Leaf gas exchange and chlorophyll fluorescence measurements were used as indices for evaluating heat tolerance among five species of birch: paper (Betula papyrifera), European (B. pendula), Japanese (B. platyphylla var. japonica `Whitespire'), Himalayan (B. jacquemontii), and river (B. nigra). Measurements were conducted on individual leaves at temperatures ranging from 25C to 40C. Carbon exchange rates (CER) were depressed for all species at 40C. However, there was considerable variation in both absolute and relative (percent of maximum) CER among species at 40C; river birch maintained the highest absolute and relative CER while CER of paper birch was reduced the most. Although stomatal conductance of paper birch decreased at higher temperatures, internal leaf CO2 increased indicating that reduced stomatal conductance was not responsible for decreased CER. Stomatal conductance of river birch increased at higher temperatures which provided for enhanced uptake of CO2 and greater evaporative cooling. Variable chlorophyll fluorescence decreased similarly for both species with increasing temperatures. Measurements of dark respiration rates over the range of 25C to 40C suggested that the primary factor influencing variation in CER at higher temperatures was due to variation in respiration rates at higher temperatures.
Thomas G. Ranney
James E. Faust, Sven Verlinden, and Royal D. Heins
Rapid reduction in temperature for two to three hours starting at sunrise reduces stem elongation compared to elongation of plants maintained under constant temperatures during the day. This experiment was designed to determine if syringing plants with water at sunrise would substitute for a reduction in air temperature or enhance the response to the drop in temperature. Easter lily (Lilium longiflorum Thumb.) plants were exposed to constant 20°C or to 20°C and then 16°C for a 3-hr period following sunrise. Half the plants in each temperature regime were syringed at 30-minute intervals with 20°C water for 3 hr starting 20 minutes before sunrise. Shoot-tip temperature during the three-hr pulse time averaged 20.0 and 17.3°C for the dry plants and 17.3 and 14.7°C for the syringed plants. Total elongation for the dry plants at 20°C was 30 cm and for the temperature-pulsed plants, 4.8 cm less; for the syringed plants, 3.3 and 5.8 cm less, respectively. While shoot-tip temperature of dry plants averaged 0.9°C above air temperature during the remaining hours of the day, syringed plants averaged 1.0°C cooler than the same air temperature even though plants had dried. The data indicate the reduction in stem elongation from a low-temperature pulse at sunrise can be enhanced by evaporative cooling.
J. Ryan Stewart and Roger Kjelgren
Infrared sensors were used to quantify canopy temperature and thus detect differences in incipient water stress between a cool-season grass [Kentucky bluegrass (KBG) (Poa pratensis)] and a warm-season grass [buffalograss (BG) (Buchloe dactyloides)]. The infrared sensors, connected to a datalogger, measured average hourly leaf–air temperatures (TL–TA) 1 m above eight replicate plots of Kentucky bluegrass and eight replicate plots of buffalograss. Air temperature and relative humidity from a nearby weather station were used to calculate the average hourly vapor pressure deficit (VPD). In late July, we ceased irrigating and measured TL–TA and soil water content while allowing the turf to dry down for 5 weeks. Soil water content was measured with a neutron probe. Both species exhibited a significant relationship between TL–TA and VPD. As the VPD increased, TL–TA decreased in both species (KBG r 2 = 0.73, BG r 2 = 0.71) on the 2nd day after an irrigation during well-watered conditions. An artifact was created on the first day after an irrigation as a result of excessive surface evaporation. KBG and BG were similar under well-watered conditions. KBG had a higher TL–TA after 4 to 5 days without irrigation. By contrast, BG did not have a higher TL–TA until 25 to 30 days without irrigation. Part of BG's drought avoidance was extraction of soil water down to 0.9 m vs. 0.45 m for KBG.
