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Kenneth A. Shackel, H. Ahmadi, C. Greve, and J. Labavitch

The pressure microprobe was used to measure cell turgor (ψp) in tomato pericarp tissue, and also to sample vacuolar fluid for the measurement of cell osmotic potential (ψs) in a nanoleter freezing point osmometer. In fresh tissue, cell ψs agreed well with the ψs of frozen-thawed whole tissue measured with a vapor pressure osmometer. Under a wide range of ripeness conditions however, and for both intact fruit and discs of fruit tissue, fruit cell turgor was consistently lower than expected, based on the values of cell ψs. When tissue discs were hydrated in aerated distilled water, disc fresh weight increased substantially (20 - 50+%), and both cell turgor and tissue ψs increased. Cell ψs however, remained relatively constant. These and other observations suggest that the turgor increase during hydration was largely due to losses of solute from the apoplastic space, partly by direct losses from the tissue, and partly by cell solute accumulation.

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Paul L. Owen and Wallace G. Pill

Seeds of `Ace 55VF' tomato were soaked in solutions of -1.0 MPa Instant Ocean™ (inorganic salt mixture) or -0.6 MPa polyethylene glycol 8000 (P.E.G.) at 25 C for 1 week. `Mary Washington' asparagus seeds were soaked in the same solutions for 2 weeks. In solutions of decreasing matric or osmotic potentials, primed seeds germinated faster than untreated seeds. Germination percentages of primed seeds generally were greater than those of untreated seeds when water stress exceeded -0.5 MPa. All primed seeds, whether dried to a low moisture content or not, germinated faster than untreated seeds after storage for up to 3 months at 4 C or 20 C. Primed asparagus seeds germinated most rapidly and synchronously after storage at 4 C and high moisture content. Storage temperature and seed moisture content had no effect on subsequent germination of primed tomato seeds.

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Kenneth A. Shackel, H. Ahmadi, C. Greve, J. Labavitch, Liesbeth Verstreken, Paul Chen, and Jim Thompson

The pressure microprobe has been used to measure cell turgor and, in addition, to sample vacuolar tissues. In carrot, a rapid initial loss of tissue firmness (instron technique) occurred when the tissue was heater (cooked), and this could be entirely attributed to a loss in cell turgor. Turgor was well-correlated to firmness over the range of turgor measurements (0–0.8 MPa). In cherry and other fruits, turgor is typically 1 to 2 orders of magnitude lower than that expected based on cell osmotic potential, indicating the presence of apoplastic solutes. Cherry fruit firmness and cell turgor were well-correlated during the first 2 h of hydration at 20C, but, as fruit began to crack, tissue decreased, whereas turgor continued to increase.

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

Freeman maples (Ace×freemanii E. Murray) are marketed as stress-resistant alternatives to red maples (Acer rubrum L.). Our objective was to compare two cultivars of Freeman maple [`Jeffersred' (Autumn Blaze®) and `Indian Summer'] and five red maples [`Franksred' (Red Sunset®), `Autumn Flame', `PNI 0268' (October Glory®), `Fairview Flame', and unnamed selection 59904] for effects of flooding and water deficit on plant growth, biomass partitioning, stomatal conductance, and leaf osmotic potential. Plants grown from rooted cuttings in containers were subjected to three consecutive cycles during which root-zone water content decreased to 0.12, 0.08, and 0.02 m3·m–3, respectively. Additional plants were flooded for 75 days, while plants in a control treatment were irrigated frequently. Stomatal conductance immediately before imposing drought and after three drought cycles did not differ among genotypes and averaged 220 and 26 mmol·s–1·m–2, respectively. Differences in stomatal conductance after recovery from the first drought cycle and at the end of the second drought cycle did not vary with species. Drought reduced estimated leaf osmotic potential similarly for all genotypes; means for drought-stressed and control plants were –1.92 and –1.16 MPa, respectively. Freeman maples had a higher mean root: shoot weight ratio and a lower leaf surface area: root dryweight ratio than did red maples. Across genotypes, stomatal conductance of flooded plants initially increased by ≈20% and then fell to and remained below 50 mmol·s–1·m–2. Stomatal conductance of `Indian Summer' decreased to ≈20 mmol·s–1·m–2 after 8 days of flooding, indicating that this cultivar may be particularly sensitive to root-zone saturation.

