Temporal Dynamics of Carbon Partitioning and Rhizodeposition in Wheat

The temporal dynamics of partitioning and rhizodeposition of recent photosynthate in wheat (Triticum aestivum) roots were quantified in situ in solution culture. After a 30-min pulse of 14CO2 to a single intact leaf, 14C activities of individual carbon fluxes in the root, including exudation, respiration, and root content, were measured continuously over the next 20 h concurrently with 14C efflux from the leaf. Immediately after the end of the 14CO2 pulse, 14C activity was detected in the root, the hydroponic solution, and in root respiration. The rate of 14C exudation from the root was maximal after 2 to 3 h, and declined to one-third of maximum after a further 5 h. Completion of the rapid phase of 14C efflux from the leaf coincided with peak 14C exudation rate. Thus, exudation flux is much more rapidly and dynamically coupled to current photosynthesis than has been appreciated. Careful cross-calibration of 14C counting methods allowed a dynamic 14C budget to be constructed for the root. Cumulative 14C exudation after 20 h was around 3% of 14C fixed in photosynthesis. Partitioning of photosynthate between shoot and root was manipulated by partial defoliation before applying the 14CO2 pulse to the remaining intact leaf. Although the rate of photosynthesis was largely unaffected by partial defoliation, the proportion of new photosynthate subsequently partitioned to and exuded from the root was substantially reduced. This clearly indicates that exudation depends more on the rate of carbon import into the root than on the rate of photosynthesis.