鎳與尿素對水耕‘桃園四號’草苺生長與氮利用之影響

草苺 (Fragaria ×ananassa) 為台灣冬春季具高經濟價值之作物，由於傳統露地栽培模式體力需求高、易受土壤病蟲害影響，近年來離地介質及水耕栽培數量逐漸上升。水耕栽培中，精準供應植物所需之養分十分重要，其中，氮元素參與作物代謝，與產量關聯密切。植物根部可吸收之氮型態主要有硝酸態氮 (NO3-N)、銨態氮 (NH4-N) 及尿素三種；其中，尿素不僅為植物本身之代謝產物，亦為價格低廉之氮肥，是常用的高含氮量肥料。鎳是尿素水解酶活性部位的金屬離子，為代謝尿素所必須。然而商業栽培常用水耕液配方未添加鎳，可能是造成水耕氮源仍以無機氮為主之原因。因此，本研究目的為開發含尿素與鎳之水耕液配方，並以‘桃園四號’草苺探討鎳與氮素吸收之關聯。本研究共分為五個試驗，使用生長整齊之‘桃園四號’草苺走莖繁殖苗，洗淨根部後，定植於臺灣大學園藝系簡易型溫室內具打氣設備之浮板式水耕系統。試驗一於22%尿素為氮源之養液中添加令水耕液含0, 0.03, 0.06, 0.12, 0.24, 0.36 mgL-1鎳離子之硫酸鎳，調查五個月內之營養生長、產量、果實品質。試驗二添加令水耕液含0, 0.25, 0.5, 1, 2, 4, 8 mgL-1 鎳離子之硫酸鎳，調查鎳毒害發生時間、症狀，並進行營養要素分析；試驗三於總氮濃度7 mM之養液中，以含氮0, 0.5, 1, 1.5, 2, 2.5 mM之尿素取代部分氮源，調查營養生長、葉片尿素、硝酸態氮、銨態氮及總游離胺基酸之含量，並進行營養要素分析；試驗四以0, 2 mM尿素為氮源 (總氮為7 mM)，添加0, 0.125, 2 mgL-1鎳離子、試驗五以0, 2, 5 mM尿素為氮源 (總氮為14 mM)，添加0, 0.125 mgL-1 鎳，試驗四與五皆調查7日內水耕液中尿素、硝酸態氮、銨態氮之變化，並進行營養要素分析。 ‘桃園四號’草苺栽培於含22.2% 尿素為氮源之養液中，添加0.12-0.24 mgL-1鎳離子，能顯著提升產量與著果率，水耕液中鎳離子含量大於1 mgL-1時，根部生長開始受抑制，添加大於 2 mgL-1鎳出現新葉脈間黃化、疑似缺鐵之現象，添加8 mgL-1鎳使葉片出現壞疽、根系褐化死亡。鎳毒害造成之症狀與缺鐵相似，然而症狀發生部位鐵含量並未降低，且伴隨根部鐵之累積，由試驗結果可推測，此毒害症狀應為鎳鐵競爭作用位置，使鐵離子失去原本之功能所導致。草苺對尿素之耐受性高，甚至可能為喜尿素之植物，其原因可能與尿素、銨態氮代謝主要位置為根部有關， ‘桃園四號’草苺於經調整之1/2 Hoagland養液、2 mM尿素處理下，有最高之新生部位乾重，且添加0.125 mgL-1鎳，顯著提升尿素吸收；於經調整之Full Hoagland養液，添加 5 mM尿素，亦無毒害症狀發生，僅生長量下降。水耕液中尿素增加造成根部鎳含量上升、地上部鎳含量下降，此結果可能為鎳留在根部供應尿素分解之使用所造成。草苺對銨與尿素耐受性皆高，栽培於含尿素養液之優點為成本較低，且可降低養液電導度，缺點則為銨吸收較慢之季節，養液pH值上升速度快，然而此缺點可由改變硝銨比來改善。綜合試驗結果，水耕‘桃園四號’草苺若欲使用含尿素之養液，建議栽培於1/2 Hoagland之養液濃度，以2 mM尿素為氮源，並添加0.125 mg L-1鎳。

