Nano Zinc-Oxide Enhanced Photosynthetic Apparatus and Photosystem E ciency of Maize (Zea Mays L.) in Sandy-Acidic Soils

23 Conventional Zinc (Zn) fertilization (e.g., zinc sulfate) often leads to poor availability in 24 soils. Zinc oxide nanoparticles (nano ZnO) can be a potential solution, but their effect on crop 25 photosynthetic activity isn’t well documented. The effects of nano ZnO (50, 100, 150, 200 mg L 26 ) and application methods (seed-coating, soil-drench, and foliar-spray) in comparison with 27 ZnSO4 recommended dose were evaluated for plant height, biomass, chlorophyll pigments and 28 photosystem efficiency in a greenhouse pot experiment. 100 mg L of nano ZnO significantly 29 increased the chlorophyll (Chl.) a, b, a+b, carotenoids (x+c), a+b/x+c, SPAD, leaf Chl., total 30 chlorophyll content plant, plant height and total biological yield (by 18-30%, 33-67%, 22-38%, 31 14-21%, 14-27%, 12-19%, 12-23% 58-99%, 6-11% and 16-20%, respectively) and reduced Chl. 32 a/b (by 6-22%) over the other treatments (p<0.01) irrespective of application methods. Nano 33 ZnO applied at 100 mg L significantly increased photochemical quenching (qP) and efficiency 34 of photosystem II (EPSII) compared to 150 and 200 mg L regardless of application methods. 35 The positive correlations between Chl. a and Chl. b (r 0.90), Chl. a+b and x+c (r=0.71), SPAD 36 and Chl. a (r=0.90), SPAD and Chl. b (r=0.94) and SPAD and Chl. a+b (r=0.93) indicates a 37 uniform enhancement in chlorophyll pigments; SPAD value, qP, EPSII, and growth and yield 38 parameters. This elucidates that the application of nano ZnO at 100 mg L promotes corn 39 biochemical health and photosynthesis, irrespective of the application method. These findings 40 have a great propounding for improving plant growth through nano ZnO bio-fortification in 41 acidic Spodosols. 42

application through conventional methods (e.g., zinc sulfate) often leads to poor availability and 58 low crop uptake due to fixation reactions in soils (Elemike et al., 2019). The Zn uptake 59 efficiency is particularly low when the soil contains low organic matter, low clay content, high 60 carbonate, or low pH (Recena et al., 2021). 61 Nanoparticles, owing to their minute size and large reactive surface area, offer a potential 62 solution to improve nutrient uptake efficiency in agriculture (Rizwan et al., 2017). As they are 63 readily up-taken by plants, they possess a significant potential to improve crop growth and yield 64 (Sabir et al., 2014). Zinc oxide nanoparticles (nano ZnO) can be a viable alternative to enhance 65 Zn uptake in low organic matter, sandy, and highly acidic soils. Although using ZnO only in evaluating Zn uptake by crops but also in understanding its implications in crop 85 photosynthetic apparatus and efficiency to identify its optimal level of application.

86
In this study, the performance of nano ZnO was evaluated for corn plant photosynthetic 87 efficiency using different application modes and rates in sandy-acidic Spodosols. The major 88 objectives were to evaluate the effect of different application rates of nano ZnO on crop growth 89 as evidenced through plant growth, the robustness of photosynthetic apparatus (chlorophyll 90 pigments, carotenoids and antioxidant activity) and photosynthetic efficiency (efficiency of 91 photosystem II), as compared to conventional Zn sulfate application. Besides, Nano ZnO was 92 tested through different application methods (seed coating, soil drench and foliar spray) to 93 understand its effectiveness for photosynthetic performance, growth and Zn nutrition of corn.  Table 1). The collected soils were composited and homogenized before 100 transporting them to the laboratory, where it was air-dried and sieved through a 1-mm sieve. The 101 pre-sowing analysis of Spodosol soil to be used for this experiment is shown in Table 1

2.3.Soil analysis:
122 The soil samples were analyzed prior to the beginning of the experiment (Table 1)     with foliar application of nano ZnO over the seed coating and soil drench; and a 4% higher Chl.
190 a/b with soil drench each over the seed coating and foliar application was observed, respectively.

