Effect of shading on branch growth and fate of current photo-assimilates
Consistent with previous works on different species (Honkanen & Haukioja 1994; Ruohomaki et al. 1997; Henriksson 2001), differential shading had a slight but statistically significant effect on growth, as roughly measured by total dry matter. The use of isotopic tracers revealed another, major, effect: the carbon losses from the shaded branch-assimilated carbon were significantly higher than those from the sunlit branch-assimilated C. At this time scale, i.e. within 5 d after assimilation, carbon losses can be ascribed to respiration (Lacointe et al. 1995b), so this denotes higher relative respiratory losses in the former than in the latter case. As the metabolic requirements of the shaded branch can be assumed slightly lower than, or at most similar to, those of the sunlit branch, this is an indication for a lower absolute amount of available carbohydrate substrate, as expected if the lower photosynthetic production was not compensated for by import from other tree parts. However, the most direct evidence in support of branch autonomy was of course the very low level of between-branch photo-assimilate movements: from each source branch, regardless of light environment, only approximately 0.2% of the total exported C was recovered in the opposite branch. It can be calculated that this represented approximately 1% of the branch own diurnal net assimilation. Thus, it can be concluded that branch autonomy was nearly total regarding primary assimilate allocation.
In addition to the above-mentioned change in relative allocation to respiration, shading had another detectable effect on local carbon economy: the relative incorporation into the ‘structural’ fraction was slightly higher, which can be understood as a slight increase in the relative allocation of C to growth versus reserve storage activity. This change, however, was of limited extent, not significantly affecting the final relative carbohydrate content of the branch DM at the end of the season. In other words, the assimilate shortage affected both the growth and reserve storage rates, the latter being only slightly more affected. A similar ‘balanced covariation’ of both activities in response to different shading conditions was previously reported in young oak (Ziegenhagen & Kausch 1995) and beech (Gansert & Sprick 1998). Thus, although reserves certainly can act as short- or middle-term buffers to cope with temporary imbalance between C production and demand, such as during peak fruit growth (Ryugo et al. 1977), they should in many cases not be considered as mere passive buffers in the long term. Instead, they appear as a vital component that the tree will (if possible) ‘manage’ to keep above a critical level, possibly at the expense of growth in case of source limitation. As a practical consequence, source–sink based models of C partitioning in trees would likely benefit from assigning reserve storage a competitive ‘sink strength’, as is generally the case for growth (Lacointe 2000; Le Roux et al. 2001).
Autumn and winter dynamics
From 5 d after assimilation through leaf fall and winter, the total label incorporated in the structural fraction exhibited no change in most tree parts, namely the roots or the main stem. This is consistent with previous results (Lacointe et al. 1995b) showing that leaf export, which was the limiting step for the final pattern of spatial and biochemical partitioning of current assimilates, was completed by 90% within 5 d. In the present experiment, however, there was an increase in the structural fraction of branches, both between September and leaf fall, and even more after leaf fall. As it could in neither case be ascribed to current assimilate import and metabolism, this is an indication that some later anabolic activity occurred in current-year shoots, not only in the autumn but also in winter. Although this experiment provided no qualitative information in this respect, it might involve phenolic compounds, particularly phenol glycosides, which have been found in particular abundance in walnut tissues (Claudot, Drouet & Jay-Allemand 1992).
Regarding extractable carbohydrates, their total label decreased in most organs between September and November This is an indication for turnover, suggesting a role as mobilizable carbon source for the above-mentioned, post-primary export increase in source branch structural matter. However, as shown by the labelled carbohydrate increase in the opposite branch, part of that recirculating mobilized C was also re-deposited as carbohydrate reserves at a distance from the place of remobilization. When both fractions were added (extractable carbohydrates + structural C), the increase in the opposite branch total dry matter label exceeded 100%, so that branch autonomy before leaf fall did not appear so strict as it did when considering only primary assimilate partitioning.
Moreover, further and even more significant amounts of mobile carbon were imported into both branches during the winter, between November and April The amount of carbohydrate that has been mobilized and translocated into the shaded branch from other parts of the plant could be roughly estimated as 0.1 g carbohydrate as an order of magnitude, which is very low (approximately 0.1%) compared with the total tree reserves, but may be significant (approximately 10%) relative to the branches’ own reserves. Although this figure was yielded by a very rough calculation, it should be considered rather underestimated as it assumed that all of the imported carbon was recovered at harvest, thus ignoring any respiratory losses.
