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Georgios Psarras and Ian A. Merwin

One-year-old potted `Mutsu' apple [Malus sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.] trees on scion invigorating Malling-Merton 111 (MM.111) and scion dwarfing Malling 9 (M.9) rootstocks were grown outdoors in containers under three levels of water availability (irrigated at -20, -80, and -200 kPa) to investigate the effects of soil water availability on combined soil/root (rhizosphere) respiration rates, and developmental morphology of root systems. Rhizosphere respiration was measured with a portable infrared gas analyzer, and root biomass was estimated by electrical capacitance. These nondestructive measurements were compared with final root dry weights of harvested trees, to determine their reliability for estimating relative differences in root biomass. Water stress reduced final biomass similarly for both rootstocks, but the relative reduction in shoot growth was greater for MM.111. Root to shoot ratios were higher and average specific root respiration was lower for M.9 rootstock compared with MM.111. M.9 appeared to be more tolerant of water stress then MM.111, due to reduced canopy transpiration relative to root system mass. Water stress increased root to shoot ratios, specific root length, and the carbohydrate costs of root maintenance as indicated by specific respiration rates. Root dry weight (DW) was better correlated to rhizosphere respiration than to root electric capacitance. The observed r 2 values between root capacitance and root DW were as high as 0.73, but capacitance measurements were also influenced by soil water content and rootstock type. Electrical capacitance estimated total root biomass more accurately for M.9 than for MM.111.

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Terence R. Bates, Richard M. Dunst, Theodore Taft, and Michael Vercant

One- and 2-year-old 'Concord' (Vitis labruscana L.) grapevines were used to study the effect of soil pH on vegetative growth and nutrition. Ninety-eight, own-rooted, 'Concord' grapevines were planted in 94.6-L pots containing vineyard soil adjusted to seven soil pH levels ranging from 3.5 to 7.5. After the first growing season, seven vines from each soil pH treatment were randomly selected, destructively harvested, and measured for root and shoot growth. The remaining 49 vines over-wintered in the pots, were defruited in year two, and were destructively harvested at the end of the second growing season. There was a reduction in root biomass below soil pH of 4.5 and a greater reduction in shoot biomass leading to a higher root: shoot ratio. There were no significant differences in vegetative growth of young 'Concord' vines from a soil pH of 5.0-7.5. However, there was a trend toward lower shoot biomass and higher root: shoot ratio at the highest soil pH level. Phylloxera nodosities on roots were present in equal densities at all soil pH values. However, the negative impact of phylloxera on vine dry mass was greater on vines under nutrient stress at the highest and lowest pH treatments than on those with adequate nutrition at the mid-range soil pH values.

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D.M. Glenn and W.V. Welker

Our objectives in this study were to measure the effects of low levels of root system carbon dioxide on peach tree growth (Prunus persica L. Batsch) and nutrient uptake. Using soil and hydroponic systems, we found that increased root CO2: 1) increased root growth without increasing shoot growth, 2) increased leaf P concentration, 3) decreased leaf N concentration, and 4) reduced water use relative to air injection or no treatment.

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Monica L. Elliott and Timothy K. Broschat

A commercially available microbial inoculant (Plant Growth Activator Plus) that contains 50 microorganisms, primarily bacteria, was evaluated in a soilless container substrate to determine its effects on root bacterial populations and growth response of container-grown plants at three fertilizer rates. The tropical ornamental plants included hibiscus (Hibiscus rosa-sinensis `Double Red'), spathiphyllum (Spathiphyllum `Green Velvet') and areca palm (Dypsis lutescens). The bacterial groups enumerated were fluorescent pseudomonads, actinomycetes, heat-tolerant bacteria, and total aerobic bacteria. Analysis of the inoculant before its use determined that fluorescent pseudomonads claimed to be in the inoculant were not viable. The plant variables measured were plant color rating, shoot dry weight and root dry weight. Only hibiscus shoot dry weight and color rating increased in response to the addition of the inoculant to the substrate. Hibiscus roots also had a significant increase in the populations of fluores-cent pseudomonads and heat-tolerant bacteria. From a commercial production point of view, increasing fertilizer rates in the substrate provided a stronger response in hibiscus than did addition of the microbial inoculant. Furthermore, use of the inoculant in this substrate did not compensate for reduced fertilizer inputs.

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Peter J. Stoffella, Michele Lipucci DiPaola, Alberto Pardossi, and Franco Tognoni

Bell peppers (Capsicum annuum L. `Early California Wonder') were seeded in glass tubes on agar-based media adjusted to pH 4.1, 5.9, or 7.3 to evaluate germination, emergence, shoot growth, and root morphology for 16-day-old seedlings. Taproot lengths were measured daily from 1 to 10 days following radicle protrusion. Time from seeding to germination (radicle protrusion) differed by only one-half day among pH treatments. Peppers in a pH 5.9 medium emerged (fully expanded cotyledons) 1 day earlier than plants grown in media at pH 4.1 or 7.3. Plants grown in a pH 5.9 medium had higher shoot and root weights and longer stems than plants grown at pH 4.1 or 7.3. Shoot: root ratios were similar regardless of medium pH. However, taproot growth rate from 1 to 10 days after radicle protrusion was faster for plants grown in a pH 5.9 than in a pH 4.1 or 7.3 medium. On average, there was one basal and one lateral root per plant and they were minimally influenced by pH. The data suggest that acidic or alkaline media adversely affect early shoot and taproot development of bell peppers, but with minimal influence on time to germination or emergence, and on subsequent lateral and basal root initiation.

