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Sissel Torre, Tove Fjeld, Hans Ragnar Gislerød, and Roar Moe

Single node cuttings with one mature leaf were taken from Rosa ×hybrida `Baroness' and rooted in water culture. The plants were subjected to either 90% (high) or 70% (moderate) relative humidity (RH) in climate chambers. Single stem roses with intact roots were transferred to 40% (low) RH to investigate the stomatal response to water stress. Moderate RH plants showed decreasing leaf conductance from day 1 to day 3 during both light and dark phases, in contrast to high RH roses, which showed almost similar leaf conductances during the 3 days. Leaf samples were studied with a light microscope (LM) and a scanning electron microscope (SEM) to quantify morphological and structural changes. Epidermal imprints showed a significantly higher number of stomata and longer stomata, as well as a wider stomatal apertures on roses grown at high RH. The high RH leaves showed a reduced density of vascular tissue and thinner leaves when compared to moderate RH leaves. Enlarged intercellular air-space (ICA) was found due to a reduced number of spongy and palisade mesophyll cells. No obvious difference in shape, size, undulation or the structure of the epicuticular wax was observed in SEM between high and moderate RH grown leaves. In conclusion, roses subjected to high RH showed differences in leaf anatomy, stomatal morphology and stomatal function, which may explain the loss of water control of these plants. Stomatal ontogenesis should occur at RH conditions below 85% to secure roses with a high postharvest quality potential.

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Kourosh Vahdati, Zeinab Maleki Asayesh, Sasan Aliniaeifard, and Charles Leslie

1 ). However, stomatal aperture in leaves grown under high CO 2 was 34% narrower than stomatal aperture in controls and stomata were closed in leaves grown under high concentration of CO 2 ( Table 1 ; Fig. 2 ). Huge heterogeneity in stomatal size

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Jieshan Cheng, Peige Fan, Zhenchang Liang, Yanqiu Wang, Ning Niu, Weidong Li, and Shaohua Li

low sink demand ( Cheng et al., 2008 ; Li et al., 2001 ). Increased T leaf following a reduction in stomatal aperture has been proposed as a critical factor in regulating photosynthesis under low sink demand in fruit trees ( Li et al., 2005 ). These

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Carmen Valero Aracama, Michael E. Kane, Sandra B. Wilson, and Nancy L. Philman

surrounded by two large subsidiary cells and very small stomatal apertures. Leaves were amphistomatous and, based on visual assessment, stomatal density was greater on the adaxial surface. Yet, because stomata were present in the invaginations of the adaxial

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C.P. Sharma and Sandhya Singh

When grown in refined sand with one-twentieth normal K supply, cauliflower (Brassica oleracea L. var. botrytis L. cv. Pusi) had lower dry matter and tissue concentration of K than the controls and developed visible symptoms characteristic of K deficiency. In K-deficient plants, the specific leaf weight, diffusive resistance, and proline concentration in leaves were significantly higher and relative water content (RWC), leaf water otential (ψ), stomatal aperture, stomatal density, and transpiration rate were significantly lower than in control plants. When K-deficient plants were supplied additional Na to the extent K was deficient, Na concentration in the plants increased and the plants recovered from the K deficiency effect on free proline concentration, RWC, leaf water potential, stomatal aperture, stomatal density, specific leaf weight, diffusive resistance, and transpiration.

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Hening Hu and Darrell Sparks

Leaves of `Stuart' pecan [Carya illinoensis (Wangenh.) C. Koch] with various levels of Zn deficiency were analyzed for physiological indicators of leaf vigor. Leaf chlorophyll content, stomatal conductance, and net photosynthesis were adversely affected by Zn deficiency. In leaves with severe Zn deficiency, each of these indicators increased 3- to 5-fold with a doubling of leaf Zn concentration, but stabilized as leaf Zn approached the sufficiency range (14 μg·g-1). High intercellular CO2 associated with low net photosynthesis indicates that stomatal aperture was not the cause of the reduction of net photosynthesis under Zn deficiency.