An interesting question is the pathway for carbohydrate movements in winter. As phloem is considered not functional in winter (Aloni 1991; Aloni & Peterson 1997), this suggests the involvement of the xylem pathway. This is in agreement with other results on walnut regarding sugar exchanges between the xylem vessels and the neighbouring reserve parenchyma tissues, with major consequences on the water status and likely spring development (Améglio et al. 2001; 2004).
A total of 75% of the total September-labelled reserves in the whole tree were used up between November (leaf fall) and June (new shoots became self-sufficient), with most of the total consumption of labelled carbohydrates occurring in spring, between budbreak and new shoot self-sufficiency. This is consistent with the mobilization rates of August-labelled reserves (45%) and October-labelled (80%) as previously found by Lacointe et al. (1993) in young walnut. Of the total mobilized carbon, approximately 40% was recovered in the new shoot dry matter, which again is consistent with the recovery rates of C derived from August-labelled reserves (60%) and October-labelled (15%) as found by Lacointe et al. (1993).
Regarding the branch autonomy issue, however, the most interesting point was that the new shoots sprouted on each branch got a much higher amount of C initially labelled in the opposite branch than expected from the pre-mobilization label content of their own mother branch. This resulted in significant dilution of ‘local reserve originating C’ by ‘whole-tree reserve originating C’ in new shoot dry matter. At this point, very little was left of branch autonomy.
This dilution effect could be simulated by a simple model of the respective contribution of ‘local’ (mother-branch) versus ‘global’ (tree-wide) reserves. After fitting on the 14C data set, the model fairly well simulated the 13C data set, which can be considered not a mere replication but an independent experiment in a different situation (regarding the light environment at labelling). The results suggested that the mobilization of ‘local’ (mother-branch) reserves could be less dependent on new shoot growth than that of ‘global’ (tree-wide) reserves, which would be consistent with a rather ‘source-limited’ mobilization for local reserves whereas the tree-wide mobilization would be more ‘sink-limited’ or ‘sink-driven’. If so, the early stages of mobilization just before or at budbreak can be expected to involve preferentially ‘local’ reserves whereas the ‘global’ would be tapped on at later stages, in relation to actual growth rate; this hypothesis could be tested by frequent harvesting around and within a few weeks after budbreak.
Taken all together, these results do not support the idea of long-term branch autonomy, thus contradicting conclusions from previous defoliating or shading experiments, particularly in birch, as reported in the Introduction section. However, the contradiction may be only apparent, with at least three possible levels of explanation.
Firstly, some genetic factors, specific to each species, might be involved. Although there is a balance between growth and reserve storage in many species, including walnut, oak or beech as discussed above, this might not be the case in pioneer species such as birch. If growth indeed has priority over reserve storage as a sink in that species, local shade would lead to local significant reserve deficit (which did not occur in the present experiment in walnut). As a consequence, early bud growth might be affected, in turn hindering the subsequent growth-sink driven massive import of tree-wide originating resources as hypothesized above, etc., eventually leading to branch death as predicted by the branch autonomy principle.
Secondly, it should be emphasized that this experiment was carried out on young trees. However, as suggested by a few investigations into the response to pruning with respect to reserve dynamics (e.g. Clair-Maczulajtys & Bory 1988), within-tree C fluxes tend to become more ‘compartmentalized’ as trees grow older and larger. This would mean a higher significance of local versus tree-wide resources for local growth, resulting in better validity of branch autonomy in old than in young trees.
Thirdly, although shading systems were removed in winter both in Henriksson's (2001) and in the present experiment, light conditions were very different in spring. In the former experiment, branches were re-shaded at leaf flush. In contrast, in our experiment the shading system was not re-installed, so that not only bud break but also subsequent new shoot growth occurred in a sunlit environment, as would occur after a clearing. This could explain the observed difference, if the fate of resources from massive, tree-wide reserve mobilization is, directly or not, dependent on the local environment. Assuming sink-driven import, such dependence would arise from early sink demand of new shoots, or early growth rate, which could be stimulated by light through its own photosynthesis, or maybe through more qualitative effects on development. Through such a feedback loop, a clear light environment would not only promote import into sunlit branches, but also inhibit import of tree-wide originating resources into shaded branches, in relation to competition among sinks. This would further explain a seemingly paradoxical effect observed by Henriksson (2001), which true branch autonomy would not allow: the impact of shading, as evaluated by eventual death rate, was more pronounced on branches that were individually shaded, hence competing with sunlit ones, than on completely shaded trees.