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Umpika Poonnachit and Rebecca L. Darnell

Vaccinium corymbosum, one of the cultivated blueberry species, is not well-adapted to mineral soils, which are generally marked by high pH, the predominance of NO3-N over NH4-N, and limited iron availability. A wild species, V. arboreum, grows naturally on mineral soils, and thus may be better adapted than V. corymbosum. This adaptation may be related to the ability of V. arboreum to assimilate NO3 and/or iron more efficiently than V. corymbosum. Both species were grown in a hydroponic solution containing 5.0 mM N as (NH4)2SO4 or NaNO3, and buffered to pH 5.5. Nitrate reductase (NR) and iron reductase (FeR) activities were measured. NR activity was higher in V. arboreum compared with V. corymbosum when grown with N03-N, while no difference between species was observed when grown under NH4-N. Activity of FeR was higher in V. arboreum compared with V. corymbosum, and higher under NO3-N compared with NH4-N. After 5 months in hydroponics, Fe was removed from one-half of the solutions. The activity of NRA in both species was higher under Fe-sufficient compared with Fe-limited conditions, but in both cases, activity was higher in V. arboreum compared with V. corymbosum. FeR activity continued to be higher in V. arboreum compared with V. corymbosum, and under NO3 compared with NH4-N. Activity decreased in both species under limited Fe conditions, and there were no interactions between species and Fe. These data indicate that V. arboreum possesses higher NR and FeR activities than V. corymbosum, under both Fe-sufficient and Fe-limited conditions. This may play a role in the better adaptability of V arboreum to mineral soil conditions.

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Thomas E. Marler, Ruben dela Cruz, and Andrea L. Blas

Four papaya (Carica papaya L.) cultivars were cultured aeroponically or in perlite to determine the magnitude, timing, and root locality of Fe reductase induced by Fe deficiency. Five soybean [Glycine max (L.) Merrill] lines with a known range of Fe-deficiency chlorosis scores were cultured in perlite for comparison. Speed of inducement of Fe reductase activity was determined in plants cultured without Fe for 0 to 17 days. Location of Fe reductase activity was determined by sectioning roots from the tip to 60 to 70 mm proximal to the root tip from plants cultured without Fe for 16 to 19 days. The Fe reductase system was induced in all papaya cultivars after 7 to 11 days without Fe, and activity increased through 17 days. Iron reductase activity in all papaya cultivars was comparable to the most tolerant soybean line. The zone of highest activity was the apical 10 mm of roots. These results indicate that papaya roots are highly efficient in induced Fe reductase activity. The highest activity in root tips underscores the importance of maintaining a healthy, continually growing root system with numerous growing points when culturing papaya in alkaline substrates.

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Luis Alonso Valdez-Aguilar and David William Reed

Response to alkalinity was evaluated in two hibiscus cultivars, Bimini Breeze and Carolina Breeze, grown in a soilless growing medium and in hydroponic culture. For soilless growing medium, plants were potted in a sphagnum peat–perlite-based substrate and irrigated with solutions containing 0 to 10 mm NaHCO3 for 12 weeks. In hydroponic culture, bare-rooted plants were transferred to a 9-L tray containing a Hoagland's nutrient solution prepared with NaHCO3 at the concentrations previously indicated. In soilless growing medium, shoot dry weight was minimally affected by NaHCO3 concentration for `Bimini Breeze', but `Carolina Breeze' exhibited a significant decrease in shoot mass with increasing NaHCO3 concentration. In hydroponic culture, increasing concentration of NaHCO3 induced a decrease in shoot and root mass in both cultivars, but root mass decrease was more pronounced in `Bimini Breeze'. In soilless growing medium, increasing the concentration of NaHCO3 caused an increase in growing medium pH. The pH increase was less pronounced for `Bimini Breeze' than for `Carolina Breeze', indicating a higher capacity for root zone acidification by `Bimini Breeze'. Newly developed leaves of both cultivars showed increasing chlorosis with increasing NaHCO3 concentration. However, `Bimini Breeze' was more tolerant because, according to regression models, 5.7 mm NaHCO3 would be required to reduce chlorophyll levels by 10%, compared with 2.2 mm for `Carolina Breeze', when grown in soilless medium. Fe reductase activity decreased when `Carolina Breeze' plants were grown in 5 mm NaHCO3. However, in `Bimini Breeze', Fe reductase activity was enhanced. These observations indicate that the increased tolerance of `Bimini Breeze' to increasing alkalinity is the result of enhanced Fe reductase activity and increased acidification of the root zone.

