25 research outputs found
Role of magnesium in carbon partitioning and alleviating photooxidative damage
Magnesium (Mg) deficiency exerts a major influence on the partitioning of
drymatter and carbohydrates between shoots and roots. One of the very early
reactions of plants to Mg deficiency stress is themarked increase in the shootto-
root dry weight ratio, which is associated with a massive accumulation of
carbohydrates in source leaves, especially of sucrose and starch. These higher
concentrations of carbohydrates in Mg-deficient leaves together with the
accompanying increase in shoot-to-root dry weight ratio are indicative of
a severe impairment in phloem export of photoassimilates from source
leaves. Studies with common bean and sugar beet plants have shown that
Mg plays a fundamental role in phloem loading of sucrose. At a very early
stage of Mg deficiency, phloem export of sucrose is severely impaired, an
effect that occurs before any noticeable changes in shoot growth, Chl
concentration or photosynthetic activity. These findings suggest that accumulation
of carbohydrates in Mg-deficient leaves is caused directly by Mg
deficiency stress and not as a consequence of reduced sink activity. The role
of Mg in the phloem-loading process seems to be specific; resupplying Mg for
12 or 24 h to Mg-deficient plants resulted in a very rapid recovery of sucrose
export. It appears that the massive accumulation of carbohydrates and related
impairment in photosynthetic CO2 fixation in Mg-deficient leaves cause an
over-reduction in the photosynthetic electron transport chain that potentiates
the generation of highly reactive O2 species (ROS). Plants respond to Mg
deficiency stress by marked increases in antioxidative capacity of leaves,
especially under high light intensity, suggesting that ROS generation is
stimulated by Mg deficiency in chloroplasts. Accordingly, it has been found
that Mg-deficient plants are very susceptible to high light intensity. Exposure
of Mg-deficient plants to high light intensity rapidly induced leaf chlorosis
and necrosis, an outcome that was effectively delayed by partial shading of
the leaf blade, although the Mg concentrations in different parts of the leaf
blade were unaffected by shading. The results indicate that photooxidative
damage contributes to development of leaf chlorosis under Mg deficiency,
suggesting that plants under high-light conditions have a higher physiological
requirement for Mg. Maintenance of a high Mg nutritional status of plants is,
thus, essential in the avoidance of ROS generation, which occurs at the
expense of inhibited phloem export of sugars and impairment of CO2
fixation, particularly under high-light conditions
Reduced anthropogenic aerosol radiative forcing caused by biogenic new particle formation
The magnitude of aerosol radiative forcing caused by anthropogenic emissions depends on the baseline state of the atmosphere under pristine preindustrial conditions. Measurements show that particle formation in atmospheric conditions can occur solely from biogenic vapors. Here, we evaluate the potential effect of this source of particles on preindustrial cloud condensation nuclei (CCN) concentrations and aerosol-cloud radiative forcing over the industrial period. Model simulations show that the pure biogenic particle formation mechanism has a much larger relative effect on CCN concentrations in the preindustrial atmosphere than in the present atmosphere because of the lower aerosol concentrations. Consequently, preindustrial cloud albedo is increased more than under present day conditions, and therefore the cooling forcing of anthropogenic aerosols is reduced. The mechanism increases CCN concentrations by 20-100% over a large fraction of the preindustrial lower atmosphere, and the magnitude of annual global mean radiative forcing caused by changes of cloud albedo since 1750 is reduced by 0.22 W m-2 (27%) to -0.60 W m-2. Model uncertainties, relatively slow formation rates, and limited available ambient measurements make it difficult to establish the significance of a mechanism that has its dominant effect under preindustrial conditions. Our simulations predict more particle formation in the Amazon than is observed. However, the first observation of pure organic nucleation has now been reported for the free troposphere. Given the potentially significant effect on anthropogenic forcing, effort should be made to better understand such naturally driven aerosol processes
Molecular understanding of sulphuric acid-amine particle nucleation in the atmosphere
4 pages 359-363 in the print version, additional 7 pages online.Peer reviewe