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Elaine F. Smith, Ivana Gribaudo, Andrew V. Roberts, and John Mottley

Plantlets of Vitis vinifera L. `Moscato Bianco' were grown in vitro in cellulose plugs (Sorbarods) saturated with a modified Murashige and Skoog rooting medium. Both the inclusion of 0.5-l mg paclobutrazol/liter in the rooting medium and the use of culture vessels that reduce the relative humidity from 100% to 94% improved resistance of plantlets to wilting after transplanting. Maximum benefit was obtained with a combination of paclobutrazol and reduced humidity; it resulted in smaller stomatal apertures, shorter stems, reduced leaf area, and more and thicker roots. Chemical names used: (2RS, 3RS)-1-(4-chlorophenyl)-4,4-dimethyl-2-(1H-1,2,4-triazol-1-yl) pentan-3-01 (paclobutrazol).

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Cary A. Mitchel

Brief, periodic seismic (shaking) or thigmic (contact rubbing) stress treatments applied to plants growing in a wind-protected environment typically reduce but strengthen vegetative growth and often inhibit reproductive development. Cell division and cell enlargement both are affected. Mechanically dwarfed plants accumulate less leaf area than do undisturbed controls and undergo temporary stomatal aperture reduction following an episode of stress, leading to reduced photosynthetic productivity. Vibration or mild shaking may lead to a slight stimulation of plant growth. Most classes of phytohormones have been implicated to mediate different growth responses to mechanical stress. Physical perturbation turns on the transcription of several genes coding for calmodulin-like proteins. Calcium chelators and calmodulin inhibitors partially negate effects of thigmic stress. Growth rate responses of naive seedlings are immediate and dramatic, suggesting turgor collapse, whereas recoveries are slow and sometimes partial, suggesting reduced wall extensibility in the cell enlargement zone. Mechanical stress may be used for height control of intensively cultivated bench crops or to physically toughen bedding plants prior to outdoor transplant. Physiological hardening remains a question. Mechanical height control avoids use of chemicals but increases risk of wounding and pathogen infection.

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Pedro Perdomo, James A. Murphy, and Gerald A. Berkowitz

Understanding the factors influencing the performance of Kentucky bluegrass (Poa pratensis L.) cultivars under summer stress is necessary for developing criteria for identifying resistant germplasm. The objectives of this study were to evaluate two Kentucky bluegrass cultivars for leaf water (ψl) and osmotic potential (ψπ), stomatal resistance (Rs), leaf: air temperature differential (ΔT) and determine the relationship of these parameters to drought and heat tolerance. Stress-resistant (`Midnight') and susceptible (`Nugget') cultivars were evaluated in a field study during 1993 and 1994 under moisture-limiting conditions. Leaf water potential for `Nugget' was higher than for `Midnight' in 1993 and similar in 1994. `Midnight' had lower ψπ than `Nugget' during the evaluation period in 1994. `Midnight' maintained more open stomata (lower Rs) and lower ΔT than `Nugget' at the end of the dry down period when `Nugget' was showing visual signs of stress. `Midnight' and `Nugget' had similar root weight at the 0- to 45-cm depth zone in 1994. Lower basal osmotic potential (i.e., higher solute concentration) may be the physiological mechanism allowing larger stomatal aperture in `Midnight'. Greater transpirational cooling in `Midnight' relative to `Nugget' was correlated with higher turf quality for `Midnight'.

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Aaron J. Brown

Polyethylene glycol (PEG) was evaluated for its influence on hardening of in vitro-propagated `Fern' strawberries (Fragaria ×ananassa) when applied just before transplanting. Strawberries were micropropagated via shoot tips and grown in vitro until roots were well developed. Plantlets were then transferred onto filter paper bridges in liquid medium with 15% (w/v) of PEG-8000. After treatment in the medium for various periods, the plants were compared to the control (no PEG) for water loss from detached leaves, stomatal aperture, and survival rates after transplanting. Leaf epicuticular wax was also quantified. Overall, the in vitro PEG treatment was not successful in significantly increasing hardiness and survivability of the strawberry plants after transplanting from in vitro conditions to a soil medium. Osmotic stress was created, but apparently not for the time needed to increase survival. Further tests are needed to pinpoint the proper exposure time required to increase hardiness and survivability after transplanting plantlets. To increase survival, the time exposed to PEG should be 15, 18, or possibly 21 days.