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Thomas J. Tworkoski, Michael E. Engle, and Peter T. Kujawski

A polypropylene fabric containing control-release pellets of the herbicide, trifluralin, can be oriented in the soil to regulate the distribution of plant roots. In 1990, trenches were dug near 10-year-old red oak (Quercus rubra L.) and 10-year-old yellow poplar (Liriodendron tulipifera L.) and fabric containing trifluralin control-release pellets and polypropylene fabric alone were installed vertically to redirect root growth. Roots grew alongside trifluralin fabric and fabric alone and did not penetrate either fabric 38 months after installation. Shoot growth of yellow poplar was reduced about 47% each year by the trifluralin fabric treatment compared to control. Red oak shoot growth was not affected by trifluralin fabric. Leaf water potential was not affected by treatment in either species. Trifluralin residues in trifluralin fabric decreased from 23.3% to 22.0% from July 1990 to October 1993. During this time, trifluralin levels increased from 0.4 to 3.6 mg·kg-1 in soil sampled 0 to 15 cm below trifluralin fabric. These results suggest that controlled-release trifluralin will provide persistent inhibition of root and shoot growth of some species and will not migrate significantly in the soil. Chemical names used: α,α,α-trifluoro-2,6-dinitro-N-N-dipropyl-p-toluidine (trifluralin).

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Ami N. Erickson and Albert H. Markhart

High temperature reduces fruit set in bell pepper [Capsicum annuum L. var. annuum (Grossum Group)], and reduction of pepper productivity, resulting from high temperature, may be a direct effect of temperature or an indirect effect of water stress induced by increased vapor pressure deficits (VPDs) at high temperature. We evaluated responses of plant growth, reproduction, net photosynthesis (PN), chlorophyll fluorescence, predawn respiration, leaf water potential, and stomatal conductance of `Ace' and `Bell Boy' bell pepper to elevated temperature (33 °C) with increased VPD (2.1 kPa) or elevated temperature with no increase in VPD (1.1 kPa). VPD had no effect on flower number or fruit set and did not adversely influence the physiological processes measured. Therefore, deleterious effects of high temperature on pepper fruit set does not appear to be temperature induced water stress, but is more likely a direct temperature response. Elevated temperature decreased fruit set but not flower production. Gas exchange measurements suggest failure to set fruit was not due to reduced leaf photosynthesis.

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A. Richard Renquist, Horst W. Caspari, M. Hossein Behboudian, and David J. Chalmers

Stomatal conductance (g s) of `Hosui' Asian pear (Pyrus serotina Rehder) trees growing in lysimeters was characterized for trees in well-watered soil and after brief water deficit. The measures of water status used to interpret g s data were soil-water content, leaf water potential (ψl), and instantaneous water use (trunk sap flow by the compensation heat-pulse technique). The diurnal course and range of g s values of well-irrigated Asian pear trees were similar to those reported for other tree fruit crops. Soil moisture at the end of a midsummer deficit period was 60% of lysimeter pot capacity, and diurnal ψl reflected this deficit predawn and in the late afternoon compared to well-irrigated trees. The g s was sensitive to deficit irrigation during more of the day than ψl, with g s values <3 mm·s-1 for most of the day; these were less than half the conductances of well-irrigated trees. The reduction of g s in response to a given soil-water deficit was not as great on days with lower evaporative demand. After a water deficit, g s recovered to predeficit values only gradually over 2 to 3 days. The low g s of trees in dry soil was the apparent cause of reduced transpiration, measured by trunk sap flow, and reduced responsiveness of sap flow to fluctuations in net radiation.

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Abdul K. Janoudi, Irvin E. Widders, and James A. Flore

Cucumber plants were cultured in a greenhouse and subjected to either well-watered or water deficit conditions that reduced leaf water potential to-0.6 MPa. Leaf gas exchange measurements were conducted using an open gas exchange system. Carbon dioxide assimilation (A) attained saturation at a photon flux density (PFD) of 1000 μmol·m-2·s-1 (400-700 nm). There were no significant differences in A at ambient temperatures between 16 and 34C. Water use efficiency decreased rapidly with increasing vapor-pressure deficits to 2.5 kPa. Water stressed plants had lower stomata1 conductances and CO2 assimilation rates. The decrease in A was only partially due to stomata1 closure. The A vs. intercellular CO (Ci) relationship for stressed leaves revealed a change in the CO, compensation point, and that nonstomatal factors were contributing to the decrease in A in stressed plants. Thus, feedback inhibition of A may have occurred through photoassimilate accumulation. The concentrations of sucrose and raffinose were higher, and the concentration of stachyose was lower in leaves of stressed than of well-watered plants.

