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Martin P.N. Gent, Zakia D. Parrish, and Jason C. White

Exudation of organic acids by roots has been implicated in uptake of minerals from soil. Three cultivars within each of two subspecies of summer squash (Cucurbita pepo ssp. ovifera D. S. Decker var. ovifera and C. pepo ssp. pepo var. pepo) were grown in the field. Plants of ssp. pepo had higher concentrations of K, P, and Zn than those of ssp. ovifera. These same cultivars were grown under P sufficient and depleted conditions in hydroponics, to measure exudation of organic acids from roots. When grown in hydroponics, tissues of ssp. ovifera had similar or higher concentrations of nutrients than ssp. pepo. Therefore, differences in tissue composition of field-grown plants are likely due to differences in nutrient uptake ability, not inherent differences in tissue composition between subspecies. Phosphorus nutrition played a significant role in exudation of organic acids into the hydroponics solution. For both subspecies, P depletion resulted in exudation of more citric and succinic acid, and less oxalic and tartaric acid. Under P depletion, ssp. pepo exuded more citric acid than ssp. ovifera. When soil was eluted with solution containing root exudates, the exudates from ssp. pepo eluted more K, Mg, Fe, and Zn than did those from ssp. ovifera. Among subspecies of C. pepo, exudation of organic acids, particularly exudation of citric acid in response to P depletion, is associated with the plant's ability to accumulate more inorganic nutrients when grown in the field.

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Uttara Samarakoon, Jack Palmer, Peter Ling, and James Altland

.05 in all tanks using either phosphoric acid (pH Down; General Hydroponics, Sebastopol, CA) or potassium hydroxide and potassium carbonate (pH Up, General Hydroponics). The average preadjusted pH was 5.86 ± 0.02 for all stock tanks. Expt. 2: Effect of pH

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Mary A. Rogers

and ecology, it becomes clear that systems of crop production that eliminate soil from the system, such as hydroponics or aeroponics, cannot be considered as examples of acceptable organic farming practices” ( USDA, 2010 ). The implication is that

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Youbin Zheng, Linping Wang, Diane Feliciano Cayanan, and Mike Dixon

. 1997 Differential tolerance to copper and zinc of micropropagated birches tested in hydroponics New Phytol. 137 543 549 Zheng, Y. Wang, L. Dixon, M. 2004 Response to copper toxicity for several

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Yuan Li, Joseph Heckman, Andrew Wyenandt, Neil Mattson, Edward Durner, and A.J. Both

-specific nutrient solutions and constant aeration using an air pump (GH2716; General Hydroponics, Santa Rosa, CA) with air stones. Each hydroponic system was able to hold 64 (8 × 8) plants. The deep-flow hydroponic systems were arranged in a double

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Sergio Jiménez, Jorge Pinochet, Anunciación Abadía, María Ángeles Moreno, and Yolanda Gogorcena

hydroponics showed that root iron reducing capacities were highly negatively correlated with visual chlorosis scores from field trials ( Ellsworth et al., 1997 ) and provide a better screening ability than H + ion release ( Ellsworth et al., 1998 ). Moreover

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Rebecca L. Darnell and Gary W. Stutte

Strawberries (Fragaria xananassa Duch. .Osogrande.) were grown hydroponically with three NO3-N concentrations (3.75, 7.5, or 15.0 mM) to determine effects of varying concentration on NO3-N uptake and reduction rates, and to relate these processes to growth and fruit yield. Plants were grown for 32 weeks, and NO3-N uptake and nitrate reductase (NR) activities in roots and shoots were measured during vegetative and reproductive growth. In general, NO3-N uptake rates increased as NO3-N concentration in the hydroponics system increased. Tissue NO3-. concentration also increased as external NO3-N concentration increased, reflecting the differences in uptake rates. There was no effect of external NO3-N concentration on NR activities in leaves or roots during either stage of development. Leaf NR activity averaged ~360 nmol NO2 formed/g fresh weight (FW)/h over both developmental stages, while NR activity in roots was much lower, averaging ~115 nmol NO2 formed/g FW/h. Vegetative organ FW, dry weight (DW), and total fruit yield were unaffected by NO3-N concentration. These data suggest that the inability of strawberry to increase growth and fruit yield in response to increasing NO3-N concentrations is not due to limitations in NO3-N uptake rates, but rather to limitations in NO3 - reduction and/or assimilation in both roots and leaves.

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N.W. Osorio, X. Shuai, S. Miyasaka, B. Wang, R.L. Shirey, and W.J. Wigmore

Nitrogen (N) is often the most limiting mineral nutrient for taro growth. Two experiments were carried out under hydroponics conditions to determine the effects of varying solution N levels and N form on taro (Colocasia esculenta L. Schott cv. Bun Long) growth and foliar nutrient concentrations for 42 days. In the first experiment, taro plants were grown at six NH4NO3 levels (0, 0.25, 0.5, 1.0, 2.0, and 4.0 mm N). In the second experiment, taro plants were grown at a total N level of 3 mm with five nitrate (NO3-): ammonium (NH4+) percent molar ratios (100:0, 75:25, 50:50, 25:75, and 0:100). In the N level experiment, dry matter and leaf area increased up to 2 mm N and then decreased at the highest N level. The reduced growth of taro at the highest N level was attributed in part to a high NH4+ level that reduced uptake or translocation of cations, such as Ca2+, Mg2+, and Mn2+. Nitrogen concentration in leaf blades increased with increasing N levels. The critical foliar N concentration that coincided with 95% of maximum growth based on a quadratic model was 40.4 g·kg-1 (dry weight basis). In the N form experiment, NO3-: NH4+ ratios of 75:25 or 100:0 favored greater plant growth compared to other treatments. Taro plants grown in NH4+-rich solutions drastically acidified the solution pH, and had retarded growth and smaller leaf area compared to those grown in NO3--rich solutions.

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Elizabeth A. Wahle and John B. Masiunas

Greenhouse hydroponics and field experiments were conducted to determine how nitrogen (N) fertilizer treatments affect tomato (Lycopersicon esculentum Mill.) growth, yield, and partitioning of N in an effort to develop more sustainable fertilization strategies. In a hydroponics study, after 4 weeks in nitrate treatments, shoot dry weight was five times greater at 10.0 than at 0.2 mm nitrate. An exponential growth model was strongly correlated with tomato root growth at all but 0.2 mm nitrate and shoot growth in 10 mm nitrate. Root dry weight was only 15% of shoot biomass. In field studies with different population densities and N rates, height in the 4.2 plants/m2 was similar, but shoot weight was less than in the 3.2 plants/m2. At 12 weeks after planting, shoot fresh weight averaged 3.59 and 2.67 kg/plant in treatments with 3.2 and 4.2 plants/m2, respectively. In 1998, final tomato yield did not respond to N rate. In 1999, there was a substantial increase in fruit yield when plants were fertilized with 168 kg·ha-1 N but little change in yield with additional N. Nitrogen content of the leaves and the portion of N from applied fertilizer decreased as the plants grew, and as N was remobilized for fruit production. Both studies indicate that decreasing N as a way to reduce N loss to the environment would also reduce tomato growth.

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Toshiki Asao, Hiroaki Kitazawa, Takuya Ban, M. Habibur Rahman Pramanik, and Kenzi Tokumasa

Date, S. Terabayashi, S. Matsui, K. Namiki, T. Fujime, Y. 2002 Induction of root browning by chloramine in Lactuca sativa L. grown in hydroponics J. Jpn. Soc. Hort. Sci. 71 485 489 Feng