Phloem Mobility of Magnesium

Abstract

MNagnesium-28 was applied to specific leaves of bean (Phaseolus vulgaris) and barley (Hordeum vulgare) plants.After 24 lhours, as muchi as 7% of the absorbed Mg was exported from the treated bean leaves and ll%G was transported basipetally from the treated zoine of the barley leaves. Transport of Mg did not occur past a heat-killed section of the treated leaf, thereby indicating that translocation took place via the phloem. M-g movement in the phloem was also evident in autoradiograms of bean stein segments in which the xylem was separated fronm the phloem by a thin sheet of plastic.During the past 3 decades radioisotopes have been used to observe the behavior of mineral nutrients in plants, and, therefore, the phloem mobilities of many of the major mineral nutrients are known (2). Magnesium is an exception. The development of magnesium deficiency symptoms, which are frequently first evident in mature leaves, has been the basis of the hypothesis that magnesium is mobile in plants (10). However, this argument is often confounded by the fact that magnesium is more readily leached from mature than from younger foliage (15,17). In studies (13,14) Translocation of Foliar-appHled "Mg. One primary leaf of 9-day-old and 16-day-old bean plants each received 0.5 ml of the treatment solution which contained 25 ,uc of 21Mg. The tracer was deposited on the leaf surface in 10-,l droplets with a micropipette, the droplets being evenly distributed over the leaf blade. Twenty-one-day-old barley plants were treated by applying 0.75 ,c of the same 2"Mg treatment solution to 1-cm bands across the second leaves, one-third of the way from the ligules of the leaves. Parallel strips of lanolin, 1 cm apart, were placed across the leaves to prevent surface migration of the tracer. Prior to applying the tracer to the leaves of some plants, the petioles of the bean plants to be treated and the portions of barley leaves just basipetal to the treatment zones were killed by heat. These leaves retained their turgor throughout the experiment, indicating that the xylem elements were not constricted by the heat treatment. The relative humidity of the environment was maintained at 70% during the course of the experiment to aid foliar absorption of the tracer. The plants were harvested after 24 hr of exposure to the tracer, and the treated bean leaves and the treated portions of the barley leaves were removed and rinsed three times with 10 ml of 1 mM HCl, which was found to wash off adequately the residual tracer on the surfaces of the leaves.Autoradiograms were made by exposing x-ray film to the plants, with treated portions detached to prevent spurious transport of the 2'Mg, in a freezer chest at -12 C for 3 days. Other plants were sectioned, dried at 77 C, and digested by a nitricperchloric acid procedure (18). The 2"Mg activity in the digests was assayed by counting Cerenkov radiation with a liquid scintillation spectrometer. All count rates were corrected for decay.

Both heavy metal-amendment of soil and aphid-infestation increase Cd and Zn concentrations in phloem exudates of a metal-hyperaccumulating plant

“…The enhanced Pb concentration may result from an increase in transporter activity or other processes that are not or partially selective among Zn, Cd and Pb. Mg is involved in photosynthesis and amino acid synthesis (Shaul, 2002) and is very mobile within the phloem (Steucek and Koontz, 1970). An increase in Mg caused by aphidinfestation may be a reaction of the plant to ensure that the various aphid-induced transcriptional changes can be accomplished (Divol et al, 2005), because Mg is essential for the function of several enzymes (Shaul, 2002).…”

Transport of magnesium ions in the phloem of Ricinus communis L. seedlings

“…The constancy of the magnesium concentration in the phloem sap under very different flow rates indicates that the magnesium uptake and-or efflux system in the sieve tubes usually exerts a relatively strict control on the intracellular magnesium concentration, which can be only modulated by phosphate. The relatively constant level of magnesium, irrespective of the external incubation conditions, may explain why previously no change was found after magnesium application (Bukovac et al 1960), though 28Mg showed up in the phloem (Steucek and Koontz 1970).…”

“…Gross underestimates result if the magnesium-transport rate in the phloem is calculated using data on magnesium mobilization in leaves (0.04 lamol 9 g FW-1. h-1, Schimansky 1973) and needles (0.007~tmol.g FW-I.h -1, Schulze et al 1989), or if calculation is based on translocation of applied 28Mg in barley (0.1 pmol.g FW -1 9 h -1, Steucek and Koontz 1970).…”

“…More recently, a decrease of the magnesium content in senescing beech leaves and a simultaneous temporary increase of magnesium in the sieve elements of the petiole was shown (Eschrich et al 1988). However, it has proven difficult to demonstrate magnesium loading by adding magnesium to leaves: for example, Bukovac et al (1960) could not detect any increase of magnesium concentration in the phloem after magnesium application to the foliage of bean plants, though addition of labelled 28Mg clearly led to the appearance of radioactive magnesium in the phloem (Steucek and Koontz 1970). These controversial data indicate that phloem transport of magnesium may be either saturated at the naturally prevalent concentration of the ion or may be controlled by some factors unknown so far.…”

Selectivity of the root absorption of nutrient ions in grain sorghum

“…This element, which is known to be mobile in the phloem (22), can be redistributed, but in the case of grain sorghum, its translocation is likely to be less intense than for potassium and nitrogen.…”