Application of kinetin to ameliorate cadmium toxicity on growth and yield components of maize (Zea mays L.) plants in Saudi Arabia

S. M. Al-Amri

Department of Biological Sciences, College of Science and Art, Shaqra University, Saudi Arabia. Tel: +966 554433055   Fax: +966 1 4678467

E-mail address: phd1618@hotmail.com

ABSTRACT:

Maize is the most widely distributed crops of the world. During the past two decades in Saudi Arabia, maize becomes one of an important source of feed for human and animals. The present experiment was conducted to investigate the effect of soil drench of two concentrations (100 and 500 µmol) of cadmium chloride on growth, mineral contents, water relations and productivity of Zea mays. Also, the study was extended to evaluate the effect of grain presoaking with kinetin to ameliorate the toxicity effects exerted by CdCl₂. Generally, cadmium treatments led to significant decrease growth parameters (shoot and root dry weights, leaf area and shoot height), photosynthetic pigments (Chl. a, Chl. b and carotenoids), water relations (relative water content, transpiration rate and leaf conductance), ion contents (sodium, potassium, calcium and cadmium) and yield components (grain yield, number of grains and harvest index) of maize plants as compared to control plants. Grain presoaking in kinetin induced remarkable increases in all above studied parameters, except decrease cadmium content, of maize plants treated with different concentration of CdCl₂ as compared to treated plants with CdCl₂ alone. Total soluble sugars and protein content in yielded grains were significantly decreased in maize plants irrigated with CdCl₂, but application of kinetin alleviates or ameliorates the toxic effects of CdCl2. Of particular interest that application of kinetin was positively correlated with some growth, yield and water relations and negatively correlated with cadmium content of maize plants irrigated with CdCl₂ solution.

INTRODUCTION

Corn is considered one of the main grain crops in the world in general and Saudi Arabia in particular because it is important in feeding the human, animal and poultry. It enters the dry feed industry by up to 80% and the bread industry by 25% glucose, fructose and oil. The whole grain contains 72% carbohydrate, 13% water, 8.9% protein, 3.9% fat and 1.2% ash .The maize

grains are rich in fat, protein and starch in particular, and they form an important part of human food in many developing countries in Africa and Asia (Dahmardeh, 2011). Saudi Arabia has been interested in growing maize plants. The cultivation of maize in the soils is successful with good drainage and ventilation, ranging from acidic to moderate. It is necessary to plant many elements, including phosphorus, nitrogen and potassium. These elements, together with many other elements, are added to agricultural soils in the form of organic or chemical fertilizers before soil tillage (El- Naggar et al., 1991; Rahmat et al., 2017). In some cases, some farmers may be forced to use water from industrial waste in irrigation. This is the element of cadmium, which is considered one of the most important toxic elements, and its accumulation in the soil in large quantities has negative effects on the growth and productivity of some (Misra et al., 1994; Al-Hakmi, 2007and Singh et al., 2017).

Burstron et al. (1986) explained that ionic cadmium greatly helps to inhibit the chromosome content of leaves of lettuce plants by directly influencing the production of 5-aminolaevuli acid. The increase in cadmium concentrations in the soil leads to a decrease in the content of chlorophyll and carotenoids in addition to the water content, the transpiration rates and the ionic content of the calcium and potassium elements in the leaves of the carrot plants (Stobartet et al., 1985; Gadallah, 1999). Abo-Hamed et al. (1987) reported that diffusion of different varieties of wheat at a concentration of 50, 100, 200 ppm of kinetin significantly helps to increase the growth rates and yield of these plants under different concentrations of cadmium chloride if compared to cadmium-treated plants only. Vodnik et al. (1999) showed that this hormone plays a prominent role in increasing the ionic content of nitrogen, potassium and calcium in cadmium chloride-treated grains. Producing polysaccharides and protein content of bean seeds grown in contaminated soil with cadmium and lead was recorded by Robertson et al. (1999) and Srivastav et al. (2017). The influence of growth regulators has been studied to overcome the toxic and harmful effects caused by these heavy polluting elements of soil as a result of continuous irrigation in terms of their role in delayed aging and yellowing of plant leaves and increasing their tolerance to severe interstitial conditions (Aldesuquy, 2000) note that the role of cytokintin in plant resistance to the toxicity of heavy metals is unclear and further study is needed. This study aimed to study the effect of kinetin in reducing or eliminating the toxic effects of cadmium chloride in the growth and productivity of maize plants in Saudi Arabia.

