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A series of experiments were conducted to determine the sensitivity of radish to four light alcohols (ethanol, methanol, 2-propanol, and t-butanol) identified as atmospheric contaminants on manned spacecraft. Radish (Raphanus sativus L. `Cherry Bomb' Hybrid II) seedlings were exposed for 5 days to concentrations of 0, 50, 100, 175, 250, and 500 ppm of each alcohol and the effect on seedling growth was used to establish preliminary threshold response values. Results show a general response-pattern for the four alcohol exposures at threshold responses of 10% (T₁₀), 50% (T₅₀) and 90% (T₉₀) reduction in seedling length. There were differences in the response of seedlings to the four alcohols, with the T₁₀ for t-butanol and ethanol (25 to 40 ppm) being 3 to 5× lower than for methanol or 2-propanol (110 to 120 ppm). Ethanol and t-butanol exhibited similar T₅₀ values (150 to 160 ppm). In contrast, T₅₀ for methanol (285 ppm) and 2-propanol (260 ppm) were about 100 ppm higher than for ethanol or t-butanol. Chronic exposures to 400 ppm t-butanol, ethanol or 2-propanol were highly toxic to the plants. Radish was more tolerant of methanol, with T₉₀ of 465 ppm. Seeds did not germinate at the 500 ppm treatment of t-butanol, 2-propanol, or ethanol. There were significant differences in projected performance of plants in different environments, dependent upon the regulatory guidelines used. The use of exposure guidelines for humans is not applicable to plant systems.

To characterize CO₂, exchange, potatoes (Solanum tuberosum cv. Norland) were grown for 90 and 105 days in KSC's Biomass Production Chamber, a 116 m^-3 closed cuvette, with 0.5 strength modified Hoagland solution using recirculating NFT culture, 12/12 photoperiod, 1000 μmol mol^-1 CO₂, and approximately 900 μmol m^-2 s^-1 PPF from HPS lamps. Canopy gas exchange responses to CO, concentration, light intensity, and photoperiod were experimentally determined. CO, exchange showed a linear response to PPF (up to 1100 μmol m^-2 s^-1 max.) and a light compensation point of about 150 μmol m^-2 s^-1. Sustained exchange rates of >45 μmol CO₂, m^-2 s^-1 were obtained 50 days after planting. CO₂ saturation was approximately 1200 μmol mol^-1 CO₂ with a compensation point < 100 μmol mol^-1. A dark cycle of less than 4 hours resulted in a rapid, continuous decrease in C0₂ exchange rate which was partially reversed by a 12-hour dark cycle. There was a weak correlation CO₂ exchange and leaf starch concentration at the end of the dark cycle.

Proton Density and T2 maps were created throughout the dormant season in `Anna' and Northern Spy′ apple cultlvars. The percentige of oixels with 15-25 ms T2 time increased from 30% in both cultlvars at the beginning of dormancy to 80 and 72% respectively, by the end of dormancy. The conversion in `Anna' was rapid and in `Northern spy was slow. Growth occured only when conversion of bound to free water reached 70% of the total number of pixels. Buds entered into a transitional phase when conversion of water reached 50%. Buds in the transitional phase are willing to respond to treatments aimed to end dormancy. Thus dormancy can be divided into two major part based on the boundness of freeness of water in the bud.

Photoperiod treatments were imposed on potato (Solanum tuberosum L. cv. Norland) grown in the Biomass Production Chamber (BPC) at Kennedy Space Center under HPS lamps (670 μmol m^-2s^-1 PPF) at 1200 μmol mol^-1 CO₂. Stand A decreased with dark cycle length, which correlated with the change in leaf starch concentration during the dark cycle, but not absolute starch concentration. A series of growth chamber experiments were performed to characterize the effect of photoperiod and PPF on CO₂ assimilation and starch mobilization in single leaves. Plants grown on a 12/12 photoperiod at either low (300 μmol m^-2s^-1) or high (600 μmol m^-2s^-1) PPF were subjected to short-term photoperiod treatments of 8/16, 16/8, and 24/0 and diurnal CO₂ assimilation rates, CO₂ response curves, and leaf starch content were determined. CO₂ compensation point was not affected by either photoperiod or PPF. However, A_max (when normalized for PPF) decreased with increasing photoperiod. A_net correlated with the changes in specific leaf weight and starch content during the dark cycle.

