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

ABSTRACT

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.

Acknowledgments