Cakmak Ismail, Kirkby Ernest A
Faculty of Engineering and Natural Sciences, Sabanci University, Istanbul 34956, Turkey.
Physiol Plant. 2008 Aug;133(4):692-704. doi: 10.1111/j.1399-3054.2007.01042.x.
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.
镁（Mg）缺乏对干物质以及地上部与根部碳水化合物的分配有重大影响。植物对镁缺乏胁迫的早期反应之一是地上部与根部干重比显著增加，这与源叶中碳水化合物尤其是蔗糖和淀粉的大量积累有关。镁缺乏叶片中这些较高浓度的碳水化合物以及随之而来的地上部与根部干重比增加，表明源叶中光合同化物的韧皮部输出严重受损。对菜豆和甜菜植株的研究表明，镁在蔗糖的韧皮部装载中起基本作用。在镁缺乏的早期阶段，蔗糖的韧皮部输出就严重受损，这种影响在地上部生长、叶绿素浓度或光合活性出现任何明显变化之前就已发生。这些发现表明，镁缺乏叶片中碳水化合物的积累是由镁缺乏胁迫直接引起的，而不是由于库活性降低的结果。镁在韧皮部装载过程中的作用似乎是特异性的；给镁缺乏的植株重新供应镁12或24小时，蔗糖输出会迅速恢复。似乎镁缺乏叶片中碳水化合物的大量积累以及光合二氧化碳固定的相关损害导致光合电子传递链过度还原，从而增强了高活性氧（ROS）的产生。植物通过显著提高叶片的抗氧化能力来应对镁缺乏胁迫，尤其是在高光强下，这表明叶绿体中的镁缺乏会刺激ROS的产生。因此，已发现镁缺乏的植株对高光强非常敏感。将镁缺乏的植株暴露在高光强下会迅速诱导叶片黄化和坏死，尽管叶片不同部位的镁浓度不受遮光影响，但通过叶片部分遮光可有效延迟这一结果。结果表明，光氧化损伤导致了镁缺乏条件下叶片黄化的发展，这表明高光条件下的植物对镁有更高的生理需求。因此，维持植物较高的镁营养状况对于避免ROS的产生至关重要，ROS的产生是以糖的韧皮部输出受抑制和二氧化碳固定受损为代价的，尤其是在高光条件下。
