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Krishna Nemali* and Marc van Iersel

Subjecting bedding plants to non-lethal moisture stress is an established irrigation practice for bedding plants; however information on physiological responses of bedding plants to moisture stress is limited. We examined the CO2 exchange rates (CER) and water relations of salvia (Salvia splendens) and vinca (Catharanthus roseus) during moisture stress. Seedlings of both species were grown from seed in 7-L trays containing a soilless growing medium. After plants completely covered the trays, they were irrigated and shifted into whole-plant gas exchange chambers (27 °C and daily light integral of 7.5 mol/m2) arranged inside a growth chamber. Inside the gas exchange chambers, the growing medium was allowed to dry and plants were re-watered after wilting. Results from this study indicate that the growth rate (moles of CO2 gained by plants in a day) of salvia was higher than vinca before experiencing moisture stress; however the volumetric moisture content of the growing medium at which plant growth decreased was higher for salvia than for vinca. During moisture stress, the decrease in growth rate of salvia was gradual and that of vinca was rapid. After re-watering the plants, leaf water potential (ΨL) and growth rate of vinca revived completely, and ΨL of salvia remained low (more negative), whereas its growth rate revived completely. This study shows that bedding plant species respond differently to moisture stress, particularly with respect to the critical substrate moisture level for initiating moisture stress and the rate of development of moisture stress.

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Zhongchun Wang and Gary W. Stutte

Abbreviations: ψ P , leaf turgor potential; ψ s , leaf osmotic potential; ψ W , leaf water potential; DPM, disintegration per minute; MEOH, methanol; Pn, photosynthesis; RWC, relative water content; Rs, stomatal resistance. 1 Current address

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Tomasz Anisko and Orville M. Lindstrom

The purpose of the present study was to determine whether water stress affects tolerance of Rhododendron L. `Catawbiense Boursault' to rapid freezing. Tolerance to freezing at cooling rates of 2 or 6C/hour in stems and leaves of plants subjected to continuous and periodic water deficit stresses was examined. Under continuous stress treatments, water content of the growing medium was maintained in a range of 0.60 to 0.75, 0.45 to 0.60, or 0.30 to 0.45 m3·m–3 between 24 Aug. 1992 and 11 Feb. 1994. Under periodic stress treatments, water content of the growing medium was maintained near field capacity, i.e., 0.6 to 0.8 m3·m–3, for the duration of the study or plants were subjected to the periodic stress at various times between 15 July and 19 Feb. during 2 years. Watering of water-stressed plants was delayed until water content reached below 0.4 m3·m–3, and then was resumed to maintain water content in the range of 0.3 to 0.4 m3·m–3. Cold hardiness was evaluated in the laboratory with freeze tolerance tests on detached leaves and stem sections. In most cases, cooling at 6C/hour caused injury at higher temperature than cooling at 2C/hour. The difference in lethal temperature between the two cooling rates depended on the level of the plant's cold hardiness. In plants cold hardy to about –25C, freezing at 6C/hour caused injury at a temperature ≈3C higher than freezing at 2C/hour. The effect of cooling rate was not evident in plants cold hardy to about –18C. Subjecting plants to continuous or periodic water stress did not have an effect on this relationship.

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Mengmeng Gu, Curt R. Rom, and James A. Robbins

Poster Session 25—Stress Physiology 19 July 2005, 1:15–2:00 p.m. Poster Hall–Ballroom E/F

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Thomas R. Clarke

Irrigation scheduling can be improved by directly monitoring plant water status rather than depending solely on soil water content measurements or modeled evapotranspiration estimates. Plants receiving sufficient water through their roots have cooler leaves than those that are water stressed, leading to the development of the crop water stress index, which uses hand-held infrared thermometers as tools for scheduling irrigations. However, substantial error can occur in partial canopies when a downward-pointing infrared thermometer measures leaf temperature and the temperature of exposed, hot soil. To overcome this weakness, red and near-infrared images were combined mathematically as a vegetation index, which was used to provide a crop-specific measure of vegetative cover. Coupling the vegetation index with the paired radiant surface temperature from a thermal image, a trapezoidal two-dimensional index was empirically derived capable of detecting water stress even with a low percentage of canopy cover. Images acquired with airborne sensors over subsurface drip-irrigated muskmelon (Cucumis melo L.) fields demonstrated the method's ability to detect areas with clogged emitters, insufficient irrigation rate, and system water leaks. Although the procedure needs to be automated for faster image processing, the approach is an advance in irrigation scheduling and water stress detection technology.

