Effects of aluminum (Al) stress on nitrogen (N) metabolism of leaves and roots in two Citrus species with different Al tolerance

Citrus are mostly planted in tropical/subtropical acidic soil areas, and are susceptible to aluminum (Al) toxicity, which affects tree growth and fruit quality. However, the effects of Al stress on the nitrogen (N) metabolism in different Al-tolerant Citrus varieties, and whether N metabolism is related to Al-tolerance in different Citrus varieties, were remained unveiled. In the current study, Al-tolerant "Xuegan" (Citrus sinensis) and Al-sensitive "Sour pummelo" (C. grandis), were irrigated with different nutrient solutions with or without Al. Al decreased the dry weights (DW) of leaves, stems and roots, as well as the N content, the N accumulation, total soluble proteins (TSPs) and total free amino acids (TFAAs) of different tissues of C. grandis, but it increased the ratio of root DW to shoot DW. In contrast, Al did not change the parameters mentioned above, except for decreasing the content and accumulation of N in C. sinensis roots. Al did not change the content of NO3– in different position leaves and roots, but decreased the content of NH4+ in C. grandis. Al decreased the content of NO3– in different position leaves and roots, but except for the lower leaf, Al did not change that of NH4+ in C. sinensis. Although Al affected the activities and gene expression levels of N metabolism–related enzymes in the leaves and roots of these two Citrus species, the effects of Al on these parameters were more pronounced in C. grandis than in C. sinensis. Results from correlation analysis (CoA) and principal components analysis (PCA) indicated that the increased root DW/shoot DW and NO3--N content in the roots and middle/upper leaves might be the related indicators of Al toxicity, whereas the enhanced activity of leaf GOGAT might be related to the Al–tolerance in Citrus plants. In conclusion, this study preliminarily revealed that the impact of Al on N metabolism and the response of N metabolism–related indicators to Al stress in Citrus, providing scientific reference for high-yield and high-quality cultivation of Citrus grown in acidic soil areas.