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D. Joseph Eakes, Robert D. Wright, and John R. Seiler

Abbreviations: EC, electrical conductivity; MSC, moisture stress conditioning; P L , leaf turgor potential; PV, pressure-volume; RWC, relative leaf water content; SWC, symplastic water content; ψ L , leaf water potential; π 100 , π 0 osmotic

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Robert A. Saftner, William S. Conway, and Carl E. Sams

Changes in tissue water relations, cell wall calcium (Ca) levels and physical properties of Ca-treated and untreated `Golden Delicious' apples (Malus×domestica Borkh.) were monitored for up to 8 months after harvest. Pressure infiltration of fruit with CaCl2 solutions at concentrations up to 0.34 mol·L-1 reduced both fruit softening and air space volume of fruit in a concentration-dependent manner. Turgor potential-related stress within the fruit persisted during storage and was higher in Ca-treated than in untreated fruit. Fruit that were pressure infiltrated with CaCl2 solutions between 0.14 and 0.20 mol·L-1 and then waxed to reduce water loss during storage showed no peel injury. Calcium efflux patterns from apple tissue disks indicated two distinct Ca compartments having efflux kinetics consistent with those for cell wall Donnan-phase bound and water free space soluble Ca. At Ca concentrations up to 0.20 mol·L-1, cell wall bound Ca approached saturation whereas soluble Ca showed a linear dependence. At higher external Ca concentrations, only soluble Ca in the tissue increased. During 8 months of cold storage, cell wall Ca-binding capacity increased up to 48%. The osmotic potential of apples harvested over three seasons ranged between-1.32 and -2.33 MPa. In tissue disks, turgor potential changes caused by adjusting the osmolality of the incubation solution with CaCl2 or sorbitol were accompanied by changes in the osmotic and water potentials of the tissue. In CaCl2 solutions up to 0.34 mol·L-1, turgor potential was ≥0.6 MPa in tissue incubated in 0.14 or 0.17 mol·L-1 solutions of CaCl2 and was more than 3 times higher than in tissues incubated in low (≤0.03 mol·L-1) or high (≥0.27 mol·L-1) concentrations of CaCl2. At osmotically equivalent concentrations, turgor potential was up to 40% higher in Ca-than in sorbitol-treated tissue. The results suggest that postharvest treatment with 0.14 to 0.20 mol·L-1 solutions of CaCl2 are best for maintaining fruit water relations and storage life of `Golden Delicious' apples while minimizing the risk of salt-related injuries to the fruit. While higher concentrations of CaCl2 may better maintain firmness, these treatments adversely affect fruit water relations and increase the risk of fruit injury.

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Cindy Tong, Darryl Krueger, Zata Vickers, David Bedford, James Luby, Ahmed El-Shiekh, Kenneth Shackel, and Hamid Ahmadi

Many studies of apple (Malus ×domestica Borkh.) softening have been done using cultivars that eventually become mealy. We wanted to determine whether observations in these studies would be seen in a cultivar that maintains its crispness. In this paper, we compared the texture, ultrastructure, and some physiological parameters of Honeycrisp, an apple cultivar introduced in 1991 by the Minnesota Agricultural Experiment Station, with its parents and Delicious. Sensory evaluations and instrumental texture measurements showed that `Honeycrisp' maintained a crisp texture from harvest through 6 months of cold storage, whereas its parents, `Macoun' and `Honeygold', softened over the same time period. Turgor potential, cell wall composition, and ultrastructural comparisons of the fruit were made. Cell turgor potentials of `Honeycrisp' and `Delicious' were similar and greater than those of `Macoun' and `Honeygold', and clearly correlated with firmness. There were no differences in cell wall neutral sugar composition, except for arabinose, which was not highly correlated with crispness. `Honeycrisp' fruit maintained cell wall integrity after 6 months of storage, while cell walls of `Macoun' and `Honeygold' deteriorated. These data show that it is important to compare more than one cultivar when studying crispness. Honeycrisp is a cultivar that maintains its crispness through long storage without controlled atmosphere conditions. After 6 months of storage, this crispness can be attributed to a maintenance of high turgor potential and cell wall integrity.

