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Toshio Shibuya, Akihito Sugimoto, Yoshiaki Kitaya, Makoto Kiyota, Yuichiro Nagasaka, and Shinya Kawaguchi

The leaf vapor conductance ( g l ) is a useful index for the management of plant water status. The value of g l is often estimated using porometry (e.g., Bakker, 1991 ). Bunce (2006) noted, however, that porometry is not suitable to evaluate

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C.L. Mackowiak, R.M. Wheeler, and N.C. Yorio

Leaf stomatal conductance was monitored with a steady-state porometer throughout growth and development of soybean and potato plants grown at 500, 1000, 5000, and 10,000 (potato only) μmol mol-1 carbon dioxide (CO2). All plants were grown hydroponically with a 12-hr photoperiod and 300 μmol m-2 s-1 PPF. As expected, conductance at 1000 was < 500 μmol mol-1 for both species, but conductance at 5000 and 10,000 μmol mol-1 was ≥ that at 500 μmol mol-1. Subsequent short-term (24-hr) tests with potato and wheat plants grown at 1000 μmol mol-1 showed that raising CO2 to approx. 10,000 μmol mol-1 or lowering CO2 to 400 μmol mol-1 increased conductance compared to 1000 μmol mol-1 for potato, while only lowering CO2 to 400 μmol mol-1 increased conductance for wheat. Furthermore, raising the CO2 to 10,000 μmol mol-1 increased dark-period conductance in comparison to 1000 μmol mol-1 for potato, while dark-period conductance for wheat leaves was low regardless of the CO2 concentration. Results suggest that very high CO2 levels (e.g. 5000 to 10,000 μmol mol-1) may substantially increase water use of certain crops.

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Leon H. Allen Jr., Mary P. Brakke, and James W. Jones

A water flow model was developed which uses irradiance, leaf-to-air vapor concentration difference, and soil water potential to establish stomatal conductance. Water flow to the roots was computed using a linear approximation of radial flow through the soil toward the axis of the roots across concentric shells. Root length density and soil rooting volume within four separate layers or compartments were included in the model. The simulation was executed in small time step iterations. A small increment of transpiration was translated to a water content deficit at the root and then sequentially through the concentric shells to simulate water uptake and change of soil water potential. The change in soil water potential was used to increment changes in stomatal conductance and transpiration. The output of the model simulated the pattern of diurnal stomatal behavior observed in other types of experiments, as well as the total soil water extraction patterns of young potted citrus trees.

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Steve C. Yuza, Art L. Youngman, and John C. Pair

This study examined physical factors and physiological responses of five different ecotypes and cultivars of Acer saccharum and A. nigrum. The objective was to determine variations in leaf conductance and xylem water potential and correlations associated with their natural geographic distribution. Compared were two ecotypes of sugar maple, Caddo and Wichita Mountains, native to Oklahoma with cultivars Green Mountain and Legacy, plus black maple seedlings from Iowa. Measurements taken included leaf conductance, xylem water potential and soil water potential in a replicated block of 15-year-old trees. The two ecotypes had consistently higher photosynthetic rates, stomatal conductance and transpiration rates than other selections. Xylem water potentials were significantly higher for Caddo maples than Green Mountain, Legacy and Acer nigrum in both predawn and midday samples. This difference in water availability can be associated with a tendency for Caddo to vary its stomatal conductance. The other tree types maintained stable stomatal conductances.

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F. Liu and H. Stützel

This study was designed to quantify the responses of leaf expansion, stomatal conductance, and transpiration of four genotypes of vegetable amaranth [Amaranthus tricolor L. (Hin Choi), A. tricolor L. (Co. 2), A. blitum L. (WS80-192), and A. cruentus L. (RRC 1027)] to soil drying. Two greenhouse experiments were conducted during 1999 and 2000. Soil water status was expressed as the fraction of transpirable soil water (FTSW). Leaf expansion rates, stomatal conductances, and transpiration rates of the stressed plants were determined relative to those of nonstressed plants, and expressed as relative leaf expansion (RLE), relative stomatal conductance (RSC), and relative transpiration (RT), respectively. The rate of soil water extraction differed among genotypes, with RRC 1027 depleting soil water fastest and Hin Choi slowest. Whereas in 1999 all genotypes were equally efficient in soil water use, RRC 1027 extracted a greater volume of transpirable soil water than the other genotypes in 2000. The responses of RLE, RSC, and RT to FTSW were well described by linear-plateau models which allowed calculation of soil-water thresholds for leaf expansion (CL), stomatal conductance (CS), and transpiration (CT). Values for CL were higher than for CS and CT. CL was similar for the four genotypes in each year, whereas, CS and CT differed among genotypes. CS and CT was lowest for Hin Choi and highest for WS80-192. Differences of CL, CS, and CT between the two experiments might have been due to the different soils used in the experiments and the different evaporative demands during the drought cycles. Under drought stress, the reduction of transpiration of vegetable amaranth was due mainly to reduction of stomatal conductance, not to reduction of leaf expansion. The relative reduction of dry weight caused by drought stress was positively correlated with CS or CT across the four genotypes. Variation in CS and CT among amaranth genotypes revealed different responses to drought stress, which could make them suitable for different drought situations.

