Testing the branch autonomy theory: a ¹³C/¹⁴C double‐labelling experiment on differentially shaded branches

Abstract

The impact of a heterogeneous within-crown light environment on carbon allocation was investigated on young walnut trees trained on two branches: one left in full sunlight, the other shaded until leaf fall resulting in 67% reduction in photosynthetically active radiation. In September, the two branches were separately labelled with 14 CO 2 and 13 CO 2 , respectively, so that the photosynthates from each branch could be traced independently at the same time. Although some carbon movements could be detected within 5 d in both directions (including from the shaded branch to the sun branch), between-branch carbon movements were very limited: approximately 1% of the diurnal net assimilation of a branch. At this time of the year branch autonomy was nearly total, leading to increased relative respiratory losses and a moderate growth deficit in the shaded branch. The ratio of growth to reserve storage rate was only slightly affected, indicating that reserves acted not as a mere buffer for excess C but as an active sink for assimilates. In winter, branch autonomy was more questionable, as significant amounts of carbon were imported into both branches, possibly representing up to 10% of total branch reserves. Further within-plant carbon transfers occurred in spring, which totally abolished plant autonomy, as new shoots sprouted on each branch received significantly more C mobilized from tree-wide reserves than from local, mother-branch located reserves. This allowed great flexibility of tree response to environment changes at the yearly time scale. As phloem is considered not functional in winter, it is suggested that xylem is involved as the pathway for carbohydrate movements at this time of the year. This is in agreement with other results regarding sugar exchanges between the xylem vessels and the neighbouring reserve parenchyma tissues.

Maintenance mechanisms of the pipe model relationship and Leonardo da Vinci’s rule in the branching architecture of Acer rufinerve trees

“…The results described here indicate that the branch autonomy principle does not hold, especially for a long term such as a year, as has been pointed out by Lacointe et al (2004). Instead, the source-sink balance would be an important determinant of the patterns in the leaf production, shoot elongation and the diameter growth.…”

“…The share of downward translocation of the photosynthates would depend not only on the local environment of the branch itself but also on the status of the branch relative to other branches. Further, maintenance respiration and development of new shoots rely on stored carbohydrates that do not necessarily originate from the branch (Lacointe et al 2004). Therefore, the branch is not perfectly autonomous when carbon flow in the long term, such as a year, is considered.…”

Carbon allocation and carbon isotope fluxes in the plant-soil-atmosphere continuum: a review

“…13 C-depleted lipids or 13 C-enriched carbohydrates; see 2.2). Similarly, leaf growth in deciduous trees relies on stored C (mainly starch) during the first phase of leaf development (Lacointe et al, 2004;Kagawa et al, 2006a;Asaeda et al, 2008), which in some species can be rather short (Keel and Schädel, 2010). Damesin and Lelarge (2003) have documented the switch from 13 C-enriched starch to more 13 C-depleted assimilates for young beech leaves.…”

Leaf Physiological and Morphological Responses to Shade in Grass-Stage Seedlings and Young Trees of Longleaf Pine

“…were autonomous. In young English walnut (Juglans regia L.) trees, carbon movement between branches subjected to shading treatments was only 1% of the diurnal net assimilation of a branch [21]. Branches have been found to be essentially autonomous with respect to water supply as well [22].…”