Effect of Nickel supply on Urease Activity and N-metabolism and Urea fertilizer efficiency in Wheat (Triticum aestivum, L.) and Corn (Zea maize, L.) by using 15N

Urea is the most common forms of combined N-fertilizer used in agricultural practices worldwide. To be in plant metabolism, urea must be first hydrolyzed to ammonia and carbon dioxide by urease. Urease activation is the only known function of nickel (Ni) ion in higher plants. In this study, the effects of various Ni levels on growth, chlorophyll content, urease activity in wheat (Triticum aestivum, L. cv. Pishtaz) and corn (Zea maize L. cv. 704) plant were studied during 2006- 2011. Plants were either supplied with ammonium nitrate or urea as the sole nitrogen sources. Plants were grown in nutrient solutions in the facilities provided by Iran Nuclear Research Center. Experiment I: To find the optimum Ni levels for corn and wheat plants growth, two separated experiments each arranged in completely randomized with three replications were designed. Treatments included nutrient solution cultures containing either urea or ammonium nitrate as two different N-sources with three Ni levels (0, 0.1, 1 mg Ni L-1). Plants were allowed to grow for 6 weeks and at the end of this period, shoots and roots dry weights and the amount of cations uptake were determined. Wheat shoot and root dry weights were decreased when plants were supplied with various Ni levels in both types of solutions. However, the shoot and root dry weights of urea-fed corn plants increased at the 2nd Ni level and were decreased at the 3rd Ni levels In plants supplied with ammonium nitrate, these parameters decrease at all Ni levels. When wheat plants were supplied with various Ni levels, the rates of mono and divalent cations absorption by root and their accumulation in the shoots were decreased in both nutrient solutions. The rate of mono and divalent cations uptake by corn roots and their accumulation in shoots were also decreased when supplied with various Ni levels in nutrient solutions containing ammonium nitrate as N-source. However, in urea-fed corn plants cations uptake and accumulation were more or less compatible with plants growth conditions. Ni accumulation increased significantly with the increase in Ni-supply. The rates of Ni accumulation in roots of both plants were higher than those of shoots. In corn plants grown in nutrient solutions supplied with urea, the optimum Ni level for plant growth was 0.1 mg L-1. However, corn plants supplied with urea or ammonium nitrate, Ni application had negative effects on plants growth. Experiment II: Finding the optimum Ni levels for plants growth from the first experiment, the 2nd experiment was designed to investigate the effects of optimum Ni level on growth, chlorophyll content and its interactions with other cations in both plant species. Experiments were designed as completely randomized blocks with three replications. Treatments included two growth media the nitrogen of which was either urea or ammonium nitrate and 4 Ni levels (0, 0.01, 0.05 and 0.1 mg L-1). Shoots and roots dry weights and leaves chlorophyll content of urea-fed wheat plants increased significantly at the 3rd Ni levels, but they were decreased at the 4th level. In plants supplied with ammonium nitrate, these parameters increased up to 2nd Ni level and started to decrease with further increase in Ni supply. Both shoot dry weight and leaves chlorophyll content of the urea-fed corn plants increased significantly up to the 4th Ni level. However, in these plants, root dry weight increased up to the 2nd Ni level and started to decrease with further increase in Ni supply. Shoot dry weight and leaves chlorophyll content of the ammonium nitrate-fed corn plants increased up to the 3rd Ni level. However, in the same media, plants root dry weight increased up to the 2nd Ni level and then started to decrease with further increase in solutions Ni content. Cation absorption and accumulation except for copper were in accordance with plant growth for both wheat and corn plants. In both plant groups, copper uptake was reduced with increase in Ni supply. Plants Ni content increased significantly with the increase in Ni supply the increase was higher in the roots of plants supplied with urea. Experiment III: In this experiment, the effects of optimum Ni level on urease activity in roots and leaves of both plant species were studied. The experiments were designed as completely randomized blocks with three replications. Treatments included two growth media the nitrogen of which was either urea or ammonium nitrate, four Ni levels (0, 0.01, 0.05 and 0.1 mg L-1) and three dates sampling times (second, fourth and sixth weeks). Urease activity in leaves of wheat plants supplied with urea increased up to the third Ni level on the 4th week sampling date significantly. Further Ni supply and sampling date resulted in a decrease in urease activity. Plants supplied with ammonium nitrate, urease activity increased up to the 2nd Ni level at the 2nd week sampling date. Further increase in Ni supplies and sampling dates reduced urease activity. Urease activity in the roots of wheat grown in both media increased up to the 3rd Ni level on the 2nd week sampling date and started to decrease with further increase in Ni supplies and sampling dates. Urease activity in leaves of corn supplied with urea increased significantly with the increase in Ni supply till the end of 6th week sampling date, however in those supplied with ammonium nitrate, urease activity increased up to 3rd Ni level and 4th week of sampling date but was redused at 4th Ni level on 6th week. Urease activity in the roots of corn plants supplied with urea was the highest at 2nd Ni level at the end of 2nd week. Increase in Ni levels and date of sampling resulted in a decrease in urease activity. However, in ammonium nitrate-fed plants urease activity on the 2nd week of sampling date increased up to the 4th Ni level and for other sampling dates the activity increased up to 2nd Ni level. Further increase in Ni supply and date of sampling resulted in a decrease in urease activity. In