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Esmaeil Fallahi

at 70% of ET c . Rainfall in both experiments was comparable. Table 1. Precipitation, evapotranspiration (ET), depth of applied water, and total volume of applied water per tree in ‘Pacific Gala’ apple from 2004 to 2007. Effects of rootstock on leaf

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Rolston St. Hilaire, Cathleen F. Feser, Theodore W. Sammis, and Anderson S. St. Hilaire

Accurate measurement of evapotranspiration (ET) is difficult and expensive for large, in-ground container (pot-in-pot) plants. We engineered and used a simple and inexpensive system to determine evapotranspiration of in-ground container trees. The system was shop-assembled and used a block and tackle system attached to a collapsible tripod. A unique container harness system attached to the block and tackle system was used to lift containers that were sunken in the ground. Containers were weighed with a battery-operated balance that was accurate to 1 g (0.04 oz) at its maximum load capacity of 60 kg (132.3 lb). One person operated the system, and the weight of the system exclusive of the balance was 17.5 kg (38.50 lb). Gravimetric water use data obtained with the system werecombined with meteorological data to compute crop coefficients (Kc) of mexican elder (Sambucus mexicana). The system detected small changes in daily water use of mexican elder trees grown in 76-L (20-gal) in-ground containers. Crop coefficients ranged from 0.17 to 0.71. The acquisition of evapotranspiration data from relatively large, containerized landscape plants may be facilitated because the system is simple, inexpensive, and accurate.

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Fabián Robles-Contreras, Raul Leonel Grijalva-Contreras, Manuel de Jesus Valenzuela-Ruiz, and Rubén Macias-Duarte

The main crops in Caborca, Sonora, Mexico, in the Sonoran Desert, are grapes, asparagus, olive, and vegetables, such as melons and green beans. The growers in this area normally do not use methods to calculate water requirements of the crops, although in a desert region water use management needs to be as efficient as possible. A reason for using some of the models is the need to fit water requirements to regional conditions, or some stress could be caused in the crops. Recently, studies have been made in northwestern Mexico that indicate it is feasible to adapt the use of the Makkink model. Three models for estimated potential evapotranspiration (Eto) were evaluated: Penman-Monteith, Hargreaves, and Makkink, and were compared with the Class A pan method (control). Measured variables were the correlation coefficient (R 2) between the models and the control; and the daily average difference between the models and the control. The results indicate that a low R 2 exists between the three evaluated models and the control (0.55, 0.67, and 0.54 for Hargreaves, Penman-Monteith, and Makkink, respectively). The average difference of the daily Eto in the course of 75 days of evaluation was of 0.62, 0.5, and 0.81 mm/day for Hargreaves, Penman-Monteith, and Makkink, respectively. This represents almost 6–10 cm less of potential evapotranspiration with the models, for a cycle crop of 120–130 days.

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J.B. Beard, R.L. Green, and S.I. Sifers

Cultivar selection is one method used for the conservation of irrigation water. The primary objective of this research was to evaluate the evapotranspiration (ET) rates of 24 well-watered, turf-type bermudagrass (Cynodon spp.) genotypes under field conditions and established on a fritted clay root zone contained in plastic minilysimeter pots. A secondary objective was to correlate ET rate to leaf extension rate, a potential rapidly assessed predictor of the amount of leaf surface area present for ET. ET rates were determined by the water-balance method. Both the overall ET and leaf extension rate differed significantly among genotypes. ET rates were not correlated with leaf extension rates in individual years. Our data indicated a potential for water savings based on bermudagrass cultivar selection that was similar to the reported potential water savings based on warm-season turfgrass species selection.

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Jason McAfee and Curt Rom

Pesticides and alternative fruit thinners are needed for certified organic fruit growers. Transient reductions in photosynthesis (Pn) have proven an effective technique for fruit thinning. Pesticides can be detrimental to plant growth by Pn reduction. This study was developed to measure plant response to foliar applications of essential oils at 2% concentrations. Treatments were applied to vegetative apple trees grown under controlled environment conditions to study photosynthetic effects. There was no significant effect on Pn for treatments; however, clove oil was very phytotoxic and defoliated all trees in this study. Cinnamon oil and cedarwood oil significantly decreased evapotranspiration and stomotal conductance 1 day after treatment. Differences in plant growth were not significantly different for all treatments excluding clove oil. Studies on concentration effects may determine horticultural usefulness of these compounds.

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R.L. Green, J.B. Beard, and D.M. Casnoff

The objectives of this investigation were to determine the stomatal frequencies of 12 perennial cool-season turfgrasses, encompassing nine species, and their associated evapotranspiration (ET) rates under nonlimiting soil moisture and controlled environmental conditions. Significant differences in stomatal density were found among the 12 cool-season turfgrasses on both the abaxial (P > F = 0.0008) and adaxial (P > F = 0.0009) leaf surfaces. Significant differences (P > F = 0.0007) in ET rates also were found among the 12 cool-season turfgrasses. The Kentucky bluegrass (Pea pratensis L.) cultivars exhibited the highest ET rates, while the fine-leafed fescues (Festuca rubra and longifolia L.) exhibited the lowest rates, except for `Big Horn' sheep fescue (Festuca ovina L.), which exhibited an intermediate ET rate. No significant correlation was found between ET rate and either adaxial or abaxial stomatal density. It was concluded that, under nonlimiting soil moisture conditions, stomatal density was not reliably associated with ET rate.

