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held at the 92nd ASHS Annual Meeting Montreal, Quebec, Canada 31 July 1995 sponsored by the Seedling Establishment Working Group Seed Research Working Group Vegetable Crops Management Working Group Root Growth and Rhizosphere Dynamics Working Group

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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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Bernard A. L. Nicoulaud and Arnold J. Bloom

We examined root ammonium absorption by tomato seedlings (Lycopersicon esculentum Mill. `T-5') after first exposure of the roots to ammonium. Some plants received a nutrient medium containing nitrate as the sole N source. In a second treatment, the leaves were sprayed daily with a urea solution, while the roots were in N-free medium. The last two treatments were initially grown in medium that contained ammonium nitrate, but then either were shifted to a N-free medium for 10 days or had their roots excised and were rerooted in N-free medium for 21 days. Root ammonium absorption remained constant after first exposure to ammonium for the plants exposed to nitrate alone, whereas root ammonium absorption declined with time for the other three treatments. These results indicate that for tomato a) ammonium in the rhizosphere does not induce root ammonium absorption and b) some product of ammonium metabolism represses root ammonium absorption.

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Nancy W. Callan and Don E. Mathre

Biological seed treatment offers a safe, environmentally responsible option for protection of seeds and seedlings from attack by soilborne pathogens. Most effective biological seed treatments have used either bacterial or fungal agents. The efficacy of a biological seed treatment depends upon the ability of the biocontrol agent to compete and function on the seed and in the rhizosphere under diverse conditions of soil pH, nutrient level, moisture, temperature, and disease pressure. Seed treatment performance may be improved through application and formulation technology. An example of this is the bio-priming seed treatment, a combination of seed priming and inoculation with Pseudomonas aureofaciens AB254, which was originally developed for protection of sh-2 sweet corn from Pythium ultimum seed decay. Bio-priming has been evaluated for protection of seed of sweet corn and other crops under a range of soil environmental conditions.

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Clauzell Stevens, Victor A. Khan, Theresa Okoronkwo, Ah-Yin Tang, Mack A. Wilson, John Lu, and James E. Brown

Soil polarization for 98 days in 1985 resulted in a 91% reduction of weeds present in collard greens (Brassica oleracea acephafa L.) plots during 1986. Soil solarization was more effective in controlling weeds in collard green plots when compared to an application of Dacthal-75W herbicide in nonsolarized plots. Collard green plants grown in solarized soil showed an increase in yield and other growth responses. Soil samples from the rhizosphere of plants grown in solarized soil showed higher population levels of bacteria and thermotolerant fungi than from nonsolarized soil. There were significant negative responses in marketable yield and root growth of collard greens and in soil microflora in solarized soil in response to Dacthal-75W herbicide application. Chemical name used: dimethyltetrachloroterephthalate (Dacthal-75W).

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X. Fontanet, V. Estaún, A. Camprubí, and C. Calvet

Prior to the commercial use of arbuscular mycorrhiza (AM) in the nursery, the effects of commonly used pesticides on symbiosis must be evaluated. Metalaxyl and propamocarb are two fungicides added to potting substrates to prevent diseases caused by phycomycetes. Both fungicides were incorporated into the potting substrate before the inoculation and planting of the peach-almond hybrid rootstock GF677 (Prunus persica L. × Prunus dulcis Mill. D.A. Webb). The AM fungus used were Glomus mosseae (Nicol. & Gerd.) Gerdemann & Trappe and Glomus intraradices Schenck & Smith. Glomus intraradices was not affected by either fungicide; however, metalaxyl adversely affected root colonization by G. mosseae and decreased rhizosphere activity as measured by esterase activity. Chemical names used: N-(2-methoxyacetyl)-N-(2,6-xylyl)-DL-alaninate) (metalaxyl); propyl 3(dimethylamino)propylcarbamate (propamocarb).

