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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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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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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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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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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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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.

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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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A 2-year study was conducted to quantify the actual evapotranspiration (ETa) of three woody ornamental trees placed under three different leaching fractions (LFs). Argentine mesquite (Prosopis alba Grisebach), desert willow [Chilopsis linearis (Cav.) Sweet var. linearis], and southern live oak (Quercus virginiana Mill.) (nursery seedling selection) were planted as 3.8-, 18.9-, or 56.8-liter container nursery stock outdoors in 190-liter plastic lysimeters in which weekly hydrologic balances were maintained. Weekly storage changes were measured with a portable hoist-load cell apparatus. Irrigations were applied to maintain LFs of +0.25, 0.00, or -0.25 (theoretical) based on the equation irrigation (I) = ETa/(1 - LF). Tree height, trunk diameter, canopy volume, leaf area index, total leaf area (oak only) and dry weight were monitored during the experiment or measured at final harvest. Average yearly ETa was significantly influenced by planting size (oak and willow, P ≤ 0.001) and leaching fraction imposed (P ≤ 0.001). Multiple regressions accounting for the variability in average yearly ETa were comprised of different growth and water management variables depending on the species. LF, trunk diameter, and canopy volume accounted for 92% (P ≤ 0.001) of the variability in the average yearly ETa of oak. Monthly ETa data were also evaluated, with multiple regressions based on data from nonwater-deficit trees, such that LF could be ignored. In the case of desert willow, monthly potential ET and trunk diameter accounted for 88% (P ≤ 0.001) of the variability in the monthly ETa. Results suggest that irrigators could apply water to arid urban landscapes more efficiently if irrigations were scheduled based on such information.

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Abstract

Mature peach trees [Prunus persica (L.) Batsch] grown in weighing lysi-meters were subjected to soil moisture stress. Evapotranspiration (ET) was not affected by rapid changes in soil moisture until soil matric potential reached -1500 kPa. When the trees reached permanent wilt, there was a sharp decline in water use.

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The influence of K nutrition (25, 75, 150, 300, 450, and 600 mg K/liter) and moisture stress conditioning (MSC) (exposing plants to four sublethal dry-down cycles) on leaf water relations, evapotranspiration, growth, and nutrient content was determined for salvia (Salvia splendens F. Sellow `Bonfire'). Potassium concentration and MSC had an interactive influence on osmotic potential at full (π100) and zero (π0) turgor. Differences in osmotic potential between MSC and non-MSC plants for π100 and π0 increased with increasing K concentration. Increasing K concentration and MSC resulted in active osmotic adjustment and, consequently, increased cellular turgor potentials. Foliar K content increased with increasing K concentration and MSC. High K concentrations and MSC both reduced plant evapotranspiration on a per-plant and per-unit-leaf-area basis. Greatest shoot dry weight occurred for plants grown with 300 mg K/liter and non-MSC. Total leaf area increased with increasing K concentration, but MSC had little effect.

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