The contribution of nano-zinc to alleviate salinity stress on cotton plants

To investigate the effect of nano-zinc fertilizer on growth, yield and mineral status of cotton plants grown under salt stress, a pot experiment was set up in the greenhouse of the National Research Centre. The treatments were as follows: (I) diluted seawater: 10% (S1), 20% (S2) and tap water as a control (S0), (II) 100 ppm (NZn1), 200 ppm (NZn2) nano-zinc and distilled water as a control (NZn0). Irrigation with 10 and 20% seawater decreased dry weight (DW) of leaves by 11.53 and 43.22%, while decreases in bolls were 15.50 and 71.65%, respectively. Except for root DW and top/root ratio, the measured growth parameters were increased as nano-zinc concentration increased. As for the interaction between treatments, the highest DW of stem, leaves and bolls resulted from the addition of NZn2 under normal condition, followed by NZn2 x S1 and the next was NZn2 x S2. The foliar application of 200 ppm nano-Zn led to mitigating the adverse effect of salinity and confirmed that diluted seawater could be used in the irrigation of cotton plant. However, phosphorus fertilizer should be added with nano-Zn application to avoid P/Zn imbalance. Some elements’ status and their ratios were recorded.


Introduction
Ecosystem processes changed by the climate change by increasing both biotic and abiotic stress [1]. Increased salinity of agricultural land is expected to have destructive global effects, resulting in loss of up to half of arable lands by the middle of the twenty-first century [2]. The adverse effects of salinity have been attributed to the increase of sodium and chloride that are considered the most important ions which induced several disorders in physiological processes of different plants [3]. Salinity leads to plant death by ionic and osmotic stress, which causes nutrient 2018 The Authors. Published by the Royal Society under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/4.0/, which permits unrestricted use, provided the original author and source are credited.

Effect of nano-zinc fertilizer
The measured growth parameters increased with increasing zinc concentration in the form of nano, except for root dry weight, or T/R ratio (table 4). The highest DW of root was obtained by spraying 100 ppm nano-zinc oxide. T/R ratio decreased with the first nano-zinc concentration and tended to increase markedly by using 200 ppm nano-zinc to be clearly more than the control. Pronounced increases in growth parameters, i.e. stem, leaves, bolls and whole plant DW, were shown with the increases in nano-fertilizer rates up to the highest level (NZn2) compared to plants receiving distilled water (NZn0).
Data in table 5 reported that N, K, Ca, Na and Zn concentrations augmented with nano-Zn treatments, while P% slightly increased with NZn1 and sharply decreased with NZn2. This may be due to the antagonistic effect between P and Zn.
The content of leaves of all studied elements (mg/plant) follows the same trend of dry weight of leaves, where they increased significantly with the addition of nano-zinc compared to control and with increasing the foliar solution's concentration from 100 to 200 ppm (figure 2). The increasing percentages of adding NZn1 and NZn2 were 56.5 and 82.6% for N, 82.0 and 85.2% for P, 60.1 and 90.1% for K, 93.5 and 132.5% for Ca, 50.3 and 95.7% for Na and 53.2 and 85.6% for Zn. Interestingly, calcium is the most influenced nutrient with nano-Zn application.
Examination of data in table 6 shows that Na/Ca and K/Ca ratios decreased with Zn nano-spraying without significant differences between its application rates, but the opposite was true for Ca/(K + Na) ratio. Zinc application did not affect Na/K ratio; meanwhile, P/Zn ratio was increased by the NZn1 and decreased with the highest level of nano-Zn. Also, its value became less than that of the control.
Growth parameters of savory plant; height, leaf number, leaves fresh and DW, chlorophyll, essential oil and phosphorus content were improved by nano-zinc application [21]. The height, fresh and dry weights of treated cotton plants increased in control < mineral Zn < Zn-chelate < nano-Zn chelate in that order [22]. Foliar Zn application increased Zn concentration and protein, carbohydrate metabolism, but decreased P% in grains of wheat compared with Zn alone [23]. The positive response of nano-ZnO compared to ZnSO 4      oxygen species) levels, which led to lower lipid peroxidation, MDA (malondialdehyde), activity of prominent antioxidant enzymes and superoxide dismutase as discussed by Burman et al. [24]. Also, zinc addition affects auxin (growth regulator) biosynthesis positively; this can promote mineral absorption, cell division and thus enhance plant growth [25,26]. It also enables the plant to maintain the plasma membrane integrity [27]. In tomato plants, inadequate Zn is correlated with a reduction in IAA content which tends to increase after zinc is resupplied [9]. In cotton, Rezaei & Abbasi [22] reported that application of nano-chelate zinc improves physiological processes in cotton plant; increases chlorophyll content and antioxidant activity of catalase, peroxidase and polyphenol oxidase.