Intact apple (Malus domestica Borkh.) buds were examined by magnetic resonance imaging (MRI). MRI did not excite water in unchilled apple buds and could not image it. When chilling was satisfied, images were produced. We interpret this difference to mean that water is in bound and/or structured form in dormant apple leaf buds before the chilling requirement is satisfied. Conversion of bound to free water occurred equally in the low-chilling-requirement cultivar Anna and the high-chillingrequirement cultivar Northern Spy only after 600 and 4000 hours of chilling, respectively. It appears that processes involved in satisfying chilling requirement are also converting water in buds from bound to free form. Absence of free water in dormant buds during the winter signifies endodormancy, whereas when the water is in free form, buds are ecodormant. Thidiazuron, a dormancy-breaking agent, applied to partially chilled buds is instrumental in converting water to the free form within 24 hours. Summer-dormant buds contain free water, and they could be classified only as paradormant. Based on proton profiles, ecodormant and paradormant buds cannot be distinguished but endodormant buds can be readily identified.

Tight control of growth media moisture content is needed when plant growth systems employ shallow root zones or for cultivating fast-growing plants (i.e., crops). Poor control of moisture can affect both growth rate and plant quality by either excessive watering (waterlogging) or drought events. We evaluated the performance of two types of moisture sensors: tensiometers and heat-pulse moisture sensors. The output from each sensor type was evaluated as a function of volumetric moisture content in 1 to 2 mm Turface. The tensiometers were more sensitive between 30% and 60% volumetric moisture content, and their output was nonlinear because they measure water potential directly. In contrast, both the sensitivity and the output of the heat-pulse moisture sensors, as a function of volumetric moisture content, were linear. The heat-pulse moisture sensors were used to control moisture content in a shallow root zone, whereby water was added or removed from the media through a porous tube using peristaltic pumps. Moisture content in the media could be maintained within ±2% of setpoint for moisture contents ranging from 20% to 100% volumetric moisture content. The heat-pulse sensors were better suited for controlling media moisture because of their linear output and because of their constant sensitivity as a function of volumetric moisture content.

The maintenance of pH in unbuffered nutrient solutions has important consequences for the hydroponic industry and proposed nutrient delivery systems for plants in space. The requirement for charge balance by a model plant system, dwarf wheat (Triticum aestinum cv. Yecora rojo), is largely a function of the uptake ratio of four cation species ( ${\text{NH}}_4^{+}$ , Ca²⁺, and Mg²⁺) and two anion species ( ${\text{NO}}_3^{-}$ and ${\text{SO}}_4^{2-}$ ) up to anthesis. The change in electrical conductivity (EC) and pH of the nutrient solution over time integrates the overall influx:efflux process of the plants. Solutions with three different NH₄:NO₃ ratios were sampled at 15-min intervals over a 12-h period at 9, 10, 16, 17, 23, 24, 37, and 38 days after planting. Exhaustion of N in the solution at all stages of ontogeny resulted in a 2- to 3-fold reduction in ΔpH/Δt, despite high plant tissue N and irrespective of the concentration of other charge balance ions in solution. These data, combined with a plant nutrient uptake database (normalized for plant relative growth rate per mole PPF), suggest that a system can be developed to control pH by direct supply of various alternative nutrient stock solutions, rather than by the addition of H⁺ or OH^– from acid or base.

