Differential uptake of soluble organic and inorganic nitrogen by two fruit species: Dimocarpus longan Lour. and Eriobotrya japonica Lindl.

The aim of this study was to investigate whether two subtropical fruit species, Dimocarpus longan Lour. (Wulonglin) and Eriobotrya japonica Lindl. (Zaozhong 6), could directly take up soluble organic nitrogen (N), and if there was differential uptake of different forms of N between the two species. The stable isotopic tracing experiment was carried out using three N-15-labeled tracers [glycine-2-C-13-N-15, ((NH4)-N-15)(2)SO4, and K((NO3)-N-15)] to examine the short-term (2, 6, and 72 h) uptake and distribution pattern of N within the plant in 1-year-old seedlings of both fruit species. After glycine-2-C-13-N-15 application, the ratios of C-13 to N-15 concentration in the roots and whole seedlings of both Wulonglin and Zaozhong 6 were close to 1:1 at 2 and 6 h, respectively, showing that both fruit species could directly take up intact glycine. The (NH4)-N-15 (+) absorbed by Wulonglin and Zaozhong 6 whole seedlings in the ((NH4)-N-15)(2)SO4 treatment were 13.2 and 9.2 times higher than glycine-derived-N-15, while (NO3)-N-15 (-) absorbed in the K((NO3)-N-15) treatment were 27.7 and 7.7 times higher than glycine-derived-N-15 in the glycine-2-C-13-N-15 treatment, respectively. This indicated that soluble organic N might not be the dominant N source taken up by both fruit species. Results also showed that Wulonglin preferred NO3 (-), while Zaozhong 6 preferred NH4 (+). The greater uptake of (NH4)-N-15 (+) and (NO3)-N-15 (-) than glycine-derived-N-15 by both fruit species might be related to their long-term adaptation to the supply of large quantity of inorganic N via fertilization. To elucidate the mechanisms responsible for the differential uptake of different forms of N by these two fruit species, further studies on mycorrhizal associations, transporter genes, and glycine metabolisms are warranted. This study has demonstrated that both D. longan Lour. and E. japonica Lindl. seedlings were able to take up intact glycine directly, while the NO3 (-) and NH4 (+) were still dominant N forms absorbed by both fruit species.