Introduction

Corn is a warm season crop and is of special interest due to its high yield per input (water, fertilizer, etc.) and numerous uses in human, livestock, and poultry nutrition, and various products in industry, and is referred to as the king of grains. Critical periods of stress in corn include seedling establishment, rapid growth, pollination, and grain filling, and because each stage involves different physiological processes, the effect of stress on yield can be different. Drought stress is an important factor that prevents proper growth and development of corn plants (Nelson, 2002) and severely reduces plant yield (Turhan & ). Superabsorbent hydrogels are hydrophilic hydrocarbon polymer networks (Baser, 2004). These materials absorb and retain several times their own weight in water, and as the environment dries, the water inside the polymer is gradually discharged, and thus the soil remains moist for a long time without the need for re-irrigation (Monnig, 2005). An experiment on corn plants showed that the number of grains per plant was significantly affected by the use of minimal superabsorbent. However, at medium and higher levels of superabsorbent application, the number of grains per plant increased by 31 and 45 percent, respectively. Other experimental results showed (Mao et al., 2011) that the trend of increasing dry weight of corn plants under the influence of the application of superabsorbent polymer amounts was linear, which can be attributed to the improvement of soil properties and, as a result, the increase in usable moisture in the soil due to the application of superabsorbent polymers (Moezin Ghamsari et al., 2009). Kohestani et al. (2009) reported that the use of superabsorbent hydrogels in corn plants at a level of 300 kg/ha improved grain yield, yield components, and fresh weight. They also showed that with increasing drought stress, the effect of superabsorbent hydrogels on increasing grain yield was greater. They concluded that the effect of superabsorbent polymers at lower moisture levels was more noticeable. Superabsorbent polymers increased water retention in the soil and reduced the number of irrigations by 50% in corn plants. In another study (Nazarli et al., 2010), the relationship between the use of superabsorbent polymers and the increase in plant water availability was investigated by Wu et al. (2005). The results of this study showed that with the use of superabsorbent, 68% more water remained in the soil than the control. In another study, by investigating the effect of superabsorbent moisture materials in reducing the effect of drought stress in corn with two irrigation cycles of seven, 10 and fourteen days, it was found that plant height, dry matter percentage and yield were significantly affected by superabsorbent treatments, and with the use of both levels of 40 and 80 kg/ha of superabsorbent, growth traits increased significantly in a 14-day irrigation cycle. Also, Mao and Jahan et al. (2010) found that (2013) concluded that the number of seeds per plant was not affected by the application of minimum superabsorbent application, but at medium and higher levels of superabsorbent application, the number of seeds per plant increased by 31 and 45%, respectively.

Nitrogen is one of the important nutritional elements for plant growth. This element is the basis for the formation of proteins and nucleic acids. Given the importance of this element, it is very necessary to provide the required amount for the plant. This element is usually prepared and consumed in the form of chemical fertilizers, and its excessive use in this way is one of the reasons for groundwater pollution. In addition, their production is also expensive and costly, while replacing them with biological fertilizers plays an important role in increasing productivity and reducing its negative environmental effects. Nitroxin biological fertilizer (Chandrasekar et al., 2005)
Contains nitrogen-fixing bacteria of the genera Azotobacter and Azospirillum, whose number of viable cells is 108 per gram of carrier material from each of the bacterial genera, and is produced and supplied with the highest technology and based on international standards. In addition to fixing atmospheric nitrogen and balancing the absorption of essential macronutrients and micronutrients required by plants, the bacteria in Nitroxin biofertilizer synthesize and secrete plant growth promoters such as various regulatory hormones (Sturz & Christie, 2003). In sustainable agricultural systems, biofertilizers are of particular importance in crop production and maintaining sustainable soil fertility (Sharma, 2003). In addition to increasing the bioavailability of soil elements through biological fixation of nitrogen, solubilization of phosphorus and potassium, and inhibition of pathogens, they increase the yield of crop plants by producing plant growth promoter hormones (Sturz & Christie, 2003; Shaharoona et al., 2006; Violent & Portugal, 2007), considering the high water requirement of corn and its greater sensitivity to stress. Drought, proper irrigation management, and proper nutrition in the field can play an important role in increasing water use efficiency. In the climatic conditions of the country and Hamedan province, the most important limitation of corn production is the lack of irrigation water and drought stress at important stages of growth. Therefore, applying appropriate management methods in corn irrigation and nutrition can minimize the damage caused by this stress and be useful in improving crop production (Nador et al., 2005). The use of water superabsorbents and biofertilizers are appropriate tools for water management and plant nutrition that, in addition to optimizing the efficiency of water and soil resources, increase the competitive power of the crop by creating optimal growth and avoiding stress at critical stages of growth, and can lead to an increase in the quantity and quality of the crop by creating more desirable growth and avoiding stress at critical stages of growth. Therefore, the aim of this study was to investigate the effects of separate and combined use of nitroxin biofertilizer and super moisture absorber under moisture stress conditions on corn growth and yield.

