Abstract

Optimizing the use of nitrogen (N) for food production is a major challenge in agricultural systems. The transformation of N into crop production results from intricate pathways, depending on plants, as well as the environment and fertilization regimes, which affect the N-use efficiency (NUE) of plants. In this context, lemon trees [Citrus limon (L.) Burm. f.] attain maximum harvest index at lower leaf N concentrations compared with sweet orange trees [Citrus sinensis (L.) Osbeck], and the processes governing these plant responses are not well known. The aim of this study was to understand how the higher NUE in lemons trees is constructed based on growth and biomass partitioning evaluation, as well as photochemical and biochemical characteristics of photosynthesis. To attain this goal, we evaluated growth, photosynthesis and biochemical characteristics in lemon and sweet orange trees under two different N levels over 14 months. We hypothesized that higher NUE in lemon trees is affected by plant capacity to grow with economy on nutrient resources. Furthermore, lemon trees could be more efficient in CO₂ assimilation in non-limiting environmental conditions. We found that higher NUE in lemon trees was explained in part by the ability of trees to invest greater biomass in leaves instead of roots, even though this species exhibited lower relative recovery efficiency of N from the substrate than the sweet orange. We also found that lemon trees had a higher relative growth rate than sweet oranges, despite the fact that net CO₂ assimilation and dark respiration were similar between the two species. As a consequence, we suggested that lemons could exhibit a lower biomass construction cost than oranges. Because lemon presented lower N concentration than sweet orange trees, the former exhibited better photosynthetic N-use efficiency (PNUE: 55–120 mmol CO₂ g N⁻¹ day⁻¹) compared with the sweet orange (PNUE: 31–68 mmol CO₂ g N⁻¹ day⁻¹). Lemon trees also exhibited a higher relative rate of electron transport per unit of chlorophyll (ETR/chlor: 350–850) compared with orange trees (ETR/chlor: 300–550) at both low and at high N supply. These characteristics were likely associated with transport facilitation of CO₂ to the catalytic sites of plants. In fact, improved growth of lemon trees results from an array of events explained mostly by increase in leaf area and associated low construction cost despite N supply.

中文翻译：

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生物质分配和光合作用以寻求柑橘树种的氮利用效率

摘要

在粮食生产中优化氮（N）的使用是农业系统中的主要挑战。氮到农作物生产的转化取决于复杂的途径，具体取决于植物以及环境和施肥方式，这会影响植物的氮利用效率（NUE）。在这种情况下，柠檬树[柑橘柠檬（L.）Burm。F。]在较低的叶片N浓度的甜橙树[比较达到最大收获指数脐橙（L.）Osbeck]，以及控制这些植物反应的过程尚不清楚。这项研究的目的是了解如何基于生长和生物量分配评估以及光合作用的光化学和生化特征构建柠檬树中较高的NUE。为了实现此目标，我们评估了14个月内两种不同氮水平下柠檬和甜橙树的生长，光合作用和生化特性。我们假设柠檬树中较高的NUE受植物随着养分资源经济增长的能力的影响。此外，柠檬树的CO ₂效率更高在非限制性环境条件下的吸收。我们发现，柠檬树中较高的NUE可以部分解释为树木将更多的生物量投入叶片而不是根部的能力，尽管该物种相对于甜橙而言，其从基质中回收的N相对效率较低。我们还发现，尽管两个物种之间的净CO ₂同化和暗呼吸相似，但柠檬树的相对生长速率比甜橙高。因此，我们认为柠檬的生物量建设成本可能低于橙子。因为柠檬比甜橙树的氮含量低，所以前者表现出更好的光合氮利用效率（PNUE：55–120 mmol CO ₂ g N ^-1 天^-1）与甜橙（PNUE：31–68 mmol CO ₂ g N ^-1 天^-1）相比。在低氮供应和高氮供应下，柠檬树每单位叶绿素的相对电子传输率（ETR /氯：350-850）都比橙树（ETR /氯：300-550）高。这些特征可能与促进CO ₂到植物催化部位的运输有关。实际上，柠檬树生长的改善是由一系列事件导致的，这主要是由于尽管有氮供应，但叶面积的增加以及相关的较低的建造成本。