Magnesium Constituents

Magnesium is present in the plant in several biochemical forms. In studies with forage grasses, magnesium was measured in water-soluble, acetone-soluble, and insoluble constituents (18). These forms are present in the phloem, xylem, cytoplasm (water-soluble fraction), chlorophyll (acetonesoluble fraction), and cell wall constituents (insoluble fraction).

Distribution in Plants

The quantity of magnesium accumulated will differ for various plant organs, with a tendency toward greater allocation of magnesium in transpiring organs such as leaves and flowers, rather than the roots (186–188); however, this translocation to different plant parts may be affected by the status of other elements in the plant (143,164,189). Similarly, the ability of magnesium to remobilize and translocate out of a particular plant organ may vary among plant organs (186,187).

In cucumber, magnesium concentrations were seven times higher in the shoots (70 μmol g^-1 fresh weight) than in the roots (10 μ mol g^-1 fresh weight) (190). In native stands of 13-year-old Hooker’s Banksia (Banksia hookeriana Meissn.), magnesium was distributed to different plant organs as follows (mg g^-1 dry weight): 0.99 in stems, 1.41 in leaves, and 0.73 in reproductive structures, which account for 54, 21, and 25% of the total magnesium content, respectively (191). In walnut (Juglans regia L.), magnesium remobilization from catkins was less than that from leaves (186,187). Additional studies (192) indicate that the magnesium concentration in the seeds of several halophytes ranged from 0.22 to 0.90% for forbs and 0.07 to 0.97% for grasses (192). In corn (Zea mays L.), less magnesium was translocated from the roots to the shoots for iron-deficient plants than with plants with sufficient iron (143). In a similar manner for hydroponically grown tomatoes, increasing potassium concentrations of nutrient solutions resulted in decreased magnesium concentration in leaves and roots, but increased magnesium concentrations in fruits and seeds (193).

Although magnesium accumulates to higher levels in aboveground organs than in belowground organs, there may also be spatial differences in magnesium accumulation within a particular organ (194). In corn leaves, magnesium concentration decreased from the leaf tip to the leaf base (194). The relative distribution of magnesium within plants may be altered by magnesium fertilization rates as well as the fertilization rates of other nutrients. Other environmental stresses, such as iron deficiency, have also been shown to modify the spatial gradient of magnesium concentrations along the leaf blade of corn (194).

Seasonal Variations

In perennial ryegrass (18) and walnut (186,187), magnesium concentration increased throughout the growing season. For field-grown soybeans, there was an indication that magnesium was remobilized from stems and leaves and translocated to developing pods later in the growing season (195), since stems and leaf tissue magnesium concentrations decreased from approximately 0.70% to less than 0.50% as pod magnesium concentrations increased from 0.48 to 0.51%, indicating a remobilization of magnesium from vegetative to reproductive tissue. However, the degrees of differences were affected by soil type and irrigation frequency (195).

Physiological Aspects of Magnesium Allocation

Physiologically, certain stages of plant development, such as flowering and fruiting, may make plants more susceptible to magnesium deficiencies. In camellia (Camellia sasanqua Thunb. ‘Shishi Gashira’), magnesium deficiency may be expressed after flowering, as the first vegetative flush commences in the spring (56). This expression appears to be attributed to the large flowers of ‘Shishi Gashira’ acting as sinks for magnesium. After flowering, when magnesium reserves in the plants are low, plants may be markedly susceptible to magnesium deficiency and may develop typical magnesium deficiency symptoms if sufficient magnesium is not available in the soil for uptake. Similarly, in cucumber, magnesium concentration in leaves increased with leaf age, until flowering and fruiting, at which point concentrations increased in the younger leaves (190). In grapes (Vitis vinifera L.), the magnesium concentration (10.1 mg/cluster) of ripening berries of ‘Pinot Blanc,’ a cultivar that is susceptible to lime-induced chlorosis during ripening, was significantly higher than the magnesium concentration (7.1 mg/cluster) for berries of the lime-tolerant cultivar ‘Sauvignon Blanc’ (145). However, in blades and petioles, there were no differences in magnesium concentrations (145). In other grape cultivars (‘Canadian Muscat’ and ‘Himrod’) that are susceptible to berry drop and rachis necrosis, spray applications of magnesium were shown to increase berry yield through the alleviation of rachis necrosis and berry drop (196). A similar observation was noted on grapefruit (Citrus paradisi Macfady) trees by Fudge (197). As fruit and seed development occurred, a depletion of magnesium from leaves near to the fruits was apparent, as only the leaves in proximity to the fruits expressed magnesium deficiency symptoms.