3.2 Biuret uptake by rice seedlings
Prior to the evaluation of biuret uptake by rice plants, we first examined biuret decomposition in culture solutions. Containers filled with 1 L of culture solution supplemented with 0.3 or 1.0 mmol L-1 biuret were placed in the growth chamber, and aliquots of culture solutions were collected from the containers at 0, 12, 24, 48, and 72 h. The biuret concentrations were allowed to remain constant for up to 72 h. This result indicated that biuret was not hydrolyzed in the culture solution within this period.
Then, we carried out 48-h uptake experiments using15N-labelled biuret. Biuret can be measured as a colored chelation complex with cupric ions or by HPLC analysis combined with UV-detection, which we used for the determination of biuret concentrations in the culture solution. However, the sensitivity of the colorimetry process was too low to determine biuret concentrations in plant samples. Additionally, the peak of biuret could not be separated from UV-absorbing metabolites of rice plants in our system.
When 19-day-old Nipponbare seedlings were allowed to take up15N-labelled biuret for 48 h, biuret-derived15N was detected both in the shoots and roots of the seedlings (Table 1). This result indicated that biuret was taken up by rice roots and possibly translocated into rice shoots. The biuret-derived 15N concentrations in the shoots and roots were equal to 4.2 and 1.8 µmol biuret g-1 dw, respectively. Then, based on the amount of biuret in whole seedlings, the uptake rate of biuret with an external supply of 0.3 mmol L-1 was calculated at 0.5 mmol mg-1root DW h-1.