MATERIAL AND METHODS

Plant and growth conditions

This study was conducted at the Research Station of the Faculty of Science, King Saud University, during the last week of April to the last week of September 2016. Healthy uniformed maizegrainswere surface sterilized in a 7% sodium hypochlorite solution for 5 minutes and washed directly with distilled water and dried on sterile filter paper which was divided into two sets. First set: the grains soaked in the distilled water for 3 hours. Second set: the grains soaked in a 50-ppm solution of kinetin (based on previous preliminary experiments) for 3 hours. Immediately after the period of soaking, the seeds were washed with sterile water and 5 grains were placed in each plastic pot (30 cm in size) containing 7 kg soil (sand:clay, 2: 1 v/v).

After two weeks of planting, the plants were thinned in each pot to one plant, leaving the homogenous plants separated. The plants in the first and second sets were divided into three groups: 1^st was irrigated with normal water (control), 2^nd irrigated with a 100 µM CdCl₂ solution, the 3^rd set was irrigated with 500µM CdCl₂ solution.  The experiment was left under normal environmental conditions in the greenhouse. After three weeks of planting, all plants were fertilized with 35 g N of ammonium nitrate (N / m2) and 35 g P of super phosphate (P / m2). Harvesting (ten plants per each treatment) was carried out 45 and 90 days after planting.

Growth measurements

At each harvest, dry weights of shoots and roots were determined. Leaf area was measured using a leaf area meter (Licor-460, NE, USA). The length of the plant was measured from the surface of the earth directly to the top of the plant.

Yield and yield components

The average grain weight of the five plants was calculated in each treatment and the harvest index was calculated according to the following equation:

Harvest index = {Economic yield (grain yield) / above ground dry matter *100

Determination of Photosynthetic Pigments

Photosynthetic pigments were extracted and determined from the leaves of wheat plants by using the method of Hare (1997) method. The extraction was measured against a blank of pure 95% ethyl alcohol at three wavelengths of 452, 644 and 663 nm. The absorbance readings were followed with a spectrophotometer (Unico UV-2100 spectrophotometer).

Relative water content (RWC)

RWC was estimated from fresh, turgid and dry weight of leaf discs, as described by the methods of  Srivastava et al. (2010).

Measurements of transpiration rate and total leaf conductance

Transpiration rate and total leaf conductance of the Zea mays leaves were measured using Li-Cor, 6400XT, Lincoln, NE, USA.

Elements content analysis

A known weight of the dry grounded material of maize leaves for each treatment was digested in a digestion flask containing a triple acid mixture (HNO3:H2SO4:60% HCl4, with a ratio of 10:1:4; respectively) for analysis of sodium, calcium and potassium. Potassium (K) sodium (Na) and calcium (Ca) was assayed using a Flame photometer (Corning 400, UK). Cadmium (Cd) was estimated using an atomic absorption device.

Determination of total soluble sugars

Total soluble sugars (TSS) in dry grounded material of grains were determined after the extraction of dry seeds for each treatment in 10 ml of 80% (v/v) ethanol at room temperature with periodic shaking for 24 hours. TSS was determined using the method of Bradford (1976) with some modifications of the method of Abdel-Fattah et al. (2002). A calibration curve using pure glucose was made, which is linear over the tested range, with correlation coefficients higher than 0.999.

Determination of total soluble protein

Total protein in extracted grounded material of grains of each treatment was determined according to the method of Allam et al. (2003). A calibration curve was constructed using bovine serum albumin (BSA) and the data were expressed as µg BSA/mg dry matter.

Statistical analysis

All subjected to one-way ANOVA using the SPSS 10.0 software program. Means and standard errors were calculated for ten replicate values. Means were compared by the Duncan's multiple range test and statistical significance was determined at 5% level.