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J.G. Norcini, P.C. Andersen, and G.W. Knox

Leaf physiology and plant growth of Photinia x fraseri Dress were assessed when grown under full sunlight or (100% sun) or polypropylene shadecloth with a light transmittance of 69%, 47%, or 29% sun. Plants in 69% or 47% sun usually had the highest midday net CO2 assimilation rates (A). Net CO, assimilation rate was most dependent on photosynthetic photon flex (PPF R2 = 0.60), whereas stomata] conductance to water vapor was primarily influenced by vapor pressure deficit (R2 = 0.69). Stomatal conductance was often inversely related to sun level, and intercellular CO2 concentration was often elevated under 29% sun. Midday relative leaf water content and leaf water potential were unaffected by light regime. Light-saturated A was achieved at ≈ 1550 and 1150 μmol·m-2·s-1 for 100% and 29% sun-grown plants, respectively. Under 29% sun, plants had a lower light compensation point and a higher A at PPF < 1100 μmol·m-2·s-1. Total growth was best under 100% sun in terms of growth index (GI) increase, total leaf area, number of leaves, and dry weight (total, stem, leaf, and root), although plants from all treatments had the same GI increase by the end of the experiment. Plants in all treatments had acceptable growth habit (upright and well branched); however, plants grown in 29% sun were too sparsley foliated to be considered marketable. There were no differences in growth among the four treatments 7 months after the Photinia were transplanted to the field.

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Tessa M. Mills, M. Hossein Behboudian, and Brent E. Clothier

Information on fruit water relations is scant for apple trees, especially under deficit irrigation. Here we discuss plant and fruit responses to deficit irrigation. Three-year-old potted `Braeburn' trees were studied in a glasshouse. The treatments were: well-watered control (C), early deficit (D1), and late deficit (D2). The latter two were, respectively, water stressed during 61–183 and 109–183 days after full bloom (DAFB). The final harvest was at 183 DAFB. Photosynthesis, stomatal conductance, and trunk circumference were lower in D1 and D2 than in C. Leaf area and shoot growth was reduced only in D1. Root length remained the same for all treatments. Fruit were smaller in D1 than in C; however, fruit growth was less sensitive to deficit irrigation than was vegetative growth. Fruit growth in D2 was the same as in C. Fruit concentrations of K+, fructose, sorbitol, total sugars, and titratable acidity were higher in D1 than in D2 and C. Total soluble solids were higher in D1 and D2 than in C. Although fruit water potential was lower in D1 than in C, a concomitant lowering of osmotic potential in D1 fruit led to maintenance of turgor potential, indicating osmotic adjustment. This could have been effected, at least partially, through accumulation of K+ and soluble sugars. Water relations of D2 fruit were not affected by deficit irrigation, although leaf water potential was lower than in C. Fruit water relations and fruit growth are therefore less sensitive to deficit irrigation than are those of vegetative parts.

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Hui-lian Xu, Laurent Gauthier, and André Gosselin

Tomato plants (Lycopersicon esculentum Mill. cv. Capello) were grown in peat bags, rockwool slabs, and NFT in a greenhouse to examine the effects of nutrient solution electrical conductivity (EC) and potential evapotranspiration (PET)-dependent EC variation on plant water relations. Peat bags were irrigated by a PET-dependent irrigation system. EC was varied from 1 to 4 mS·cm-1 according to PET under –5 and –9 kPa of substrate water potential setpoints (SWPS). The plants in rockwool and NFT were treated with ECs of 2.5, 4, and 5.5 mS·cm-1. Peat bags and rockwool slabs were overwatered once a week to wash out the accumulated salts. Leaf water potential (ψ1) and relative water content (θ) were measured before and after plants were overwatered. Turgor (P) and osmotic π potentials were estimated from the pressure-volume method. Before plants were overwatered, ψ1 was significantly lower in the plants with high EC and low SWPS treatments and also lower in variable EC-treated plants, but P maintained close to the control value. After plants were overwatered, ψ1 recovered close to the control level and P became higher because of the lower π in the treatments of high EC, variable EC, and/or low SWPS. At a given ψ1 the plants with high EC, variable EC, and/or low SWPS maintained higher θ. The analysis of the pressure-volume curve showed that the leaves treated with high EC, variable EC, and/or low SWPS had higher turgid water content, higher symplasmic (osmotically active) water content, lower apoplasmic (osmotically inactive) water content, and lower θ point of zero turgor (incipient plasmolysis). Maintenance of P after overwatering was directly proportional to photosynthetic capacity. We suggest that osmotic adjustment occurs in response to high EC, low SWPS, or both and that overwatering substrates and varying EC can not only avoid salinity stress, but also improve turgor maintenance.