關鍵字

鎳；尿素；銨態氮；硝酸態氮；營養要素；草苺

並列摘要

Strawberry (Fragaria ×ananassa) is a high value crop planted in winter and spring of Taiwan. The traditional cultivate system is open-field, high-hill system. Due to high labor demand and soil diseases, soilless and hydroponic cultivation has increased in recent years. In hydroponic, it is important to supply nutrients accurately, while nitrogen is a critical element because it involves in numerous of metabolisms and is closely related to yield. Plants can uptake nitrate (NO3-N), ammonium (NH4-N) and urea as nitrogen source. Among them, urea is not only a metabolite of plant, but also a low-cost nitrogen fertilizer. In plants, urea is metabolized by urease, which is activated by nickel. However, common hydroponic solution protocols do not contain nickel, and the nitrogen sources are mainly nitrate and ammonium. Therefore, the purpose of this research is to develop a hydroponic solution protocol containing urea and nickel, and to clarify the relation between nickel and nitrogen absorption with 'Taoyuan No. 4' strawberry. The research was divided into five experiments. In each parts, well-grown 'Taoyuan No. 4' strawberry seedlings (propagated form runner) was planted in deep flow technic (DFT) with air pump in green house of National Taiwan University, department of Horticulture and Landscape. In the 1st experiment, 0, 0.03, 0.06, 0.12, 0.24, 0.36 mgL-1 nickel was added to the nutrient solution containing 22% urea as nitrogen source. Vegetative growth, yield and fruit quality were investigated within five months. In 2nd experiment, 0, 0.25, 0.5, 1, 2, 4, 8 mgL-1 nickel were added in solution. Time and symptoms of nickel toxic were recorded, while mineral elements were also analyzed. In 3rd experiment, a part of nitrogen was replaced by 0, 0.5, 1, 1.5, 2, 2.5 mM urea in nutrient solution containing 7 mM nitrogen. Vegetative growth, leaf urea, nitrate, ammonium, total free amino acid and other mineral nutrition content were measured. In 4th experiment, 0, 0.125 or 2 mgL-1 nickel were added in solution containing 0 or 2 mM urea as nitrogen source (Total nitrogen: 7 mM); while in 5th experiment, 0 or 0.125 mgL-1 nickel were added in solution containing 0, 2 or 5 mM urea as nitrogen source (total nitrogen: 14 mM). The changes of urea, nitrate and ammonium content in solution were measured within 7 days and mineral elements were analyzed in both experiments. Adding small amount of nickel was considered beneficial to 'Taoyuan No. 4' strawberry grown in urea-containing solution. When 22% of inorganic nitrogen was substituted by urea, the suitable nickel content would be 0.12-0.24 mgL-1, which had largest yield and highest percentage of fruit set. Inhibition of root growth and swelling root tips were observed in solution contained more than 1.0 mgL-1 nickel. When exposed to more than 2.0 mgL-1 nickel, yellowing of intervals was observed in new leaves. While adding 8 mgL-1 nickel caused necrosis on leaves and browning of roots. The symptoms caused by nickel toxic are similar to those of iron deficiency. However, the iron content of affected site was not decreased, accompanied by iron accumulation in roots. This result indicated that toxicity symptom may be caused by competition between nickel and iron at new formed leaves. Nickel may substitute the function site of iron, which induced iron deficiency symptoms. Strawberry is urea-tolerant plant, and may even prefer urea as part of nitrogen source. This might be caused by the metabolism site of urea and ammonium. The highest dry weight of 'Taoyuan No. 4' strawberry occurred in treatment containing 2 mM urea (total N: 7 mM). While adding 0.125 mgL-1 nickel increased urea absorption significantly. No toxic symptom except growth decrease was observed even in 5 mM urea solution (total N: 14 mM). With more urea in solution, the content of nickel rose in root but declined in shoot, which suggests that nickel might be left in root for urea metabolism. Strawberry is highly tolerant to ammonium and urea. Thus, it is suitable to grown in urea-containing nutrient solution. The advantage of urea-containing solution is lower cost and lower electricity conductivity (EC). Prompt raise of solution pH in season which has slower ammonium absorption would be a shortage. However, this can be improved by adjusting NH4:NO3 in solution. According to the result, it is recommended to cultivate strawberry in modified 1/2 Hoagland solution containing 2 mM urea as nitrogen source, and add 0.125 mg L-1 nickel.