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W. Cao and T.W. Tibbitts

The effects of various NH4-N/NO3-N ratios on growth and mineral accumulation in potatoes (Solanum tuberosum cv. Norland) were investigated using a nutrient film technique. Plants were grown for 35 days after transplanting at six NH4-N/NO3-N mixtures of 0/100%, 20/80%, 40/60%, 60/40%, 80/20%, and 100/0% with the same total N concentration of 4 mM. All mixed N treatments significantly increased total and tuber dry weights, plant size, leaf area, and specific leaf area as compared to either NH4 or NO3 alone. Plant growth was better with NO3 alone than with NH4 alone. Compared with mixed N treatments, total N concentrations in shoots were lower with either N form alone whereas total N in roots was lower only with NO3 alone. With increased percentages of NH4, root nitrate N concentrations decreased, and reduced N increased. The NO3 alone treatment increased concentrations of Ca, Mg, Fe and Mn, and reduced concentrations of P, S, Cl, B, Zn and Cu in shoots as compared with NH4 and mixed N treatments. It is concluded that a proper maintenance of both NH4 and NO3 forms can potentially promote growth and yield in potatoes.

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T.W. Tibbitts and W. Cao

Plants of three potato (Solanum tuberosum L.) cultivars, Denali, Norland, Russet Burbank, were grown under CO2 concentrations of 500, 1000, 1500, 2000 ppm at each of 16 and 20C temperature levels. In all three cultivars, total plant dry weight on day 35 after transplanting was greater under 1000, 1500, and 2000 ppm CO2 than under 500 ppm CO2 at both 16 and 20C, and greater at 20C than at 16C under each of the CO2 concentrations. At 20C total dry weight was highest under 2000 ppm CO2 for all cultivars whereas at 16C total dry weight was highest under 1000 ppm CO2 for Denali and Norland, but highest under 1500 ppm CO2 for Russet Burbank. The similar pattern was seen with tuber dry weight except that in Russet Burbank the weight was greater at 16C than at 20C under 500, 1000, and 1500 ppm CO2. Also, for all cultivars specific leaf weight (SLW) under 1000, 1500, and 2000 ppm CO2 was much higher than under 500 ppm CO2 at 16C, but only slightly higher than under 500 ppm CO2 at 20C. The SLW was higher at 16C than at 20C under all CO2 concentrations. This study demonstrates that growth responses of potatoes to CO2 concentrations differ with temperature.

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T.W. Tibbitts and W. Cao

A nutrient delivery system developed for plant growth in space provides a unique system for maintaining a constant, slightly-negative water tension for plant research. The system involves the use of multiple porous stainless steel tubes positioned 4 cm apart in shallow trays (44 cm long, 32 cm wide and 8 cm deep), and then covered with a 4 cm layer of fine medium. Nutrient solution is recirculated through the porous tubes under -5 cm (water head) of negative pressure maintained with a siphoning procedure. Potatoes grown with negative pressures were compared to growth in similarly constructed trays that were maintained on a slant and solution added to the upper end of the trays and drained from the lower end. The same nutrient solution was recirculated through the trays of each treatment and maintained at a pH of 5.6. A microcultured plantlet of Norland cv. was transplanted into each tray. The negative pressure produced plants with less total plant dry weight, leaf area, branches, and stolons but increased biomass partitioning into tubers. The data suggest that this small constant negative water pressure regulates assimilate partitioning to encourage production of tubers.

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W. Cao and T.W. Tibbitts

A modified nutrient film technique (NFT) with a shallow granite medium was developed to control the flow rate and concentration of nutrients to which potato plants were subjected. Flow rates were 2, 4, and 8 ml per minute with balanced nutrient concentrations at 25, 50, and 100% (0.6 to 2.4 dS m-1 conductivity) of modified Hoagland's solution that was not recycled. Potato growth was greatest and about equal at 4 ml of 50% solution and at 8 ml of 25% solution. In shoots, accumulation of P, Fe, and Mn increased with both increasing concentrations and increasing flow rates. Zn accumulation decreased with increasing concentrations, and Ca, Mg, and Cu accumulation decreased with increasing flow rates. Accumulation of K, S, and B differed little with either concentrations or flow rates. In tubers, the differences resulting from variations in concentrations and flow rates were less than in shoots but accumulation patterns were similar except Ca and Mg accumulation did not decrease with increasing flow rates and K accumulation increased with both increases in concentration and increases in flow rate.

