaneously accompanied with improvement in photosynthetic pigments, the probable conclusion can be that partial increasing concentration of photosynthesis pigments (chlorophyll a and b) may depend on maintaining ultrastructure and orderliness of chloroplast or improving of chlorophyll biosynthetic pathways as a consequence of Si-related up-regulation of antioxidant system and phenol components.
Several previous reports have shown silicon can regulate the activities of main photosynthetic enzymes of Calvin cycle [5, 11]. Adatia and Bestford (1986) [5] reported that the Si addition to nutrient solution enhanced carboxylase activity (RubisCO) of cucumber leaves under normal conditions. Similar findings were reported for barley [27] and Spartina densiflora [11] under saline and Cu toxicity stresses. Silicon application increased the activity of phosphoenol pyruvate carboxylase in wheat under drought conditions [32]. Hence it seems that in this experiment, part of the Si influence has been linked with activity regulation of key enzymes in non-photochemical photosynthetic processes. Moreover, up-regulation endogenous phytohormones such as GAs, IAA, and cytokines in Si-treated plants as reported for mango under drought stress [33] or GA1 in soybean leaves under normal hydroponically conditions [28] mentioned in previous studies. In conclusion, it is likely that enhance of chlorophyll content and Pn were correlated with an increase of growth regulators in Si-treated potato leaves.
Results of this study indicated that by increasing Si concentration, Chl b content was significantly increased at 1.6 and carotenoids increased at 3.2 mmol L− 1 of Si. The Chl b and carotenoids are considered as an antenna and auxiliary pigments for Chl a reaction centers. Therefore, an increase in carotenoids and Chl b can be helpful for the absorption of light energy for electron transport photosystems in Chl a [34]. It is possible to suggest a positive role of Si in controlling photoinhibition of potato leaves. Since the absorption spectrum of Chl a and Chl b are different, it seems that an increase in the Chl a/b ratio can determine the quality of light-harvesting by leaf. Moreover, the finding of Kitajima and Hogan (2003) verified that improvement of Chl a/b accompanied by an increase in the electron transport rate in reaction centers of Chl a and rubisco carboxylation capacity which are in agreement with our results that shown a positive correlation between Pn with Chl a and Chl a/b in two scales of Si particle treatments (Table 6).
Based on this study results, an increase of Pn was simultaneously accompanied with decrease of Tr in Si-treated leaves at the tuberization growth stage. Nevertheless, the Gs was not affected by Si treatments (Tables 1 and 2). If the limitation of Tr was due to stomatal closure, there should be a decrease in Ci. However, Ci had a slight increase in Si-treated leaves with 2.4 and 3.2 mmol L− 1. Therefore, the decrease in Tr may be due to nonstomatal restrictions which are in agreement with the results reported for S. densiflora [11] (Mateos-Naranjo et al. 2015) and tomato [35] treated with Si under salinity stress in greenhouse conditions. Various studies have described that by applying of silicon, the silica cuticle double layer was formed on the leaf epidermis which may reduce water loss through the cuticles of plants [11, 14, 36]. Although, the cuticular transpiration rate is lower than the stomatal transpiration rate, it can perform an important role in leaf water loss. Therefore, it seems that the decrease of Tr in Si-treated potato leaves most likely has been due to the reduction of cuticular Tr. Asmar et al. (2013) [14] suggested that the Si accumulation in epidermal tissue can have a positive effect on water relations in leaves during acclimatization under humidity changes in greenhouse conditions. Moreover, physical strength caused by Si deposition may develop mechanical protection to infection pathogens in crop leaves [18]. Therefore, it seems that foliar application of Si particles can be useful for the growth and health of potato plantlets are transferred to soilless culture. Our results also indicated that stomatal conductance did not change (Table 1) and it has not limited the CO2 diffusion into sub-stomata chamber or CO2 assimilation in chloroplasts. Accordingly, an increase of Pn with Si may associate with the photosynthetic enzymatic process and chloroplast function.
Overall, silicon particles levels improved water use efficiency. Previous studies are in agreement with these results indicated that Si can enhance water use efficiency at the normal conditions in tomato [8, 36]. Our results showed that a decrease in transpiration rate was accompanied by an increase of Pn in Si-treated leaves. Therefore, improvement of water use efficiency was predictable. A positive effect of Si application on mesophyll conductance of potato leaves was also in agreement with Haghighi and Pessarakli (2013) [8] results in cherry tomato. Our results showed that all Si levels had a positive effect on mini-tuber yield. The ability of Si to increase yield production has been demonstrated in cucumber [5] and tomato [35].
According to this study, although net photosynthesis, pigment content, and yield measured in Si treatments were enhanced. These changes in nano-Si treated plants were more than ionized-Si. These results also indicated that potato leaves could foliar uptake of Si and nanoscale particles showed more efficiency in response to this method. The higher influence of nanoparticles may be due to unique characteristics [8, 12, 37] and facility uptake by leaf stomata because of their smaller size. These observations are in agreement with Tripathi et al. (2015) [12], who achieved the beneficial influence of nano-Si on growth and dry weight of Pisum sativum in normal and Cr toxicity. Siddiqui and Al-Whaibi (2014) [36] also confirmed that nano-Si increased the germination characteristics, which enhanced the seedling dry weight of tomato. However, Haghighi and Pessarakli (2013) [8] showed that although Si addition mitigated adverse effects of salinity in gas exchange parameters and dry weight of cherry tomato, no difference between root application of nano and bulk Si was observed. The comparison of silicon nanoparticles and silicate in Fenugreek by Nazaralian et al. (2017) [6] also indicated that the influence of the added nanoparticles in nutrient solution declined over time. Moreover, the results of Abdel-Haliem et al. (2017) [37] showed that application of nano and ions Si in rice seedling under saline conditions increased growth, antioxidant activity, and biochemical traits such as soluble carbohydrates and amino acids, but the difference between particle sizes of Si was not noticeable.
Stomatal uptake can be a main pathway for the foliar uptake of mineral nutrients and other solutes [34] (Taiz et al., 2015), which can not able to penetrate through the surface of the epidermal cells. There are very fine pores with a diameter on a nanoscale on adaxial and abaxial leaf surface which are called ectodesmota with an approximate density of 1010 per cm− 2 leaf area. Moreover, inside of this pores are covered with polygalacturonic acids, which only allow positive particles to enter [38] (Marschner 1995). It can be concluded that since in our study silicon solutions were acidic (pH = 5) and ectodesmota diameter was nanoscale, it seems that the high influence of nano-Si treatments may be due to an increase of their foliar uptake via ectodesmota in potato leaves.