Materials and methods

This study was conducted in 2015 in a field located in Hamedan Province, Bahar County and Hossein Abad Village to investigate the effects of nitroxin biofertilizer and super absorbent on the morphological characteristics and yield of corn cobs under water deficit stress conditions. The physical and chemical properties of the soil of the tested field are listed in Table 1. In this study, soil water deficit as the main factor and nitroxin biofertilizer and super absorbent polymer as the secondary factor were studied as a split plot experiment in a randomized complete block design with three replications. The factors studied included the irrigation factor at three levels of normal irrigation (irrigation at a soil water potential of -3 bar), moderate stress (irrigation at a soil water potential of -7 bar), and severe stress (irrigation at a soil water potential of -11 bar). Superabsorbent polymer and nitroxin biofertilizer were considered as secondary factors in four levels of using superabsorbent and nitroxin separately, combining the two, and not using them.
Biofertilizer was used as a seed dressing at a rate of 200 grams per 25 kilograms of seed. Superabsorbent polymer Stakosorb was also purchased from Atiyeh Energy Talash Company and was used at a rate of 100 kilograms per hectare according to the company’s recommendation. Soil preparation operations began with one plow and two perpendicular discs in early June, and then the land was furrowed and ridged using a furrower. After preparing the land and creating ridges, the ridges were broken up with a hand plow and the superabsorbent polymer was placed at a desired ratio at a depth of 5 to 10 centimeters from the ridge surface and under the seedbed. Then, nitroxin biofertilizer was applied as seed and soaked with corn seeds and the seeds were sown at a depth of 4 to 5 centimeters from the soil surface. Each plot consisted of 4 rows of 6 meters long with a distance of 75 centimeters and a distance of 20 centimeters between seeds on the row. The seeds were sown on June 30 using a corn row-planting machine. The corn variety studied in this study was Hybrid 678, which is a mid-season variety. The growth period of this variety is 115 to 120 days, and the emergence of the cobs occurred in 50 to 55 days after sowing.

