INTERACTION BETWEEN ASSIMILATIONS OF FIXED CARBON AND NEWLY ABSORBED NITRATE ESTIMATED BY 13C AND 15N TRACING IN INTACT SPINACH.

To investigate the relations between assimilations of photosynthetically fixed carbon (C) and newly absorbed nitrate, 13CO2 and 15NO3- were fed to five-week-old hydroponically grown spinach (Spinacia oleracea L. cv. Lead) plants, and the assimilation of 13C and 15N into biochemical fractions was followed at the whole plant level during six hours. After 75-min pulse feeding with 13CO2 and 90-min initial feeding with 15NO3- (designated as 0 h), the photosynthetically fixed 13C was largely found as the form of sugars in leaf blades and the newly absorbed 15N as nitrate in roots. During a further six hours of chase feeding with non-labeled CO2 and continuous 15NO3- supply, the amounts of 13C in sugars, organic acids, undefined C and amino acids gradually decreased in leaf blades while those in C compounds (except sugars and undefined C) increased in petioles and roots. 15N in the examined N compounds increased in all three organs. At 6 h, 13C in each organ was present as sugars (12–19%), undefined C (28–44%) or insoluble compounds (29–45%) and 15N as nitrate (15–24%) or undefined N (44–62%). During these six hours the assimilation of 15NO3- to reduced 15N seemed correlated with the utilization of 13C-labeled sugars in the three organs. [ABSTRACT FROM AUTHOR]