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.

We aimed to quantify the separate effects of photosynthetic and postphotosynthetic carbon isotope discrimination on d 13 C of the fast-turn-over carbon pool (water soluble organic carbon and CO 2 emitted from heterotrophic tissues), including their diel variation, along the pathway of carbon transport from the foliage to the base of the stem.For that purpose, we determined d 13 C in total and water-soluble organic matter of the foliage plus d 13 C and d 18 O in phloem organic matter of twigs and at three heights along the stem of Pinus sylvestris over a nine-day period, including four measurements per day. These data were related to meteorological and photosynthesis parameters and to the d 13 C of stem-emitted CO 2 .In the canopy (foliage and twigs), the d 13 C of soluble organic matter varied diurnally with amplitudes of up to 1.9%. The greatest 13 C enrichment was recorded during the night/early morning, indicating a strong influence of starch storage and remobilization on the carbon isotope signatures of sugars exported from the leaves. 13 C enrichment of soluble organic matter from the leaves to the twig phloem and further on to the phloem of the stem was supposed to be a result of carbon isotope fractionation associated with metabolic processes in the source and sink tissues. CO 2 emitted from the stem was enriched by 2.3-5.2% compared with phloem organic matter. When day-to-day variation was addressed, water-soluble leaf d 13 C and twig phloem d 18 O were strongly influenced by c i /c a and stomatal conductance (G s ), respectively.These results show that both photosynthetic and postphotosynthetic carbon isotope fractionation influence d 13 C of organic matter over time, and over the length of the basipetal transport pathway. Clearly, these influences on the d 13 C of respired CO 2 must be considered when using the latter for partitioning of ecosystem CO 2 fluxes or when the assessment of d 13 C in organic matter is applied to estimate environmental effects in c i /c a .

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.

Post-photosynthetic carbon isotope fractionation might alter the isotopic signal imprinted on organic matter (OM) during primary carbon fixation by Rubisco. To characterise the influence of post-photosynthetic processes, we investigated the effect of starch storage and remobilisation on the stable carbon isotope signature (δ 13 C) of different carbon pools in the Eucalyptus delegatensis R. T. Baker leaf and the potential carbon isotope fractionation associated with phloem transport and respiration. Twig phloem exudate and leaf water-soluble OM showed diel variations in δ 13 C of up to 2.5 and 2‰, respectively, with 13 C enrichment during the night and depletion during the day. Damped diel variation was also evident in bulk lipids of the leaf and in the leaf wax fraction. δ 13 C of nocturnal phloem exudate OM corresponded with the δ 13 C of carbon released from starch. There was no change in δ 13 C of phloem carbon along the trunk. CO 2 emitted from trunks and roots was 13 C enriched compared with the potential organic substrate, and depleted compared with soil-emitted CO 2 . The results are consistent with transitory starch accumulation and remobilisation governing the diel rhythm of δ 13 C in phloem-transported OM and fragmentation fractionation occurring during respiration. When using δ 13 C of OM or CO 2 for assessing ecosystem processes or plant reactions towards environmental constraints, post-photosynthetic discrimination should be considered.

PDT with photofrin II has an excellent effect on patients with centrally located early-stage lung cancer who have limited tumor invasion extending over a small area (< or = 1 cm).