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W. Cao and T.W. Tibbitts

Three nutrient culture experiments were conducted to determine the responses of potatoes (Solanum Tuberosum L.) to various solution pH levels with NO3, NH4, and mixed NO3/NH4 (1/1) at the same total N of 4 mM. The pH levels were maintained at 4, 5, 6, and 7 with NO3 or NH4, and at 4, 4.5, 5, 6, 6.5, 7 with mixed N. In each of the experiments, Norland plants were grown for 28 days after transplanting. With mixed N, plant growth as total dry weight, leaf area and tuber number was essentially similar at pH 4.5 to 7, and decreased only at pH 4. However, with either NO3 or NH4 growth peaked at a particular pH level, pH 5 and 6 respectively, and was significantly reduced at other pH levels with severe stunting at pH 7. With mixed N, the concentrations of total N in shoots were similar at pH 4 to 7 whereas, with either N form, the concentrations of total N were higher at particular pH levels, pH 4 and 5 with NO3 and pH 7 with NH4. The concentrations of P, S, Ca, Mg, and Mn in shoots were similar at pH 4 to 7 with mixed N, but varied at certain pH levels with either NO3 or NH4. The results indicate that the useful pH range for nutrient uptake and plant growth is broader with mixed N than with either NO3 or NH4.

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K. E. Cushman and T. W. Tibbitts

Chlorosis and necrotic spotting develop on expanding leaves of particular cultivars of potato (Solanum tuberosum L.) when grown under constant light and temperature conditions. Plantlets of a constant-light sensitive cultivar, Kennebec, were planted into peat:vermiculite and established at 18C for 10 d under a 12 h light: 12 h dark photoperiod. Plants were then exposed to constant light and sprayed with 1 ml of either 0.5 mM silver thiosulfate (STS), an ethylene-action inhibitor, or water (as a control) every 2 days. Specific `target' leaflets, 5-10 mm in length at the beginning of the constant-light period, were harvested on days 5-9 of constant light, during injury development, and placed in bags made of Teflon film for IO-15 minutes to collect ethylene. Ethylene release and necrotic spotting increased as days of constant light increased for both water and STS-treated leaves, though STS-treated leaves produced slightly less ethylene and significantly less necrotic spotting than water-treated leaves. Ethylene release was correlated with extent of necrotic spotting. STS-treated plants exhibited greater dry weight and leaf area then water-treated plants. The results indicate that ethylene is not only produced by injured leaf tissue but, in addition, that ethylene may have a role in the development of constant-light injury symptoms.

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M.E. Hoenecke, R.J. Bula and T.W. Tibbitts

Light-emitting diodes (LEDs) with high-intensity output are being studied as a photosynthetic light source for plants. High-output LEDs have peak emission at ≈660 nm concentrated in a waveband of ±30 nm. Lettuce (Lactuca sativa `Grand Rapids') seedlings developed extended hypocotyls and elongated cotyledons when grown under these LEDs as a sole source of irradiance. This extension and elongation was prevented when the red LED radiation was supplemented with more than 15 μmol·m-2·s-1 of 400- to 500-nm photons from blue fluorescent lamps. Blue radiation effects were independent of the photon level of the red radiation.

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K.C. Cushman, T.W. Tibbitts, M. Anderson, X. Fu and W. Zeltner

The performance of a reactor designed to convert volatile hydrocarbons to carbon dioxide and water by a combination of surface chemistry and UV radiation was tested under conditions relevant to horticulture. Air containing 65 to 1100 nL·L–1 ethylene gas passed through a bed of catalyst crystals at a rate of 0.1 to 2.0 L·min–1. The catalyst bed consisted of 14 g of zirconia-titania particles, 0.50 to 0.75 mm in size, that occupied the space between a 4-W UV lamp and a stainless-steel housing. Dew-point temperatures of the air passing through the reactor ranged from 5 to 22°C and internal reactor temperatures ranged from 20 to 80°C. Increasing internal reactor temperature, ethylene concentration, or air flow resulted in increasing ethylene photocatalysis by the reactor. Increasing dewpoint temperature of the air stream resulted in decreasing ethylene photocatalysis by the reactor. Operation of the reactor over a 120-day period showed that reactor design and catalyst performance were stable and robust during continuous duty. Our results demonstrate that the reactor performed well over a wide range of conditions and may be useful for applications in horticulture. This research was, in part, NASA sponsored, and a reactor similar in design to that used in our studies has been used for plant growth in space.

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R.J. Bula, R.C. Morrow, T.W. Tibbitts, D.J. Barta, R.W. Ignatius and T.S. Martin

Development of a more effective radiation source for use in plant-growing facilities would be of significant benefit for both research and commercial crop production applications. An array of light-emitting diodes (LEDs) that produce red radiation, supplemented with a photosynthetic photon flux (PPF) of 30 μmol·s-1·m-2 in the 400- to 500-nm spectral range from blue fluorescent lamps, was used effectively as a radiation source for growing plants. Growth of lettuce (Lactuca sativa L. `Grand Rapids') plants maintained under the LED irradiation system at a total PPF of 325 μmol·s-1·m-2 for 21 days was equivalent to that reported in the literature for plants grown for the same time under cool-white fluorescent and incandescent radiation sources. Characteristics of the plants, such as leaf shape, color, and texture, were not different from those found with plants grown under cool-white fluorescent lamps. Estimations of the electrical energy conversion efficiency of a LED system for plant irradiation suggest that it may be as much as twice that published for fluorescent systems.