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Kuo-Tan Li, Jackie Burns, Luis Pozo, and Jim Syvertsen

To determine the effects of abscission compounds 5-chloro-3-methyl-4-nitro-1H-pyrazole (CMNP) and ethephon on citrus leaf function and water relations, we applied CMNP at 0, 200, 500, 1000, or 2000 ppm, or ethephon at 400 or 800 ppm, to canopies of fruiting potted and field citrus trees during the harvest season. Both compounds induced fruit and leaf drop after 3 days of application, especially at high concentrations. Low concentrations of CMNP (0, 200, or 500 ppm) or either ethephon treatments did not affect leaf photosystem II efficiency, as indicated by leaf chlorophyll fluorescence (Fv/Fm). High concentrations of CMNP (1000 or 2000 ppm) immediately reduced photosystem II efficiency in leaves and fruit peel. However, Fv/Fm of leaves remaining on the trees was gradually restored and close to the level of control after 4 days of treatment. Both compounds had little effect on chlorophyll content, ratio of chlorophyll a to chlorophyll b, leaf water content, and mid-day leaf water potential. The results suggest that CMNP at recommended concentrations (200 to 500 ppm) effectively reduced fruit attachment force with little herbicidal effect on leaves.

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Said Ennahli and Sorkel Kadir

Partial root-zone drying (PRD) irrigation management has been developed for grapevines as an efficient method to control excessive growth, improve fruit quality, and save water without compromising yield. PRD is based on knowledge of the mechanisms that control transpiration and requires slow dehydration of half of the plant root system, whereas the other half is irrigated. A study was conducted in the field to evaluate the effect of PRD on physiological characteristics, growth, yield, and fruit quality of three grape cultivars. The wetting and drying cycle of the PRD-vine root system is alternated on a 10–14 day schedule. Significant reduction in vigor was observed in treated plants compared with control plants. Root biomass was not affected, but fine roots significantly increased in PRD-treated plants, compared with that of the control. This contributed to the ability of PRD-treated plants to maintain leaf water potential similar to that of the control. Stomatal conductance of PRD plants was significantly reduced when compared with that of the control plants. Abscisic acid (ABA) concentration in leaves of PRD vines increased significantly when compared to the control vines. PRD treatment significantly increased yield and fruit quality when compared with the control treatment. PRD significantly increased water use efficiency (pruning weight per unit of water applied). This study shows that PRD stimulated ABA production in the drying roots, which caused reduction in stomatal conductance and transpiration rate, leading to a substantial reduction in vegetative growth without compromising yield and fruit quality.

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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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Madhulika Sagaram and Leonardo Lombardini

Pecan is a riparian species distributed over an area of geographic and climatic variation; such a wide distribution produces exposure to varied environmental conditions, providing a potential for genetic adaptation within the cultivars. Genotypes can be screened in order to obtain more drought tolerant cultivars using indirect screening parameters (chlorophyll fluorescence, osmotic adjustment, and abscisic acid assay) based on physiological responses of plants to abiotic stress conditions. A study was established at Texas A&M University, College Station, using a mixture of fritted clay (Quick dry) and pure sand in 1:1 (by weight) ratio to study the effects of drought on pecan rootstocks. The experiment was set up with the three water potential levels as treatments (–0.033 MPa, –0.1 MPa, –0.3 MPa) in a randomized complete-block design with three blocks. Measurements will include leaf water relations (relative water content, leaf water potential, osmotic adjustments, etc.), gas exchange parameters [net carbon dioxide assimilation rate (A), transpiration rate (E), stomatal conductance (gs)], chlorophyll fluorescence measurements [minimum (Fo), maximum (Fm), and variable fluorescence (Fv), quantum efficiency], water use efficiency, and abscisic acid assay on roots. Statistical analysis systems (SAS) package will be used for analysis. PROC GLM of the SAS will be used for statistical analysis of study involving plant response to water potential levels.

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Roger Kjelgren and Mike Foutch

Growth and water relations of seedlings grown in protective tree shelters were investigated during establishment in a field nursery. Shelters, 1.2 m high, were placed over 0.5 m Kentucky coffee tree seedlings following spring transplanting in a field experiment. Predawn leaf water potential (ψ) and stomatal conductance (gs) were monitored periodically through the season and growth was measured in late summer. In a second experiment diurnal microclimate, and seedling water relations and use, in the shelters were studied under controlled conditions. In the shelters, leaf and air temperature, humidity, and gs exceeded non-sheltered levels while solar radiation was 70% lower. Despite greater gs, normalized water use was 40% lower in the sheltered trees. While midday gs was similarly high in the field-grown trees, no differences in predawn ψ were detected through the season. Sheltered trees in the field had four times more shoot growth but 40% less caliper growth. Sheltered trees had leaf thickness lower than control trees, and together with the growth and radiation pattern, indicated that they were shade acclimated. Shelters can improve height growth and reduce water loss during establishment, but may not allow sufficient trunk development or taper for upright support

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Dariusz Swietlik

Sour orange seedlings were grown in water culture to which one of seven aromatic compounds, associated with allelopathic effects, was added to produce concentrations ranging from 0.5 to 2.0 mM. Leaf water potential (ψ1), leaf stomatal conductance (gs), and whole plant transpiration (T) were measured during a 7-day treatment period. At the end of that period, the total and average leaf surface area, shoot elongation, and fresh weight gain of seedlings were determined. Solutions of vanillic, coumaric, and ferulic acids of 2mM concentration reduced ψ1, gs, and T. Reductions of gs, and T but not (ψ1) occurred when vanillic acid of 1mM concentration was applied. Solutions of vanillic (0.5; 1.0; 2.0mM), coumaric (1; 2mM), cinnamic (1mM), or chlorogenic (1; 2mM) acids reduced fresh weight gain of seedlings. Only the coumaric and chlorogenic acids treatments of 2mM concentration reduced shoot elongation. No treatment affected total or individual leaf area. Gallic and caffeic acids had no effect on sour orange water relations and growth.

