Search Results

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.

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.

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.

As part of NASA's effort with bioregenerative life support systems, the growth of candidate crops is being investigated in controlled environments. Peanut (Arachis hypogaea L.) was selected for the high oil and protein content of its seed. Peanut cvs. Pronto and Early Bunch were grown from seed, using recirculating nutrient film technique (NFT) in 6-cm-deep, trapazoidal culture trays. The trays were fitted with slotted covers, which allowed developing pegs to reach the root zone. Use of a separate moss-filled pegging compartment above the root zone (tray within a tray) had little effect on seed yield, but resulted in a 60% increase in the nitric acid requirements for pH control. Yields from both cultivars were equivalent to field values on an area basis; however, harvest indices were lower than field values due to the luxuriant canopy growth under controlled environment conditions. Proximate analysis of seeds was similar to field values, with the exception of fat, which was ≈15% lower, and ash, which was ≈30% greater under controlled environment conditions, regardless of cultivar.

Bean (Phaseolus vulgaris L.) cv. Etna, a dry bean variety, and cv. Hystyle, a snap bean variety, were grown at 400 and 1200 μmol·m^-2·s^-1 CO₂ to determine the effects of CO₂ enrichment on plant growth and stomatal conductance. Plants were grown in controlled environment chambers for 70 days at each CO₂ level using nutrient film technique hydroponics. An 18-h light/6-h dark photoperiod was maintained for each test, with a corresponding thermoperiod of 28 °C/24 °C and constant 65% RH. Diurnal stomatal conductance measurements were made with a steady-state porometer at 28 days after planting (DAP) and 49 DAP. As expected, plant growth and yield was consistently increased for each cultivar when plants were grown at 1200 μmol·m^-2·s^-1 CO₂ compared to 400 μmol·m^-2·s^-1 CO₂. Stomatal conductance measured during the light period showed an expected decrease for each cultivar when grown at 1200 μmol·m^-2·s^-1 CO₂ compared to 400 μmol·m^-2·s^-1 CO₂. However, during the dark period, stomatal conductance was higher for each cultivar grown at 1200 μmol·m^-2·s^-1 CO₂. These results suggest a stomatal opening effect in the dark when plants are exposed to enriched levels of CO₂. Tests are underway to investigate the effects of CO₂ levels greater than 1200 μmol·m^-2·s^-1 on the growth and stomatal conductance of bean.

Radish (Raphanus sativus cv. Giant White Globe) and lettuce (Lactuca sativa cv. Waldmann's Green) plants were grown for 25 days in growth chambers at 23 °C, ≈300 μmol·m-²·s^-1 PPF, and 18/6 photoperiod, and four CO₂ concentrations: 400, 1000, 5000, and 10,000 μmol·mol^-1. Average total dry mass (g/plant) at the 400, 1000, 5000 and 10,000 μmol·mol^-1 treatments were 6.4, 7.2, 5.9, and 5.0 for radish and 4.2, 6.2, 6.6, and 4.0 for lettuce. Each species showed an expected increase in yield as CO₂ was elevated from 400 to 1000 μmol·mol^-1, but super-elevating the CO₂ to 10,000 μmol·mol^-1 resulted in suboptimal growth. In addition, many radish leaves showed necrotic lesions at 10,000 μmol·mol^-1 by 17 days and at 5000 μmol·mol^-1 by 20 days. These results are consistent with preliminary tests in which radish cvs. Cherry Belle, Giant White Globe, and Early Scarlet Globe were grown for 16 days at 400, 1000, 5000, and 10,000 μmol·mol^-1. In that study, `Giant White Globe' produced the greatest total dry mass at 1000 (3.0 g/plant) and 5000 μmol·mol<sup>-1</sup> (3.0 g/plant), and the least at 10,000 μmol·mol<sup>-1</sup> (2.2 g/plant). `Early Scarlet Globe' followed a similar trend, but `Cherry Belle' showed little difference among CO₂ treatments. Results suggest that super-elevated CO₂ can depress growth of some species, and that sensitivities can vary among genotypes.

Peanut (Arachis hypogaea L.) plants were grown hydroponically, using continuously recirculating nutrient solution. Two culture tray designs were tested; one tray design used only nutrient solution, while the other used a sphagnum-filled pod development compartment just beneath the cover and above the nutrient solution. Both trays were fitted with slotted covers to allow developing gynophores to reach the root zone. Peanut seed yields averaged 350 g·m^-2 dry mass, regardless of tray design, suggesting that substrate is not required for hydroponic peanut production.