The internal wiring of an existing stem or trunk flow gauge was redesigned to obtain greater accuracy of the gauge itself, eliminate errors due to signal loss in connecting cables, and reduce the number of channels and of the computing required of the datalogger. Tests of the gauge conducted on bald cypress (Taxodium distichum) and Ficus retusa (L.) Nitida trees, in a greenhouse and in an urban backyard, and under well‐watered and dry conditions gave daily sap mass flow rates that were within 5% of those obtained by direct weighing.

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Liquid behavior under reduced gravity conditions is of considerable interest for various components of life‐support systems required for manned space missions. High costs and limited opportunities for spaceflight experiments hinder advances in reliable design and operation of elements involving fluids in unsaturated porous media such as plant growth facilities. We used parabolic flight experiments to characterize hydraulic properties under variable gravity conditions deduced from variations in matric potential over a range of water contents. We designed and tested novel measurement cells that allowed dynamic control of water content. Embedded time domain reflectometry probes and fast‐responding tensiometers measured changes in water content and matric potential. For near‐saturated conditions, we observed rapid establishment of equilibrium matric potentials during the recurring 20‐s periods of microgravity. As media water content decreased, the concurrent decrease in hydraulic diffusivity resulted in limited attainment of equilibrium distributions of water content and matric potential in microgravity, and water content heterogeneity within the sample was influenced by the preceding hypergravity phase. For steady fluxes through saturated columns, we observed linear and constant hydraulic gradients during variable gravity, yielding saturated hydraulic conductivities similar to values measured under terrestrial gravity. Our results suggest that water distribution and retention behavior are sensitive to varied gravitational forces, whereas saturated hydraulic conductivity appears to be unaffected. Comparisons between measurements and simulations based on the Richards equation were in reasonable agreement, suggesting that fundamental laws of fluid flow and distribution for macroscopic transport derived on Earth are also applicable in microgravity.

From June 1993 through February 1994, the removal of NH4-N was evaluated in constructed wetlands at the TVA constructed wetland research facility in Muscle Shoals, AL. The objectives were to determine rates for NH4-N removal and speculate on potential mechanisms for removal. Nine constructed wetland cells were used with approximate dimensions of 9.1 × 6.1 × 0.6 m3 and a recirculating subsurface flow system in a gravel base. Treatments consisted of an unplanted (WO=control) and two polycultural planting schemes (P1=Scirpus acutus, Phragmites communis and Phalaris arundinacea; P2=Typha sp., Scirpus atrovirens georgianus and Scirpus cyperinus) replicated 3 times. Salt solutions were added and recirculated in each cell resulting in initial concentrations of 50 and 300 mg l−1 of NH4-N and COD, respectively, when fully diluted with wetland water. Salts were added to wetlands approximately every 6 weeks with the first addition on June 1, 1993 and the last addition on February 9, 1994 for a total of 6 time periods (times I, II, III, IV, V and VI). The COD of the waters was removed at rates ranging from 5.5 to 10 g/m2/d during times I through IV with no discernible difference amongst the planting treatments. Wetland cells with P1 were more efficient at removing NH4-N (1.1 g/m2/d) than P2 (0.6 g/m2/d) or WO (0.5 g/m2/d) at time I with differences decreasing by time IV (0.3 to 0.7 g/m2/d). During the winter (times V and VI), there were no differences in NH4-N removal amongst planting treatments with an average removal rate of 0.35 g/m2/d. There was a seasonal change in NH4-N removal in all the treatments, with the change most noticeable in the planted cells. The removal of NH4-N in WO was speculated to be due to a combination of sorption onto gravel, microbial assimilation, and nitrification at the air-water interface. The extra NH4-N removal in the planted cells diminished in the winter because the removal was most likely due to a combination of enhanced nitrification from O2 transport and NH4-N uptake mediated by seasonal macrophyte growth.

Water transport through a microporous tube-soil-plant system wa.s investigated by measuring tbe response of soil and piant water status to step cbange reductions in the water pressure within the tubes. Soybeans were germinated and grown in a porous ceramic 'soii' at a porous tube water pressure of -0-5 kPa for 28 d. During tbis time, tbe soii matric potential was nearly in equilibrium witb tube water pressure. Water pressure in tbe porous tubes was then reduced to eitber -l-O, -1-5 or -2-{) kPa. Sap flow rates, leaf conductance and soil, root and leaf water potentials were measured before and after this change. A reduction in porous tube water pressure from -0-5 to -1-0 or -1-5 kPa did not result in any significant cbange in soil or plant water status. A reduction in porous tube water pressure to -2-0 kPa resulted in significant reductions in sap flow, leaf conductance, and soil, root and leaf water potentials. Hydraulic conductance, calculated as the transpiration rateiAy/ between two points in tbe water transport patbway, was used to analyse water transport through the tube-soil-plant continuum. At porous tube water pressures of-0-5 to -1'5 kPa soil moisture was readily available and bydraulic conductance of tbe plant limited water transport. At -2-0 kPa, hydraulic conductance of the bulk soil was the dominant factor in water movement.