Maintaining pH to optimize nutrient availability in unbuffered nutrient solutions is important for closed spaceflight hydroponic systems and in agriculture. Total nutrient uptake is reflected by electrical conductivity (EC) measurements, while pH reflects the net imbalance of cation and anion absorption. The pH of nitrate-only (0 ${\text{NH}}_4^{+}$ : 100 ${\text{NO}}_3^{-}$ ) nutrient solutions normally increases, whereas with equimolar (50 ${\text{NH}}_4^{+}$ : 50 ${\text{NO}}_3^{-}$ ), solutions, pH decreases. However, when solution pH was controlled to 5.8 by a mixed N sources (25 ${\text{NH}}_4^{+}$ : 75 ${\text{NO}}_3^{-}$ ), plant yields of semi-dwarf wheat (Triticum aestivum cv. `Yecora Rojo') were equal to the control (0 ${\text{NH}}_4^{+}$ : 100 ${\text{NO}}_3^{-}$ ) system. When nutrient uptake was monitored at 15-min intervals, it was found that ${\text{NH}}_4^{+}$ and ${\text{NO}}_3^{-}$ were taken up simultaneously. Uptake of ${\text{NH}}_4^{+}$ was more rapid than ${\text{NO}}_3^{-}$ . The change in pH and EC was primarily a function of the absorption of three ions, namely ${\text{NH}}_4^{+}$ , ${\text{NO}}_3^{-}$ , and K⁺. A significant amount of the K⁺ uptake was highly correlated (P < 0.001) to the presence of ${\text{NO}}_3^{-}$ in solution. When the daily N requirement was supplied as a 25 ${\text{NH}}_4^{+}$ : 75 ${\text{NO}}_3^{-}$ mixture, comparatively little change in solution pH occurred, with reduced K⁺ uptake by the plants. Thus, by knowing the daily crop N requirement from the relative growth rate, the pH fluctuations within hydroponic nutrient solutions can be reduced with daily additions of a balanced nutrient solution with a 25 ${\text{NH}}_4^{+}$ : 75 ${\text{NO}}_3^{-}$ mixture of N.

Requirements for water, nutrient replenishment and acid (for pH control) were monitored for stands of wheat, soybean, potato, and lettuce grown in a recirculating hydroponic culture using a modified 1/2 Hoagland solution with NO₃-N. Water use at full canopy cover for all crops ranged from 4 to 5 L m^-2 day^-1. When averaged over the entire crop cycle, nutrient stock solution (∼0.9 S m^-1) use varied from 0.2 L m^-2 day^-1 (lettuce: to 0.75 L m^-2 day^-1 (wheat), while acid use varied from 6 mmol m^-2 day^-1 (lettuce and soybeans) to over 40 mmol m^-2 day^-1 (wheat). Water-per unit biomass was highest for soybean and lettuce (0.3 to 0.4 L g DW), and least for wheat and potato (0.15 L g DW). Nutrient replenishment per unit biomass was highest for lettuce, 34 mL g^-1 DW, with other crops ranging from 21-26 mL g^-1 DW. Acid requirements were highest for wheat, 1.2 mmol g^-1 DW, and lowest for potato, 0.7 mmol g^-1 DW. On a PAR basis, acid needs were highest for wheat, 0.6 mmol mol^-1 photons, with all other crops near 0.4 mmol mol^-1. Acid data suggest that wheat nutrient uptake favors anions more strongly than the other species, or that more nitrate loss (e.g., denitrification) may occur during wheat growth.

Critical levels of nutrients in leaf tissue are influenced by plant metabolism, environment, and nutrient availability. In this study, we measured the elemental concentrations in fully expanded, upper canopy potato (Solunum tuberosum cv. Norland) leaves throughout growth and development in a controlled environment. Plants were grown hydroponically (NFT) in NASA's Biomass Production Chamber using a complete nutrient solution with the electrical conductivity maintained continuously at 0.12 S m^-1. Photoperiod and air and root zone temperatures were changed midseason to promote tuberization, while CO₂ levels were maintained at 1000 μmol mol^-1 throughout growth. During vegetative growth, leaf nutrient concentrations remained relatively constant, except for a decline in Ca. During tuber enlargement and plant maturation, overall nutrient uptake decreased. Concentrations of the less mobile nutrients such as Ca, Mg, and B increased in the leaf tissue during mature growth, but somewhat surprisingly, highly mobile K also increased. Leaf concentrations of P, Zn, and Cu decreased during maturation.