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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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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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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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John M. Ruter

A study was conducted with Prunus × incamp `Okame' to evaluate the effects of a pot-in-pot production system compared to a conventional above-ground system and cyclic irrigation on plant growth and water loss. Plants were grown in #7 (26-L) containers with a 8:1 pinebark:sand (v/v) substrate. Cyclic irrigation provided the same total volume of water, but was applied one, three, or four times per day. Final plant height and stem diameter, shoot and root dry weight, total biomass, and root:shoot ratio were all increased for plants grown pot-in-pot compared to above-ground. Multiple irrigation cycles increased stem diameter, shoot dry weight, and total biomass, compared to a single irrigation application. Multiple irrigation cycles decreased the root:shoot ratio. Evapotranspiration was influenced by production system, irrigation, and date. Amount of water lost as leachate was influenced by irrigation and date. Cyclic irrigation resulted in a two-fold decrease in leachate volume. Soluble salts and nitrate-nitrogen in the leachate were influenced by an interaction between production system, irrigation, and date.

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Enrique Troyo-Diéguez and José Luis León

Mango (Mangifera indica L.) has been grown since the beginning of the century in Baja California Sur, one of the most arid states of México. Since water is a very scarce resource in this area, the estimation of water consumption by popular crops becomes a relevant aspect of hydrological research. Actual (ETa) and potential (ETp) evapotranspiration of mango c.v. Kent were estimated in the Experimental Station of CIB, located 17 Km west La Paz city. Trees under study were three years old; irrigation frequency was 14 days and depth of applied water was 0.15 m, a common amount in the region. Estimates for ETp were carried out through two indirect methods (Blaney-Criddle and Penman equations), and ETa by a direct method (a diffusive porometer). Data were fitted according to the total leaf area (TLA). Estimates from the indirect methods were 31 and 25% respectively greater than those given by the porometer. Main results (ETa = 0.5 cm day-1, or 0.07 m H20/14 days) suggest that a 40 to 50% reduction in the applied water depth is feasible in the region.

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Robert L. Green, Ki S. Kim and James B. Beard

The objective of this study was to determine the effects of two plant growth regulators (PGR) and two soil moisture levels (SML) on the evapotranspiration (ET) rate, leaf extension rate (LER), and visual turfgrass quality of `Texas Common' St. Augustinegrass [Stenotaphrum secundatum (Walt.) Kuntze] grown under glasshouse conditions in black plastic minilysimeters. Treatments included mefluidide at 0.42 kg·ha-1, flurprimidol at 0.84 kg·ha-1, and no PGR, each grown under optimal (– 0.01 MPa) or suboptimal (– 0.8 MPa) SML. Both flurprimidol and mefluidide significantly affected ET rate, LER, and turfgrass quality, whereas the durations of the responses to both PGR treatments were affected by SML. For both SML, the durations of significant reduction in ET rate, LER, and turfgrass quality were longer for flurprimidol than for mefluidide. Application of either PGR at either SML caused a significant reduction in ET rate averaging 18% and a significant reduction in LER averaging 83%. Flurprimidol was more effective than metluidide in terms of ET rate and LER reduction. However, the considerably longer duration of reduced turfgrass quality of flurprimidol-treated turf was a negative effect. Chemical names used: α-(1 -methylethyl)- α -[4-trifluoromethoxy)phenyl]-5-pyrimidinemethanol (flurprimidol) and N- [2,4-dimethyl-5-[[(trifluoromethyl) sulfonyl]amino]phenyI] acetamide (mefluidide).

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

Water is a very limited resource in the Sonoran Desert region of Caborca, Sonora, Mexico. For an efficient use of irrigation water, a method of calculating water requirements of the crops is needed. Potential evapotranspiration (Eto) value obtained with the Penman-Monteith model from a regional weather station was not dependable, since some parameters, such as sensible heat flux in the soil, are estimated from a fixed rate with net radiation (Rn), also an estimated value. The weather station did not have a sensor for heat flux in its network. Studies in northwestern Mexico have indicated that it is feasible to adapt the use of the Makkink model, because a single measurement of solar radiation and temperature would be required. We compared the daily Makkink Eto against the Class A pan method (control) Eto during 75 days and found a value of 0.81 mm/day less with the Makkink model. To fit the Makkink model to regional conditions, we ran the Makkink model varying the value of C constant (from 0.5 to 0.95), and found that a value of C = 0.87 substituted for C = 0.65 (original value) has an daily average difference of 0.09 mm/day less with respect to the control. This could be because there are few clouds in the region, and a greater proportion of global radiation arrives at the surface from the earth or the crops in form of net radiation.

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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.