Variations in the accumulation, localization and rate of metabolization of selenium in mature Zea mays plants supplied with selenite or selenate.

Quantification of selenium bioavailability from foods is a key challenge following the discovery of the antioxidant role of this micronutrient in human health. This study presents the uptake, accumulation and rate of metabolization in mature Zea mays plants grown in hydroponic solution supplemented with selenate or selenite. Selenium content was lower in plants supplemented with selenate and accumulated mainly in the leaves compared with selenite-treated plants where the selenium was retained in the roots. Selenite-treated grains accumulated more selenium. Selenate was metabolized less than selenite in whole plants, but in grains selenium was present exclusively as organic selenium compounds. For humans, the bioavailability of organic selenium was evaluated at 90% compared with only 50% for inorganic forms. Our results show that the potential for selenium bioavailability is increased with selenite treatment.

Developmental priming of stomatal sensitivity to abscisic acid by leaf microclimate.

Plant water loss and CO2 uptake are controlled by valve-like structures on the leaf surface known as stomata. Stomatal aperture is regulated by hormonal and environmental signals. We show here that stomatal sensitivity to the drought hormone abscisic acid (ABA) is acquired during leaf development by exposure to an increasingly dryer atmosphere in the rosette plant Arabidopsis. Young leaves, which develop in the center of the rosette, do not close in response to ABA. As the leaves increase in size, they are naturally exposed to increasingly dry air as a consequence of the spatial arrangement of the leaves, and this triggers the acquisition of ABA sensitivity. Interestingly, stomatal ABA sensitivity in young leaves is rapidly restored upon water stress. These findings shed new light on how plant architecture and stomatal physiology have coevolved to optimize carbon gain against water loss in stressing environments.

Variability of 13C-labeling in plant leaves.

RATIONALE: Plant tissues artificially labeled with (13)C are increasingly used in environmental studies to unravel biogeochemical and ecophysiological processes. However, the variability of (13)C-content in labeled tissues has never been carefully investigated. Hence, this study aimed at documenting the variability of (13)C-content in artificially labeled leaves. METHODS: European beech and Italian ryegrass were subjected to long-term (13)C-labeling in a controlled-environment growth chamber. The (13)C-content of the leaves obtained after several months labeling was determined by isotope ratio mass spectrometry. RESULTS: The (13)C-content of the labeled leaves exhibited inter- and intra-leaf variability much higher than those naturally occurring in unlabeled plants, which do not exceed a few per mil. This variability was correlated with labeling intensity: the isotope composition of leaves varied in ranges of ca 60‰ and 90‰ for experiments that led to average leaf (13)C-content of ca +15‰ and +450‰, respectively. CONCLUSIONS: The reported variability of isotope composition in (13)C-enriched leaves is critical, and should be taken into account in subsequent experimental investigations of environmental processes using (13)C-labeled plant tissues.

Reducing and correcting for contamination of ecosystem water stable isotopes measured by isotope ratio infrared spectroscopy.

Concern exists about the suitability of laser spectroscopic instruments for the measurement of the (18)O/(16)O and (2)H/(1)H values of liquid samples other than pure water. It is possible to derive erroneous isotope values due to optical interference by certain organic compounds, including some commonly present in ecosystem-derived samples such as leaf or soil waters. Here we investigated the reliability of wavelength-scanned cavity ring-down spectroscopy (CRDS) (18)O/(16)O and (2)H/(1)H measurements from a range of ecosystem-derived waters, through comparison with isotope ratio mass spectrometry (IRMS). We tested the residual of the spectral fit S(r) calculated by the CRDS instrument as a means to quantify the difference between the CRDS and IRMS δ-values. There was very good overall agreement between the CRDS and IRMS values for both isotopes, but differences of up to 2.3‰ (δ(18)O values) and 23‰ (δ(2)H values) were observed in leaf water extracts from Citrus limon and Alnus cordata. The S(r) statistic successfully detected contaminated samples. Treatment of Citrus leaf water with activated charcoal reduced, but did not eliminate, δ(2)H(CRDS) - δ(2)H(IRMS) linearly for the tested range of 0-20% charcoal. The effect of distillation temperature on the degree of contamination was large, particularly for δ(2)H values but variable, resulting in positive, negative or no correlation with distillation temperature. S(r) and δ(CRDS) - δ(IRMS) were highly correlated, in particular for δ(2)H values, across the range of samples that we tested, indicating the potential to use this relationship to correct the δ-values of contaminated plant water extracts. We also examined the sensitivity of the CRDS system to changes in the temperature of its operating environment. We found that temperature changes ≥4 °C for δ(18)O values and ≥10 °C for δ(2)H values resulted in errors larger than the CRDS precision for the respective isotopes and advise the use of such instruments only in sufficiently temperature-stabilised environments.

Concentration effects on laser-based δ18 O and δ2 H measurements and implications for the calibration of vapour measurements with liquid standards.

Recently available isotope ratio infrared spectroscopy can directly measure the isotopic composition of atmospheric water vapour (δ(18) O, δ(2) H), overcoming one of the main limitations of isotope ratio mass spectrometry (IRMS) methods. Calibrating these gas-phase instruments requires the vapourisation of liquid standards since primary standards in principle are liquids. Here we test the viability of calibrating a wavelength-scanned cavity ring-down spectroscopy (CRDS) instrument with vapourised liquid standards. We also quantify the dependency of the measured isotope values on the water concentration for a range of isotopic compositions. In both liquid and vapour samples, we found an increase in δ(18) O and δ(2) H with water vapour concentration. For δ(18) O, the slope of this increase was similar for liquid and vapour, with a slight positive relationship with sample δ-value. For δ(2) H, we found diverging patterns for liquid and vapour samples, with no dependence on δ-value for vapour, but a decreasing slope for liquid samples. We also quantified tubing memory effects to step changes in isotopic composition, avoiding concurrent changes in the water vapour concentration. Dekabon tubing exhibited much stronger, concentration-dependent, memory effects for δ(2) H than stainless steel or perfluoroalkoxy (PFA) tubing. Direct vapour measurements with CRDS in a controlled experimental chamber agreed well with results obtained from vapour simultaneously collected in cold traps analysed by CRDS and IRMS. We conclude that vapour measurements can be calibrated reliably with liquid standards. We demonstrate how to take the concentration dependencies of the δ-values into account. Copyright © 2010 John Wiley & Sons, Ltd.