Nitrate nitrogen (NO3−) and ammonium N (NH4+) are the two main forms of nitrogen (N). The effects of the NO3−:NH4+ ratio (0:100, 25:75, 50:50, 75:25, and 100:0) in the nutrient solution on growth, pigment content, photosynthetic parameters, chlorophyll (Chl) precursor content, and chloroplast mitochondrial ultrastructure of lettuce leaves were evaluated to determine the appropriate N ratio for the growth of lettuce. The results showed that a ratio of nitrate to ammonium of 75:25 compared with the single N form resulted in significantly higher fresh weights in the shoots and the lower parts of the lettuce. The growth rate, as indicated by the CO2 response curve, was increased because of the carboxylation efficiency and dark respiration rate (Rday) of the lettuce seedlings, and the difference between the light compensation point and CO2 compensation point was reduced under single nitrate N treatment. The contents of δ-aminolevulinic acid (ALA), porphobilinogen, uroporphyrinogen III, protoporphyrin IX, and Mg-protoporphyrin IX in seedlings at 75:25 nitrate:ammonium were higher than those in seedlings under the other treatments. The contents of Chl a and b were also the highest when the ratio of nitrate ammonium was 75:25. The chloroplast structure of lettuce leaves was intact, the cell membrane was completely closed to the cell wall, and the organelles were closely arranged on the cell membrane. The chloroplast was spindle-shaped and full of vesicles. The whole mitochondria were rich, the base particles were rich, the base granules were clear and rich, and the mitochondria exhibited the overall best state with a complete structure, rich sputum, and dense inclusions. The above results show that the nutrient solution with a ratio of nitrate to ammonium of 75:25 was more conducive to Chl synthesis and the promotion of photosynthesis and root growth in lettuce seedlings than the other treatments. The single nitrate N nutrient solution reduced Chl synthesis and photosynthesis in lettuce seedlings, while the single ammonium N nutrient solution affected the growth of lettuce seedlings, resulting in lower growth of lettuce shoots and roots. Therefore, the nutrient solution ratio of nitrate to ammonium of 75:25 is the most suitable for the lettuce seedling growth.

中文翻译：

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不同NO3−:NH4+比例对生菜幼苗光合作用和超微结构的影响

硝酸盐氮 (NO3-) 和铵态氮 (NH4+) 是氮 (N) 的两种主要形式。营养液中 NO3−:NH4+ 比例（0:100、25:75、50:50、75:25 和 100:0）对生长、色素含量、光合参数、叶绿素 (Chl) 前体含量的影响和生菜叶的叶绿体线粒体超微结构进行了评估，以确定适合生菜生长的 N 比率。结果表明，与单一 N 形式相比，75:25 的硝酸盐与铵的比例导致芽和生菜下部的鲜重显着更高。由于生菜幼苗的羧化效率和暗呼吸速率 (Rday)，如 CO2 响应曲线所示，生长速率增加，单一硝态氮处理降低了光补偿点与CO2补偿点的差异。75:25硝酸盐:铵盐处理的幼苗中δ-氨基乙酰丙酸(ALA)、胆色素原、尿卟啉原III、原卟啉IX和Mg-原卟啉IX的含量高于其他处理。当硝酸铵的比例为75:25时，Chl a和b的含量也最高。生菜叶片叶绿体结构完整，细胞膜与细胞壁完全闭合，细胞器紧密排列在细胞膜上。叶绿体呈纺锤形，充满小泡。整个线粒体丰富，基粒丰富，基粒清晰丰富，线粒体整体呈现最佳状态，结构完整，痰液丰富，包裹体致密。以上结果表明，硝酸盐与铵盐比为75:25的营养液比其他处理更有利于生菜幼苗叶绿素的合成和促进光合作用和根系生长。单一的硝酸盐N营养液降低了生菜幼苗的Chl合成和光合作用，而单一的铵态N营养液影响了生菜幼苗的生长，导致生菜芽和根的生长缓慢。因此，75:25的硝酸盐与铵盐的营养液比例最适合莴苣幼苗的生长。25比其他处理更有利于叶绿素的合成和促进生菜幼苗光合作用和根系生长。单一的硝酸盐N营养液降低了生菜幼苗的Chl合成和光合作用，而单一的铵态N营养液影响了生菜幼苗的生长，导致生菜芽和根的生长缓慢。因此，75:25的硝酸盐与铵盐的营养液比例最适合莴苣幼苗的生长。25比其他处理更有利于叶绿素的合成和促进生菜幼苗光合作用和根系生长。单一的硝酸盐N营养液降低了生菜幼苗的Chl合成和光合作用，而单一的铵态N营养液影响了生菜幼苗的生长，导致生菜芽和根的生长缓慢。因此，75:25的硝酸盐与铵盐的营养液比例最适合莴苣幼苗的生长。导致生菜芽和根的生长缓慢。因此，75:25的硝酸盐与铵盐的营养液比例最适合莴苣幼苗的生长。导致生菜芽和根的生长缓慢。因此，75:25的硝酸盐与铵盐的营养液比例最适合莴苣幼苗的生长。