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Valerie M. Jonas and Kimberly A. Williams

A series of experiments were conducted to determine the ranges of irrigation frequency and N and P fertilization regimes that produce ivy geranium (Pelargonium peltatum L.) plants of optimum commercial quality. Two cultivars, `Sybil Holmes' and `Amethyst', were grown. Data collected included fresh and dry weights, ratings, leaf area, height, width, ratings, and nutrient tissue content. Individual pots were weighed daily and irrigated when weight of pots dropped by 15%, 30%, 45%, or 60% of container capacity (CC). Leaf water potential was measured using a pressure chamber. At both mid and end of crop, plants irrigated when pot weight dropped by 30% of CC were under least water stress (e.g., water potential of –7.0 to –4.7 MPa). Irrigation frequencies at 15%, 45%, or 60% of CC had similar water potentials (e.g., –9.9 to –9.1 MPa). At 15%, a plausible explanation of the stress is that oxygen was limiting in the root zone due to water-logging; at 45% and 60%, water was the limiting factor. Single factor experiments with N at five concentrations ranging from 2 to 32 mm and P at five concentrations ranging from 0.08 to 2.56 mm were conducted. Quadratic equations were fit to curves of growth responses plotted against concentration of N or P applied. As an example of results, N fertilizer rates of 16 and 32 mm for `Amethyst' resulted in similar, commercially acceptable dry weights (37g), but different N tissue concentrations of 3.4% and 3.9% respectively. For `Sybil Holmes', N fertilizer rates of 10 and 26 mm resulted in similar dry weights (21g) but different tissue concentrations of 2.8% and 3.4%, respectively.

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James A. Zwack, William R. Graves, and Alden M. Townsend

Freeman maples (Acer × freemanii E. Murray) are marketed as stress-resistant alternatives to red maples (Acer rubrum L.), but few data from direct comparisons of these species are available. As a first step in comparing the stress resistance of red maple and Freeman maple, responses to drought were studied in Acer × freemanii `Autumn Fantasy', `Celebration', and `Marmo'. Plants grown from rooted cuttings were treated by withholding irrigation through four drought cycles of increasing severity that were separated by irrigation to container capacity. Drought reduced shoot dry mass, root dry mass, and height growth by 64%, 43%, and 79%, respectively, over all cultivars. Predawn leaf water potential was reduced by 1.16 MPa over all cultivars, and stomatal conductance data indicated water use was more conservative over all root-zone moisture contents after repeated cycles of drought. Specific mass of drought-stressed leaves increased by 25% for `Autumn Fantasy', and microscopy to determine leaf thickness and cellular anatomy is ongoing. `Autumn Fantasy' also had the lowest ratio of leaf surface area to xylem diameter, and `Autumn Fantasy' and `Celebration' had higher ratios of root to shoot mass than `Marmo'. Pressure-volume curve analysis revealed osmotic potential of drought-stressed plants at full turgor was 0.24 MPa more negative than controls, and droughted plants had a greater apoplastic water percentage than controls. Although osmotic adjustment during drought was similar among cultivars, differences in specific mass of leaves and in ratios of transpiring and conducting tissues suggest cultivars of Freeman maple vary in resistance to drought in the landscape.

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N.S. Lang, R. Smithyman, L. Mills, R.L. Wample, J. Silbernagel, and E.M. Perry

Blackleaf (a.k.a. chocolate leaf) is of worldwide concern in Vitis due to its negative impact on fruit ripening, yield reduction and overall stress on grapevines. Research suggests blackleaf is induced by high levels of UV radiation and overall light intensity, which induce color changes (purple-brown-black) in exposed leaves, resulting in >50% reduction in photosynthesis. The ability to detect blackleaf symptoms before expression can provide insight into metabolic stresses and the possibility of the use and/or timing of management practices to reduce its impact. Remotely sensed imagery and spatial analysis may elucidate reflectance-related processes and symptoms not apparent to the un-aided eye. In this research we mapped canopy growth (leaves/shoot and shoots/vine), metabolic triggers (photosynthesis, leaf water potential, soil moisture), and percent blackleaf expression within vineyards using global positioning system (GPS), infrared gas analyzer, and digital remotely-sensed images. Each image and data record was stored as an attribute associated with specific vine location within a geographical information system (GIS). Spatial maps were created from the GIS coverages to graphically present the progression of blackleaf across vineyards throughout the season. Analysis included summary statistics such as minimum, maximum, and variation of green reflectance, within a vineyard by image capture date. Additionally, geostatistics were used to model the degree of similarity between blackleaf values as a function of their spatial location. Continuing research will be aimed at identifying spectral characteristics of early season stresses due to UV light, water stress, and reduced photosynthetic capacity. Spatial relationships between early season stress and later blackleaf expression will be assessed using joint spatial dependence measures. Overall, information obtained through digital image and spatial analysis will supplement site level information for growers.