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Jason D. McAfee and Curt R. Rom

Pesticides and alternative fruit thinners are needed for certified organic fruit growers. Transient reductions in photosynthesis (Pn) have proven an effective technique for fruit thinning. Pesticides can be detrimental to plant growth by Pn reduction. A two-part study was developed to measure plant response to foliar applications of sulfur compounds. In study 1, 2% concentrations of various sulfur compounds were observed for potential physiological or pesticidal effects. Foliar treatments were applied to vegetative apple trees grown under controlled environment conditions to study photosynthetic effects. No treatments significantly reduced CO2 assimilation (A) and stomatal conductance (gs). Copper sulfate, ammonium sulfate, and potassium sulfate significantly reduced evapotranspiration (Et) 7 days after treatment. No significance was observed for plant growth. In study 2, a 2% potassium sulfate concentration significantly reduced A 22 days after treatment; however, no differences were observed for Et and gs. Differences in plant growth were not significantly different among treatments.

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J.D. Mcafee and C.R. Rom

Alternative fruit thinners and pesticides are needed for certified organic fruit growers. A transient reduction in photosynthesis has proven to be an effective technique used for fruit thinning. Conversely, pesticides, which reduce Pn may be detrimental to plant growth. This study was developed to measure plant response to foliar applications of various organic acids as potential horticultural chemicals Treatments were applied to vegetative apple trees under controlled environmental conditions to study photosynthetic effects. CO2 assimilation significantly decreased temporarily 3 days after treatment with citric acid. Decreased trends of evapotranspiration were observed for all treatments 1 day after foliar application; however, not significant. Salicylic acid significantly decreased stomatal conductance 1 and 15 days after treatment. Average leaf area was not significantly affected but oxalic acid increased plant stem growth while acetic acid application reduced growth. This model system for screening new and alternative compounds will be a basis to study agents that may have potential to be used as certified pesticides or fruit thinning agents.

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J. Norrie, M.E.D. Graham, and A. Gosselin

The use of potential evapotranspiration (PET) estimates to identify irrigation timing for greenhouse tomatoes (Lycopersicon esculentum Mill.) grown in peat-based substrate was evaluated for a spring and fall crop. PET (using the Penman equation) was calculated from leaf, wet and dry bulb temperatures, and incident and reflected photosynthetic photon flux. Substrate matric potential (SMP) was monitored continuously using electronic tensiometers. Two irrigation starting setpoints (-4.5 and -6.5 kPa SMP) and two nutrient solution electrical conductivity (EC) treatments (1.5 and 3.0 dS·m-1) were factorially combined in a completely randomized design. Irrigation frequency was greater in treatments irrigated at -4.5 than at -6.5 kPa. The integral of calculated PET values was correlated with SMP for both experiments. Accumulated PET values were higher at the start of irrigation in the -6.5-kPa treatments for spring and fall crops. Nutrient solution EC did not influence irrigation frequency. Leaf pressure potential (LPP) was correlated to PET-predicted LPP (r 2 > 0.56) in plants subjected to high EC, low (-6.5 kPa) matric potential setpoint, or both treatments. PET and electronic tensiometer technology can be used jointly to improve irrigation management for tomatoes grown in peat-based substrates by more accurately responding to crop needs for water and nutrients.

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Xin Zhao and Edward E. Carey

High tunnels, unheated greenhouses, have been shown to be a profitable season-extending production tool for many horticultural crops. Production of cool-season vegetables during hot summer months can be achieved using shaded high tunnels. Microclimate in high tunnels and open field was monitored during summer trials of leaf lettuce, in which unshaded tunnels and shaded tunnels (39% PAK white shadecloth) were used, respectively, in 2002 and 2003. Wind speed was consistently lower in high tunnels. Compared to open field, daily air temperature was about 0.7 °C higher in unshaded high tunnels, and 0.5 °C lower in shaded high tunnels. Relative humidity was slightly lower in unshaded tunnels, but tended to increase in shaded tunnels, in comparison to the open field. When using shadecloth, soil temperature was lowered by 1∼3 °C and the leaf surface temperature was significantly reduced by 1.5∼2.5 °C. In shaded high tunnels, PAR light dropped by at least 50% relative to the outside, where the maximum PAR light intensity reached 1800 μmol·m-2·s-1. Overall, shaded high tunnels resulted in higher quality lettuce, with less bolting and bitterness. Reference crop evapotranspiration (ET0) was estimated from meteorological data on a daily basis using the FAO-56 method. ET0 was lowest in shaded high tunnels, and highest in the open field. Relatively lower ET0 in high tunnels indicated a likely lower water requirement and therefore improved water use efficiency compared with the open field.