Penetration of Soybean Root Systems by Abscisic Acid Isomers

Abstract

The penetration of soybean (Glycine max L. cv. Ransom) root systems by exogenously applied isomers of abscisic acid was monitored by measuring the concentration of the chemical in the xylem exudate of root systems exposed to a three bar hydrostatic pressure difference. The cis-trans isomer penetrated more readily than the trans-trans isomer, however, up to 6 hours was needed to reach steady-state values. Exogenous abscisic acid also decreased volume flux through the root system and increased total carbon dioxide efflux from the vessel containing the root system.Recent evidence indicates an important role of ABA2 in plant resistance to environmental stress (13, 19). Exogenously applied as well as endogenously produced ABA hardened plants to exposure to low temperatures (18,19). ABA is known to cause stomatal closure (16,20) and has been reported to alter root permeability to water (5,(7)(8)(9)13). In experiments where ABA is added to the irrigation water (18), it is often unclear whether the hormone acts on the root or is transported to the shoot and directly effects the stomata.Despite the potential importance of exogenously applied plant growth regulators, there is a paucity of information on the uptake and transport of hormones to sites of action. The experiments presented here investigate the penetration of soybean root systems by ABA geometric isomers. This information is essential to understanding the mechanism of action of ABA and to the future use of exogenous ABA to ameleorate plant damage due to environmental stress. Additionally, a convenient accurate method of general use for measuring the transport of these compounds through root systems is described. MATERIALS AND METHODSSoybean (Glycine max L. cv. Ransom) plants were grown hydroponically in 3-L plastic pots containing half-strength Hoagland solution for 20 to 40 days in controlled environment chambers at the Duke University Phytotron as described previously (14 Twenty 1d aliquots were analyzed directly with an Altex Model 310 high performance liquid chromatograph with a UV (254 nm) detector for the presence of ABA. Separation of the two geometric isomers was achieved with a solvent system of methanol:H20: acetic acid (40:60: 1), a LiChrosorb RP-18 column, and a flow rate of 1 ml/min. Standard curves of peak height versus concentration were linear between 10-6 and l0-4 M and were used to determine exudate ABA concentration. The ABA concentration of the ambient solution was also monitored during the experiment. The exudate ABA concentration expressed as a fraction of the ambient concentration was used as a relative measure of the penetration of the isomers.Exudation Rates. Exudation rates were measured as the volume of exudate flowing from the cut stem per unit time. Rates are expressed on a whole root basis.Respiration. Root respiration was monitored by measuring the CO2 concentration of the gas passing through the pressure vessel with an IR gas analyzer operated in the absolute mode. Rates were expressed as a percent of the rate at the time...

Abscisic Acid Transport Coefficients of Phaseolus Root Systems

“…2), a quantity of exudate equal on average to about 10 times the total root volume was expressed. This relative volume is similar to what Markhart (4) observed in soybean. The causes for the shape of the time course are related to the transient changes in total volume fluxes described earlier (3).…”

ABSCISIC ACID AND DORMANCY IN ROOT‐NODULATED ALNUS GLUTINOSA (L.) GAERTN.

“…It is not known whether these relatively high values for cis ABA arose because the shoots metabolized trans ABA more successfully than the roots, or whether only a relatively small amount of trans ABA was translocated from the roots to the shoots, most being conjugated in the root system. Markhart (1982) presented evidence which supports this last suggestion since, in soybeans fed ABA via the root solution, cis ABA in the xylem sap of decapitated plants was three times that of the trans isomer.…”

“…It has been suggested that ABA treatment may restrict plant water supply by causing a decrease in hydraulic conduction ofthe roots -an explanation proposed by Markhart (1982) for the decreased xylem exudation rate of decapitated soybeans, treated via the rooting solution with ABA -but equally, because of the relatively long period in which alders were treated with ABA the observed change in water content could be a secondary effect associated with the onset of bud dormancy.…”

Relationship between Stress-Induced ABA and Proline Accumulations and ABA-Induced Proline Accumulation in Excised Barley Leaves

“…Freeze-dried tissue (1 g) was homogenized in 20 ml of methanol-ethyl acetate-acetic acid, 50:50: (1). Isocratic elution was with 40:60:1 (v/v/v) methanol:water:acetic acid at 2.5 ml/min (9). Fractions with retention times (29 to 31 min) corresponding to standard ABA (A254) were pooled and dried under air.…”