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Weixing Cao and Theodore W. Tibbitts

Plants of the potato (Solanum tuberosum L.) cultivars Denali, Norland, Haig, and Kennebec were grown for 42 days under three temperature cycling periods (thermoperiods) with continuous irradiation in two repeated experiments to help determine if temperature cycling might be varied to optimize tuber development of potatoes in controlled environments. Thermoperiods of 6/6 hours, 12/12 hours and 24/24 hours were established with the same temperature change of 22/14C and same controlled vapor pressure deficit of 0.60 kPa. The thermoperiod of 24/24 hours significantly promoted tuber initiation but slowed tuber enlargement in all four cultivars, compared to the thermoperiods of 6/6 hours and 12/12 hours. `Denali' produced the highest tuber and total dry weights under the 6/6 hours thermoperiod. `Kennebec' produced the highest tuber dry weight under the 12/12 hours thermoperiod. Thermoperiods had no significant effect on shoot and root dry weights of any cultivars. The major effect of thermoperiod was on initiation and enlargement of tubers.

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Weixing Cao and Theodore W. Tibbitts

A system maintaining continuous water tension on a nutrient solution has been developed to control root zone moisture levels for plants in microgravity conditions. This study was conducted in a growth chamber to characterize potato (Solanum tuberosum L.) responses to constant water tension compared to plants grown with no tension using a free-water technique. In three separate experiments, plants were grown in trays filled with a 4-cm layer of 1-mm-diameter isolite (porous ceramic) particles. Ten porous stainless-steel tubes, 4 cm apart, were buried in the medium, and nutrient solution was drawn through the porous tubes under a constant water tension of -0.5 kPa maintained with a siphon system. For the free-water treatment, trays were slanted, and solution was supplied along the upper end of trays, passed under the medium, and then collected at the lower end and recirculated. The same nutrient solution was recirculated through both treatments at a flow rate of 150 ml·min-1 through each tray and maintained at a pH of 5.6. Uniform micropropagated `Norland' potato plantlets were transplanted into replicate trays and maintained at 18C, 70% relative humidity, and a continuous photosynthetic photon flux (PPF) of 450 μmol·m-2·s-1. Water tension reduced total plant dry weight, leaf area, leaf number, and stolon number by >75%, but specific leaf weight increased compared to free water. However, tubers enlarged more rapidly with water tension, and plants consistently partitioned a greater fraction of biomass into tubers (than into shoots). Tuber weight was greater with water tension than in the free-water culture in Expt. 1 harvested 37 days after transplanting, however it was less in Expt. 2 when plants were grown to a larger size for 52 days before harvest. Leaf CO2 assimilation rate, stomatal conductance, and transpiration were reduced with water tension, although the relative water content of leaves was not significantly affected. Also, with water tension, concentrations of N, P, Zn, and Cu in leaf tissues decreased, whereas tissue Fe increased compared to plants grown with free water. The results in this study demonstrate that constant water tension significantly affects potato plant growth and shifts biomass partitioning toward tubers.

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Weixing Cao and Theodore W. Tibbitts

This study determined the responses of potato (Solanum tuberosum L., cv. Norland) plants to various patterns of air temperature changes over different growth periods (phasic temperature changes). In each of two experiments under controlled environments, eight treatments of temperature changes were carried out in two growth rooms maintained at 17 and 22C and a constant vapor pressure deficit of 0.60 kPa and 14-hour photoperiod. Plants were grown for 63 days after transplanting of tissue culture plantlets in 20-liter pots containing peat-vermiculite mix. Temperature changes were imposed on days 21 and 42, which were essentially at the beginning of tuber initiation and tuber enlargement, respectively, for this cultivar. Plants were moved between two temperature rooms to obtain eight temperature change patterns: 17-17-17, 17-17-22, 17-22-17, 22-17-17, 17-22-22, 22-17-22, 22-22-17, and 22-22-22C over three 21-day growth periods. At harvest on day 63, total plant dry weight was higher for the treatments beginning with 22C than for those beginning with 17C, with highest biomass obtained at 22-22-17 and 22-17-17C. Shoot dry weight increased with temperature increases from 17-17-17 to 22-22-22C during the three growth periods. Tuber dry weight was highest with 22-17-17C, and lowest with 17-17-22 and 17-22-22C. With 22-17-17C, both dry weights of stolons and roots were lowest. Total tuber number and number of small tubers (<2.5 cm) were highest with 17-17-17 and 17-17-22C, and lowest with 17-22-22 and 22-22-22C, whereas number of medium tubers (2.5-5.0 cm) was highest with 22-17-22C, and number of large tubers (>5.0 cm) was highest with 22-17-17C. This study indicates that tuber development of potatoes is optimized with a phasic pattern of high temperature during early growth and low temperature during later growth.

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Weixing Cao and Theodore W. Tibbitts

The physiological responses of four potato (Solanum tuberosum L.) cultivars to continuous irradiation were `determined in a controlled environment. Under a constant 18C and a constant photoperiod of 470 μmol·s-1·m-2 of photosynthetic photon flux, `Denali' and `Haig' grew well and produced large plant and tuber dry weights when harvested 56 days after transplanting. `Kennebec' and `Superior' were severely stunted, producing only 10% of the plant dry matter produced by `Denali' and `Haig'. The differences in leaf chlorophyll concentration and stomatal conductance were not consistent between these two groups of cultivars. The leaf net CO2 assimilation rates in `Kennebec' and `Superior' were lower, and intercellular CO2 partial pressures were higher than in `Denali' and `Haig'. These results indicate that inhibition of net CO2 assimilation in `Kennebec' and `Superior' was not due to a limiting amount of chlorophyll or to CO2 in the leaf tissues. Concentrations of starch in leaflets of `Kennebec' and `Superior' plants were only 10% of those in `Denali' and `Haig' plants, although soluble sugar concentrations were similar in the four cultivars. Therefore, the lower net CO2 assimilation rates in stunted `Kennebec' and `Superior' plants were not associated with an excess carbohydrate accumulation in the leaves.

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Waylen Y. Wan, Weixing Cao and Theodore W. Tibbitts

Because tuberization in potatoes (Solarium tuberosum L.) reportedly is inhibited when stolons are immersed in liquid, this study was conducted to determine the effect of intermittent pH reductions of the nutrient solution on tuber induction of potatoes in solution culture. Tissue-culture potato plantlets were transplanted into solutions maintained at pH 5.5. The pH of the nutrient solution was changed to 3.5 and 4.0 for 10 hours on each of three dates (30, 35, and 40 days after transplanting). For the pH 3.5 treatment, tubers were observed first on day 42 and averaged 140 tubers per plant at harvest on day 54. For the pH 4.0 treatment, tubers were observed first on day 48 and averaged 40 tubers per plant at harvest. At a constant pH 5.5, tubers were observed on day 52 and averaged two tubers per plant at harvest. Plants with the intermittent pH 3.5 had smaller shoots and roots with shorter and thicker stolons compared to constant pH 5.5. With the intermittent pH 4.0, plants were of similar size, but stolons were shorter and slightly thickener compared to those from pH 5.5. Mineral composition of leaf tissues at harvest was similar for the three pH treatments. These results indicate that regulation of solution pH can be a useful technique for inducing tuberization in potatoes.