191
A significant correlation between Chl. a and b (r 2 = 0.90), a+b and x+c (r 2 = 0.71) was observed.

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Interaction between the Zn treatments and application methods for Chl. a and Chl. a+b    214 Results ( in proline, statistically similar to the control, significantly (p<0.01) higher (by 162%) than 150 227 mg L -1 and 6% and 60% higher than the nano ZnO 100 mg L -1 and 200 mg L -1 ZnO NP 228 treatments, however, they were statistically similar (Table 2).

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Application methods were non-significant for x+c and proline but their impact was 230 significant on Chl. a+b / x+c ratio. Soil drench had an edge (p>0.05) of 1.7% and 5% in x+c and 231 57% and 50% in proline over seed-coating and foliar application, respectively. Foliar application 232 was significantly (p<0.05) higher in Chl. a+b / x+c ratio (by 10%) than soil drench but had an 233 edge of 6% (p>0.05) over the seed coating of the nano ZnO while the latter two methods were 234 statistically similar ( Table 2). The interactions between Zn treatments and their application 235 methods affecting the x+c and proline concentration were non-significant but highly significant 236 (p<0.01) for Chl. a+b / x+c ratio. Data (Fig. 1) revealed that the performance of the nano ZnO 237 100 mg L -1 was significantly (p<0.01) higher compared to other treatments applied at either 238 method except for the nano ZnO 150 mg L -1 applied as foliar. In soil drench, the higher nano   Results further indicated the impact of the application methods and their interaction with 259 Zn treatments on the SPAD, leaf Chl. and total Chl. plant -1 of the corn crop was non-significant 260 (Table 3). However, seed coating of nano ZnO showed up to 8% (p>0.05) and 5% (p>0.05) 261 improvement in total Chl. plant -1 over the soil drench and foliar application methods. SPAD 262 value had a significantly higher correlation with extractable Chl. a (r 2 =0.90), Chl. b (r 2 =0.94) and 263 Chl. a+b (r 2 =0.93) and support the data recorded for Chl. a, Chl. b and Chl. a+b with 264 spectrophotometer (Fig 2).  266 Zinc treatments significantly (p<0.05) improved the efficiency of photosystem II (EPS II) as 267 well as the quantum photosynthetic yield of PSII (Y) over the control. As for the EPS II, the 100 268 mg L -1 , 150 mg L -1 nano ZnO and the ZnSO 4 were statistically similar but significantly (p<0.05) 269 higher than the control, however, nano ZnO 100 and 150 mg L -1 higher by 0.14% and 0.16%, with Zn treatments on the Y(II) and EPS II was non-significant.

321
Zinc is associated with enzymes' activation, structural and catalytic components of proteins. The Chl. a/b ratio is a marker of pigments functionality and adoption of a photosynthetic 347 system to light (Lichtenthaler et al., 1981). Chloroplast, which is responsible for photosynthesis,  Higher SPAD values and total chlorophyll content for nano ZnO over the ZnSO 4 (Table   361 3) indicate significant (p<0.05) improvement in the crop's physiological conditions. In contrast, 362 the results for nano ZnO 100 mg L -1 (Table 3)   indicates other stress factors present in soil, such as heavy metals like Al (Table 1)  below 100 mg L -1 nano ZnO (Table 3) confirm the concentration-dependent impact on crop's 406 physiological parameters and tallies well with SPAD (Table 3) and extracted chlorophyll data 407 (Table 2). Significant correlation between Chl. a and photochemical quenching (r 2 =0.45) and

408
Chl. b and photochemical quenching (r 2 = 0.57) (Fig. 2) confirmed that increased Chl. a and Chl.   mitigation on the plant, as is evident from significantly lower proline content (Table 3)