A. Erez, Z. Yablowitz, and R. Korcinski
Fresh fruits of stone fruit species are either lacking or in short supply in the months of March and April on the world market. This results from the absence of late-maturing cultivars in most of these species and from their poor storage capabilities. In March–April, supply from the Southern Hemisphere is dwindling or finished, while supply from the Northern Hemisphere has not started yet. A horticultural system was developed in Israel to get early fruit ripening of stonefruit species. The system developed is based on early completion of dormancy requirements followed by greenhouse tree growing. The system uses the following elements: 1) Improve chilling accumulation in winter by using evaporative cooling, to prevent chilling negation by high day temperatures. 2) Monitor salt accumulation level to prevent damage to branches and buds. 3) Monitor bud temperature and evaluate dormancy development according to the dynamic model. 4) Use dormancy breaking chemicals to compensate for part of the chilling not applied. 5) Cover of the trees with polyethylene having the appropriate characteristics of light filtering. 6) Keep the temperature in the greenhouse lower than the maximal temperature allowed, at every specific stage of development by ventilation. By using this system, fruit ripening was advanced by 4 to 6 weeks in peaches and nectarines (harvested from late March) and by 4 weeks in sweet cherries (harvested from April 11). Yields of 20 to 30 tons/ha were obtained in both species with high fruit quality.
Thomas L. Davenport, Stanley P. Burg, and Thomas L. White
U.S. regulations prevent importation of fresh horticultural commodities that have not received an approved quarantine treatment assuring 100% mortality of potentially invasive insect pests. Because imported mangoes are likely to be infested by the Caribbean fruit fly (Anastrepha suspensa Loew) and other tropical fruit flies such as A. ludens Loew, A. striata, A. distincta, A. fraterculus, A. serpentina, or A. oblique, they must be hot-water treated prior to shipment in order to satisfy quarantine requirement. Hot water treatment often damages the fruit, especially if it is not fully mature. Hypobaric (low pressure = LP) intermodal shipping containers developed by the VacuFreshSM Corp. preserve fresh commodities, such as horticulturally mature mangoes, far longer than is possible using other technologies. We tested the ability of over 70,000 Caribbean fruit fly eggs and larvae to survive a simulated optimal hypobaric condition for shipment of mangoes (15 mm Hg, 98% RH, at the lowest, safe non-chilling temperature, 13 °C). A. suspensa eggs or larvae were maintained on agar media, flushed with one air change per hour at the storage pressure, and shielded with Mylar to prevent radiant heat uptake and limit evaporative cooling. Nearly 98% of the eggs and larvae were killed within 1 week at 15 mm Hg in eight replicated experiments. All eggs were killed at that pressure by 11 days, whereas a significant number survived at ambient pressure. Shipment of fresh produce using this technology promises to provide quarantine control while preserving the freshness of fully mature tropical fruits and vegetables.
Thayne Montague, Roger Kjelgren, and Larry Rupp
Research was conducted to investigate how energy balance of bark mulch and turf surfaces influence gas exchange and growth of recently transplanted trees. On several occasions over a 3-year period, stomatal conductance and leaf temperature were measured throughout the day on `Emerald Queen' Norway maple (Acer platanoides L.) and `Greenspire' littleleaf linden (Tilia cordata Mill.) trees growing over each surface. Tree water loss was estimated using a general transport flux equation applied to the tree crown apportioned between sunlit and shade layers. Microclimate variables were measured over each surface with a permanent weather station. Tree growth data were collected at the end of each growing season. Soil heat flux data revealed that a greater portion of incoming radiation was prevented from entering the soil below mulch than below turf. Due to this insulating effect, and consequent lack of evaporative cooling, mulch surface temperature was greater, and emitted more longwave radiation, than turf. Leaves over mulch intercepted more longwave radiation, had greater leaf temperature, and greater leaf-to-air vapor pressure difference than leaves over turf. As a result, leaves over mulch had greater stomatal closure than leaves over turf. Estimated tree water loss varied between surface treatments and with climatic conditions. Trees over turf had greater shoot elongation and leaf area than trees over mulch. These data suggest that gas exchange and growth of recently transplanted trees in an arid climate may be reduced if planted over nonvegetative, urban surfaces.
