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.