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.
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Claims 15
Claim · model-written summary
Beech rapidly transported one sucrose fraction into sink tissues without major storage-pool exchange.
Verbatim
e0“In beech one part of the sucrose was rapidly transported into sink tissues without major exchange with storage pools”
Claim · model-written summary
Another beech sucrose fraction exchanged strongly with possibly stored unlabeled sucrose.
Verbatim
e0“another part of sucrose was strongly exchanged with unlabeled possibly stored sucrose”
e1“In beech one part of the sucrose”
Claim · model-written summary
Ash storage and allocation patterns depended on the identity of transported sugars.
Not reportedOrgan / stageDirection
Verbatim
e0“the storage and allocation patterns in ash depended on the identity of the transported sugars”
Claim · model-written summary
RFO were the principal transport sugars with the highest turnover in shoot compartments.
Verbatim
e0“RFO were the most important transport sugars that had highest turnover in all shoot compartments”
e1“patterns in ash depended on the identity”
Claim · model-written summary
Root RFO turnover in ash was uncoupled from shoot RFO turnover.
Verbatim
e0“the turnover of RFOs in the roots was uncoupled from the shoot”
e1“patterns in ash depended on the identity”
Claim · model-written summary
In ash, stem-base and root sucrose amounts were significantly related (r2 = 0.50; p = 0.001).
Not reportedDirection
Verbatim
e0“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”
Claim · model-written summary
In ash, stem-base and root sucrose isotopic contents were significantly related (r2 = 0.47; p = 0.01).
Not reportedDirection
Verbatim
e0“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”
Claim · model-written summary
The negative relation of sucrose amounts suggested active transport into ash roots.
Verbatim
e0“The negative relation of the amounts suggested an active transport of sucrose into the roots of ash”
Claim · model-written summary
Root sucrose concentration best explained root RFO concentration in ash.
- Taxon
- ashe1
- Organ / stage
- roote0
- Outcome
- concentration of RFOs in the rootse0
- Qualifier
- Sucrose concentratione0
Not reportedDirection
Verbatim
e0“Sucrose concentration in the root also best explained the concentration of RFOs in the roots”
e1“roots of ash was found”
Claim · model-written summary
RFO in ash roots may be resynthesized from sucrose.
Verbatim
e0“RFO in the roots of ash may be resynthesized from sucrose”
Claim · model-written summary
Only sucrose appeared to directly enter fine roots in both tree species.
Verbatim
e0“in both tree species only sucrose directly entered the fine root system”
e1“We compared beech”
e2“with ash that uses active loading”
Claim · model-written summary
Ash RFOs appeared to enter fine roots only indirectly.
Verbatim
e0“in ash RFOs are transported indirectly into the fine roots only”
Claim · model-written summary
Direct sucrose transport might be passive in beech.
Verbatim
e0“The direct transport of sucrose might be passive in beech”
Claim · model-written summary
Direct sucrose transport might be active in ash.
Verbatim
e0“but active in ash (sustained active up- and unloading to co-cells)”
Claim · model-written summary
Carbohydrate transport between shoot and root may be discontinuous and controlled by phloem unloading.
Verbatim
e0“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”