The osmotic agent polyethylene glycol and the stomate regulator phenylmercuric acetate have been routinely used to affect the water economy of experimental plants. One or both of these compounds have, however, also been observed to alter plant growth (2,8,10), nutrition (3, 6), photosynthesis (11, 13), and various physical characteristics of the plant (6, 9, 10, 11). Enumeration of such effects is necessary to a critical evaluation of the chemicals. Helpful too would be information on whether the chemicals interfered directly with vital processes such as photosynthesis and nutrition, or whether such processes responded to water stress induced by the chemicals. In the current study the effects of PEG2 and PMA on phosphorus and water influx were measured with these points in mind. MATERIALS AND METHODSMeasurements were made of phosphorus and water passage across intact bean (Phaseolus vulgaris L., cv. Red Wade) roots to the xylem. These measurements rested on earlier findings that the processes of water and phosphorus movement to the xylem were not linked (5). Phosphorus passage controlled amounts of the element delivered to the xylem stream and to the plant top, and was insensitive to water flow. The well documented effect of PMA and PEG of restricting water flow would have the opposite effect. The ratio of solute to solvent would be increased and the xylem stream concentration raised. The extent to which water flow countered the effects of phosphorus passage on stream concentration was used here as a relative criterion of water flow.The measurement for phosphorus in the xylem stream was made in a trilayer environment control system described elsewhere (4). In brief, roots and foliage of test plants were maintained in carefully controlled functional environments while a section of the hypocotyl was held at 0 C. Radiophosphorus was added to the roots, and the amounts which completed passage to the xylem stream were noted by measuring the radioactivity of the chilled hypocotyl. The reading represented uP in the xylem sap of the hypocotyl section as well as UP which had moved laterally from the sap to peripheral tissues. Mathematical correction was possible for the extra-xylem fraction. Experiments with derooted plants in the manner described in reference 4 established that the extra-xylem fraction was not influenced by PMA or PEG in the xylem sap in the concentrations used here.In all experiments the test bean plants were grown in a hydroponics medium under controlled environment regimes to provide healthy, uniform, rapidly growing plants. Plants were ready for testing after 13 days growth from seed, when the first trifoliate leaves were approximately 2 cm long. Plants were taken approximately midpoint in the photoperiod and sealed in the trilayer environment system, with roots in 50 ml of the growth solution. After a 30-min conditioning period, the solution was replaced in rapid sequence with distilled water and then with 50 ml of 0.1 mM KH'2P4, which yielded approximately 3000 cpm/ml. Radiosolutions also containe...

The sequence of events involved in passage of ions from the ambient root medium to final residence in the plant top can be broken down into three overall steps; (a) lateral penetration of ions across the root to the stele, (b) acropetal movement of ions in the stem, and (c) concentration of ions into metabolic and chemical sinks in the plant top.Step (b) has historically been pictured to occur by mass flow in the transpiration stream or in the ascent of solution by root pressure. Recent work suggests that movement for at least calcium may involve exchange processes as well as mass flow; also, that some ascent may take place outside the xylem duct 1 e. These findings encourage work on this topic with the chemically related strontium ion.Investigation was made of non-metabolic deposition of strontium in the tops of bean plants. Attention was confined to the nonmetabolic fraction since it is believed that upward movement of ion primarily involves this fraction. Uptake solutions accordingly contained 2,4-dinitrophenol (DNP) as a metabolic inhibitor. Root influences were avoided by severing the roots from the plants, and allowing direct access of solution into the stem.

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The effects of time, rate of the water flow, and ambient pH on centripetal passage of radiophosphorus across intact bean roots to the xylem were studied. Isotope which completed passage and entered the xylem stream, as well as amounts delivered to the plant top, served to measure centripetal passage. were: temperature 28 C, relative humidity 55%, light 1500 ft-c at plant height. During the 6-hr dark period temperature was 24 C. Plants were started from seed on a damp blotter and transferred to an aerated solution supplied with all nutrients essential for rapid healthy development. Plants were taken for treatment 13 days from seed when the first trifoliate leaves were approximately 2 cm long.The amount of radiophosphorus which reached the xylem sap following addition of the isotope to the root medium measured centripetal passage. Radioactivity of the sap was determined in a specially designed environment control system described in detail elsewhere (6) and shown diagrammatically in Figure 1. Each of three isolated compartments housed a distinct part of the test plant. Root and foliage compartments were held at rigidly controlled functional environments. The stem was held at 0 C to arrest metabolic accumulation of isotope in the stem.Plants were taken about midpoint in the photoperiod and secured in the apparatus. Roots were submerged in 30 ml of the old growth solution, and the plants were allowed to equilibrate in the new environment for 30 min. The old solution was then replaced with 30 ml of 0.1 mm KH2PO4 containing 32P as phosphoric acid at about 32 x 103 cpm/ml. Solutions also contained 0.3 mm Ca(NO3)2. The test solutions were renewed every 15 min to minimize depletion. Radioactivity of the chilled stem was recorded as a continuous graph over a 2-hr period following introduction of the radiophorphorus solution. All measurements were replicated at least 12 times.Total radioactivity of the chilled stem (Z) represented isotope in the xylem sap (S) as well as temperature-insensitive building of isotope in the stem (X). X was a direct and linear function of S and time t according to the expression X = 2.42 St. S was derived from the expression S = Z/(2.42 t + 1). These calculations were reviewed in an earlier report (6).After the 2-hr uptake period, total isotope delivered to the plant top was determined. The stem was severed at a point slightly above the solution meniscus, and the plant top was dried, ashed, and radioactivity was determined.

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