Direct phloem transport and pressure concentration waves in linking shoot and rhizosphere activity.

Two recent reviews raised a fundamental question: what is the time lag between photosynthetic C uptake and CO2 efflux from soil. Both reviews, however, in describing the linkages between CO2 assimilation and CO2 efflux from soil, were unable to evaluate the significance of two mechanisms i) the direct transport of assimilates to the roots and rhizosphere and ii) phloem pressure concentration waves. This uncertainty led to a further discussion about the suitability of labeling when evaluating time lags. Here, we estimated the importance of the direct transport of assimilates to the rhizosphere and the importance of phloem pressure concentration waves by various approaches, and reveal further differences between both reviews. We show that the pressure concentration waves only briefly play a role for CO2 efflux. In contrast, the direct transport influence lasts longer and so, it is more important for rhizosphere processes and for CO2 efflux. To evaluate the significance of these two mechanisms, we suggested a new approach based on regressions between the time lag and tree height, which confirms significance of pressure concentration waves only for trees, smaller than 2.5 m.