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Amy J. Compton and Paul V. Nelson

Many plug seedling growers complain about the inadequacy of substrate testing as a measure of nutritional status because results are too variable. We conducted two experiments to test a model system of sampling substrate at a set time after fertilization. Petunias (Petunia×hybrida Hort. ex Vilm. var. multiflora `Primetime White') were grown in 288-cell plug trays. Six fertilizer regimes were used consisting of a factorial arrangement of three fertilizer cycles (at each, every other, and every third irrigation) and two leaching fractions (0% and 20%). Fertilizer or water was applied at 0900 HR daily, and then 24 hours later in Expt. 1, and 1 hour later in Expt. 2, substrate solutions were sampled and analyzed. Samples taken after waterings were used to assess the dilution and leaching effects of water on substrate nutrient concentrations. In Expt. 2, additional substrate samples were taken at various hours after fertilizing to test the effect of plant depletion of the substrate. Substrate nutrient concentration curves constructed from data drawn at a fixed time after fertilizations, but not after waterings, were logical and could be interpreted. When data from a fixed time after fertilizations and waterings were plotted together, the curves could not be interpreted. Data from samples taken at various hours after fertilization in Expt. 2 revealed large reductions in concentrations, often after only 4 hours. Overall, leaching and dilution effects from watering in combination with the increased time span from fertilizing to sampling resulted in nutrient concentrations that could not be interpreted. Substrate testing can be effective for plug seedling production, but samples need to be taken 1 to 2 hours after fertilizations.

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Carl Rosen, Wenshan Wang, and David Birong

A 2-year field study was conducted on a low- to medium-K testing sandy soil 1) to evaluate the effects of various K management strategies on potato (cv. Russet Burbank) yield and quality and 2) to calibrate a petiole sap test for determining plant K status. Treatments included banded applications of potassium chloride fertilizer at planting with K ranging from 0 to 300 kg·ha–1 in 75 kg·ha–1 increments. Comparisons of preplant broadcast + banded applications and evaluation of in-season applications of potassium nitrate also were made. In both years, tuber yield increased with increasing banded K fertilizer up to 150 kg ha-1 K the first year and 225 kg ha-1 K the second year. In-season applications of potassium nitrate increased tissue K levels, but at equivalent K application rates, timing of K application had no effect on yield. Petiole K concentrations, measured on a dry weight and sap basis, increased with increasing K fertilizer application. Potassium concentrations in nondiluted sap determined with the Cardy K electrode were ≈200 to 2500 ppm lower than those determined by flame emission. The greatest discrepancy occurred at the higher K sap concentrations. Potassium concentrations determined with the Cardy electrode in sap diluted with aluminum sulfate or deionized water were much closer to those determined by flame emission. These results suggest that dilution of the sap is necessary to obtain accurate K concentrations in petiole sap.

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Bruce Schaffer, Anthony W. Whiley, Christopher Searle, and Robert J. Nissen

The effects of atmospheric CO2 enrichment and root restriction on net CO2 assimilation (A), dry mass partitioning, and leaf mineral element concentrations in `Kensington' and `Tommy Atkins' mango (Mangifera indica L.) were investigated. Trees were grown in controlled-environment glasshouse rooms at ambient CO2 concentrations of 350 or 700 μmol·mol-1. At each CO2 concentration, trees were grown in 8-L containers, which restricted root growth, or grown aeroponically in 200-L root mist chambers, which did not restrict root growth. Trees grown in 350 μmol·mol-1 CO2 were more efficient at assimilating CO2 than trees grown in 700 μmol·mol-1 CO2. However, total plant and organ dry mass was generally higher for plants grown at 700 μmol·mol-1 CO2 due to increased A as a result of a greater internal partial pressure of CO2 (Ci) in leaves of plants in the CO2 enriched environment. Root restriction reduced A resulting in decreased organ and plant dry mass. In root-restricted plants, reduced A and dry matter accumulation offset the increases in these variables resulting from atmospheric CO2 enrichment. Atmospheric CO2 enrichment and root restriction did not affect dry mass partitioning. Leaf mineral element concentrations were generally lower for trees grown at the higher ambient CO2 concentration, presumably due to a dilution effect from an increased growth rate.

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J.B. Million, J.E. Barrett, T.A. Nell, and D.G. Clark

