δ13C of bulk organic matter and cellulose reveal post-photosynthetic fractionation during ontogeny in C4 grass leaves.

The 13C isotope composition (δ13C) of leaf dry matter is a useful tool for physiological and ecological studies. However, how post-photosynthetic fractionation associated with respiration and carbon export influences δ13C remains uncertain. We investigated the effects of post-photosynthetic fractionation on δ13C of mature leaves of Cleistogenes squarrosa, a perennial C4 grass, in controlled experiments with different levels of vapour pressure deficit and nitrogen supply. With increasing leaf age class, the 12C/13C fractionation of leaf organic matter relative to the δ13C of atmosphere CO2 (ΔDM) increased while that of cellulose (Δcel) was almost constant. The divergence between ΔDM and Δcel increased with leaf age class, with a maximum value of 1.6‰, indicating the accumulation of post-photosynthetic fractionation. Applying a new mass balance model that accounts for respiration and export of photosynthates, we found an apparent 12C/13C fractionation associated with carbon export of -0.5‰ to -1.0‰. Different ΔDM among leaves, pseudostems, daughter tillers, and roots indicate that post-photosynthetic fractionation happens at the whole-plant level. Compared with ΔDM of old leaves, ΔDM of young leaves and Δcel are more reliable proxies for predicting physiological parameters due to the lower sensitivity to post-photosynthetic fractionation and the similar sensitivity in responses to environmental changes.

Overestimated gains in water-use efficiency by global forests.

Increases in terrestrial water-use efficiency (WUE) have been reported in many studies, pointing to potential changes in physiological forcing of global carbon and hydrological cycles. However, gains in WUE are of uncertain magnitude over longer (i.e. >10 years) periods of time largely owing to difficulties in accounting for structural and physiological acclimation. 13 C signatures (i.e. δ13 C) of plant organic matter have long been used to estimate WUE at temporal scales ranging from days to centuries. Mesophyll conductance is a key uncertainty in estimated WUE owing to its influence on diffusion of CO2 to sites of carboxylation. Here we apply new knowledge of mesophyll conductance to 464 δ13 C chronologies in tree-rings of 143 species spanning global biomes. Adjusted for mesophyll conductance, gains in WUE during the 20th century (0.15 ppm year-1 ) were considerably smaller than those estimated from conventional modelling (0.26 ppm year-1 ). Across the globe, mean sensitivity of WUE to atmospheric CO2 was 0.15 ppm ppm-1 . Ratios of internal-to-atmospheric CO2 (on a mole fraction basis; ci /ca ) in leaves were mostly constant over time but differed among biomes and plant taxa-highlighting the significance of both plant structure and physiology. Together with synchronized responses in stomatal and mesophyll conductance, our results suggest that ratios of chloroplastic-to-atmospheric CO2 (cc /ca ) are constrained over time. We conclude that forest WUE may have not increased as much as previously suggested and that projections of future climate forcing via CO2 fertilization may need to be adjusted accordingly.

Estimation of intrinsic water-use efficiency from δ13C signature of C3 leaves: Assumptions and uncertainty.

Carbon isotope composition (δ13C) has been widely used to estimate the intrinsic water-use efficiency (iWUE) of plants in ecosystems around the world, providing an ultimate record of the functional response of plants to climate change. This approach relies on established relationships between leaf gas exchange and isotopic discrimination, which are reflected in different formulations of 13C-based iWUE models. In the current literature, most studies have utilized the simple, linear equation of photosynthetic discrimination to estimate iWUE. However, recent studies demonstrated that using this linear model for quantitative studies of iWUE could be problematic. Despite these advances, there is a scarcity of review papers that have comprehensively reviewed the theoretical basis, assumptions, and uncertainty of 13C-based iWUE models. Here, we 1) present the theoretical basis of 13C-based iWUE models: the classical model (iWUEsim), the comprehensive model (iWUEcom), and the model incorporating mesophyll conductance (iWUEmes); 2) discuss the limitations of the widely used iWUEsim model; 3) and make suggestions on the application of the iWUEmes model. Finally, we suggest that a mechanistic understanding of mesophyll conductance associated effects and post-photosynthetic fractionation are the bottlenecks for improving the 13C-based estimation of iWUE.

[Mesophyll conductance to CO2: Methods and current knowledge].

Mesophyll conductance (gm), the total conductance of CO2 diffusion from substomatal cavity to the site of carboxylation within chloroplast, is a major limiting factor for photosynthesis and a key parameter for improving photosynthetic resource use efficiency of crops. Online 13C discrimination method is an important method for plant eco-physiological studies and a well-established method for measuring gm of C3 plants, although it has not been widely used due to challenges in methodology and high demands on experimental facilities. In this review, we summarized the characteristics of commonly used methods for gm, introduced the basic theory of the online 13C discrimination method, namely Farquhar's photosynthetic 13C discrimination model; systematically introduced the practical measurements, equations and the components of facilities; and reviewed the drivers for variation in gm of C3 plants. At the last part, we discussed the outlook of the development of methodology, new experimental protocols, and applications in measurement scenarios.

[Anti-angiogenesis of interferon-alpha2b in chronic myeloid leukemia in vitro].

This study was aimed to investigate the anti-angiogenesis of IFN-alpha2b in chronic myeloid leukemia (CML) in vitro by using K562 cell line and human umbilical vein endothelial cells (HUVEC). The levels of vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF) in culture supernatant of K562 cells were determined by ELISA; the expressions of VEGF and bFGF mRNA after treating K562 cells with 10(3), 10(2) and 10 U/ml IFN-alpha2b for 24, 36, 48 hours were detected by real-time RT-PCR; the effects of K562 cell culture supernatant and IFN-alpha2b on proliferation, migration and differentiation of HUVEC in vitro were assayed by MTT, Transwell chamber and tubule formation assay respectively. The results showed that the K562 cells expressed and secreted VEGF and bFGF. The culture supernatant of K562 cells significantly promoted the proliferation, migration and tubule formation of HUVEC in vitro in a concentration-dependent manner. After treating K562 cells with IFN-alpha2b 10 U/ml for 24, 36 and 48 hours, the expression levels of VEGF and bFGF mRNA were 1.64+/-0.18, 1.49+/-0.14, 1.31+/-0.05 and 1.53+/-0.10, 1.29+/-0.15, 0.79+/-0.13 respectively (p=0.002), but the expression levels of VEGF and bFGF mRNA were not significantly different along with increasing of IFN-alpha2b concentration. It is concluded that the angiogenesis exists in CML. The K562 cell expresses and secrets VEGF and bFGF, which promotes the proliferation, migration and differentiation of HUVEC. The IFN-alpha2b displays anti-angiogenesis by inhibiting the proliferation, migration and tubule formation in vitro of HUVEC and down regulating the expression of VEGF and bFGF mRNA.