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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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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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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.

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Rolston St. Hilaire and William R. Graves

Selection of sugar maples (Acer saccharum Marsh.) and black maples (Acer saccharum Marsh. ssp. nigrum Desm. or Acer nigrum Michx. f.) that will be more resilient than existing cultivars in managed landscapes could be facilitated by defining relationships between geographic origin and foliar traits critical to leaf function. We examined variation in leaf morphology and anatomy of both taxa, known collectively as hard maples, near 43 °N latitude and tested for relationships between foliar traits and the longitude of origin from 70 ° to 94 °W longitude. Leaves exposed to direct solar radiation were sampled from up to 20 trees indigenous at each of 42 sites during 1995 and 1996. All leaves from east of 75.84 °W and from 92.73 °W and further west expressed morphological characters associated with sugar maple and black maple, respectively; leaves with intermediate traits were found between these two longitudes. Leaves from 90 ° to 94 °W had the highest surface area due to increases in the areas of middle and proximal portions of laminae. Up to 1162 trichomes/cm2 were present on the abaxial surface of laminae from west of 85 °W, while laminae from further east were glabrous or had ≤300 trichomes/cm2. Laminae from western habitats also had relatively high stomatal frequency, and stomatal apertures of laminae west of 91 °W were particularly narrow. Longitude did not affect specific weight and thickness of laminae, which averaged 5.5 mg·cm-2 and 90 μm, respectively. Principal component analysis of laminar traits showed existence of two clusters. A large group dominated by data from trees in New England also contained data from trees as far west as ≈93 °W longitude; data for trees further west were clustered separately. Although phenotypic continua were defined, laminae west of 93 °W were distinct, which suggests trees selected there may function differently in managed landscapes than trees selected from native populations further east.

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Ben Hong Wu, Hai Qiang Huang, Pei Ge Fan, Shao Hua Li, and Guo Jie Liu

roles in photosynthetic reduction with weakened sink demand ( Li et al., 2001 , 2005 ). The depression in Pn was related to reduced stomatal aperture and increased Tl, suggesting that stomatal aperture might be considered the trigger or promoter and

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Geoffrey M. Weaver and Marc W. van Iersel

lose turgor pressure and reducing stomatal aperture ( Grabov and Blatt, 1998 ; Kim et al., 2010 ). This hormone may also indirectly induce stomatal closure by decreasing the hydraulic conductivity of leaf vascular tissue ( Pantin et al., 2012

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Frank G. Bethea Jr., Dara Park, Andrew Mount, Nick Menchyk, and Haibo Liu

-applied solutions. Eichert and Goldbach (2008) found significant differences in N uptake with stomatal aperture and stomatous vs. astomatous leaf surfaces, indicating the role of stomata in foliar uptake. The use of adjuvants has also been demonstrated to increase

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Donavon Sonnenberg, Patrick A. Ndakidemi, Ambrose Okem, and Charles Laubscher

leads to closure of stomatal aperture and this affects plant water status depending on atmospheric evaporative demand ( Silber et al., 2013 ). This phenomenon was reported in an earlier study, where stomatal functioning was greatly reduced as a result of

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David Jespersen and Brian Schwartz

stomatal aperture, density of stomata, as well as canopy characteristics such as leaf texture, length, and density all play important roles in regulating transpiration rates. Rooting characteristics are another major aspect affecting drought avoidance

Open access

Yang Li, Juanqi Li, Guoxiu Wu, Yanman Li, Aimin Shen, Deli Ma, and Shengli Li

perpendicular to the dumbbell-shaped guard cell the widest value. The stomatal aperture measurement was determined by measuring the aperture width of stomata. The stomatal aperture ratio is defined as the ratio of the opening stomatal number to the total