Biomass partitioning and photosynthesis in the quest for nitrogen- use efficiency for citrus tree species.

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.

Photosynthesis is differently regulated during and after copper-induced nutritional stress in citrus trees.

Antioxidant enzymatic responses in Citrus leaves under Cu-induced stress depends on rootstock genotypes. However, there is a lack of information about how woody plants recover growth capacity after exposure to elevated Cu and whether growth is affected by the redistribution of the metal to new vegetative parts and consequently whether photosynthesis is affected. Therefore, the biomass of plants and Cu concentrations in new leaf flushes were determined in young citrus trees grafted onto contrasting rootstocks [Swingle citrumelo (SW) and Rangpur lime (RL)]. Photosynthetic rate, chlorophyll fluorescence and antioxidant enzymatic systems were evaluated in plants previously grown in nutrient solution with Cu varying from low to high levels and with no added Cu. Both rootstocks exhibited reduced plant growth under Cu toxicity. However, trees grafted onto RL exhibited better growth recovery after Cu excess, which was dependent on the modulation of antioxidant enzyme activities in roots and leaves that maintained the integrity of the photosynthetic apparatus. In contrast, plants grafted onto SW exhibited a lower photosynthetic rate at the lowest available Cu concentration. Although the highest accumulation of Cu occurred in citrus roots, the redistribution of the nutrient to new vegetative parts was proportional to the Cu concentration in the roots.