Phloem Pressure Differences and ¹⁴C-Assimilate Translocation in Ecballium elaterium

Abstract

The role of phloem turgor pressure in 14C-assimilate translocation in Ecballium elaterium A. Rich was studied. The direction of translocation was manipulated by two methods: darkening, or defoliation, of the upper or lower halves of the shoots. After 24 hours of labeled assimilate movement, sieve tube turgor levels were measured with the phloem needle technique. Distribution of label, determined by autoradiography and counting, revealed a direct correlation between the direction of assimilate transport and the pressure difference. Phloem turgor levels always decreased in the stem of darkened shoots; this resulted in greater pressure differences in the stem between the source leaf receiving '4CO2 and treated regions.Opinions have been divided for a long time as to the mechanism of phloem translocation. Most plant physiologists agree that there is a mass flow of sieve tube sap, solutes, and water moving as a stream through mature sieve tubes. This flow is thought to be "powered" by turgor gradients which are osmotically produced (2, 17). The pressure flow mechanism is supported by abundant evidence obtained from studies of sieve tube exudation (2,20,21,26,28,29); movement of viruses (5, 6), dyes (1,3,7,8,21), '4C-assimilates, and plant regulators (4,16,27). Although the importance of pressure gradients is minimized by some workers (9, 25), the general view is that a pressure difference between source and sink is essential for phloem conduction.Little has been done in the way of direct measurement of turgor gradients in sieve tubes. Hammel (13) provided direct evidence of relatively high phloem pressure in the source region of Quercus rubrum by using the "phloem needle," but there is an obvious need for more quantitative data, using plants grown under controlled conditions. The objective of the present study was to measure the phloem turgor pressure in different parts of the plant and to determine the relation of pressure differences to "4C-assimilate movement in this tissue. Phloem pressure measurements were made by slowly pushing the needle into the stem to a depth of about 2 to 3 mm. On penetration of a phloem bundle by the needle tip, the air column in the capillary was instantaneously compressed by a surge of the dye solution. The pressure value was obtained by reading the length of the compressed air column and referring to the calibration curve. Further technical details can be found in Hammel's paper (13). Not all measurements were successful, but in the majority of attempts, positive readings were obtained. Occasionally a test failed because the inner or outer phloem of the bundle was missed, or because the needle tip was blocked. Usually the needle entered the external phloem. A rapid compression of the air column was considered to indicate a good reading, and only these readings were included in the data. MATERIALSTracer Experiments and Turgor Measurements. Radioactive CO2 was prepared from "4C-sodium bicarbonate solution and applied to a mature leaf attached to a median node. Approximately half ...

Phloem as a possible major determinant of rapid cavitation reversal in stems of Laurus nobilis (laurel)

“…4C; Table 2) refilled rapidly only when a radial pressure was applied and showed parallel, significant decrease in the fractional amount of both HSC-ray cells and HSC-VC (from 91.4 to 59.3% and from 86.1 to 25.6%, respectively). Phloem hydrostatic pressures from 0.03 to over 1.00 MPa were measured in classical studies on trees and herbs (Sheikholeslam and Curier 1977;Wright and Fisher 1980;Sovonic-Dunford et al 1981) and more recently by Gould et al (2004) in Sow thistle (Sonchus oleraceus L.). Therefore, the radial pressure applied to embolised stems in the present study could well simulate physiological phloem pressures possibly diverted from the prevailing axial direction to the radial one.…”

Hydrodynamics of steady state phloem transport with radial leakage of solute

“…According to the Münch hypothesis, higher values of turgor and osmotic pressure gradients were expected for the flows and sieve element anatomy of oak. Later, using pressure transducers and the phloem needle technique of Hammel (1968), other species were examined: white ash trunk ( Fraxinus americana L.) (Sovonick-Dunford et al, 1981; Lee, 1981a,b); Manna ash ( Fraxinus ornus ) (Milburn, 1980); squirting cucumber stems ( Ecballium elaterium ) (Sheikholeslam and Currier, 1977a,b). By using a pressure bomb and Bourdon-type gauge Milburn and Zimmermann (1977) measured pressures in coconut palm inflorescences ( Cocos nucifera L.).…”

Real-time measurement of phloem turgor pressure in Hevea brasiliensis with a modified cell pressure probe

“…The phloem turgor pressure gradients change consistently due to tree stomata opening and leaf water deficit, but surprisingly the measured gradient is not along the mass flow direction. The essentially same methods were also used to measure the phloem turgor pressure of red oak ( Quercus rubrum ) by (Hammel 1968 ), squirting cucumber ( Ecballium elaterium) by Sheikholeslam and Currier ( 1977 ), white ash ( Fraxinus americana) by Susan et al ( 1981 ) and Lee ( 1981 ). Applying a more advanced technology, Wright and Fisher ( 1980 ) and Gould et al ( 2004 , 2005 ) established a more reliable methodology with pressure probes glued onto severed aphid stylets.…”