Plantlets grown in vitro with a mixed nitrogen source utilize sucrose and CO2 as carbon sources for growth. However, it is very difficult to obtain the correct proportion of assimilated nitrate, ammonium, sucrose and CO2 for plantlets. Consequently, the NH4+/NO3- use efficiency for carbon fixation derived from the assimilation of sucrose/CO2 is still unclear for plantlets.
The bidirectional stable nitrogen isotope tracer technique was employed to quantify the proportions of nitrate and ammonium utilized at different NH4+/ NO3- ratios, and the proportions of sucrose and CO2 assimilation were quantified by the foliar δ13C values of plantlets. There was an obvious difference in the assimilation of nitrate and ammonium under different NH4+/NO3- ratios for Brassica napus (Bn) plantlets. Increasing the supply of nitrate contributed to enhancing the assimilation of nitrate and ammonium simultaneously. The nitrate utilization coefficients of the Bn plantlets had no distinct change with increasing nitrate concentration, while the ammonium utilization coefficients of the Bn plantlets increased obviously with increasing nitrate concentration. The proportion of sucrose/CO2 assimilation depended on the NH4+/NO3- ratios of the Bn plantlets. Both nitrate and ammonium assimilation were independent of sucrose/CO2 assimilation. Based on the proportion of CO2, sucrose, nitrate and ammonium utilization, the nitrate/ammonium use efficiency (as indicated by the C/N ratio) for carbon fixation derived from the assimilation of sucrose/CO2 can be quantified for Bn plantlets.
Quantifying the utilization proportions of nitrate and ammonium can reveal the difference in nitrate and ammonium utilization among plantlets at different NH4+/NO3- ratios. Foliar δ13C value in combination of the foliar δ15N value of plantlets can be used to quantify the nitrate/ammonium use efficiency for the carbon fixation derived from the assimilation of sucrose/CO2, which contributes to knowing the coupling process of carbon and nitrogen in plantlets and provides an alternate way to optimize the supply of inorganic nitrogen in culture media.