Fertilization operations were carried out according to the results of soil tests, 150 kg of nitrogen from urea source and 100 kg of phosphate from triple superphosphate source per hectare. One third of nitrogen fertilizer and all phosphorus fertilizer were applied at the time of planting and the rest of nitrogen fertilizer was applied to the soil in two stages, at 7-leaf stage and before the emergence of male inflorescences. After planting, irrigation was carried out in the form of drip irrigation with a drip tape with a drip distance of 20 cm in all the treatments and in all irrigation regimes. Applications of corn began at 7-leaf stage. In – Irrigation treatments from stage 8 of this study, irrigation treatments included: 1- normal irrigation (after each irrigation, when the soil water potential reached -3 bar, the next irrigation was carried out), 2- moderate drought stress (after each irrigation, when the soil water potential reached -7 bar, the next irrigation was carried out), and 3- severe drought stress (after each irrigation, when the soil water potential reached -11 bar, the next irrigation was carried out). To determine the soil moisture of the field, first the pressure plate curve of the field soil was determined by the device and then for each irrigation, the soil water potential was determined daily using the weight method and after the soil water potential reached the desired value, irrigation was carried out in the control treatment and drought stress treatments. Calculation of the amount of water consumption in the control treatment (Cakir, 2004) was determined based on the allowable moisture discharge equal to 50% of the available soil water and the corresponding curve was drawn. Then, by adjusting the constant pressure in the system and measuring the volume of water output through a volumetric meter, the amount of water consumed was applied. Calculation of the volume of water consumed in the stress treatments was also carried out by adjusting the constant pressure in the system and based on the control treatments in each irrigation period. When the male spike was completed, three plants were randomly harvested and leaf area was measured with a surface area meter (Delta-T Devices Ltd, Cambridge, UK). In order to determine the final yield, at the full maturity stage on October 15, 3 square meters were harvested from the middle rows of each experimental plot, observing the margins from the top and bottom of the rows, and they were kept in the open air for one week for final drying. Before separating the seeds from the ear, the total weight of the plants (leaves, stems, ear and seeds) was determined and the biological yield was determined in tons per hectare. After separating the seeds from the ear, the weight of the seeds was weighed with a precision laboratory balance with an accuracy of one thousandth of a gram and the grain yield was calculated in tons per hectare. Then, the harvest index was obtained by dividing the grain yield by the biological yield. The thousand-grain weight was counted after selecting a random sample of seeds obtained from each treatment. and calculated in grams. In the final stage of growth, plant height, ear length, ear circumference, number of rows of seeds per ear, and number of seeds per row were recorded. The harvest index was obtained using the following equation:
:(Hopkins & Huner, 2004)
HI= EY/BY×100

EY and BY are the harvest index, HI is the biological and economic performance in kilograms per hectare, respectively.

All statistical calculations, including analysis of variance and comparison of means, were performed according to the statistical design using software 9.2 and comparison of means using Duncan’s test at the SAS probability level of 5%. Excel software was also used to draw graphs.

Results and discussion

The results of analysis of variance showed that the main effects of irrigation levels, nitroxin, and superabsorbent on all growth traits and yield components as well as total yield were significant. Also, in all growth traits, yield components, and grain yield, the interaction effect of irrigation levels and the use of superabsorbent and nitroxin was significant (Table 2). By comparing the mean data, it was determined that the control treatment (no use of nitroxin and superabsorbent) and the use of superabsorbent in normal irrigation had the same status in terms of plant height and did not differ significantly from each other. The use of nitroxin and superabsorbent and their combination with normal irrigation and moderate water stress significantly increased plant height. In the same way, they increased plant height significantly compared to the control treatment. Under severe stress conditions, although the use of nitroxin and superabsorbent and their combination increased plant height, this value was significantly higher with the combined use of superabsorbent and nitroxin compared to their separate use (Figure 1).

