Extensive sucrose cycling through the fructan pool of a C3 grass across a 200 to 800 μmol mol-1 atmospheric CO2 gradient

Fructan stores constitute the most important reserve carbohydrate pools of cool season (C3) grasses, but it is largely unknown how the variation of atmospheric CO2 concentration ([CO2]) between past and projected future levels affects their function in source (photosynthesis) – sink (growth and respiration) relationships. This study addressed this question with perennial ryegrass (Lolium perenne L.) grown at [CO2] of 200, 400 or 800 μmol mol-1 with growth-limiting nitrogen fertilization. Sixty-six days-old vegetative stands were labelled with 13CO2/12CO2 mixtures for 7 days and the 13C-tracer dynamics in whole-shoot sucrose, fructans, glucose and fructose pools evaluated with a four-pool compartmental model of central carbohydrate metabolism. Increasing [CO2] from 200 to 800 μmol mol-1 increased shoot mass 1.6-fold and fructan concentration from 35% to 50% of dry weight, while decreasing the nitrogen nutrition status of stands. Conversely, [CO2] had no effect on the half-lives of water-soluble carbohydrate pools (fructans, ∼7.7 days; sucrose, glucose and fructose, 2.3-4.5 hours). Carbon cycling through the fructan pool was enhanced by [CO2], increasing the mean residence time of carbon in the carbohydrate system from 9 to 14 days between 200 and 800 μmol mol-1 [CO2]. Sucrose resynthesis from breakdown products of fructans (fructose) was extensive and corresponded to 65% and 113% of concurrent sucrose neo-synthesis (or sucrose consumption in growth and respiration) at 200 and 800 μmol mol-1 [CO2], respectively. These results imply a strong constitutive buffering of sucrose availability in the source-sink system of perennial ryegrass (and likely most other C3 grasses) by fructan metabolism.