This study was performed to test if alternative carbon sources besides recently photosynthetically fixed CO 2 are used for isoprene formation in the leaves of young poplar (Populus 3 canescens) trees. In a 13 CO 2 atmosphere under steady state conditions, only about 75% of isoprene became 13 C labeled within minutes. A considerable part of the unlabeled carbon may be derived from xylem transported carbohydrates, as may be shown by feeding leaves with [U-13 C]Glc. As a consequence of this treatment approximately 8% to 10% of the carbon emitted as isoprene was 13 C labeled. In order to identify further carbon sources, poplar leaves were depleted of leaf internal carbon pools and the carbon pools were refilled with 13 C labeled carbon by exposure to 13 CO 2 . Results from this treatment showed that about 30% of isoprene carbon became 13 C labeled, clearly suggesting that, in addition to xylem transported carbon and CO 2 , leaf internal carbon pools, e.g. starch, are used for isoprene formation. This use was even increased when net assimilation was reduced, for example by abscisic acid application. The data provide clear evidence of a dynamic exchange of carbon between different cellular precursors for isoprene biosynthesis, and an increasing importance of these alternative carbon pools under conditions of limited photosynthesis. Feeding [1,2-13 C]Glc and [3-13 C]Glc to leaves via the xylem suggested that alternative carbon sources are probably derived from cytosolic pyruvate/ phosphoenolpyruvate equivalents and incorporated into isoprene according to the predicted cleavage of the 3-C position of pyruvate during the initial step of the plastidic deoxyxylulose-5-phosphate pathway.Isoprene (2-methyl-1,3-butadiene), an unsaturated C-5 hydrocarbon, is emitted in vast amounts from photosynthesizing leaves of many plant species, particularly by trees (Kesselmeier and Staudt, 1999). With a global atmospheric carbon flux of approximately 450 million tons of carbon per year, isoprene emissions are a major contributor to the total biogenic volatile organic compound (BVOC) flux of 1,200 million tons of carbon per year (Guenther et al., 1995). Current interest in understanding the biochemical and physiological mechanisms controlling isoprene formation in plants comes from the important role isoprene plays in atmospheric chemistry. Isoprene rapidly reacts with hydroxyl radicals in the atmosphere (Thompson, 1992). In the presence of nitric oxides (NO X ), the oxidation of isoprene contributes significantly to the formation of ozone, a dominant tropospheric air pollutant (Biesenthal et al., 1997). Moreover, isoprene also contributes to the regulation of tropospheric hydroxyl radicals concentration and thus plays an important role in determining the abundance of atmospheric methane, an important greenhouse gas (Chameides et al., 1988).Light controls isoprene emission through the production of photosynthetic metabolites and the supply of ATP/NADPH to the chloroplastidic deoxyxylulose-5-phosphate (DOXP) pathway (Eisenreich et al., 20...

The rates of photosynthesis and transpiration, as well as the concentrations of organic compounds (total soluble non‐protein N compounds [TSNN], soluble carbohydrates), in the xylem sap were determined during two growth seasons in one‐year‐old Quercus robur saplings. From the data, the total C gain of the leaves, by both photosynthesis and the transpiration stream, was calculated. Large amounts of C were allocated to the leaves by the transpiration stream; depending on the time of day and the environmental conditions the portion of C originating from xylem transport amounted to 8 to 91% of total C delivery to the leaves. Particularly under conditions of reduced photosynthesis, e.g., during midday depression of photosynthesis, a high percentage of the total C delivery was provided to the leaves by the transpiration stream (83 to 91 %). Apparently, attack by phloem‐feeding aphids lowered the assimilate transport from roots to shoots; as a consequence the portion of C available to the leaves from xylem transport amounted to only 12 to 16 %. The most abundant organic compounds transported in the xylem sap were sugars (sucrose, glucose, fructose) with concentrations of ca. 50 to 500 μmol C ml‐1, whereas C from N compounds was of minor significance (3 to 20 μmol ml‐1 C). The results indicate a significant cycling of C in the plants because the daily transport of C with the transpiration stream exceeded the daily photosynthetic CO2 fixation in several cases. This cycling pool of C may sustain delivery of photosynthate to heterotrophic tissues, independent of short time fluctuations in photosynthetic CO2 fixation.

In the present study, important components of carbon metabolism of mature leaves of young poplar trees (Populus x canescens) were determined. Carbohydrate concentrations in leaves and xylem sap were quantified at five different times during the day and compared with photosynthetic gas exchange measurements (net assimilation, transpiration and rates of isoprene emission). Continuously measured xylem sap flow rates, with a time resolution of 15 min, were used to calculate diurnal balances of carbon metabolism of whole mature poplar leaves on different days. Loss of photosynthetically fixed carbon by isoprene emission and dark respiration amounted to 1% and 20%. The most abundant soluble carbohydrates in leaves and xylem sap were glucose, fructose and sucrose, with amounts of approx. 2 to 12 mmol m(-2) leaf area in leaves and about 0.2 to 15 mM in xylem sap. Clear diurnal patterns of carbohydrate concentration in xylem sap and leaves, however, were not observed. Calculations of the carbon transport rates in the xylem to the leaves were based on carbohydrate concentrations in xylem sap and xylem sap flow rates. This carbon delivery amounted to about 3 micromol C m(-2) s(-1) during the day and approx. 1 micromol C m(-2) s(-1) at night. The data demonstrated that between 9 and 28 % of total carbon delivered to poplar leaves during 24 h resulted from xylem transport and, hence, provide a strong indication for a significant rate of carbon cycling within young trees.

Bronchopulmonary dysplasia (BPD) is the chronic lung disease of preterm infants and still represents a major burden of prematurity. Several clinical risk factors for the onset of the disease are already known. In addition, some candidate genes have recently been identified. We set out to determine clinical as well as genetic risk factors for the development of BPD in the German population.155 infants born with a gestational age ≤ 28 at the tertiary neonatal Centre, Freiburg, were recruited. Clinical data were recorded from hospital charts. 47 children developed moderate or severe BPD. For genetic analyses, 37 polymorphisms within sixteen genes were genotyped on all children.The strongest epidemiological risk factor for BPD was birth weight, followed by low gestational age. Genetic association was detected with single polymorphisms within Tumour necrosis factor alpha, Toll like receptor 10 and vascular endothelial growth factor. The former two genes showed also association with BPD in haplotype analyses. In conclusion, association of BPD was far more convincingly found with a few clinical factors than with genetic polymorphisms. This underscores the genetic complexity of the disease. Furthermore, the identification of predisposing genetic polymorphisms might be hampered by the complex interaction between clinical and genetic factors.

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