鎳與尿素對水耕‘桃園一號’蕹菜生長、要素及硝酸鹽含量之影響

Abstract

尿素雖為農業主要施用之氮肥，但因尿素用於水耕時氮同化效率較差，鮮少作為水耕氮源，此可能與配方未考慮鎳添加有關。作物對尿素同化過程需經脲酶(urease)水解成銨態氮後得以利用。鎳為脲酶之中心離子，故鎳可能改善作物對養液尿素的同化效率。以尿素取代養液部分硝酸氮源，可降低生產蔬菜之硝酸鹽含量，避免其過量攝入之風險。本研究特別以鎳之添加，重新評估含尿素蔬菜水耕養液配方之開發，期望運用此配方生產低硝酸鹽蔬菜。‘桃園一號’蕹菜(Ipomoea aquatica)以湛水式水耕系統進行栽培，養液參考Hoagland配方。總氮維持不變，以尿素取代部分硝酸鹽並輔以鎳之添加，探究含尿素水耕配方對蕹菜之生長、要素含量以及硝酸鹽含量之影響。本研究共分為四個試驗，試驗一為7 mM尿素態氮搭配0至85.3 μM共六鎳變級之鎳毒害試驗。水耕液過量鎳(> 8.6 μM)抑制蕹菜生長，產生新葉黃化徵狀，蕹菜葉片鐵含量較低，顯示過量鎳除本身造成毒害，可能阻礙蕹菜對鐵之吸收，並占據以鐵為中心之酵素，造成類似缺鐵之徵狀。試驗二則改以0至4 μM共五個鎳變級與兩個尿素變級(0, 8 mM)以求得適宜鎳添加範圍。0.5、1 μM鎳之添加，可改善含尿素水耕蕹菜的生長，且尿素的存在，使蕹菜有較高根部鎳累積。試驗三則以0至10 mM共七個尿素變級及適量鎳之添加(0、1 μM)，來探討尿素取代硝酸態氮的程度對蕹菜之反應。高尿素濃度下，蕹菜仍可正常生長。適量鎳添加可提升6、8 mM尿素組之生長，且蕹菜地上部硝酸鹽含量較無尿素對照下降21%。高尿素下蕹菜鈣、鐵及鋅含量較低，但錳含量則較高。其中蕹菜根部鐵含量於高尿素下，降低幅度較大，此應為蕹菜應用於含尿素養液與傳統養液栽培之間的差異。於相同適量鎳下(1 μM)，蕹菜根部鎳含量與養液尿素濃度成正相關，與試驗二之結果呼應。試驗四則以三個尿素變級(2、4、10 mM)與鎳添加(0、1 μM)來探討蕹菜，吸收尿素態氮、銨態氮及硝酸態氮之情形。鎳添加可提升蕹菜對養液尿素態氮之吸收速率，並降低銨態氮之吸收速率，鎳添加則僅降低第一週蕹菜對硝酸態氮之吸收速率。蕹菜於高尿素養液下仍能正常生長，且所生產的蕹菜硝酸鹽含量顯著較低，表示含尿素水耕配方具一定應用潛力，蕹菜對鎳需求與尿素之存在有關，並適量鎳可改善6、8 mM尿素組下之生長，增進蕹菜對養液中尿素的吸收，顯示含尿素水耕配方之開發，鎳為配方中扮演不可或缺之必要關鍵成分。

Urea is the most widely used and low-cost nitrogen fertilizer in agriculture. However, it is seldom used as a nitrogen source in hydroponic culture due to its poor nitrogen use efficiency. These results could be related to the lose sight of Ni supplement. Plants utilized urea via urease to hydrolysis it into ammonia for further assimilation process. Ni is of importance for activation of urease and hence for promoting plants to utilize urea in urea-based nutrients. Partially replacing nitrate with urea in hydroponic culture could reduce nitrate content in vegetable crops and reduce the risk of excessive nitrate intake. Therefore, the objective of this study was to develop urea-containing nutrient formula especially in view of Ni addition. We hope this nutrient formula could be applied on low-nitrate vegetable crops production. In order to investigate the effect of Ni on urea-based solution on plant growth, nitrate and nutrient content. Water spinach (Ipomoea aquatic cv. Taoyuan No. 1) were grown in deep flow technic system (DFT) with nutrient based on Hoagland solution. NO3-N were partially replaced with urea-N, while the total-N were the same. The research was divided into four experiments. In the 1st experiment, we used six Ni concentration (0-85.3 μM) with 7 mM urea-N to observe the Ni toxic effect on water spinach. Excess Ni (> 8.6 μM) inhibited plant growth. Young leaves had chlorosis or necrosis symptoms and Fe content in leaf were aslo decreased. Ni might affect Fe uptake and occupy Fe-related enzyme, and cause symptoms that similar to Fe deficiency. Then in the 2nd experiment, five Ni concentration (0-4 μM) with two urea-N level (0, 8 mM) were used to find the suitable Ni dosage in nutrients. With appropriate Ni addition (0.5, 1 μM), plant growth were improved and the Ni content in roots were higher in the presence of urea. In the 3rd experiment, seven urea levels (0-10 mM) combined with suitable Ni dosage (0, 1 μM ) to evaluate the degree of substitution urea for nitrate on plants response. Water spinach could growth well under high urea-N dosage (10 mM). Ni addition could increase water spinach growth under 6, 8 mM urea-N and drcreased nitrate content 21% compared to no urea control. The content of Ca in whole plant, Fe and Zn in root decreased under high urea dosage, while Mn in roots increased under high urea dosage. Among these nutrients, root Fe content significantly declined as urea dosage increased, which showed the distinct difference compared to traditional no-urea formula. Under the same 1 μM Ni addition, root Ni content in plants were positively correlated to urea concentration in nutrients. These trend were in agreement with the result of the 2nd experiment. The 4th experiment was to monitor the absorption of urea-N, NH4-N and NO3-N from nutrients. Nutrients contained three levels of urea-N (2, 4, 10 mM) and two levels of Ni (0, 1 μM). Ni addition significantly increased water spinach urea-N uptake rate and decreased NH4-N uptake rate. While Ni addition only decreased NO3-N uptake rate in the first week under Ni addition. Water spinach could grow well and effectively reduce nitrate content in shoots under high urea-N dosage. These advantages provided some potential of applying this urea-based formula on water spinach. Appropriate addition of Ni promoted water spinach urea absorption and improve growth in 6, 8 mM urea-N. The demand of Ni for water spinach was also higher in the existence of urea. These evidences demonstrated the essentiality of Ni on designing a urea-based hydroponic culture system.