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Baolin Zhang and Douglas D. Archbold

Plants of F. chiloensis cv. BSP14 (FC) and F. virginiana cv. NCC85-13V (FV) were stressed until wilting, then watered for 2 days prior to measurement. Diurnal measurements of leaf conductance and water relations were conducted. Leaf conductance of stressed FC plants was generally lower, than that of controls at most times, but there wee no difference between the two in FV. Leaf conductance and transpiration rates had not fully recovered to pre-stress levels within this recovery period, Leaf wafer potential declined from predawn to midday, more in stressed than control plants of both species. Leaf osmotic potential averaged 0.4 and 0.2 MPa lower in stressed than control FC and FV plants, respectively, Greater differences occurred at midday than predawn. Leaf pressure potential of stressed plants was higher predawn than midday, 1.4 vs. 0.7 MPa, in FC; it was not different for FV at most times. The difference in water relations between these two species may be explained by a greater residual effect from the osmotic adjustment in FC es compared to FV that occurred during prior water deficit stress.

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

We compared two putative Freeman maples [`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 on stomatal conductance. A method for quantifying changes in leaf color that occurred on flooded plants also was developed. Potted plants grown from rooted cuttings in a greenhouse were subjected to 75 days of root-zone inundation (flood treatment) or were irrigated frequently (control treatment). Across genotypes, stomatal conductance of flooded plants initially increased by about 20% and then fell to and was sustained below 50 mmol·s–1·m–2. Stomatal conductance of flooded plants of `Indian Summer' decreased to 20 mmo·s–1·m–2 after 8 days of inundation, and two of three flooded `Indian Summer' plants died during treatment. Other genotypes required at least twice this time to display a similar reduction in stomatal conductance, indicating `Indian Summer' may be particularly flood sensitive. Intensities of red, green, and blue color at a consistent interveinal position were analyzed with Visilog software by using scanned leaf images of the youngest fully expanded leaf of each plant in both treatments. A genotype × irrigation interaction existed for the ratio of green to red intensity. This method provided numerical data that corresponded well to differences among genotypes we observed visually. For example, while flooding did not alter the color of `Autumn Flame' leaves, the ratio of green to red was three times greater for controls of Autumn Blaze® than for the flooded plants of this cultivar.

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William L. Bauerle* and Joe E. Toler

A multiplicative model of stomatal conductance was developed and tested in two functionally distinct ecotypes of Acer rubrum L. (red maple). The model overcomes the main limitation of the commonly used Ball-Berry model by accounting for stomatal behavior under soil drying conditions. It combined the Ball-Berry model with an integrated expression of abscisic acid-based control mechanisms (gfac). The factor gfac = exp(-β[ABA]L) incorporated the stomatal response to abscisic acid (ABA) concentration in the bulk leaf tissue [ABA]L into the Ball-Berry model by down-regulating the slope and coupled physiological changes at the leaf level with those of the root. The stomatal conductance (gs) down regulation is pertinent in situations where soil drying may modify the delivery of chemical signals to leaf stomates. Model testing results indicated that the multiplicative model was capable of predicting stomatal conductance under wide ranges of soil and atmospheric conditions in a woody perennial. Concordance correlation coefficients (rc) were high (between 0.59 and 0.94) for the tested ecotypes under three different environmental conditions (aerial, distal, and minimal stress). The study supported the use of the gfac factor as a gas exchange function that controlled water stress effects on gs and aided in the prediction of gs responses.

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Paolo Sabbatini and James A. Flore

The naturally occurring carbon isotope composition (or 13C: 12C ratio, expressed with the notation d13C) of plant tissue may be used as an indicator of water use efficiency during plant growth. d13C has been shown to be an effective tool to study physiological response of plant to environmental conditions, especially water stress. The objective of this work was to test if d13C could be an indicator of carbon limitations or a low source: sink ratio. Trees of `Imperial Gala'/Bud 9 (n = 12), 6-years-old, field grown at the Clarksville Horticultural Research Station (Clarksville, Miss.), were assessed with different crop load (LCL = Low Crop Load, 0.76 ± 0.44 fruit per trunk sectional area (TCA); NCL = Normal Crop Load, 7.25 ± 1.83 fruit/TCA; HCL = High Crop Load, 15.83 ± 1.76 fruit/TCA) and leaf: fruit ratio (LCL: 52.78 ± 8.55, NCL: 13.33 ± 3.06, HCL; 4.31 ± 0.68) immediately following June drop. Net photosynthetic rate of leaves were monitored during the season and elevated rates were observed in NCL and HCL and correlated with the fruiting process. Photosynthesis was inhibited in LCL more in the afternoon (from 20% to 42% in relation to NCL) than in the morning (from 5% to 20%) and this was positively correlated with crop sink strength. Variations of the stable carbon isotope composition of roots (fine and coarse), fruit, leaves, and current-year stems were examined. The d13C varied by tissue (fruit > shoot and leaf > root) and in relation to the level of crop load (d13C‰ in fruit: LCL –23.513 ± 0.248, NCL –24.891 ± 0.594; and HCL –24.935 ± 0.375). These results may have implications for analysis of isotopic signals in carbohydrate stress and fractionation steps will be discussed.

Open access

Xiaotao Ding, Liyao Yu, Yuping Jiang, Shaojun Yang, Lizhong He, Qiang Zhou, Jizhu Yu, and Danfeng Huang

-visible spectrophotometer (Shimadzu ultraviolet-2700; Japan) at wavelengths of 663, 654, and 470 nm. Leaf photosynthesis measurements. The net photosynthesis rate (P n ), stomatal conductance ( g S ), intercellular CO 2 concentration (C i ), and transpiration rate (T r