Experiments were conducted with four kinds of flowering plants to compare one-time vs. continuous application of paclobutrazol in subirrigation water. When a crop reached the stage at which it required growth regulator treatment, four concentrations of paclobutrazol were applied via subirrigation either one-time or continuously until the crop was terminated. Based upon regression equations, concentrations resulting in 30% size reduction for one-time applications of paclobutrazol were 0.01 mg·L-1 for Begonia ×semperflorens-cultorum `Cocktail Gin', 0.09 mg·L-1 for Impatiens wallerana Hook. `Super Elfin White', 0.2 mg·L-1 for Dendranthema ×grandiflorum (Ramat.) Kitamura `Tara', and 2.4 mg·L-1 for Petunia ×hybrida Vilm.-Andr. `Plum Crazy'. Respective optimal values for continuous application were 0.005, 0.02, 0.06, and 0.4 mg·L-1. Increasing the concentration for continuous application had a greater effect on paclobutrazol efficacy than did increasing the concentration for a single application. In a trial with impatiens `Super Elfin Salmon Blush', the paclobutrazol concentration was reduced 0%, 25%, 50%, 75%, or 100% (single application) for each successive subirrigation event following an initial application of 0.1 mg·L-1 of paclobutrazol. The 50%, 75%, and 100% reduction treatments provided similar levels of size control. Dilution was more important when the reduction rate was less than 50%. Chemical name used: (±)-(R*,R*)-β-[(4-chlorophenyl)methyl]-α-(1,1-dimethyl)-1H-1,2,4-triazole-1-ethanol (paclobutrazol).

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Ted E. Bilderback, Stuart L. Warren, James S. Owen Jr., and Joseph P. Albano

Many research studies have evaluated potential organic and mineral container substrate components for use in commercial potting substrates. Most studies report results of plant growth over a single production season and only a few include physical properties of the substrates tested. Furthermore, substrates containing predominantly organic components decompose during crop production cycles producing changes in air and water ratios. In the commercial nursery industry, crops frequently remain in containers for longer periods than one growing season (18 to 24 months). Changes in air and water retention characteristics over extended periods can have significant effect on the health and vigor of crops held in containers for 1 year or more. Decomposition of organic components can create an overabundance of small particles that hold excessive amounts of water, thus creating limited air porosity. Mineral aggregates such as perlite, pumice, coarse sand, and calcined clays do not decompose, or breakdown slowly, when used in potting substrates. Blending aggregates with organic components can decrease changes in physical properties over time by dilution of organic components and preserving large pore spaces, thus helping to maintain structural integrity. Research is needed to evaluate changes in container substrates from initial physical properties to changes in air and water characteristics after a production cycle.

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Graham H. Barry, William S. Castle, and Frederick S. Davies

Juice quality of `Valencia' sweet orange [Citrus sinensis (L.) Osb.] trees on Carrizo citrange [C. sinensis × Poncirus trifoliata (L.) Raf.] or rough lemon (C. jambhiri Lush.) rootstocks was determined for fruit harvested by canopy quadrant and separated into size categories to ascertain the direct role of rootstock selection on juice soluble solids concentration (SSC) and soluble solids (SS) production per tree of citrus fruit. SS production per fruit and per tree for each size category was calculated. Juice quality was dependent on rootstock selection and fruit size, but independent of canopy quadrant. Fruit from trees on Carrizo citrange had >20% higher SSCs than fruit from trees on rough lemon, even for fruit of the same size. Large fruit accumulated more SS per fruit than smaller fruit, despite lower juice content and SSC. Within rootstocks, SS content per fruit decreased with decreasing fruit size, even though SSC increased. Rootstock effect on juice quality was a direct rather than an indirect one mediated through differences in fruit size. The conventional interpretation of juice quality data that differences in SSC among treatments, e.g., rootstocks or irrigation levels, or fruit size, are due to “dilution” of SS as a result of differences in fruit size and, hence, juice volume, is only partly supported by these data. Rather, accumulation of SS was greater for fruit from trees on Carrizo citrange than rough lemon by 25% to 30%.

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J.P. Syvertsen, J.W Grosser, and L.S. Lee

We grew three diploid (2X) Citrus rootstock seedlings and their autotetraploids (4X) at elevated CO2 to obtain insights into limitations on growth and net gas exchange that have been associated with tetraploidy. Well-nourished Volkamer lemon (Volk), Troyer citrange (Troy), and Cleopatra mandarin (Cleo) were grown in greenhouses at ambient or twice ambient CO2 for 3 months. We measured plant growth, water relations, mineral nutrition, and net gas exchange characteristics of leaves. Overall, tetraploid roots were thicker as 4X had lower root length: dry weight ratio or specific root length (SRL) than 2X roots. Tetraploid plants were smaller and had higher root/shoot ratios, shorter fibrous roots, and lower whole plant transpiration than 2X. Tetraploids also had lower leaf N and P concentrations on a dry weight basis. Since 4X leaves had thicker leaves (more dry weight per area) than 2X leaves, these nutrient differences disappeared when expressed on an leaf area basis. Elevated CO2 increased plant growth but decreased leaf N, P, and K apparently by a growth dilution effect. Elevated CO2 also increased fibrous root thickness, leaf thickness, and net assimilation of CO2 (ACO2) but decreased stomatal conductance and transpiration such that leaf water use efficiency increased. There was no effect of ploidy level on ACO2 but 4X Volk and Troy had lower rates of ACO2 than their diploids at elevated CO2. Hydraulic conductivity of intact root systems (measured in a pressure pot) was correlated to total plant growth but variability obscured effects of CO2 or ploidy on root conductivity. The low SRL of tetraploids were correlated with lower rates of water use and lower leaf nutrient concentrations, which may be operative in determining the growth characteristics associated with tetraploidy.