The interaction effect between the examined parameters
As for the interaction effect between nano-zinc application under two levels of salinity when compared with irrigation with fresh water as a control, it could be stated that the addition of nano-zinc is more effective in the whole plant dry mass under 20% diluted seawater treatment than that of 10% diluted seawater treatment or freshwater irrigation (    different cotton plants as well as total dry mass was improved by the application of nano-fertilizer under different salinity levels. The highest values of all the measured growth parameters were produced by the interaction S0 × NZn2 except for root and stem dry weight that produced their highest values by the second interaction S1 × NZn1 and S1 × NZn2. The interaction effect of nano-fertilizers and salinity on the mineral concentration of cotton plants is illustrated in table 8. All mineral concentrations were not affected significantly by nano-zinc treatments under applied salinity levels except for P that produced its highest value by the application of NZn1 with the medium salinity degree. Nitrogen percentage decreased similarly with both salt concentrations, so its highest value resulted from irrigation with tap water plus application of NZn1. Addition of the high level of nano-zinc (NZn2) without salinity stress (S0) enhanced the other values of determined element concentrations. Figure 3a,b illustrates the interaction effect of nano-fertilization and salinity on the mineral content of cotton leaves. This interaction significantly affected the content of minerals in leaves except for nitrogen. It is clear that salt stress decreased the content of minerals and vice versa for nano-zinc treatment. The lowest values of N, P and K content were produced by S2 × NZn1, while the lowest values of Ca, Na and Zn contents were produced by S2 × NZn0.     The interaction effect was not significant except for the P/Zn ratio, which decreased by Zn treatment. Furthermore, this ratio was increased by the first Zn treatment and decreased with the second one under either moderate salinity or the highest rate (table 9). In another study on cotton, addition of nano-Zn, nano-Si and a combination of them had no significant effect on any measured parameters [28].
Increasing density and reactivity of the specific surfaces of nanoparticles led to enhanced plant physiology and performance, thus increasing its ability to mitigate salinity. Moringa accumulates lower concentration of Na + and Cl − and higher amount of N, P, K, Ca, Mg, Fe and Zn upon foliar application of nano-ZnO to Hogland solution compared with those receiving Hogland solution only, under salt stress condition [19]. Arough et al. [29] reported that under high salinity level (5.55 dS m −1 ), biofertilizer and 0.8 g l −1 nano-zinc oxide increased grain yield of Triticale (a hybrid of wheat and rye) by 39% compared with the control (without any additions). Application of nano-zinc at rates of 25 or 50 mg l −1 caused significant changes in fresh and dry weight as well as in relative water content of rice [30] in biomass production of sunflower [31], in grain yield of wheat under salinity stress [32] and in yield of maize under drought [33]. These enhancements in growth, yield and quality of different plant kinds with nano-fertilizer addition may be due to (1) increasing the nutrient use efficiency [34,35]

Conclusion
The foliar application of nano-Zn led to mitigate the adverse effect of salinity and confirmed that diluted seawater could be used in the irrigation of the cotton plant. Also, nano-Zn enhanced cotton growth parameters and yield under stress condition. However, increasing the application rate of nano-Zn may reduce P absorption and translocation to leaves and consequently reduce the P/Zn ratio. It should be mentioned that an additional dose of phosphorus fertilizer with nano-Zn could be used to avoid the P/Zn imbalance. Further studies on the effect of nano-Zn on health and the environment of users are required.
Data accessibility. All data are in the tables and figures in the main text.