RESULTS AND DISCUSSION

Growth criteria

The results shown in Table (1) revealed that irrigation of maize plants with CdCl₂ had a significant decreased on growth rate parameters (shoot height, leaf area, dry weight of shoots and roots) as compared with control experiment. These effects increased with increasing cadmium chloride concentrations. On the other hand, when the maize grains were soaked in 50 ppm of kinetin, a significant increase in both the dry weight of the root and the stem as well as the length of the plant and the leaf area of ​​the irrigated plant and non-irrigated with cadmium chloride compared with the other non-treated treatments.

These results showed that the effect of irrigation with cadmium chloride was more influential in the second harvest after 90 days of plantingin all studied treatments. Statistical analysis confirmed that these increases are significant. These findings are consistent with some previous studies by scientists (Gadalla and Ghorbal, 1999; Srivastav et al. 2017; Hashem, 2014 and Singh et al. 2017). Cadmium is one of the most toxic elements in terms of its effect on the growth and productivity of important agricultural crops (Bazzaz et al., 1992; Toppi and Gabrielli, 1999 and Yousefi et al., 2018). The negative effect of cadmium ion on plant growth is due to its negative effect on the water content of the plant as indicated by previous studies on different types of plants (Zhang et al., 2013 and Benavides et al., 2005). The use of kinetin to reduce the toxic effects of cadmium through the positive cycle in increasing the area of leaves and food content and increase the enzymes of oxidation and reduction in the leaves of plants. On the other hand, the treatment of the kinetin grains led to increased growth rates of maize-treated maize plants compared to plants treated with cadmium chloride only (Heikal et al., 1982; Robertson et al., 1999and Al-Hakimi, 2007).

Photosynthetic pigments

The results indicated in Table (2) showed that irrigation of maize plants with CdCl₂ (100 and 500μmol) had a negative effect on the content of plant pigments (chlorophyll a, chlorophyll b, carotenoids and total chlorophyll) in maize plant leaves during two stages of growth as compared to plants not treated with CdCl₂ (control). On the other hand, the presoaked of Maize grains inkinetin resulted in a significant increase in the content of photosynthetic pigments of maize plants when compared to plants treated only with CdCl₂.

This increase was markedly significant in chlorophyll a.The results of this study showed that the kinetin plays an important role in reducing the toxicity of CdCl₂ on the content of photosynthetic pigments of maize plant leaves during plant growth stages. These results were agreed with (Gadallah, 1999 and Singh et al., 2017).They found that kinetin has a direct relationship with increased rates of transpiration as well as water content and nutrient content as confirmed by the findings reached during this study.

Water relations

Kinetin has an important role in controlling the opening and closing of holes, as well as increasing the water content and increasing the degree of root connection of plants. In general, presoaking of maize grains with kinetin at 50 ppm induced a drastic increase in relative water content, total leaf conductance and transpiration rate of maize leaves treated with or without cadmium chloride when compared to control plants (Table 3). These effects were apparent with high concentrations of cadmium chloride. In this connection, these results have been interpreted by scientists Zhang and Schmidt, 1999; Yousefi et al., 2018)

Table 1. Effect of kinetin on the growth criteria of Zea mays plants growing under different levels of Cd toxicities

Table 2. Effect of kinetin on the photosynthetic pigments content of the of maize plants leaves grow under different levels of CdCl₂

Table 3. Effect of kinetin on water relationships of maize leaves plants growing under different CdCl₂ levels.

Table 4.Effect of kinetin on ionic content of the Zea maize leaves growing under different concentrations of CdCl₂

Water relations

Table 5.Effect of kinetin on productivity and  yield components of growing maize plants under different concentrations of CdCl₂

Table 6.Effect of Kinetin on the amount of soluble total sugars and protein content in yielded grains of maize plants growing under different concentrations of CdCl₂