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Andrew D. Cartmill, Fred T. Davies Jr., Alejandro Alarcon, and Luis A. Valdez-Aguilar

Sustainable horticultural production will increasingly have to rely on economically feasible and environmentally sound solutions to problems associated with high levels of bicarbonate (HCO - 3) and associated high pH in irrigation water. The ability of arbuscular mycorrhizal fungi (AMF; GlomusZAC-19) to enhance plant tolerance to HCO3 - was tested on the growth, physiology and nutrient uptake of Rosamultiflora Thunb. ex J. Murr. cv. Burr (rose). Arbuscular mycorrhizal colonized and noninoculated (non-AMF) plants were treated with 0, 2.5, 5, and 10 mm HCO - 3. Increasing HCO - 3 concentration and associated high pH and electrical conductivity (EC) reduced plant growth, leaf elemental uptake and acid phosphatase activity (ACP), while increasing alkaline phosphatase activity (ALP). Inoculation with AMF enhanced plant tolerance to HCO - 3 as indicated by greater plant growth, leaf elemental uptake (N, P, K, Ca, Fe, Zn, Al, Bo), leaf chlorophyll content, higher mycorrhizal inoculation effect (MIE), lower root iron reductase activity, and generally lower wall-bound ACP (at 2.5 mm HCO3 -), and higher soluble ALP (at 10 mm HCO3 -). While AMF colonization (arbuscules, vesicles, and hyphae formation) was reduced by increasing HCO - 3 concentration, colonization still occurred at high HCO - 3. At 2.5 mm HCO3 -, AMF plant growth was comparable to plants at 0 mm HCO3 -, further indicating the beneficial effect of AMF for alleviation of HCO3 - stress.

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Ryan W. Dickson, Paul R. Fisher, and William R. Argo

greater ability to reduce iron at root surfaces ( Bienfait, 1988 ; Marschner, 2012 ). Albano and Miller (1996) found that marigold responded to low substrate-iron concentration by acidifying the root zone and increasing iron reductase activity, which

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Ryan W. Dickson, Paul R. Fisher, Sonali R. Padhye, and William R. Argo

). Other iron-efficiency mechanisms were not measured in this experiment, such as physiologically “active” iron levels in shoot tissue, root iron reductase activity, and root efflux of organic compounds which solubilize iron ( Albano and Miller, 1996

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

, photosynthetic gas exchange, chlorophyll fluorescence and antioxidant defense in two peach rootstocks differing in Fe deficiency tolerance J. Plant Physiol. 163 176 185 Moog, P.R. Brüggemann, W. 1994 Iron reductase systems

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Wan-Yi Yen, Yao-Chien Alex Chang, and Yin-Tung Wang

that the root releases, it also secretes a H + , resulting in pH decline in the rhizosphere ( Kraffczyk et al., 1984 ). Some dicots, under low iron (Fe) conditions, release H + to the rhizosphere to activate the iron reductase on the cell membrane to

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Paul R. Fisher, William R. Argo, and John A. Biernbaum

balance, acid exudation by proton pumps, and iron reductase activity ( Albano and Miller, 1996 ; Marschner, 1995 ; Taylor et al., 2008 ). Literature Cited Albano, J.P. Miller, W.B. 1996 Iron deficiency stress influences physiology of iron acquisition in

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Charalambos I. Siminis and Manolis N. Stavrakakis

mesophyll cells. Two possible explanations are raised as a consequence of the results. The first is that Fe is required as a constituent of the reducing enzymes, because ferric iron reductase is a flavocytochrome ( Robinson et al., 1999 ). The second is that