LAUSR

Optimizing the use of N for food production is a major challenge in agricultural systems. The transformation of nitrogen (N) into crop production results from intricate pathways, depending on plants, as well environment and fertilization regimes, which affect the N use efficiency (NUE) of plants. In this context, lemon trees (Citrus limon) attain maximum harvest index at lower leaf N concentrations compared to sweet orange trees (Citrus sinensis), 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 fourteen 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 higher relative growth rate than sweet oranges, despite net CO2 assimilation and dark respiration were similar between 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 nitrogen-use efficiency (PNUE: 55─120 mmol CO2 g N-1 day-1) compared to 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 to orange trees (ETR/chlor: 300-550) either at low or 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.