Yuguang Zhao, George C.J. Fernandez, Daniel C. Bowman, and Robert S. Nowak
Cumulative evapotranspiration (ETcum) patterns of 10 commercially available cool-season turfgrass species and cultivars were evaluated under progressive water stress in the semi-field conditions using a gravimetric mass balance method in three studies. At the end of water stress, the cultivars were visually scored for green appearance on a 0 (no green) to 10 (100% green) scale. A Gompertz nonlinear model gave a best fit to ETcum vs. days adjusted for pan evaporation variation. Two of the ETcum attributes (ti, the time during which the rate change in ET is zero, and ETmax, the maximum ET rate) estimated from the Gompertz model appeared to reflect efficient water-use attributes in the turfgrass. Among the physiological screening techniques studied, electrolyte leakage, relative water content, and the difference between canopy and air temperature appeared to separate cultivars by drought resistance and water use efficiency (WUE). These physiological attributes were also relatively easy to measure and had high correlations with color score and WUE. Biplot display is a graphical technique in which the interrelationships between the cultivars and water-use attributes can be displayed together. Based on ti, ETmax, color score, and physiological attributes, `Wabash' and `Bristol' Kentucky bluegrass (Poa pratensis L.), `Aurora' hard fescue (Festuca ovina var. duriuscula L. Koch.), and `FRT-30149' fine fescue (F. rubra L.) were identified as cultivars with higher WUE.
Tracy Dougher, Toby Day, Paul Johnson, Kelly Kopp, and Mark Majerus
The ongoing drought in the Intermountain West has brought a great deal of attention to water conservation over the past several years. During that time, turfgrass irrigation has been targeted as a source for large potential water savings. Some communities promote downsizing turfgrass areas as the best water conservation measure. In reality, turfgrass controls erosion, reduces evaporation from a site, and provides a safe surface for human activities. One alternative to elimination would be wider use of low water-use-grasses appropriate to the area. However, many questions arise regarding the choice of such grasses and their management. Our research addresses these questions. Plots have been established at Montana State University, Bozeman; Utah State University, Logan; and USDA-NRCS Plant Materials Center, Bridger, Mo. The grasses considered include 12 single species and 12 mixed species stands of `Cody' buffalograss, `Foothills' Canada bluegrass, `Bad River' blue grama, sheep fescue, sandberg bluegrass, muttongrass, and wheatgrasses `Sodar' streambank, `Road Crest' crested, `Rosana' western, and `Critana' thickspike with Kentucky bluegrass and tall fescue as controls. Line source irrigation allowed the plots to be evaluated at a number of levels of irrigation. Experimental measurements on the plots included growth response as determined by clipping yield and quality ratings, and species composition. Fescues and wheatgrasses retained their color, texture, and density throughout the growing season, regardless of moisture level. Warm-season grasses performed well in June, July, and August only, and worked poorly in mixtures as the green cool-season grasses could not mask the brown dormant leaves in cooler weather.
Fan-Hsuan Yang, David R. Bryla, Scott T. Orr, Bernadine C. Strik, and Yanyun Zhao
include the use of kaolin-based particle film, shade nets, and evaporative cooling. Kaolin particle film is not an option for blueberry because the surface residue is difficult to remove ( Spiers et al., 2003 ). Shade net successfully reduces fruit surface
Ahmad Shirazi and Arthur C. Cameron
A method was developed to measure transpiration rates and apparent water-vapor permeability coefficients (P'H2O) of detached fruit using an analytical balance equipped with a humidity chamber, wide-range humidity-generating and sensing devices, and a datalogger. The system was designed to monitor weight changes with time and, hence, weight loss of individual fruit during exposure to specific relative humidities (RHs) and temperatures. Weight loss was corrected for loss due to respiratory exchange of 02 and CO2 before calculating P'H2O. Values of P'H2O for tomatoes obtained using this method over periods of 5 minutes to 24 hours ranged from 3 to 12 nmol·cm-2·s-1·kPa-1 at 20C, depending on the experimental conditions. These values are similar to previously published values and to those obtained in a conventional weight-loss experiment, which involved intermittent weighing. P'H20 for tomatoes dropped ≈15% in 24 hours. P'H20 increased with a transient increase in RH; the extent of the increase was variable from fruit to fruit, ranging from 5% to 100% over 30% to 90% RH. The change was reversible in that P'H2O increased and decreased within minutes following shifts in RH. Similar changes were found for strawberry P'H20. The increase in P'H2O may be due, in part, to a direct effect of water vapor on the water transport properties of the cuticular polymer and surface temperature depression as a result of evaporative cooling. At 50% RH and 20C, water vapor diffuses from tomatoes 50 times faster than O2 enters on a molar basis. This information will be useful for modeling RH changes in modified-atmosphere packages.