Effect of homogeneous and heterogeneous supply of nitrate and ammonium on nitrogen uptake and distribution in tomato seedlings

Abstract

To evaluate the effects of homogeneous or heterogeneous supply of different sources of nitrogen (N) on N uptake and distribution in tomato seedlings, hydroponic experiments with 15N tracers were conducted. Under homogeneous condition, NO3 − and NH4 + were supplied as the ratio of 100:0 (100-0NA), 75:25 (75-25 NA), and 50:50 (50-50NA) for all roots of tomato seedlings. Heterogeneous supply of N (localized N supply) was conducted by a split-root experiment with three treatments: both half of the roots supplied with NO3 − (N|N, as the control), a half of the roots supplied with NH4 + (N|A), and a half of the roots supplied with NO3 − and NH4 + at a ratio of 75:25 (N|AN). The results showed that (1) homogeneous supply of NO3 − and NH4 + at the ratio of 75:25 led to the highest root and shoot biomass and a higher total N uptake. 15NO3 − absorption was not affected by decreased NO3 − percentage from 100 to 75 %, and increase of NH4 + ratio from 25 to 50 % showed no effect on 15NH4 + absorption. Similar amount of 15NH4 +-N was absorbed by roots under N|A and N|AN treatments in the localized supply. Spatially separated supply of NO3 − and NH4 + led to a more than 3 times higher 15N accumulation in NO3 −-fed root than NH4 +-fed root in N|A. Half-roots supplied by mixed NO3 − and NH4 + at the ratio of 75:25 in N|AN resulted in higher 15N accumulation in the shoot. (2) In plant tissues, the amount of 15N deriving from NO3 − in root, stem, and leaves was all decreased by 50 % NH4 + in the whole-root system. In N|A treatment in the split-root system, however, the amount of leaf 15N deriving from sole NO3 − was increased by 66 % and that coming from sole NH4 + decreased by 73 %, in comparison with N|N. The amount of 15N coming from the mixed N source was found to be more than 30 % higher in stem, leaves, and flower. Taken together, our results suggested that whether homogeneous or heterogeneous supply of NO3 − and NH4 + significantly influenced the N uptake and distribution in the plant and combined application of NO3 − and NH4 + was approved to be superior to plant growth and N uptake.