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Leslie A. Weston

Allelopathy can be defined as an important mechanism of plant interference mediated by the addition of plant-produced secondary products to the soil rhizosphere. Allelochemicals are present in all types of plants and tissues and are released into the soil rhizosphere by a variety of mechanisms, including decomposition of residues, volatilization and root exudation. Allelochemical structures and modes of action are diverse, and may offer potential for development of future herbicides. In the past, allelopathy was described by the Romans as a process resulting in the “sickening” of the soil; in particular, chickpea (Cicer arietinum) was described as problematic when successively cropped with other species. Other early plant scientists, such as De Candolle in the 1800s, first described the ability of plant roots to produce toxic exudates. More recently, research has focused on development of weed management strategies using allelopathic crop residues, mechanism of allelochemical action, and gene regulation of allelochemical production. This paper briefly describes a variety of weed and crop species that establishes some form of potent allelopathic interference, either with other crops or weeds, in agricultural settings, in the managed landscape, or in naturalized settings. Recent research suggests that allelopathic properties can render one species more invasive to native species and thus potentially detrimental to both agricultural and naturalized settings. In contrast, allelopathic crops offer strong potential for the development of cultivars that are more highly weed suppressive in managed settings. A new challenge that exists for plant scientists is to generate additional information on allelochemical mechanisms of release, selectivity and persistence, mode of action, and genetic regulation. Armed with this specific information, we can further protect plant biodiversity and enhance weed management strategies in a variety of ecosystems.

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Joseph P. Albano and William B. Miller

Iron chelate photodegradation is a problem in tissue culture where limited soluble Fe in agar reduces callus tissue growth. Our objectives were to determine if Fe chelate photodegradation occurs in commercial fertilizers used in greenhouse plant production and, if so, the effects on plant Fe acquisition. Commercial 20N–10P–20K soluble fertilizers containing Fe-EDTA were prepared as 100x stocks based on a 100 mg N/liter (1x) concentration. A modified Hoagland's solution with Fe-DTPA was prepared as a 10x stock based on a 200 mg N/liter (1x) concentration. Samples then were kept in darkness or were irradiated with 500 μmol·m–2·s–1 from fluorescent and incandescent sources for ≤240 hours. Soluble Fe in the irradiated commercial fertilizer solutions decreased 85% in 240 h. Soluble Fe in the Hoagland's solution, prepared in the lab, decreased 97% in 72 h. There was no loss in soluble Fe in any dark-stored treatment; demonstrating photodegradation of Fe-chelates under commercial settings. Excised roots of marigold (Tagetes erecta L.), grown hydroponically in the irradiated solutions, had Fe(III)-DTPA reductase activity 2 to 6 times greater than roots of plants grown in solutions kept in darkness. Plants growing in irradiated solutions acidified the rhizosphere more than plants growing in solutions kept dark. The increase in Fe reductase activity and rhizosphere acidification are Fe-efficiency reactions of marigold responding to the photodegradation of Fe-chelates and subsequent decrease in soluble Fe in both commercial fertilizers and lab-prepared nutrient solution.

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Working Group Postharvest Working Group Root Growth and Rhizosphere Dynamics Working Group Water Utilization and Management Working Group Mineral Nutrition Working Group Plant Dormancy Working Group published by the American Society for Horticultural

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Dharmalingam S. Pitchay*, Jonathan M. Frantz, and James C. Locke

Currently, formulation of inorganic fertilizers is based on cation amounts such as NH4, K, Mg, Ca, Fe, MN Cu, and Zn, whereas anion species and amounts are viewed, with few exceptions, as necessary fillers. The delivery of cations in the nutrient solution is associated with an anion such as Cl, SO4, NO3, PO4 or CO3. These anions at higher concentrations can result in different growth responses by altering the rhizosphere pH, soluble salts, and influencing the uptake of both cations and anions. The impact of these anions has not been extensively studied in the formulation of inorganic fertilizers. Several experiments assessed the effect of SO4 and Cl on root and shoot growth and development of bedding plants represented by petunia, impatiens, and vinca. In all treatments, plant height, shoot and root dry weight, and flower number decreased with an increase in Cl concentration. Root morphology was marked by fewer total roots and shorter primary and secondary roots when grown with Cl anions compared to the plants grown with SO4 anions. This indicates that anions have a larger role in determining optimum fertilizer formulation than previously believed. This information provides an additional tool in formulating fertilizers for greenhouse bedding plant production.