In Brazilian Northeast, most of citrus orchards are submitted to seasonal water deficiency, because they are grown under rainfed condition. Grafting sweet orange trees onto drought-tolerant rootstock, such as ‘Rangpur’ lime, is the most used strategy to cope water deficit. However, some studies showed that adequate leaf nitrogen levels can reduce water stress symptoms in citrus. The purpose of this study was to investigate the influence of nitrogen (N) rates on proline and nitrogen leaf contents and fruit production in 4–9 years-old ‘Pera’ sweet orange onto three citrus rootstocks, grown without irrigation. Three simultaneous experiments were carried out with sweet orange trees grafted on ‘Rangpur’ lime, ‘Sunki Tropical’ mandarin or ‘San Diego’ citrandarin during five years. Aiming to that, proline, chlorophyll, and total nitrogen leaf contents, photosynthetic attributes, total tree height, canopy volume, N-use efficiency in fruit yield and productive performance were evaluated in response to N rates. After five years, effects on fruit production could be verified just in trees on ‘Rangpur’ and ‘Sunki Tropical’. Trees on ‘San Diego’ presented changes in tree growth and fruit quality. In contrast, proline accumulation varied with rootstock, season, nitrogen leaf contents and N rates. The greatest content was verified in the fifth year of N rates application, particularly on the driest season. In response to N rates, ‘Sunki Tropical’ induced proline accumulation up to a maximum in areas with moisture availability, whereas ‘Rangpur’ and ‘San Diego’ induced increasing increases. The results indicated that proline and total N leaf contents are interdependent, closely related to N fertilization and dependent on rootstock and water availability. It is concluded that the N rates favor increases in PRO content independent to the rootstocks, ‘Sunki Tropical’ mandarin favors greater proline accumulation and water savings in years with longer periods of water deficit, ‘Rangpur’ lime, followed by ‘San Diego’, induces the greatest PRO contents in years with frequent short dry periods, which suggests greater resilience to changing climate, and ‘San Diego’ demands less N fertilizer than ‘Rangpur’ lime and ‘Sunki Tropical’ mandarin to fruit yield.

中文翻译：

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氮肥用量 - 对雨养条件下生长的 3 种砧木上 'Pera' 甜橙幼苗的脯氨酸和总氮积累以及果实产量的影响


在巴西东北部，大多数柑橘园都处于季节性缺水状态，因为它们是在雨养条件下生长的。将甜橙树嫁接到耐旱的砧木上，例如 'Rangpur' 石灰，是应对水分短缺最常用的策略。然而，一些研究表明，充足的叶片氮水平可以减轻柑橘的水分胁迫症状。本研究的目的是调查氮 （N） 用量对 4-9 年生 'Pera' 甜橙的脯氨酸和氮叶含量以及水果产量的影响，这些砧木是三种无灌溉生长的柑橘砧木。在五年内，用甜橙树嫁接 'Rangpur' 酸橙、'Sunki Tropical' 柑橘或 'San Diego' 柑橘树同时进行了 3 次实验。为此，对 N 用量的响应评估了脯氨酸、叶绿素和总氮叶含量、光合属性、总树高、树冠体积、果实产量的 N 利用效率和生产性能。五年后，仅在 'Rangpur' 和 'Sunki Tropical' 的树木中就可以验证对水果生产的影响。“San Diego”上的树木表现出树木生长和果实质量的变化。相比之下，脯氨酸积累随砧木、季节、氮叶含量和 N 用量而变化。在 N 费率申请的第五年验证了最精彩的内容，尤其是在最干旱的季节。作为对氮肥用量的响应，“Sunki Tropical”诱导脯氨酸积累在有水分供应的地区达到最大值，而“Rangpur”和“San Diego”诱导的增加增加。结果表明，脯氨酸和总氮叶含量是相互依赖的，与氮肥密切相关，并取决于砧木和水的可用性。 得出的结论是，氮含量有利于独立于砧木的 PRO 含量增加，'Sunki Tropical' 柑橘在缺水时间较长的年份有利于更多的脯氨酸积累和节水，'Rangpur' 石灰，其次是 'San Diego'，在频繁的短干涸期的年份诱导 PRO 含量最高，这表明对气候变化的适应能力更强， “圣地亚哥”对果实产量的需求比“Rangpur”石灰和“Sunki Tropical”柑橘需要更少的氮肥。