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Alvan G. Gaus and George M. Greene II

Water stress in mature `Redhaven' / Lovell peach [Prunus persica (L.) Batsch] trees was imposed, during the 1988 growing season. Trickle irrigation was reduced from 100% to 25% of a calculated weekly evaporation amount on 22 June, 11 July, and 8 and 27 Aug. Trees were isolated from rainfall by tents under the canopy and from horizontal water movement between root systems on 4 sides to a depth of 0.5 m by a water-proof barrier. Canopy to air temperature differentials monitored throughout the growing season were developed into 3 stress indexes: crop water stress index (CWSI); cumulative crop water stress index (CCWSI); and postharvest cumulative crop water stress index (PCCWSI). CWSI values varied from 0 to 0.6, while both CCWSI and PCCWSI increased through late Sept. Mean PCCWSI of the 22 June 25% treatment increased at a greater rate than the other treatments. Significant linear regressions were found with some of the indexes and net photosynthesis or stomatal conductance; however, the r-square values were low. In general, no linear relationships were found between either CCWSI of PCCWSI and the Index of Injury for cold hardiness.

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Tomasz Anisko and Orville M. Lindstrom

89 POSTER SESSION 13 Temperature Stress/Cross-Commodity

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Aliza Benzioni, David A. Palzkill, and John M. Nelson

Flower bud dormancy and anthesis patterns, ABA concentration, and bud survival following frost were studied in eight jojoba [Simmondsia shinensis (Link) Schneider] clones grown under two irrigation regimes (water stress and well irrigated). Several clones broke dormancy in the autumn. Anthesis in the field before winter occurred only in one clone (a male) in the well-irrigated treatment. Buds on water-stressed plants broke dormancy earlier than those on well-watered plants, but anthesis in the field occurred later in the year. Buds on water-stressed plants were less affected by a severe frost than those on control plants (43% vs. 10% survival). There were large differences among clones in the amount of frost damage. ABA levels did not correlate with dormancy patterns or with the amount of frost damage. Chemical name used: S-(Z,E)-5-(1-hydroxy-2,6,6-trimethyl-4-oxo-2-cyclohexen-1-yl)-3-methyl-2,4-pentadienoic acid (ABA).

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Llngxiao Zhang

The effect of water stress on photosynthesis was investigated in strawberry plants to see responses of different aged-leaves within the same plant. Preliminary results indicated that, under severe stress (SS) conditions, young leaves had lower water potentials and higher photosynthetic CO2 assimilation rates than old leaves had, due to higher stomatal conductance in young leaves. This situation was not found in moderately stressed or well–watered plants, probably because of the higher non-stomatal limitation in old leaves under SS condition. Under SS condition, old leaves had a higher intracellular CO2 concentration. Osmotic adjustment or acclimation might occur during slow drying process, so that the young leaves could adjust their stomata and still remain open under low water potentials.

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Barbara Kocourkova, Craig S. Charron, Charles Graves, Norman K. Lownds, and Carl E. Sams

High glucosinolate content in brassica meal is a limiting factor in consumption of rapeseed. In recent years canola cultivars of rapeseed with decreased glucosinolate content have been developed. However, environmental and nutritional factors are also believed to influence glucosinolate content. This study was conducted to determine the relationships among water stress, B nutrition, and glucosinolate content in canola. Two canola cultivars (`Cyclone' and `American A112') were grown in a continuously recirculating hydroponic system with modified Hoagland solution (0.6 ppm B). Water stress was induced gradually (2% per day using polyethylene glycol 8000) starting when plants were 4 weeks old. Osmotic potential was maintained at –0.1 MPa (high stress level), –0.085 MPa (medium stress), or 0.05 MPa (control). Treatments were arranged in a randomized incomplete-block design, with three blocks, four replications, two cultivars, and three treatments. Upper leaves (no. 15 and higher) were collected and analyzed by inductively coupled plasma emission spectrometry for B content. Total and indole glucosinolate content of seeds were measured colorimetrically and by HPLC. The leaf B content of stressed plants decreased by 55% in `Cyclone' and 29% in `American A112'. Total glucosinolate content increased 28% and 12%, respectively, in stressed plants of `Cyclone' and `American A112'. Indole glucosinolate content was 44% and 13% higher in the same plants. The interaction between cultivar and water stress was not significant for glucosinolate content but was significant for B content of the leaves.