In situ 13 CO 2 pulse labelling of field-grown eucalypt trees revealed the effects of potassium nutrition and throughfall exclusion on phloem transport of photosynthetic carbon

Abstract : Potassium (K) is an important limiting factor of tree growth, but little is known of the effects of K supply on the long-distance transport of photosynthetic carbon (C) in the phloem and of the interaction between K fertilization and drought. We pulse-labelled 2-year-old Eucalyptus grandis L. trees grown in a field trial combining K fertilization (+K and -K) and throughfall exclusion (+W and -W), and we estimated the velocity of C transfer by comparing time lags between the uptake of (CO2)-C-13 and its recovery in trunk CO2 efflux recorded at different heights. We also analysed the dynamics of the labelled photosynthates recovered in the foliage and in the phloem sap (inner bark extract). The mean residence time of labelled C in the foliage was short (21-3 h). The time series of C-13 in excess in the foliage was affected by the level of fertilization, whereas the effect of throughfall exclusion was not significant. The velocity of C transfer in the trunk (0.20-0.82 h(-1)) was twice as high in +K trees than in -K trees, with no significant effect of throughfall exclusion except for one +Ka'W tree labelled in the middle of the drought season that was exposed to a more pronounced water stress (midday leaf water potential of -2.2MPa). Our results suggest that besides reductions in photosynthetic C supply and in C demand by sink organs, the lower velocity under K deficiency is due to a lower cross-sectional area of the sieve tubes, whereas an increase in phloem sap viscosity is more likely limiting phloem transport under drought. In all treatments, 10 times less C-13 was recovered in inner bark extracts at the bottom of the trunk when compared with the base of the crown, suggesting that a large part of the labelled assimilates has been exported out of the phloem and replaced by unlabelled C. This supports the 'leakage-retrieval mechanism' that may play a role in maintaining the pressure gradient between source and sink organs required to sustain high velocity of phloem transport in tall trees.
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Tree Physiology, Oxford University Press (OUP): Policy B - Oxford Open Option B, 2015, 36 (1), pp.6 - 21. 〈10.1093/treephys/tpv090〉
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Soumis le : jeudi 6 juillet 2017 - 09:07:45
Dernière modification le : vendredi 9 novembre 2018 - 11:40:03

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Daniel Epron, Osvaldo Machado Rodrigues Cabral, Jean-Paul Laclau, Masako Dannoura, Ana Paula Packer, et al.. In situ 13 CO 2 pulse labelling of field-grown eucalypt trees revealed the effects of potassium nutrition and throughfall exclusion on phloem transport of photosynthetic carbon. Tree Physiology, Oxford University Press (OUP): Policy B - Oxford Open Option B, 2015, 36 (1), pp.6 - 21. 〈10.1093/treephys/tpv090〉. 〈hal-01557275〉

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