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Donald J. Merhaut and Rebecca L. Darnell

Nitrogen uptake and N and C partitioning were evaluated in `Sharpblue' southern highbush blueberries fertilized with different N forms. Plants were grown in acid-washed silica sand and fertilized with a modified Hoagland's solution supplemented with 5.0 mm N as NH4 + or NO3 -. Nutrient solution pH was adjusted to 3.0 and 6.5 for the NO3 - and NH4 +-treated plants, respectively. After 12 months of growth, plants were dual labeled with 14CO2 and 10% enriched 15N-N as either NaNO3 or (NH4)2SO4 and harvested 12 hours after labeling. Fertilization with NO3 --N increased leaf, stem, and root dry weights compared to NH4 + fertilization. Total 15N uptake did not differ between N fertilization treatments, thus whole plant and root 15N concentrations were greater in NH4 +-fertilized vs. NO3 --fertilized plants. Fertilization with NO3 --N increased C partitioning to new shoots compared to NH4 +-fertilized plants. However, C partitioning to other plant parts was not affected by N form. Although NO3 - uptake in blueberry appears to be restricted relative to NH4 + uptake, this limitation does not inhibit vegetative growth. Additionally, there appears to be adequate available carbohydrate to support concurrent vegetative growth and N assimilation, regardless of N form.

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Dilma Daniela Silva and Richard C. Beeson Jr.

An experimental system that allows imposition of precise irrigation treatments with easy and quick observations of unrestricted root growth of woody plants was developed. The system mimics natural deep soil percolation and facilitates rapid assessment of large root populations. It was designed to be relatively inexpensive to build so that treatments could be efficiently replicated. Designs for this star-shaped rhizotron were developed and evaluated with the goals of: 1) optimizing volume and shape for minimal physical restriction and use with mature woody plants; 2) developing a drainage system comparable to natural deep soils; and 3) facilitating ease, accuracy, and duration of data acquisition. The final design allows efficient root observation, uses a wick-type drainage system to provide a near-uniform profile of soil moisture, and is easily manageable for precise long-term data acquisition. This rhizotron has eight independent viewing/sampling windows and holds 0.16 m3 of soil. An associated lightweight and compact camera positioning frame was developed that facilitates acquisition of digital photographs of soil profiles for time-series assessment of morphological and architectural parameters.

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David Bubenheim, Kanapathipillai Wignarajah, Wade Berry, and Theodore Wydeven

Recycling wastewater containing soaps and detergents for plant growth is highly desirable when fresh water is limited. This is especially true during times of drought and is imperative in some specialized situations such as a regenerative space habitat. To regenerate food, water, and air, the National Aeronautics and Space Administration's Controlled Ecological Life Support System (CELSS) must recycle wastewater commonly known as gray water. The anionic surfactant Igepon is the principal ingredient of many detergent formulations and soaps and is a prime candidate for use in a space habitat. To determine if gray water would have phytotoxic effects on crops grown in a CELSS, `Waldmann's Green' lettuce (Lactuca sativa L.) was grown in nutrient solutions containing varying concentrations of Igepon TC-42. Igepon concentrations of 250 mg·L-1 or higher in nutrient solutions resulted in phytotoxic effects in lettuce. Thus, the toxic threshold of Igepon is <250 mg·L-1. Toxicity symptoms include browning of the roots within 4 hours of exposure to Igepon followed by suppression of root dry mass within 24 hours. Plant death never resulted from exposure to Igepon used in these experiments, although roots were killed. The phytotoxic effect of Igepon was not persistent; plants initially displaying acute toxicity show clear signs of recovery within 3 days of initial exposure. Further, when fresh plants were exposed to these same nutrient solutions 3 days or more following initial Igepon addition, no phytotoxic effect was observed. The elimination of the phytotoxicity was associated with a decrease in fatty acid components in the nutrient solution associated with Igepon. The degradation of phytotoxicity appears to be associated with microbes present on the surface of the roots and not directly due to any plant process or instability of the surfactant.

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Fengyun Zhao, Junli Sun, Songlin Yu, Huaifeng Liu, and Kun Yu

used for aeration without extensive increases in cost. Rhizosphere soil microorganisms are the most important and active parts of soil ecosystems ( Abbott and Murphy, 2003 ; Zhu et al., 2014 ) and reflect varieties in soil quality in a sensitive

Open access

Li Liu, Lin Jin, and Qiaosheng Guo

and evolution of soil, but also maintain the stability of the soil ecosystem. In the microecosystem of rhizosphere soil, these microorganisms also play important roles in the growth of plants. Previous work found that increasing the richness and