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Abstract:
We investigated soluble carbohydrate transport in trees that differed in their phloem loading
strategies in order to better understand the transport of photosynthetic products into the
roots and the rhizosphere as this knowledge is needed to better understand the respiratory
processes in the rhizosphere. We compared beech, which is suggested to use mainly passive
loading of transport sugars along a concentration gradient into the phloem, with ash
that uses active loading and polymer trapping of raffinose family oligosaccharides (RFOs).
We pulse-labeled 20 four-year old European beech and 20 four-year old ash trees with
13CO2 and tracked the fate of the label within different plant compartments. We extracted
soluble carbohydrates from leaves, bark of stems and branches, and fine roots, measured
their amount and isotopic content and calculated their turnover times. In beech one part of
the sucrose was rapidly transported into sink tissues without major exchange with storage
pools whereas another part of sucrose was strongly exchanged with unlabeled possibly
stored sucrose. In contrast the storage and allocation patterns in ash depended on the identity
of the transported sugars. RFO were the most important transport sugars that had highest
turnover in all shoot compartments. However, the turnover of RFOs in the roots was
uncoupled from the shoot. The only significant relation between sugars in the stem base
and in the roots of ash was found for the amount (r2 = 0.50; p = 0.001) and isotopic content
(r2 = 0.47; p = 0.01) of sucrose. The negative relation of the amounts suggested an active
transport of sucrose into the roots of ash. Sucrose concentration in the root also best
explained the concentration of RFOs in the roots suggesting that RFO in the roots of ash
may be resynthesized from sucrose. Our results interestingly suggest that in both tree species
only sucrose directly entered the fine root system and that in ash RFOs are transported
indirectly into the fine roots only. The direct transport of sucrose might be passive in beech
but active in ash (sustained active up- and unloading to co-cells), which would correspond to
the phloem loading strategies. Our results give first hints that the transport of carbohydrates
between shoot and root is not necessarily continuous and involves passive (beech) and
active (ash) transport processes, which may be controlled by the phloem unloading.