With these results, it was determined that the use of superabsorbent under conditions of 217 cm in / normal irrigation with an average height of 216 cm – 67 / compared to the control treatment (with an average height of 11 meters), did not have positive effects on increasing the height of the plants, and with sufficient water supply, the corn plant used the maximum photosynthetic capacity, and the use of superabsorbent was practically unable to increase this capacity. However, with a decrease in the amount of irrigation and an increase in the intensity of stress, because the photosynthetic capacity of the plants is reduced, the growth and height of the plants also decrease. It seems that moderate stress and with a higher intensity of severe stress accelerate flowering and reduce the height of the corn plant. Therefore, the ability of nitroxin and superabsorbent to increase the absorption of water and nutrients and reduce the effects of stress is more evident with increasing the degree of water stress, and the use of both has increased the height of the stem. In addition, their combined use with increasing stress intensity in corn plants had a higher synergistic effect. Allahdadi Jahan et al. (2007) and also Jan et al. (2013) in studying the effect of superabsorbent polymer on the growth characteristics of forage corn found that the use of superabsorbent had positive effects on plant height and dry matter accumulation under low irrigation conditions.
The use of superabsorbent in normal and conventional irrigation did not have a significant positive effect on the leaf area index (with an average of 0.08). However, in both low-irrigation treatments (moderate and severe stress) and normal irrigation (no stress), the use of nitroxin and the combination of nitroxin and superabsorbent significantly increased the leaf area. The trend of increasing the leaf area index in nitroxin treatments and their combination was not uniform, and the combined use of nitroxin and superabsorbent with severe stress was more significant than their separate use and showed a significant difference. In contrast, in moderate stress, the three treatments () had a relatively more uniform state (Figure 1). Leaf development in corn plants is one of the processes that is greatly affected by the water conditions of the plant. The reduction in water absorption due to the reduction in root and stem growth and activity
Reduction in the rate and efficiency of photosynthesis due to reduced stomatal exchange and reduced photosynthetic enzyme activity (Molla et al., 2001), which ultimately reduce plant growth, are factors affecting the reduction of leaf area under drought conditions. The most important effect of drought stress is to limit the rate of leaf development and reduce the rate of leaf growth (Blum, 1974). This reduction is due to a decrease in the rate of cell division and/or cell elongation (Borrell et al., 2000).
The reason for the decrease in leaf area index under low irrigation conditions compared to optimal irrigation conditions can be attributed to the decrease in photosynthetic materials for the growth and development of leaf cells and the increase in leaf senescence under water deficit stress conditions. Also, the increase in leaf area (Betran et al., 2003) in plants treated with nitroxin under low irrigation conditions (especially under severe stress) compared to the control treatment can be attributed to the decrease in leaf senescence due to the increase in chlorophyll production or its reduction in its destruction (photooxidation) and in fact the improvement in the water status of the plant due to the increase in nitrogen fixation and absorption by the biological fertilizer nitroxin. (Boomsma & Vyn, 2008) In low irrigation treatments, the leaf area index of corn was significantly affected by the use of superabsorbent. The more positive effects of the use of superabsorbent were more noticeable with increasing stress levels. Thus, by using superabsorbent and under severe stress, the average leaf index increased to 2.5, which showed a significant difference compared to the control treatment (no use of superabsorbent and at the same stress level) with an average of 9. Similar results were obtained with the application of superabsorbent on the surface of corn leaves under low irrigation conditions by the research of Moezin Ghamsari et al. (2009). (Jahan et al., 2013)
The increase in corn leaf area index under the influence of superabsorbent polymer application in different irrigation cycles may be the result of the continuation of the pressure potential necessary for leaf growth and the reduction of the effect of drought stress in the plant as a result of the use of this material.
In the use of nitroxin and superabsorbent and their combination in the length and circumference of the ear, a relatively similar situation was created with plant height, and with increasing water stress intensity, positive effects were created from the use of these materials. In addition, more positive effects were achieved with the combined use of two materials, superabsorbent and nitroxin, in severe stress compared to mild stress in the ear length (Figure 2).

Alavi Fazel et al. (2008) also reported in a study conducted on corn that the length of the ear decreases with increasing stress intensity. Nitroxin fertilizer is able to increase the growth power of the reproductive organs and the volume of the ear in corn plants for two reasons: firstly, by increasing the activity of nitrogen-fixing bacteria, it makes it more accessible and easier to absorb from the soil solution, and on the other hand, by directly affecting the synthesis of plant phytohormones, especially auxin and cytokinin, it provides more favorable conditions for stimulating cell divisions and the growth of the reproductive organs. Superabsorbents, in other ways, by increasing the water retention capacity of the soil, while increasing the capacity of readily available water and also making nutrients available, lead to an increase in the photosynthetic capacity and the absorption and utilization of the materials obtained from it, and as a result, better growth power of the reproductive organs, length and circumference of the ear. In general, it has been found in corn that with increasing irrigation intervals and stress intensity, ear circumference decreases. Nadur et al. (2005) reported that there is a direct relationship between increasing irrigation rates and ear diameter in corn. The results of their research showed that with decreasing irrigation intervals, cell division and their number increase and the ear diameter and consequently the ear circumference increase. The number of rows of grains in the ear and also the number of grains in the ear and also the grains in the row in the use of superabsorbent were 32 and 31 and 68, respectively, with an average of 52, 33, 28 and 12, 67, 14, 56, respectively, and the differences were statistically significant in terms of 35, 26 and 65, respectively. In addition, in moderate irrigation stress, their separate and combined use were at the same level with a much better situation than the control treatment. In severe stress, although their use increased significantly compared to the control treatment, the effects of their combined use were far superior to their separate use (Figure 3).

