The solubility of 02 in polyethylene glycol 4000 and 6000 solutions of varying concentrations was determined iodimetrically (titrimetrically) and electrochemically using a rotating glassy carbon electrode and a PAR Model 174 Polarograph.The titrimetric determination resulted in the formation of an unexpected precipitate at 2% (w/v) polyethylene glycol corresponding to the approximate critical micelie concentration of the two polyethylene glycol homologs. Beyond 5% polyethylene glycol, 02 concentration was inversely proportional to polyethylene glycol concentration, and was higher in polyethylene glycol 4000 solutions than in polyethylene glycol 6000. The electrochemical data are a direct measure of 02 transport to the electrode surface, rather than 02 activity or concentration. Results indicate that even at relatively high H20 potentials, the transport of 02 to the root surface might be insufficient to meet the plant's respiratory requirements. MATERIALS AND METHODSThe oxygen solubilities of PEG 4000 (J. T. Baker Chem. Co., average mol wt = 3000-3700) and PEG 6000 (J. T. Baker Chem. Co., average mol wt = 6000-6700) in airsaturated aqueous solutions were determined polarographically with a PAR Model 174 polarographic analyzer. Solutions were adjusted to about pH 11.8 with NaOH pellets and maintained at 25 C in a water bath. The wave pattern of 02 was measured with a rotating (60 rps) glassy carbon electrode referenced against a saturated calomel electrode. The polarograph was adjusted to a range of 1.5 v, a rate of 20 mv/ sec, an initial potential of -200 mv, and a current range of 0.1 milliamperes (full scale). Current for the reduction of 02 to peroxide was measured at -550 mv.Oxygen concentrations of various PEG 4000 and 6000 solutions were also determined iodimetrically using the modified micro-Winkler technique (27). RESULTSTitrimetric determination of the oxygen concentration of airsaturated PEG 4000 and 6000 solutions by the micro-Winkler method revealed that any addition of PEG reduced the 02 concentration of the solution when compared to water (Fig. 1)

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Three ectomycorrhizal fungi, Cenococcwn graniforme, Suillus luteus, and Thelephora terrestris were grown in artificial nutrient media. Water potential of the media was varied by the use of the osmoticum polyethylene glycol (PEG) 4000 and measured by thermocouple psychrometry. Cenococcum graniforme was very tolerant of low water potentials and exhibited maximum growth at a potential of −15 bars. Maximum growth of S. luteus and T. terrestris occurred at −5 bars. The water potential of solutions containing PEG 4000 appears to consist of both an osmotic and matric component, making PEG 4000 ideally suited for simulation of soil moisture stress. It was neither metabolized nor readily absorbed by C. graniforme as inorganic salts or sugars might be.

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Little is known about the root zone soil water dynamics in irrigated pecan [Caryu illinoinensis (Wangenh.) K.Koch] within and outside tree canopies. Simulations were performed using the HYDROS-1D model to quantify isothermal and thermal water fluxes in the unsaturated zone of a mature pecan orchard in Las Cruces, NM, with and without root water uptake. Simulated water contents and soil temperatures correlated well with measured data at each depth. Isothermal water flux dominated the soil water movement in bare soil immediately after irrigation, while the contribution of vapor flux increased with increasing soil drying because of upward isothermal and much smaller thermal water and vapor fluxes within the 20-cm depth. In contrast, isothermal water flux was predominant throughout the under-canopy soil profile. Actual evaporation from bare soil displayed two distinct stages, immediately after irrigation and after evaporation continued to fall oS with soil drying. Immediately after irrigation, trends of under-canopy actual and potential evaporation rates were similar. Ví'ith the depletion of surface soil water, evaporation losses were lower and actual transpiration due to root water extracrion substantially contributed to actual evapotranspirarion. Relative evapotranspiration (actual/potential ratio) correlated [P < 0.05) with the pecan stem water potential. The root water uptake pattern followed the root length density distribution immediately after irrigation. Patterns of uncompensated (water stress index [u] = 1.0) and compensated (0.0 < j < 1.0) root uptake were similar during early periods after irrigation. Compensated uptake remained higher under water-stressed conditions, and the compensation from the deeper soil proHle generally increased ft)r lower values of u, although not consistently for 0.1 < uj < 0.5.Abbreviations: DOY, day of the year; ET, cvapotranspiratlon; LAI, leaf area index; RLD, root length density; SWP, stem water potential.

Quantitative information about the spatial and temporal patterns of compensatory root water uptake (RWU) in flood-irrigated pecan orchard is limited. We evaluated spatio-temporal compensated and uncompensated RWU patterns of mature pecan tree in a silty clay loam orchard using the HYDRUS (2D/3D) model. HYDRUS (2D/3D) simulations, which agreed well with measured water contents and temperatures at different soil depths and horizontal distances from the tree trunk, suggested that while both compensated and uncompensated RWU varied with soil depth they did not do so laterally because of similar spatial vertical distributions of root length density (RLD) for the under-canopy and the tree canopy dripline locations. Considering compensated RWU resulted in an increase in actual transpiration by 8%, and a decrease in evaporation and drainage by 5% and 50%, respectively, during a growing season. Simulated transpiration and relative transpiration (a ratio between actual and potential transpiration) values were correlated with measured transpiration and plant-based water stress indicators (stem and leaf water potentials), respectively. Overall, our results of the spatio-temporal compensatory RWU provide support to use HYDRUS (2D/3D) as a tool for managing efficient water use of pecan.

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