11C-PET imaging reveals transport dynamics and sectorial plasticity of oak phloem after girdling

Abstract

Carbon transport processes in plants can be followed non-invasively by repeated application of the short-lived positron-emitting radioisotope 11C, a technique which has rarely been used with trees. Recently, positron emission tomography (PET) allowing 3D visualization has been adapted for use with plants. To investigate the effects of stem girdling on the flow of assimilates, leaves on first order branches of two-year-old oak (Quercus robur L.) trees were labeled with 11C by supplying 11CO2-gas to a leaf cuvette. Magnetic resonance imaging gave an indication of the plant structure, while PET registered the tracer flow in a stem region downstream from the labeled branches. After repeated pulse labeling, phloem translocation was shown to be sectorial in the stem: leaf orthostichy determined the position of the phloem sieve tubes containing labeled 11C. The observed pathway remained unchanged for days. Tracer time-series derived from each pulse and analysed with a mechanistic model showed for two adjacent heights in the stem a similar velocity but different loss of recent assimilates. With either complete or partial girdling of bark within the monitored region, transport immediately stopped and then resumed in a new location in the stem cross-section, demonstrating the plasticity of sectoriality. One day after partial girdling, the loss of tracer along the interrupted transport pathway increased, while the velocity was enhanced in a non-girdled sector for several days. These findings suggest that lateral sugar transport was enhanced after wounding by a change in the lateral sugar transport path and the axial transport resumed with the development of new conductive tissue.

Light on phloem transport (an MRI approach)

“…However, those models were developed for trees, not for herbaceous plants like tomato. The difference between the models and our observation could be due to a different mechanism in phloem loading between woody and herbaceous species (De Schepper et al, 2013). On the other hand, our observation agrees with the model developed for carbon status in tomato where higher sucrose concentration in the phloem is predicted during the light period (De Swaef et al, 2013).…”

“…For example, it has been recognised that the loading and unloading of photo-assimilates into and from the phloem pathway take place along the whole stretch of transport phloem and occurs by a variety of combinations of direct symplasmic diffusion, apoplasmic transport, or polymer trap mechanisms (De Schepper et al, 2013). Our view of the sieve tubes has also changed since the time of Münch who envisaged them as simple semi-permeable pipes.…”

“…To be available, the photo-assimilates have to be transported from photosynthetically-active tissue (the source) to energy-demanding tissue (the sink) (van Bel, 2003;Lemoine et al, 2013). This transport is accommodated in phloem vascular tissue and is driven by an osmoticallygenerated moderate pressure gradient (Münch, 1930;Thompson, 2006;Knoblauch & Oparka, 2012;De Schepper et al, 2013). Both source and sink activity regulate this dynamic system (Björkman & Holmgren, 1963;Wolswinkel, 1984;Wardlaw, 1993;Marcelis, 1996;Farrar, 1996;Murchie et al, 2009;Turgeon, 2010).…”

“…Xylem transport takes place in xylem vessels or tracheids, and is responsible for the transport of water and nutrients from the roots into the aerial parts of the plant. Phloem transport is accommodated in sieve tubes and distributes photoassimilates from source to sink tissues (van Bel et al, 2002;van Bel, 2003a;De Schepper et al, 2013). Phloem transport is one of the major determinants of plant growth and therefore its thorough understanding may help in optimising plant growth conditions in order to increase crop yield.…”

“…Although hypotheses about xylem and phloem transport mechanisms exist for almost a century (i.e., cohesion tension theory and Munch pressure flow), the measurement of both, but particularly phloem transport, has remained experimentally elusive (Minchin & Thorpe, 2003;De Schepper et al, 2013;Hubeau & Steppe, 2015). The major drawback of most reported techniques lies in their invasiveness, which can precipitate an immediate defence reaction in the phloem transport pathway or may lead to embolism within xylem conduits.…”