During drought stress, especially in the final stages of growth, the plant reduces the completion of terminal grains by regulating the allocation of nutrients from photosynthesis and allocating more nutrients to the roots and other organs in order to resist stress, and as a result, the number of grains in the row decreases. However, the use of superabsorbents moderates the effects of stress and increases the number of grains in the row compared to not using it under water stress conditions. It has been reported that the use of superabsorbents increases the number of grains in the ear. It seems that superabsorbent, by providing water and subsequently helping to absorb some nutrients at the critical stage of grain formation, reduces abortion and consequently increases the number of fertilized grains (Fazli, Rostampour et al., 2010). In normal irrigation, the use of nitroxin and also the combined use of nitroxin and superabsorbent had a significant and uniform effect on biological yield, hundred-grain weight and total yield. The average biological yield, hundred-grain weight and total yield with the use of nitroxin and the combined use of nitroxin and superabsorbent and at normal irrigation level were 28.29 and 18.02 tons per hectare, 18.88 and 94.88 tons per hectare, respectively, which was 11.11 and 22.35 grams per hectare compared to the two control treatments (no use of nitroxin and superabsorbent) and also with the use of Superabsorbent alone showed a significant difference in normal irrigation. However, under normal irrigation conditions, the use of superabsorbent alone did not have a significant effect on these traits. With low irrigation and stress, the effects of using superabsorbent on 100-grain weight, total yield, and biological performance were significant. Under moderate and severe stress, the use of superabsorbent and the combined use of superabsorbent and nitroxin had a better effect than the use of nitroxin alone on 100-grain weight, total yield, and biological performance (Figures 4 and 5).

Corn plants, when exposed to drought stress, try to shorten their life cycle in order to escape the effects of stress. Therefore, due to the shortening of the grain filling period, the final weight of the grains decreases. The increase in grain weight in the superabsorbent and nitroxin treatments can be due to the longer grain filling period and the durability of the leaf surface. It has also been shown that the increase in yield in crop plants is positively correlated with the increase in leaves and the absorption of nutrients. Therefore, under such conditions, the production of photosynthetic materials increases and the transfer of these materials to the reservoirs (seeds) causes an increase in the weight of a thousand grains under the conditions of nitroxin application.
Fazeli Rostampour et al. (2010) reported that the use of superabsorbents causes an increase in the weight of a hundred grains, which is consistent with the results of this study. Therefore, by increasing the soil water retention capacity, superabsorbents can increase the relative water content of the plant and consequently the water potential of the cells and the reservoir strength during the seed filling stage, and increase the seed weight. And (Shaharoona et al., 2006 Violent & Portugal, ) also Violent & Portugal 2007) stated that Azospirillum and Azotobacter, which are considered molecular nitrogen-fixing microorganisms, are present in the biofertilizer Nitroxin and, in cooperation with the roots of plants, enhance their growth.
In addition, by increasing the absorption of nutrients in the plant, they increase the growth and development and biochemical activities of the plant, and this increases the biological performance. It has been reported that the use of superabsorbent polymers increases the performance of crops (Kuhestani et al., 2009). The reason for this could be an increase in the capacity to hold water and nutrients for a long time in the soil, a decrease in nutrient leaching, rapid and favorable root growth with nutrient storage and better soil aeration. It seems that water-soluble compounds with low molecular weight, such as nutrient cations, can be absorbed by this material and, with gradual release, absorbed by the plant root.

Overall conclusion

In general, the results of this study showed that nitroxin biofertilizer was able to stimulate corn growth under both normal irrigation conditions and stress conditions, increasing yield by 1 ton/1 and 98/1, respectively, and increasing yield by 67 grains/hectare. Also, super absorbent moisture helps to increase the osmotic regulation ability of corn plants by increasing water retention capacity in the soil and the possibility of absorbing and accumulating more mineral salts, and enables the plant to absorb more water in more negative soil potentials, and as a result, helps to moderate the effects of water shortage stress in the soil. In addition, their combined use increases their positive effects.