Phosphorus is known to be exported from leaves of apple (8,9,10), bean (4,6,11,12,13), chrysanthemum (1, 2), corn (11,12), tomato (12) and squash (71). The present work with bean plants compares export following application by spray, leaf vein injection (4) and droplet (13) methods. Other factors investigated which affect absorption and subsequent translocation of the spray-applied phosphorus include: 1) wetting agents, 2) phosphorus concentration of the spray, 3) leaf surface (upper vs lower surface), 4) different phosphorus compounds (pH and cation), 5) time, 6) hygroscopic agent, 7) size of area sprayed, 8) age and position of sprayed leaf, and 9) phosphorus level of the plant. METHODSRed Kidney bean plants grown in one-half strength Hoagland solution under fluorescent lights on a 6:00 A.2I. to 6:00 P.NI. day were used throughout. The light intensity, temperature and relative humidity were 1000 ft-c (as measured half-way up the stem), 240 ± 1°C and 60 % + 4 %, respectively. At least two plants were used for each treatment. To obtain more uniform plants, the axillary buds were removed before vascularization was pronounced, 2 and 3 days before treatment.The phosphorus solutions containing 0.2 ml of P32-labeled phosphorus of known specific activity were sprayed onto the upper surface of the terminal leaflet of the first trifoliate leaf, unless stated otherwise, between 9:00 and 11:00 A.NM. when the plants were 18 days old (12 days after the hypocotyls had straightened). To prevent contamination of the remainder of the plant the leaflet receiving the spray was enelosed in a clear plastic box which was removed several minutes after treatment. Except in the time eourse of uptake experiments, the plants were harvested and sectioned 24 hours after the application of the tracer. Usually each plant was sectioned into three parts: 1) the sprayed leaflet, 2) the remaining two leaflets and petiole of the first trifoliate leaf, 3) and the remainder of the plant. In some cases the remainder of the plant was divided at the node of the first trifoliate leaf to obtain the upward and (lownward movement. The amount of phosphorus applied was determined by summation. The percentage of applied phosphorus which moved into the nutrient solution was negligible (about 0.03 %) and was not determined in most experiments.
Perhaps the earliest suggestion that soluble materials may make a circuit within a plant was made by Hartig (16). He concluded that materials assimilated in the leaves moved downward in the bark and were stored in the parenchyma and rays. In the spring these materials were brought into solution, passed into the trachea and ascended with the moving current of water. Atkins (1) expressed essentially the same views, and later the data of 'Mason and Maskell (20) indicated that such a circuit might be made without an intervening storage period. The latter investigators suggested that nitrogen, phosphorus, potassium, and other minerals ascended the stem primarily in the xylem, and any excess not currently used in the leaves was re-exported downward in the phloem. The ratios of N, P, and K to the carbohydrate moving downward in the phloem appeared to be in excess of that required for growth of the roots, so it was suggested that the excess was liberated into the xylem sap to re-ascend the stem. Biddulph (2) showed the rapidity of movement of phosphorus in the phloem of the bean plant and the possibility of a more or less continual circulation of this element throughout the plant. Helder (17) has confirmed the circulation of phosphorus in barley and bean plants. A free circulation of phosphorus is to be expected on the basis of its mobility in the phloem and its rapid metabolic turnover in many cellular reactions which serve to maintain a supply of inorganic phosphate in the cellular fluids.Prior to the work of Thomas et al (28) sulfur was considered to be relatively immobile in plants (Wood (29)), but the demonstration by Thomas et al of movement from the leaves on one stem of a multistemmed alfalfa plant downward to the crown and then upward in another stem, left little doubt of its mobility. Biddulph et al (5) showed sulfur to be freely mobile in the phloem of the bean plant, moving at rates comparable to other mobile substances. It is suspected that the earlier views of immobility may stem from the rapidity with which sulfur may be metabolically incorporated into newly formed protein, leaving little free to re-circulate. The present investigation is intended to test this concept.Calcium has been considered immobile in the phloem since the early investigations of the withdrawal of minerals from leaves prior to their abscission in the autumn (22). Bledsoe et al (9) recently provided a very direct demonstration of the immobility of calcium in the phloem of the peanut plant, and subse-
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.
Bean, corn, and tomato plants were grown in a nutrient solution labeled with 32P, 4"Ca, or 35S and varying concentrations of AgNO3. Following a 6-hour treatment period, plants were harvested and analyzed. A low Ag' concentration (50 nanomolar) inhibited the shoot uptake of the ions investigated. In the roots, Ca uptake increased whereas P and S uptake decreased.Autoradiograms of bean and corn plants, using "'Ag, showed that Ag+ was uniformly deposited in the bean shoot, but corn 13, 19). Since Ag ions have a high affinity for sulihydryl, amino, and imino groups, it is believed that the uptake of these ions results in their binding to and/or complexing with membrane constituents, and possibly active sites on some enzymes thereby altering membrane permeability (12). Pettersson (15) stated that Ag was not translocated in measurable amounts to the shoots of cucumber plants and that this was the most toxic of the 10 heavy metals he investigated. MATERIALS AND METHODSBean (Phaseolus vulgaris var. Black Valentine), corn (Zea mays hybrid sweetcorn honey and cream), and tomato (Lycopersicon esculentum var. Homestead 24) plants were germinated on wet paper towels for 2, 2, and 5 days, respectively. Young seedlings were then transferred to a nutrient solution and grown in a controlled environmental chamber with a 14-h photoperiod (300 ,uE/M2 s PAR at average leaf height, cool-white fluorescent light), 23 ± 1 C, and RH 50 ± 5%. The basic nutrient solution contained the following salts in Am: Ca(NO3)2, 500; KNO3, 500; MgSO4, 200; KH2PO4, 50; K2HPO4, 25; FeSO4, 13.5; H3BO3, 15; MnCl2, 0.9; ZnCl2, 0.15; MoO3, 0.10; NaCl, 0.15. Individual plants were maintained in 4 liters of aerated nutrient solution (pH 6) and changed every 3 or 4 days.Most treatments were conducted at the following ages: 12 days for bean; 14 days for corn; and 30 days for tomato. Three plants were used for each treatment, with Ag added as nitrate at 0, 0.1, 0.25, and 0.5 j.M (plus 0.05 Am for the S experiment). Unless stated otherwise, the experimental solution volume was 4 liters. Approximately 4 ,uCi/plant of 32P and 10 ,uCi/plant of 45Ca and 3S were used in the tracer studies. Following a 6-h uptake period, plants were harvested, roots rinsed in deionized H20, plants sectioned (roots and shoots systems separated), and dried overnight at 80 C. Individual root and shoot systems were then weighed. For Ca and P studies, plant samples were dry ashed at 600 C, the ash dissolved with dilute HCI and made to 25-ml volume with distilled H20. For the S studies, plant samples were digested with HNO3 and HC104. Five-ml aliquots were then analyzed by liquid scintillation spectrophotometry (Cerenkov method for 32p and a LS cocktail of 10 ml of aqueous counting scintillant for 45Ca and 35S). Uptake rates were determined as mg P (Ca or S) per g dry weight of tissue.Analysis for the "0'°Ag concentration of the nutrient solution and distribution in the translocation and phloem mobility studies was done by solid scintillation spectrometry.Phloem mobility of Ag was ...
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