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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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Robert Savé, Josep Peñuelas, Oriol Marfà, and Lydia Serrano

Field-grown strawberry (Fragaria × annanasa Duch. cv. Chandler) plants were subjected to two irrigation regimes from Nov. 1989 to July 1990 to evaluate the physiological and morphological effects of mild water stress. Irrigation was applied when soil matric potential reached -10 and-70 kPa for the wet and dry treatments, respectively. During the spring, these regimes did not promote significant changes in plant water relations, transpiration rates, plant morphology, or canopy architecture. However, during the summer, after several stress cycles, significant differences between treatments were observed. Pressure-volume curves of dry-treatment plants indicated that leaf osmotic potentials, measured at full and zero turgor, decreased 0.2 to 0.4 MPa. This decrease in osmotic potential also was accompanied by a 50% increase in the modulus of elasticity for these water-stressed plants compared to well-watered plants. Dry-treatment plants also showed stress avoidance mechanisms in changes of whole-plant morphology and canopy architecture, from monolayer to polylayer leaf distribution and leaf orientation from south to north. Despite what would appear to be useful drought-resistance strategies, there was significantly lower fruit production by plants grown under the dry treatment.

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Baolin Zhang and Douglas D. Archbold

Abbreviations: FC, Fragaria chiloensis (L.) Duch. `BSP14'; FV, Fragaria virginiana (L.) Duch. `NCC85-13V'; RWC, relative water content; Ψ water potential; Ψ P , turgor potential; Ψ p osmotic potential; Ψπ 100 , osmotic potential at full turgor

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Terence L. Robinson and Bruce H. Barritt

In unstressed apple seedlings (Malus domestics Borkh.), concentrations of free abscisic acid (ABA) decreased in order from apical stem sections, immature expanding leaves, mature stem sections, and mature leaves. PEG-induced water stress stimulated a 2- to 10-fold increase in free ABA concentrations 1 day after treatment, depending on the amount of stress and the tissue. By the 3rd day of stress, free ABA concentrations were nearly the same as the unstressed treatment and remained low for the remainder of the 21-day stress period. Bound ABA concentrations were an order of magnitude lower than free ABA and were not influenced dramatically by water stress. Shoot growth rate, leaf expansion rate, and leaf emergence rate were reduced by water stress in relation to the severity of the stress; this reduction was associated with the initial increase in ABA. However, there was no increase in shoot or leaf growth rates associated with the decline in ABA concentrations by day 3 as growth rates remained depressed on water-stressed plants throughout the 21-day stress period. Water stress reduced evapotranspiration rate and midshoot leaf water potential (ψW)after 1 day, but leaf osmotic potential (ψS) adjusted more slowly, resulting in a loss of leaf turgor. The reduction in leaf turgor pressure (ψP) was highly correlated with decreased shoot growth rate and increased ABA concentrations on day 1 after treatment. By the 3rd day of water stress, ψP bad recovered even in the most severe treatment, and the recovery of turgor was associated with the drop in ABA concentrations. However, the increase in midshoot ψP and the decline in ABA were not associated with any increase in shoot growth rate. The continued inhibition of shoot growth was probably not related to ABA or turgor pressure of mature leaves but may have been related to turgor pressure in the growing tip.

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Douglas D. Archbold

Maintenance of positive cell turgor is an essential factor in cell, and fruit, expansion. Since apple fruit partition carbohydrates between the starch and soluble pools to maintain turgor, variation among cultivars in this osmoregulatory aspect may play an important role in defining cultivar-specific fruit growth rates. Cultivar-specific apple fruit growth rates were determined over a 6 week period following June drop during 2 seasons. Fruit water relations parameters and carbohydrate levels were also measured. Although cultivar differences were evident, generally, fruit absolute growth rate increased, relative growth rate (RGR) declined, water potential and osmotic potential declined, and turgor potential increased as the season progressed. Soluble carbohydrate levels increased over 6 weeks, while starch levels fluctuated. Soluble carbohydrates contributed 50 to 90% of the osmotic potential. RGR was not correlated to either turgor potential or the relative allocation of carbohydrates between the soluble and starch pools. Thus, although positive turgor was maintained, factors other than turgor per se determine fruit growth rate.

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Rémy E. Milad and Kenneth A. Shackel

Abbreviations: DFB, days after full bloom; Ψ, water potential; Ψ P , turgor potential; Ψ S , osmotic potential. 1 Graduate Student/Research Assistant. The cost of publishing this paper was defrayed in part by the payment of page charges. Under

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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.