探讨了在模拟铁缺乏条件下（高HCO3-）,分别短期供应铵态氮（NH4+-N）和硝态氮（NO3--N）对玉米（Zea mays L cv Lenz）根尖（0～3cm）质外体铁的组成和FeCN（ferricyanide）还原酶活性的影响.结果表明,玉米根尖质外体Fe（Ⅱ）的绝对浓度和相对比例均较低,分别只有20～40nmol/L和7％～13％,绝大部分是以Fe（Ⅲ）形式存在,增加NH4+-N和NO3--N的处理时间有利于提高Fe（Ⅱ）的浓度.与NO3--N相比,NH4+-N提高了玉米根尖质外体Fe（Ⅱ）和交换性铁的浓度和比例,而NO3--N使FeCN还原酶活性增加.此外,还对质外体pH的变化与铁组分和FeCN还原酶活性的关系进行了讨论.
在营养液培养条件下,以硝态氮为对照,研究了铵态氮对向日葵叶片(Helianthus annuus L cvFrankasol)质外体pH和可溶性铁的影响,探讨了铵态氮改善植物铁营养状况的作用机制.结果表明,铵态氮通过降低叶片质外体pH和提高新叶质外体可溶性铁的浓度来改善缺铁植物的铁营养状况.铵态氮处理的缺铁植株新叶质外体、细胞汁液和木质部汁液中铁的浓度显著高于硝态氮处理,新叶未出现缺铁黄化症状.新叶质外体pH受氮素形态的影响较大,不供铁时,硝态氮和铵态氮处理植株新叶质外体pH分别为6.15和5.94,供铁时,新叶质外体分别为6.43和5.50.初生叶质外体pH不受氮素形态的影响,均在6.25左右.供应硝态氮和铵态氮的缺铁植物木质部汁液pH分别为5.72和5.49,供铁使木质部汁液pH分别提高了0.27和0.16个单位.
The effect of nitrogen form on pH and concentration of soluble iron (Fe) in leaf apoplast was investigated in hydrophonically grown sunflower plants (Helianthus annuus L. cv. Frankasol), and the mechanism underlying the improved Fe nutrition by ammonium (NH4) supply was also elucidated. Ammonium supply ameliorated Fe nutrition of plants grown without Fe through decreasing apoplastic pH and increasing soluble Fe concentration in apoplastic fluid of young leaves. The soluble Fe concentration in apoplastic fluid and cell sap of young leaves, and xylem exudates of NH4 fed-plants was higher than that of nitrate (NO3) fed-plants, and no typical Fe-deficiency chlorosis in young leaves was observed in NH4 fed plant without Fe supply. The apoplastic pH was 6.15 and 5.94 in young leaves of Fe-deficient plants fed respectively with NO3 and NH4, while in Fe-sufficient plants, the apoplastic pH was 6.43 with NO3, and 5.50 with NH4 supply. In primary leaves, the apoplastic pH was around 6.25 irrespective of nitrogen form and Fe supply. The pH of xylem exudate was 5.72 in Fe-deficient plants fed with NO3 and 5.49 with NH4. Iron nutrition increased the pH of xylem exudate by 0.27 and 0.16 unit under NO3 and NH4 supply respectively.
ZOU Chunqin & ZHANG Fusuo Key Laboratory of Plant-Soil Interactions, Ministry of Education