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John E. Fucik and Dariusz Swietlik

Water extracts of cocklebur,CBX (Xanthium spinosa L.) and velvetleaf,VLX (Abutilon theophrasti Medic.) shoots and Mexican ash,AshX (Fraxinus Berlandieriana A.DC.) roots were added to 9 month-old sour orange Citrus aurantium L.) seedlings(SOs) in water culture. Final extract concentrations represented either 50 or 12.5 g. of plant material liter-1 of culture solution, i.e. 1/20 or 1/80 dilutions. Leaf water potential(ψ); stomatal conductance(gs);transpiration(T) and growth responses were measured for 13 days. After 1 day, SOs in AshX and CBX had lower ψ than controls. After 11 days SOs in CBX had higher ψ than the others. ψ responded similarly to both extract concs.. Thru day 5, AshX decreased gs vs. the controls and VLX. By day 11, gs of SOs in AshX was less than for VLX but not the others. On days 1 and 5, gs for VLX at 1/20 was lower than controls but at 1/80, gs's were the highest of all treatments. These results supported by the T rates, growth responses and others findings suggest AshX and VLX induce water stress by reducing water absorption and/or transport in addition to possibly disrupting normal root/shoot communications

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Maria G. Janssen and Albert H. Markhart III

Tepary beans (Phaseolus acutifolius Gray) are more drought tolerant and have stomata that are more sensitive to low leaf water potentials (ψ w) than common beans (P. vulgaris L.). This study was designed to examine the role of ABA in controlling stomatal behaviour in these species. Comparison of the bulk leaf ABA content does not explain why tepary stomata are more sensitive to low leaf ψ w compared to common bean (at -1.4 MPa ABA content increased 40-fold in common bean and 25-fold in tepary). We hypothesize that the greater sensitivity of tepary stomata to low leaf ψ w is related to a higher concentration of ABA in the xylem sap, and/or to a greater sensitivity of tepary stomata to ABA. Xylem sap of well-watered and water stressed plants is analyzed to determine the concentration of ABA, and whether ABA is a putative candidate serving as a chemical root signal in response to water stress in Phaseolus. To test stomatal sensitivity to ABA, epidermal strips and detached leaves are exposed to a range of ABA concentrations. The relationship between stomatal aperture and different ABA concentrations is discussed.

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Stan D. Wullschleger and Derrick M. Oosterhuis

Growth-chamber studies were conducted to examine the ability of seven vegetable crops-`Blue Lake' bean (Phaseolus vulgaris L.), `Detroit Dark Red' beet (Beta vulgaris L.), `Burgundy' okra (Abelmoschus esculentus (Moench), `Little Marvel' pea (Pisum sativum L.), `California Wonder' bell pepper (Capsicum annuum L.), `New Zealand' spinach (Spinacia oleracea L.), and `Beefsteak' tomato (Lycopersicon esculentum Mill.)–to adjust osmotically in response to water-deficit stress. Water stress was imposed by withholding water for 3 days, and the adjustment of leaf and root osmotic potentials upon relief of the stress and rehydration were monitored with thermocouple psychrometers. Despite similar reductions in leaf water potential and stomata1 conductance among the species studied, crop-specific differences were observed in leaf and root osmotic adjustment. Leaf osmotic adjustment was observed for bean, pepper, and tomato following water-deficit stress. Root osmotic adjustment was significant in bean, okra, pea, and tomato. Furthermore, differences in leaf and root osmotic adjustment were also observed among five tomato cultivars. Leaf osmotic adjustment was not associated with the maintenance of leaf growth following water-deficit stress, since leaf expansion of water-stressed bean and pepper, two species capable of osmotic adjustment, was similar to that of spinach, which exhibited no leaf osmotic adjustment.

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C. Olivella, C. Biel, R. Savé, and M. Vendrell

The effects of flooding (3 days) and recovery (3 days) on leaf gas exchange parameters (stomatal conductance, net photosynthetic assimilation rate, and leaf transpiration rate) were studied in five, 1-year-old gerbera (Gerbera jamesonii Bolus) cultivars under greenhouse conditions. Flooding reduced the measured parameters in all cultivars, which had not recovered 3 days after flooding ceased. A more detailed study was conducted with the `Beauty' cultivar, in which leaf water potential (Ψw); root hydraulic resistance (Rh); and hormonal concentrations of ABA, IAA, and CK were measured in leaves and roots. Plants were flooded for 2 days, then allowed to recover for 4 days. After 2 days of flooding, Ψw had decreased and Rh had increased. Leaf ABA content increased and IAA and CK content decreased from the start of flooding and did not change during recovery. In roots a transient increase in root ABA levels occurred during flooding and a sharp decrease was measured during recovery, which was related to root death. Similar patterns were observed in root CK concentrations. The IAA concentration in roots remained constant throughout the experiment. The results suggest that ABA and CK may act as signals of flooding stress. Also, the marked intolerance of gerbera to flooding could be a serious barrier to its culture under anaerobic conditions, and hence careful irrigation management is required. Chemical names used: abscisic acid (ABA); indoleacetic acid (IAA); cytokinin (CK).