In situ¹³CO₂pulse 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 (13)CO2 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 (21-31 h). The time series of (13)C 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 m h(-1)) was twice as high in +K trees than in -K trees, with no significant effect of throughfall exclusion except for one +K -W tree labelled in the middle of the drought season that was exposed to a more pronounced water stress (midday leaf water potential of -2.2 MPa). 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 (13)C 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.

Foliar Application of Potassium Sulfate Partially Alleviates Pre-anthesis Drought-induced Kernel Abortion in Maize

“…It has been reported that foliar applied K stimulates the export of sugars from phloem (phloem unloading) into the apoplast (Doman and Gieger, 1979). In a recent study, Epron et al (2016) showed that K fertilization-induced increased growth Considering the important role of K in phloem transport of assimilates, it was hypothesized that drought stress around pollination may induce K deficiency in source leaves. Maize takes up most of the K by flowering with a peak uptake rates during silking (Rejado, 1978).…”

Seasonal variations drive short‐term dynamics and partitioning of recently assimilated carbon in the foliage of adult beech and pine

“…labelling remained in leaves at the end of the pulse chase period (H€ ogberg et al, 2008;Ruehr et al, 2009). Only a few studies have distinguished two pools of C compounds, mobile and stable (Shibistova et al, 2012;Streit et al, 2013;Epron et al, 2016), to describe the dynamics of recently assimilated C in foliage more realistically. Compartmental models have proven effective for analysing tracer kinetics after labelling (Lattanzi et al, 2005;Lehmeier et al, 2008Lehmeier et al, , 2010.…”

“…The C export rate is driven by source export capacity and sink organ demand at tree level (Lacointe, 2000;Andersen, 2003;K€ orner, 2003;Kuptz et al, 2011a,b). Recent pulse-labelling studies demonstrated that labelled C is recovered within only a few days in root biomass and in soil CO 2 effluxes, highlighting a close link between photosynthetic assimilation of C by leaves and its use in metabolic processes by belowground organs (Carbone et al, 2007;H€ ogberg et al, 2008Endrulat et al, 2010;Blessing et al, 2015;Epron et al, 2016). The export rate of C from leaves is also driven by the equilibrium between C supply and C demand at the leaf level.…”

Climate warming and tree carbon use efficiency in a whole‐tree 13CO2 tracer study

“…Specifically, 13 C-CO 2 pulse-chase studies have been used to track the movement and respiration of labeled photosynthate in trees growing in natural settings and experimental treatments (Kagawa et al, 2006;Streit et al, 2013;Blessing et al, 2015;Heinrich et al, 2015;Thoms et al, 2017). Several studies have tracked the temporal time courses of respiration of labeled photosynthate in potted saplings (Blessing et al, 2015), small trees (Epron et al, 2016), individual branches (Keel & Schadel, 2010), and whole-tree crowns (Plain et al, 2009). Critically missing from this literature, however, are direct quantifications of the total amount of the assimilated 13 C label respired by trees in response to experimental warming.…”