Ion contents

The results recorded in Table (4) showed that soil irrigation with cadmium chloride (100 and 500μmol) resulted in a decrease in the content of potassium, sodium, calcium and increase cadmium in the leaves of maize plants as compared to control plants. This increase was significantly remarked at 500 μM cadmium chloride. In this context, the kinetin had an important role in increasing the content of potassium, sodium and calcium, and a significant decrease in cadmium in the leaves of maize plants compared to their contents of non-treated plants. Many researchers have agreed with these finding(Ibrahim, 1998; El-Naggar and Allam1991, Zhang and Schmidt, 1999;Srivastavet al. 2017and Singh et al., 2017). The increase of these elements with kinetin is due to increased ATP content and activation of ATP-ase in plasma dinners of plant cells thereby increasing the potassium and calcium content in plant cells. Vodniket al., (1999) and Hashem (2014) has shown that Kinetin has a prominent role in increasing membrane membrane fluidity. Through these results, it was found that the Kinetin plays a major role in increasing the resistance of the maize plants to the cadmium toxicity by increasing the production of protein in the plant roots (Phytochelatins) and/or by accumulating in the surface of the leaves (David et al.1995). Kinetin may increases the storage gaps of cadmium increase the vacuolar storage of Cd++ in the plant (Chardonnens et al., 1998).

Yield and yield components

The results recorded in Table (5) showed that the irrigation of plants with cadmium chloride solution had a significant decreased on the plant yield of grain number, weight, yield and harvest index of maize plants compared with non-treated cadmium chloride Zea maysplants. These results have been agreed with many investigators (Hare et al. 1997;Toppi and Gabrielli 1999; Benavides et al. 2005 and Yousefi et al., 2018) concluded that cadmium chloride has a negative impact on the productivity of most agricultural crops through direct impact on transpiration, water relationships and nutrient content ,antioxidant enzymes, and also the content of plant pigments. This was confirmed by the results obtained in this study. In contrast, David et al.(1995) and Srivastav et al.(2017) showed no significant negative effect on yield productivity with low concentrations of cadmium chloride. These differences in plant response to Cd were due to plant type and soil type as well as plant genetics.

In general, soaking of maize grain in kinetin resulted in a significant increase in the yield parameters (weight and number of grains - grain productivity - and harvest parameters) of treated plants with cadmium chlorides compared with plants treated with cadmium chloride only. These results showed that the kinetin plays an important role in increasing vegetative growth and leaf area, improving water relations an increasing the transport of nutrients such as potassium and calcium, as well as reducing cadmium transfer to the stem and leaves. This was evident from the results achieved, which was directly reflected in productivity with many researchers (Gadalla and Ghorbal, 1999; Al-Hakimi, 2007 and Singh et al. 2017).

Total protein and soluble sugars content in yielded grains

The results recorded in Table (6) showed that cadmium chloride had a negative effect on the amount of soluble sugars as well as the protein content in the grain produced by the maize-treated maize crop compared to the control plants. In the treatment of soaking grains in kinetin, the reduction of the negative effect of cadmium chloride in terms of increased soluble sugars and protein content of grains compared to plants treated with cadmium chloride only. These results have been agreed with (Gadallah and Ghorol 1999; Hashem 2014 and Singh et al. 2017). The chemistry has a significant role in reducing aging and increasing chlorophyll content, thus increasing the production of a large amount of soluble sugars and increasing the content of amino acids and nitrogen and thus increasing the protein in the grains of some plants, (Aldesuquy 2000 and Yousefi et al. 2018).

Coefficient between the effect of kinetin and some studied traits

The results shown in Table (7) show that there is a strong positive correlation coefficient between both the effect of kinetin and some vegetative traits such as dry leg weight r = 0.77 dry root weight r = 0.65 and leaf area r = 0.85, The soluble sugars r = 0.89 and soluble protein r = 0.80 and some characteristics of water relations such as water content of the paper r = o.91 and the rate of transpiration r = 0.80 and on the other hand the correlation coefficient was negative between the effect of kinetin and cadmium content r = - 0.88 in the leaves of maize growing plants under Effect of cadmium chloride toxicity.

Table 7. Correlation coefficient (π) between the effect of kinetin and some growth parameters, productivity and plant relationships of growing maize plants under different concentrations of CdCl₂

Values with * and ** are significant at P≤ 0.05 and 0.01 respectively

CONCLUSIONS

This study has clearly concluded that grain presoaking with kinetin improved plant growth, productivity of maize plants grown under different concentrations of cadmium chloride by increasing photosynthetic pigments, transpiration rate, improving relative water content and ion contents like calcium, potassium and sodium (decreasing cadmium content in plant leaves), accumulation of sugars and protein contents in yielded grains of maize plants. In future, this study will be extended to include further investigations on the effect of kinetin on some metabolic pathways, different enzymes and endogenous hormonal levels.

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