Mesophyll diffusion conductance to CO(2) is a key photosynthetic trait that has been studied intensively in the past years. The intention of the present review is to update knowledge of g(m), and highlight the important unknown and controversial aspects that require future work. The photosynthetic limitation imposed by mesophyll conductance is large, and under certain conditions can be the most significant photosynthetic limitation. New evidence shows that anatomical traits, such as cell wall thickness and chloroplast distribution are amongst the stronger determinants of mesophyll conductance, although rapid variations in response to environmental changes might be regulated by other factors such as aquaporin conductance. Gaps in knowledge that should be research priorities for the near future include: how different is mesophyll conductance among phylogenetically distant groups and how has it evolved? Can mesophyll conductance be uncoupled from regulation of the water path? What are the main drivers of mesophyll conductance? The need for mechanistic and phenomenological models of mesophyll conductance and its incorporation in process-based photosynthesis models is also highlighted.

The mechanistic understanding of isotope fractionation processes is increasing but we still lack detailed knowledge of the processes that determine the isotopic composition of the tree-ring archive over the long term. Especially with regard to the path from leaf photosynthate production to wood formation, post-assimilation fractionations/processes might cause at least a partial decoupling between the leaf isotope signals that record processes such as stomatal conductance, transpiration and photosynthesis, and the wood or cellulose signals that are stored in the paleophysiological record. In this review, we start from the rather well understood processes at the leaf level such as photosynthetic carbon isotope fractionation, leaf water evaporative isotope enrichment and the issue of the isotopic composition of inorganic sources (CO2 and H2O), though we focus on the less explored 'downstream' processes related to metabolism and transport. We further summarize the roles of cellulose and lignin as important chemical constituents of wood, and the processes that determine the transfer of photosynthate (sucrose) and associated isotopic signals to wood production. We cover the broad topics of post-carboxylation carbon isotope fractionation and of the exchange of organic oxygen with water within the tree. In two case studies, we assess the transfer of carbon and oxygen isotopic signals from leaves to tree rings. Finally we address the issue of different temporal scales and link isotope fractionation at the shorter time scale for processes in the leaf to the isotopic ratio as recorded across longer time scales of the tree-ring archive.

Summary Recent studies have highlighted a direct, fast transfer of recently assimilated C from the tree canopy to the soil. However, the effect of environmental changes on this flux remains largely unknown. We investigated the effects of drought on the translocation of recently assimilated C, by pulse‐labelling 1.5‐yr‐old beech tree mesocosms with 13CO2. 13C signatures were then measured daily for 1 wk in leaves, twigs, coarse and fine root water‐soluble and total organic matter, phloem organic matter, soil microbial biomass and soil CO2 efflux. Drought reduced C assimilation and doubled the residence time of recently assimilated C in leaf biomass. In phloem organic matter, the 13C label peaked immediately after labelling then decayed exponentially in the control treatment, while under drought it peaked 4 d after labelling. In soil microbial biomass, the label peaked 1 d after labelling in the control treatment, whereas under drought no peak was measured. Two days after labelling, drought decreased the contribution of recently assimilated C to soil CO2 efflux by 33%. Our study showed that drought reduced the coupling between canopy photosynthesis and belowground processes. This will probably affect soil biogeochemical cycling, with potential consequences including slower soil nitrogen cycling and changes in C‐sequestration potential under future climate conditions.

Summary The 13C isotopic signature (δ13C) of CO2 respired from plants is widely used to assess carbon fluxes and ecosystem functioning. There is, however, a lack of knowledge of the metabolic basis of the δ13C value of respired CO2. To elucidate the physiological mechanisms driving 12C/13C fractionation during respiration, the δ13C of respired CO2 from dark‐acclimated leaves during the night, from darkened leaves during the light period, and from stems and roots of Ricinus communis was analysed. The δ13C of potential respiratory substrates, the respiratory quotient and the activities of phosphoenolpyruvatecarboxylase (PEPc) and key respiratory enzymes were also measured. It is shown here that the CO2 evolved from darkened light‐acclimated leaves during the light period is 13C‐enriched, and that this correlates with malate accumulation in the light and rapid malate decarboxylation just after the onset of darkness. Whilst CO2 evolved from leaves was generally 13C‐enriched (but to a lesser extent during the night), CO2 evolved from stems and roots was depleted compared with the putative respiratory substrates; the difference was mainly caused by intensive PEPc‐catalysed CO2 refixation in stems and roots. These results provide a physiological explanation for short‐term variations of δ13C in CO2, illustrating the effects of variations of metabolic fluxes through different biochemical pathways.

The (13)C isotopic signature (C stable isotope ratio; delta(13)C) of CO(2) respired from forest ecosystems and their particular compartments are known to be influenced by temporal changes in environmental conditions affecting C isotope fractionation during photosynthesis. Whereas most studies have assessed temporal variation in delta(13)C of ecosystem-respired CO(2) on a day-to-day scale, not much information is available on its diel dynamics. We investigated environmental and physiological controls over potential temporal changes in delta(13)C of respired CO(2) by following the short-term dynamics of the (13)C signature from newly assimilated organic matter pools in the needles, via phloem-transported organic matter in twigs and trunks, to trunk-, soil- and ecosystem-respired CO(2). We found a strong 24-h periodicity in delta(13)C of organic matter in leaf and twig phloem sap, which was strongly dampened as carbohydrates were transported down the trunk. Periodicity reappeared in the delta(13)C of trunk-respired CO(2), which seemed to originate from apparent respiratory fractionation rather than from changes in delta(13)C of the organic substrate. The diel patterns of delta(13)C in soil-respired CO(2) are partly explained by soil temperature and moisture and are probably due to changes in the relative contribution of heterotrophic and autotrophic CO(2) fluxes to total soil efflux in response to environmental conditions. Our study shows that direct relations between delta(13)C of recent assimilates and respired CO(2) may not be present on a diel time scale, and other factors lead to short-term variations in delta(13)C of ecosystem-emitted CO(2). On the one hand, these variations complicate ecosystem CO(2) flux partitioning, but on the other hand they provide new insights into metabolic processes underlying respiratory CO(2) emission.