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A.E. Dudeck, C.H. Peacock, and J.C. Wildmon

Salt tolerance in grasses is needed due to increased restrictions on limited fresh water resources and to saltwater intrusion into groundwater. St. Augustinegrass [Stenotaphrum secundatum (Walt.) Kuntze] is used widely as a lawngrass in states bordering the Gulf of Mexico. We describe the response of four St. Augustinegrass cultivars to solution cultures differentially salinized with synthetic seawater. A sea salt mixture was added to half-strength Hoagland's No. 2 nutrient solution to provide six salinity treatments ranging from 1.1 to 41.5 dS·m-1. Adjustments in leaf water potential, leaf osmotic potential, and leaf turgor potential were measured as salt levels were increased gradually at 2-day intervals over 10 days. Salinity effects on growth of top, crown, and root of each cultivar were measured over 3 months. Turfgrasses differed in their response, but were consistent in adjustment in leaf water potential and in leaf turgor potential as salinity increased. Leaf water potential, leaf osmotic potential, and leaf turgor potential decreased linearly with increased salinity, but a positive turgor of 0.1 MPa was maintained at a salt concentration equal to that of seawater. `Seville', the most salt-tolerant St. Augustinegrass cultivar, exhibited a 50% reduction in top growth at 28.1 dS·m-1, while `Floratam', `Floratine', and `Floralawn' St. Augustinegrasses showed the same reduction in top growth at 22.8 dS·m-1. Differences between cultivars were greatest at salinity levels <10 dS·m-1, where `Seville' was twice as salt-tolerant compared to other cultivars. The grasses did not die, although top growth of all cultivars was severely reduced at a salt level equal to seawater.

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Guntur V. Subbarao, Raymond M. Wheeler, L.H. Levine, and Gary W. Stutte

Accumulation of glycinebetaine occurs in Chenopodiaceae members and is thought to assist in osmotic adjustment and protect cytoplasm from sodium toxicity. Red beet has an ability to tolerate high tissue sodium levels, which may result in increased glycinebetaine production. To test this hypothesis, two cultivars of red beet ['Scarlet Supreme' (SS) and `Ruby Queen' (RQ)] were grown under nonsaline (4.75 mM Na) and saline (54.75 mM Na) conditions in a recirculating hydroponic system for 42 days at elevated CO2 (1200 μmol•mol-1) in a growth chamber. Leaf glycinebetaine level, relative water content, and osmotic potential were measured at weekly intervals. Leaf glycinebetaine levels increased with plant age and reached a maximum of 67 μmol•g-1 dw under nonsaline and 101 μmol•g-1 dry weight (dw) under saline conditions at 42 days in SS; in RQ, the glycinebetaine levels reached a maximum of 91 μmol•g-1 dw under nonsaline and 121 μmol•g-1 dw under saline conditions by 26 days. The mean glycinebetaine levels were increased over two-thirds under saline conditions in both the cultivars. RQ accumulated significantly higher (37% more under nonsaline, and 46% more under salinity) glycinebetaine than SS. The turgid leaf osmotic potential of RQ was consistently higher than SS under nonsaline (2.23 MPa in RQ vs. 1.82 MPa in SS) and saline (2.48 MPa in RQ vs. 2.02 MPa in SS) conditions. The results indicate that higher glycinebetaine levels in the leaf could result in better osmotic adjustment, and glycinebetaine accumulation in red beet can vary among cultivars and is strongly affected by external salinity.

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Norman K. Lownds, Larry S. Kennedy, and Carl E. Sams