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Roney Ballinas-Cano, Javie Farias-Larios, Mario Orozco-Santos, J. Gerardo López-Aguirre*, and Emilio Sánchez-Arevalo

Soil solarization is used for soil born pathogens control, as a result of temperature increase in soil, around 10 °C higher than in not solarized soils. In Mexico, is mostly used to decrease cost to control of different diseases that affect to melon crop, one of them is caused by Fusarium oxysporum f. sp. melonis (L & C) Snyder & Hansen, which is characterized by wilt and yellowing in melon plants. The objective of this assay was to evaluate the effect of heat on infective capacity of F. oxysporum f. sp. melonis in melon plants and its reproduction capacity after to be under different periods of heat under laboratory conditions. Isolated was taken from melon plants from Carmelitas Ranch in the Colima Municipality. Inoculation was 1 × 10-6 conidia concentration. Cloth bags, with 20 g of inoculated soil, were introduced at 9 cm depth in metallic pots (16 cm diameter and 18 cm depth) containing 4 kg of not inoculated soil during 24, 48, 72, 96, and 120 h, with 6 replications. After each period, 1 g was taken from the cloth bags used in heat treatments, later was diluted in 50 mL of distillated water, and petri dishes containing PDA, were inoculated with 1 mL from that dilution and inoculum viability was registered at 96 h after incubation. Parameters evaluated were: mycelium growth, propagule number, and conidia number. Results showed a positive effect to control of Fusarium oxysporum f. sp. melonis, in treatments with a higher heat period, respecting to the control. Is necessary to evaluate this technique under field conditions during summer season.

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J.M. Smagula, W. Litten, and S. Dunham

In the acid podzol soils of Maine where most lowbush blueberries are grown, low availability of boron tends to keep foliar B concentration below the 24 ppm standard. To compare efficacy of soil and foliar boron application methods, 1.5 × 7.6-m treatment plots in a commer-cial lowbush blueberry field received soil-applied borate at 0, 1.1, 2.2, or 3.3 kg·ha-1 B with or without additional DAP (89 kg·ha-1 P) and ZnSO4 (3.3 kg·ha-1 Zn) or foliar-applied Solubor at 0, 0.24, 0.49, or 0.74 kg·ha-1 B with or without the additional DAP and Zn. These 16 treatments were replicated eight times in a randomized complete-block design. Leaf B concentrations were raised by all soil-applied borate treatments and by the 0.49 and 0.74 kg·ha-1 B foliar Solubor treatments, compared to the controls. When borate at 2.2 or 3.3 kg·ha-1 B was combined with DAP plus Zn a lower leaf B concentration was observed compared to B alone, possibly due to a dilution effect caused by an increase in DAP-induced growth. Leaf P deficiency (<0.125% P) was corrected when DAP and Zn were included in the fertilizer treatment. The greatest potential yield (flower buds/stem and flower bud density) was measured in treatment plots receiving a combination of DAP plus Zn and either borate at 2.2 kg·ha-1 B or Solubor at 0.74 kg·ha-1 B. With no additional treatments applied in 1999, leaf B concentrations were slightly higher in soil-treated and foliar-treated plots than in controls suggesting a small carryover from 1997-applied boron. Carryover may vary with rainfall.

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Martin P.N. Gent

Solution electrical conductivity (EC) and the supply of nitrate in proportion to other elements (nitrate supply ratio) should effect tissue composition of lettuce (Lactuca sativa L.) grown in hydroponic solution. These parameters were varied in several series of successive plantings in greenhouses in the northeast United States. In 1996, when the treatments differed only in EC, 0.65 and 0.9 dS·m-1, but not in nitrate supply ratio, leaf tissue had more nitrate and total reduced-N and lettuce grew faster in the solution with higher EC. Over four series of plantings in 1997 and 1998, the nitrate supply ratio of a low-N treatment was only 60% of that for a high-N treatment, and EC was varied from 1.2 to 2.0 dS·m-1. In 1997 and 1998, tissue nitrate was lower in the low-N treatment only when EC was less than in the high-N treatment. However, under irradiance greater than 10 MJ m-2 per day, the lower EC also slowed growth. Stepwise regression over data from all experiments showed leaf nitrate was primarily a function of EC, and a term that described the interaction between irradiance and EC. Due to selective uptake by the plants, the ratio of elements in the recirculating solution differed from the ratio in which they were supplied. Under irradiance less than 10 MJ m-2 per day and solution EC greater than 1.5 dS·m-1, nitrate accumulated in solution to a concentration greater than expected from simple dilution of the concentrates. Tissue nitrate was also related to solution nitrate, increasing by 0.08-0.09 mg·g-1 dry weight per 1 mg·L-1 increase in solution nitrate. To prevent a rise in tissue and solution nitrate under low irradiance, both solution EC and nitrate supply ratio had to be reduced by about one-third, compared to the conditions required for rapid growth under high irradiance.