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Nicole L. Waterland, John J. Finer, and Michelle L. Jones

d. Stomatal conductance readings were taken using a steady-state porometer (LI-1600; LI-COR, Lincoln, NE). Three basal leaves per plant were tagged and the same leaves were used for g S measurements at each time point. Stomatal conductance was

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James Bunce

the chloroplast, the “mesophyll conductance,” and how it may be affected by water stress and by carbon dioxide concentration as well as taking into account the well-known difficulties in leaf gas exchange analysis presented by “patchy” stomatal closure

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Steven J. McArtney, Suzanne R. Abrams, Derek D. Woolard, and Peter D. Petracek

, 2.5, 10, 25, or 50 mg·L −1 or with ABA at 0, 25, 100, 250, or 500 mg·L −1 . A non-ionic organosilicone surfactant (Break Thru) was included with all of the treatments at 0.05%. Stomatal conductance was measured 1 d after treatment on five fully

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

end products and activities of related enzymes. A positive linear relationship was observed between stomatal conductance ( g s ) and P n in previous studies, showing that stomatal closure was an important factor in the decline in photosynthesis with

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John Erwin, Tanveer Hussein, and David J. Baumler

O). Table 1. Variation in the instantaneous photosynthetic rate (P n ; μmol·m −2 ·s −1 ), stomatal conductance ( g S ; mmol H 2 O/m 2 /s), transpiration rate (E; mmol H 2 O/m 2 /s), cuvette leaf temperature (T leaf ; °C), and relative humidity after

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Octavio Arquero, Diego Barranco, and Manuel Benlloch

The effects of potassium (K) status and water availability in the growth medium on growth, water content, water-use efficiency and stomatal conductance was studied in mist-rooted `Chemlali de Sfax' olive (Olea europaea L.) cuttings grown in a perlite substrate. Potassium starvation produced dehydration of all parts of the plant, reduced shoot growth and water-use efficiency. By contrast, K starvation enhanced stomatal conductance in well-irrigated plants and, even more, in water-stressed plants. These results suggest that moderate K deficiency in olives may impair the plant's ability to regulate stomatal closure; this may account for the dehydration observed in K-starved plants, particularly in situations of water stress. This result is of great importance for agricultural practices of this crop, because K status, which may not be considered deficient, can cause disorders in olive trees.

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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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Mary Ann Rose and Mark A. Rose

A closed-loop photosynthesis system and a heat-balance sap-flow gauge independently confirmed oscillatory transpiration in a greenhouse-grown Rosa hybrids L. Repetitive sampling revealed 60-minute synchronized oscillations in CO2-exchange rate, stomatal conductance, and whole-plant sap-flow rate. To avoid confusing cyclical plant responses with random noise in measurement, we suggest that gas-exchange protocols begin with frequent, repetitive measurements to determine whether transpiration is stable or oscillating. Single measurements of individual plants would be justified only when transpiration is steady state.

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K.A. Shackel, V. Novello, and E.G. Sutter

The relative contribution of stomatal and cuticular conductance to transpiration from whole tissue-cultured apple shoots of Malus pumila Mill. M.26 was determined with a modified steady state porometer. When shoots were exposed to 90% RH and high boundary layer conductance, large (73%) and, in some eases, rapid (2 to 3 hours) reductions in leaf conductance occurred, indicating functional stomata. Stomatal closure was also observed microscopically. A maximum estimate for the cuticular conductance of these apple leaves was 18 to 40 mmol·m-2·s-1, which is lower than previous estimates and close to the upper limit of naturally occurring leaf cuticular conductances. Hence, both stomatal and cuticular restrictions of water loss appear to be of importance in determining the water balance of tissue-cultured apple loots. The pathway of water transport in relation to water stress of tissue-cultured shoots is also discussed.

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Robert C. Ebel, Xiangrong Duan, and Robert M. Augé

Mycorrhizal colonization can alter stomatal behavior of host leaves before or during soil drying, but the mechanism of influence is not always clear. We examined the possibility that mycorrhizal symbiosis might result in either altered stomatal sensitivity to abscisic acid (ABA) moving from roots to shoots in xylem sap, or altered movement of ABA in xylem as a function of soil water content (θ). Mycorrhizal colonization of Vigna unguiculata did not change the relationship between stomatal conductance (g s) and xylem [ABA] during drying of whole root systems. Stomatal conductance was higher in mycorrhizal than in similarly sized and similarly nourished nonmycorrhizal plants when soil moisture was relatively high, perhaps related to lower xylem [ABA] in mycorrhizal plants at high soil θ. Neither g s nor xylem [ABA] was affected by mycorrhizae at low soil θ. Higher g s in mycorrhizal plants was evidently not related to a mycorrhizal effect on leaf water status, as neither g s/shoot Ψ nor shoot Ψ/soil θ relationships were altered by the symbiosis. Stomatal conductance was much more closely correlated with xylem [ABA] than with soil θ or shoot Ψ. Decreased xylem [ABA] may explain why mycorrhizal colonization sometimes increases g s of unstressed mycorrhizal plants in the absence of mycorrhizae-induced changes in host nutrition. This work was supported by USDA NRICGP grant 91-37100-6723 (R.M.A).