Interaction of carbon and nitrogen assimilation estimated by l3C and 15N tracing in detached leaves of spinach

To investigate short term interaction between recently fixed carbon and assimilated nitrogen, the fates of pulse-fed 13C and continuously-applied 15N in sugar, nitrate, organic acid, amino acid and insoluble compound fractions from detached leaves of 5-weeks old spinach (Spinacia oleracea L. cv. Lead) were followed under four light/NO3− combined treatments. At the termination of a 75 min pulse 13CO2 feeding (defined as 0 h), the total amounts of the 13C recently fixed by detached leaves in the light were slightly higher for N-free and 1 mM NO3− medium than for 4 mM NO3−. In shaded leaves, 13C fixation and 15NO3− uptake, respectively, decreased to 11.2% and 71.3% of the fully illuminated leaves exposed to the same 4 mM NO3− concentration. In the light at 0 h, the incorporation of 13C was mostly into sugars, followed by undefined 13C (total 13C – sum of analyzed sub-fractions). In shaded leaves, the largest incorporation was found in the undefined C, followed by insoluble C and sugars. The uptake of 15NO3− from 0 to 3 h was 128, 665 and 620 μg g−1 DW and the percentages of 15N present as reduced forms were 80, 77 and 41, respectively, in the leaves exposed to, in mM, 1 NO3−, 4 NO3− in the light and 4 NO3− in the shaded condition; thus, shading did a small decrease in uptake but a great decrease in reduction of 15NO3−. Under light treatment, the synthesis of reduced 15N was negatively related to the reduction of 13C sugars from 0 to 3 h, but positively related to amounts of organic acid 13C at 3 h.