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Eva Bacaicoa and Jose María García-Mina

629 Bienfait, H.F. De Weger, L.A. Kramer, D. 1987 Control of the development of iron-efficiency reactions in potato as a response to iron deficiency is located in the roots Plant Physiol. 83 244 247

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

on leaf SPAD chlorophyll content, physiologically “active” leaf iron at flowering stage, and pod yield. Genotypic differences in iron efficiency has not been studied in calibrachoa, which often shows iron deficiency symptoms at high substrate pH or

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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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Joseph P. Albano and William B. Miller

Excised roots of `First Lady' marigold (Tagetes erecta L.) grown in an aerated 0 Fe nutrient solution had Fe(III)-DTPA reductase activity 14-fold greater, and an enhanced ability to acidify the rhizosphere than plants grown in a solution containing 0.018 mm (1 ppm) Fe-DTPA. Reductase activity and rhizosphere acidification of plants grown in 0.018 and 0.09 mm Fe-DTPA were similar. Manganese concentration in leaves of plants grown in the 0 Fe treatment was 2-fold greater than in leaves of plants grown in the 0.018 mm Fe-DTPA treatment. These results indicated that `First Lady' marigold is an Fe-efficient plant that possesses both an inducible or adaptive reductase system and the ability to acidify the rhizosphere, and that these Fe-efficiency reactions do not occur when Fe is sufficient. Chemical name used: ferric diethylenetriaminepentaacetic acid, monosodium salt (Fe-DTPA).

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Cameron Rees and James Robbins

The iron-efficiency of pin oak (Quercus palustris) and red oak (Quercus rubra) grown in a static solution culture system was evaluated. Treatments included nutrient solutions with no iron, an unavailable iron form (Fe2 O3), and an available iron form (FeEDDHA), each adjusted to a starting pH of 5.5 or 7.0. Both oaks grew better when the available form of iron was used than when the solution contained unavailable or no iron. There was no difference in the height or leaf color for plants of either species when grown with unavailable or no iron. Red oak grown with an available iron form significantly lowered the pH of the solution prior to a growth flush. A similar drop in solution pH was not observed for pin oak growing under similar conditions.

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Cameron Rees and James Robbins

The iron-efficiency of pin oak (Quercus palustris) and red oak (Quercus rubra) grown in a static solution culture system was evaluated. Treatments included nutrient solutions with no iron, an unavailable iron form (Fe2 O3), and an available iron form (FeEDDHA), each adjusted to a starting pH of 5.5 or 7.0. Both oaks grew better when the available form of iron was used than when the solution contained unavailable or no iron. There was no difference in the height or leaf color for plants of either species when grown with unavailable or no iron. Red oak grown with an available iron form significantly lowered the pH of the solution prior to a growth flush. A similar drop in solution pH was not observed for pin oak growing under similar conditions.

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Andrew I. Samuelsen, Sumontip Bunnag, Ruth C. Martin, David W. S. Mok, and Machteld C. Mok

Two approaches are used to improve iron utilization of fruit trees under iron-limiting conditions: 1) selection of somaclonal variants; and 2) cloning and incorporation of genes encoding Fe(III) reductases. Two somaclonal variants of quince with tolerance to low iron availability have been selected from 2000+ shoots regenerated from leaf discs. In greenhouse tests, under iron stress conditions, the variant clones continued to show improved iron utilization, having significantly higher chlorophyll concentrations in the new leaves when compared to the quince control. Cloning of Fe(III) reductase genes is based on mutant rescue utilizing a yeast mutant deficient in Fe(III) reduction (Dancis et al., PNAS 89:3869, 1992). A shuttle vector is used to incorporate a tomato root cDNA library into the yeast mutant. Complementing cDNAs, restoring growth to wild-type levels, are selected for further analyses. Cloning of genes involved in iron utilization will eventually lead to transgenic plants with improved iron efficiency.

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Joseph P. Albano and William B. Miller

Marigolds under iron deficiency stress exhibited characteristics associated with iron efficiency (e.g. induced reductase and rhizosphere acidification). Ferric reduction rates for roots of the minus Fe-DTPA treatment group was 0.97 μmol·g FW-1·h-1, 14 times greater than the 17.9 μM Fe-DTPA treatment group. Excised primary lateral roots from the minus Fe-DTPA and 17.9 μM Fe-DTPA treatment groups embedded in an Fe reductase activity gel visually confirmed an increased Fe reduction rate for the minus Fe-DTPA treatment group. The pH of the nutrient solution one week after initiation of treatments indicated that the minus Fe-DTPA treatment group was 1 pH unit lower than the 17.9 μM Fe-DTPA treatment group at 4.1 and 5.1, respectively.

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

decrease substrate-pH, possibly acidifying the root zone as an iron-efficiency mechanism ( Marschner, 2012 ). Materials and Methods The experiment was a factorial randomized complete block design with 15 species and three fertilizer NH 4 + :NO 3 − nitrogen

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

stable substrate pH over time. This is particularly challenging given the range in iron efficiency of floriculture crop species, which affect both tendencies to either raise or lower pH and susceptibility to either iron deficiency at high pH or iron