Rapid cycling brassica (RCB) plants, because of their short life cycle and ease of growth under laboratory conditions, offer a valuable tool for studying Brassica nutrition. We have been particularly interested in B nutrition in Brassica and, therefore, a hydroponic system was developed to accurately deliver micronutrient concentrations to RCB plants. RCB plants were supported in predrilled holes in the lids of brown 1-L plastic containers. Nutrients were supplied by spraying a modified Hoagland's solution onto the plant roots as they developed inside the containers. This system provided adequate solution aeration for plant growth and allowed analysis of both plant shoots and roots. RCB seeds were pregerminated for radicle emergence, then placed in the holes in the plastic container lids. The effect of B nutrient concentration on B uptake was examined using nutrient solutions containing 0.08, 0.02 and 0.00 ppm added B. Leaf B contents were 139.5, 26.1, and 7.1 g·g–1 for plants grown in 0.08, 0.02 and 0.00 ppm added B, respectively. Effects of drought stress on B uptake and distribution were studied by adjusting nutrient solution osmotic potential using polyethylene glycol (PEG) 8000. PEG-induced drought, (osmotic potential –0.1 MPa) reduced leaf and root B content ≈50% compared to plants grown in nutrient solution only (–0.05 MPa). Boron content in the shoots and pods, however, was not affected by PEG-induced drought stress. These results suggest that this system provides a reliable tool for studying nutrition and drought stress effects using RCB plants.

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Roger Kjelgren and Craig Spihlman

Limited root development of nursery stock in root-control bags facilitates harvest but without irrigation may predispose stock to water stress. The effect of bags and irrigation on growth and water relations of field-grown Malus sieboldii var. zumi were investigated following transplanting as large liners into a silty-clay soil. Predawn leaf water potential (ψ), and midday stomatal conductance (gs) and ψ, were measured periodically through the season. Late-season osmotic potential (ψπ), caliper, leaf area, and root growth were also measured. Non-irrigated treatments exhibited water stress during an extended mid-summer drought, as predawn ψ and particularly gs were less than irrigated treatments, resulting in lower vegetative growth and ψπ. For combined bagged treatments water relations did not differ, but leaf area, root growth, and ψπ, but not caliper, were less than non-bagged trees. Growth measurements and ψπ of non-irrigated bagged trees, however, were consistently lower but nonsignificant than the other treatments. Bag-induced root reduction can limit some top growth even with optimum soil water. Moreover, in terms of potential Type-II errors extrapolated over a conventional production cycle, trees grown in root-control bags in normally non-irrigated soils may be more susceptible to water stress and subjected to further cumulative growth limitation.

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D.R. Edwards and M.A. Dixon

Six-year-old trees were repeatedly conditioned by withholding irrigation until Ψpd (predawn) thresholds of either –0.9 (“mild”) or –1.4 MPa (“moderate”) were attained. After conditioning, trees were exposed to severe drought (Ψpd –2.0 MPa) and then to 10 days of well-watered conditions. Throughout the investigation, osmotic potential (Ψπ), leaf RWC, transpiration, and total water potential (Ψx) were measured. Water stress was quantified by integrating Ψx. Conditioning caused a significant, but modest, degree of osmotic adjustment (0.08 to 0.28 MPa), which persisted after a brief relief from stress and transpiration rates were reduced 35% to 50%. Osmotic adjustment was not significantly enhanced by more than one stress exposure or conditioning beyond the mild threshold of stress. During severe drought, the moderate group maintained less negative Ψx and lower transpiration rates (38%). After prolonged stress relief, Ψx was similar among all treatments and daily transpiration rates and Ψx gradually recovered. Thuja occidentalis appears to rely on increased stomatal resistance more than osmotic adjustment to tolerate drought stress.

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Shaun F. Kelly, James L. Green, and John S. Selker

The process of fertilizer diffusion was examined using KBr and NaBr salts placed at the top of columns filled with a container medium at an initial water content of 4.0, 2.5, or 1.0 g·g-1 (mass of water/mass of medium). Columns were sealed to create a protected diffusion zone (PDZ) shielding the system from water infiltration and evaporation. Bromide and water distributions were determined after 5, 10, 25, and 120 days. Using a Fickian diffusion model, effective diffusion coefficients calculated for Br- in the medium at 2.5 g·g-1 ranged from 2.7 to 4.6 × 10-6 cm2·s-1, which is 3 to 9 times less than the diffusion coefficient in water alone. Diffusion rates increased with increasing medium water content. Differences in the hygroscopicity and solubility of KBr and NaBr affected the distribution of water and diffusion rates in the columns. Redistribution of water was driven to a significant degree by vapor-phase transport, caused by large gradients in osmotic potential, and was most apparent at low water content. At high water content, water redistribution was affected by solution density gradients in the system. This significantly complicates the mathematical modeling of the system, which renders a simple Fickian diffusion model of limited predictive value in high and low water content media.