Effect of Nitrate and Ammonium Nitrogen Sources on the Growth, Photorespiration, Dark Respiration and Activities of Related Enzymes in Wheat (Triticum aestivum L.)

Abstract

Wheat seedlings were grown hydroponically with 100 ppm of either nitrate or ammonium as the N source. Plant growth, CO2 compensation concentration, dark respiration rate and activities of several related enzymes were measured and analyzed. The purpose was to study the influence of N forms on the carbon metabolism in plants. Wheat seedlings grew better in solution supplied with nitrate. The detrimental effect of ammonium on the growth could be reverted partially by increasing the pH of culture medium to 7.0. CO2 compensation concentration was 15 ppm lower for nitrate-grown plant than for ammoniu-grown plants. However, the difference was also oriented partly by the pH of culture solution. pH value of tissue extracts was not affected by N treatment despite the marked fluctuation of pH in the surrounding nutrient medium. Ammonium-grown plants had higher total titratable acidity but much lower malate content than nitrate-grown plants, suggesting that a considerable amount of CO2 was fixed in both the leaf and root tissue. This assumption is further supported by the facts that wheat seedlings supplied with ammonium nutrition had higher RuBP carboxyl-ase acitivty in the leaves and PEP carboxylase activity in the roots. Values of the latter (up to 210 μmol/gfw. hr) is at least 2-3 times higher than those normally found in the leaf blades of most C3 plants. Wheat seedling supplied with ammonium-N had higher NAD-malate dehydrogenase but similar spartate amino-transferase activity when compared with those supplied with nitrate-N. Ammonium nutrition could increase the CO2 compensation concentration of wheat seedlings with a temperature range of 21 to 35°C and under 2, 21 and 100% oxygen concentrations. Glycolate oxidase activity was slightly higher but dark respiration rate was significantly higher for plants grown in ammonium that in nitrate. The linear relationship between CO2 compensation concentration and oxygen concentration and theextrapolated finite compensation concentration at zero oxygen suggest that in addition to photorespiration, ‘dark’ respiration also contribute to CO2 efflux in the light and the increased CO2 compensation concentration observed in ammonium grown plants. 本試驗以小麥幼苗為材料，以100 ppm 之硝酸態或銨態氮為氮源，測定小麥生長、二氧化碳補償濃度、暗呼吸速率及一些酵素活性的變化，目的在瞭解氮素代謝對碳素代謝作用的影響。小麥在含硝酸態氮源的水耕液內生長較佳，唯銨態氮源對生長的不利作用可經由維持水耕液pH值於7.0而獲得部分改善。小麥生長於銨態氮源下的二氧化碳補償濃度較生長於硝酸態氮源下高15 ppm左右，但此項差異亦可經由提高水耕液pH值而減小。雖然小麥生長於不同氮源可導致水耕液pH值的明顯變化，植株萃取液的pH值卻大致相近。銨態氮源條件下生長的小麥含有極高的可滴定酸度（total titratable acidity）和極低的malate含量，顯示必需固定較多的二氧化碳以維持植株內的離子平衡（ionic balance），此項假設可由葉片內較高的RuBP carboxylase 活性與根部較高的PEP carboxylase活性得到證明，後一酵素於根部的活性可高達210 μmol/fgfw．Hr，為多數C3型植物葉片同一酵素活性的二至三倍。銨態氮源下生長的小麥同時含有較高的NAD-malate dehydrogenase活性，但aspartate aminotransferase活性則與硝酸態氮源處理者相近。在21-35°C的溫度與2-100％為氧氣濃度下，銨態氮源處理植株的二氧化碳補償濃度均顯著高於硝酸態氮處理的結果；前者的glycolate oxidase活性略高，但暗呼吸速率却顯著高於後者。二氧化碳補償濃度與氧氣濃度間有直線關係，但0％氧氣濃度下的二氧化碳補償濃度仍為正值，表示除了光呼吸作用外，暗呼吸與光照情況下的二氧化碳釋放與銨態氮營養導致之高二氧化碳補償濃度有關。