Role of magnesium in carbon partitioning and alleviating photooxidative damage

Abstract

Magnesium (Mg) deficiency exerts a major influence on the partitioning of dry matter and carbohydrates between shoots and roots. One of the very early reactions of plants to Mg deficiency stress is the marked increase in the shoot-to-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.

Magnesium mobility in soils as a challenge for soil and plant analysis, magnesium fertilization and root uptake under adverse growth conditions

“…According to the forecasts the frequency of extreme weather conditions will increase due to processes involved in climatic change (Easterling et al 2000). The roles of Mg in plant metabolism particularly under stress conditions are well known (Cakmak and Kirkby 2008). Magnesium is involved in carbohydrate formation and translocation.…”

“…When the root system as an important and heavy sink for metabolites suffers from limited carbohydrate supply the reduced root growth further enhances the risk of other nutrient deficiencies and environmental stresses (e.g. drought stress) due to less explored soil volume and, therefore, less access to soil resources (Cakmak and Kirkby 2008). This may be underlined by results of Damm et al (2011), who recently published results on the effect of a long-term combined Mg and K fertilization on soil physical properties and root penetration of several plant species with different rooting systems.…”

Changes in the Soil Magnesium and Sulphur Content after Kieserite Application into Haplic Luvisol and the Effect on Yields of Barley Biomass

“…Magnesium also favourably influences assimilation (Marschner, 2002;Mengel and Kirkby, 2001;Dorenstouter et al, 1985). Magnesium is the basic nutrient supporting nitrogen uptake and simultaneously controlling processes responsible for photosynthesis and assimilate production and partitioning among plant parts (Cakmak and Kirkby 2008;Shaul 2002).…”

Magnesium in Crop Production, Food Quality and Human Health

“…One of critical physiological roles of Mg in plants is regulation of assimilate partitioning into sink organs such as roots and seeds (Cakmak and Kirkby 2008). It was shown during the Symposium that Mg deficiency affects wheat yield by decreasing individual seed weight rather than seed number per spike.…”