Inhibition of Phosphorus and Water Passage Across Intact Roots by Polyethylene Glycol and Phenylmercuric Acetate

Abstract

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...

Semipermeable Membrane System for Subjecting Plants to Water Stress

“…The membrane (Spectrapor 1) had a minimum molecular weight cutoff that excluded the PEG 20M. (3,10,11), and the 02 content of nutrient solutions (9).…”

“…The membrane (Spectrapor 1) had a minimum molecular weight cutoff that excluded the PEG 20M. (3,10,11), and the 02 content of nutrient solutions (9).Zur (15) showed that it was not necessary to place a plant directly into a PEG solution to control its water potential. He (15) and subsequent investigators (2, 4, 16) encased soil-grown plants within semipermeable membranes which were immersed within nutrient solutions containing an osmotic agent (PEG).…”

A Simple Procedure to Overcome Polyethelene Glycol Toxicity on Whole Plants

“…It was suggested by Lawlor (12) that PEG blocks water pathway and thus induces desiccation. Toxic effects of PEG were attributed to inhibited phosphorus transport across the root to the xylem (2). Others (21,24), however, claimed that PEG may be contaminated with phosphorus, so that high phosphorus concentrations in nutrient solution lead to high rates of uptake.…”

“…Although such contaminants can be removed by ion exchange resins, gel filtration, or dialysis, toxicity was not always prevented (3,12,18,21). Plant roots are probably not completely impermeable to PEG, and its toxicity might be due to uptake (2,17,21) and translocation throughout the plant (9, 11, 12). Some investigators (11,12,17) claim that it is transported without being broken down and that molecular size will determine the rate of its transport and location (8,9,12 …”

Evaluation of a System for the Imposition of Plant Water Stress

“…Recent osmotic techniques have favored the use of PEG3 as an osmoticum (1 1,12,14,18). However, a measurable amount of PEG may be absorbed by the plant (1 1,14), and PEG has been reported to reduce phosphorous uptake (7,21,22), translocation (22), and the oxygen content of nutrient solutions (16). These difficulties have been partially overcome by separating the roots from the osmoticum with a semipermeable membrane enclosing a combination of roots and soil (5,8,28,29), or other rooting medium (26).…”