The root zone oxidation state was monitored over a period of 87 d for alpine rush (Juncus alpinus Viii.), canarygrass (Phalaris arundinacea L.), and cattail (Typha latifolia L.) growing in gravelnutrient solution culture. The dissolved oxygen concentration in the root zone of cattail and canarygrass was _<1 mg L -~, whereas in alpine rush it ranged from 0 to 2 mg L-~. All planted treatments consistently had a dissolved oxygen concentration I to 2 mg L-~ lower than gravel without plants. Redox potentials in the root zone of alpine rush were normally between 400 and 700 mV, indicating an aerobic root zone. The root zone of cattail also tended to be aerobic, although redox potentials of <400 mV were obtained 40% of the time. Canarygrass had the most reduced root zone with 85% of the redox potential measurements <400 mV. Dissolved oxygen concentrations and redox potentials in the root zone did not change significantly on a diurnal basis for any of the plant species. The results show that there was a plant species etfeet on the oxidation state of the root zone as measured by dissolved oxygen and redox potential.

Understanding the effect of gravity on hydraulic properties of plant growth medium is essential for growing plants in space. The suitability of existing models to simulate hydraulic properties of porous medium is uncertain due to limited understanding of fundamental mechanisms controlling water and air transport in microgravity. The objective of this research was to characterize saturated and unsaturated hydraulic conductivity (K) of two particle-size distributions of baked ceramic aggregate using direct measurement techniques compatible with microgravity. Steady state (Method A) and instantaneous profile measurement (Method B) methods for K were used in a single experimental unit with horizontal flow through thin sections of porous medium providing an earth-based analog to microgravity. Comparison between methods was conducted using a crossover experimental design compatible with limited resources of space flight. Satiated (natural saturation) K ranged from 0.09 to 0.12 cm s-1 and 0.5 to >1 cm s-1 for 0.25- to 1- and 1- to 2-mm media, respectively. The K at the interaggregate/intraaggregate transition was approximately 10(-4) cm s-1 for both particle-size distributions. Significant differences in log(10)K due to method and porous medium were less than one order of magnitude and were attributed to variability in air entrapment. The van Genuchten/Mualem parametric models provided an adequate prediction of K of the interaggregate pore space, using residual water content for that pore space. The instantaneous profile method covers the range of water contents relevant to plant growth using fewer resources than Method A, all advantages for space flight where mass, volume, and astronaut time are limited.

containment and liquid and gas phase separation in microgravity by using microporous membranes to control Hydroponic culture has traditionally been used for controlled enviwater delivery to plants. In the present configuration ronment life support systems (CELSS) because the optimal environment for roots supports high growth rates. Recent developments in of this method, nutrient solution flows under a slight zeoponic substrate and microporous tube irrigation (ZPT) also offer negative pressure through microporous tubes and is dehigh control of the root environment. This study compared the effect livered by capillary action directly to roots (Dreschel of differences in water and nutrient status of ZPT or hydroponic and Sager, 1989) or to solid substrate (Morrow et al., culture on growth and yield of wheat (Triticum aestivum L. cv. USU-1994; Tibbitts et al., 1995). A nearly constant matric Apogee). In a side-by-side test in a controlled environment, wheat potential can be maintained in solid substrate by conwas grown in ZPT and recirculating hydroponics to maturity. Water trolling water flow and pressure through microporous use by plants grown in both culture systems peaked at 15 to 20 L m Ϫ2 tubes. The dynamics of water transport through a microd Ϫ1 up to Day 40, after which it declined more rapidly for plants grown porous tube-solid substrate-plant system was studied in ZPT culture due to earlier senescence of leaves. No consistent by Steinberg and Henninger (1997), who showed that differences in water status were noted between plants grown in the two culture systems. Although yield was similar, harvest index was water holding and transport characteristics of solid sub-28% lower for plants grown in ZPT than in hydroponic culture. Sterile strate determine the range of viable operating pressures green tillers made up 12 and 0% of the biomass of plants grown in of the system. ZPT and hydroponic culture, respectively. Differences in biomass Little is known about the growth and yield of plants partitioning were attributed primarily to NH 4 -N nutrition of plants grown in solid substrate maintained at a nearly constant grown in ZPT compared with NO 3 -N in hydroponic nutrient solution.matric potential by microporous tube irrigation as com-It is probable that NH 4 -N-induced Ca deficiency produced excess pared to hydroponic culture. Cao and Tibbitts (1996) tillering and lower harvest index for plants grown in ZPT culture. compared biomass production and gas exchange of po-These results suggest that further refinements in zeoponic substrate tato (Solanum tuberosum L.) grown in a microporous would make ZPT culture a viable alternative for achieving high protube irrigation system containing isolite (a porous ceductivity in a CELSS.