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Emad Bsoul* and Rolston St. Hilaire

Although valued for its fall foliage color, bigtooth maple (Acer grandidentatum Nutt.) is not widely used in managed landscapes. Furthermore, information on the tolerance of bigtooth maples to drought is scant. We studied water relations, plant development, and carbon isotope composition of bigtooth maples indigenous to New Mexico, Texas, and Utah. Plants were field grown in New Mexico using a pot-in-pot nursery production system. Plants were maintained as well-irrigated controls or irrigated after the weight of pots decreased by 35% due to evapotranspiration. Drought treatment lasted 71 days. Among the drought-stressed plants, plants native to Logan Canyon in Utah (designated UW2), had the greatest root: shoot dry weight ratio (3.0), while plants with the lowest root: shoot dry weight ratio (0.9) were half siblings from a tree native to the Lost Maples State Park in Texas (designated LMP5). Among the five sources we tested, LMP5 had the greatest (1242 cm2) leaf area, while UW2 plants had the smallest (216 cm2). Regardless of the treatment, plants from LMP5 had the highest shoot dry weight (25.7 g). Plants showed no differences neither among sources nor between treatments in relative water content, specific leaf weight, xylem diameter, root dry weight, plant dry weight, relative growth rate, and carbon isotope discrimination, which averaged - 26.53%. The lack of differences in these parameters might be due to selection of these sources from provenances we deemed to be the most drought tolerant. Our selection was based on the results of a previous greenhouse study of 15 bigtooth maple sources. We conclude that these sources, and in particular, plants from LMP5 in Texas, might hold promise for use in areas prone to drought.

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Emad Bsoul, Rolston St. Hilaire and Dawn M. VanLeeuwen

Although bigtooth maple (Acer grandidentatum Nutt.) is an ornamental plant that might thrive in managed landscapes in arid and semiarid regions, little information on the drought tolerance of bigtooth maples appears to be available. We studied water relations, plant development, and carbon isotope composition of bigtooth maples indigenous to New Mexico, Texas, and Utah that were field-grown in New Mexico using a pot-in-pot nursery production system. Plants were maintained as well-irrigated controls or irrigated after the weight of pots decreased by 35% due to evapotranspiration. Bigtooth maples subjected to drought had more negative predawn leaf water potentials (−0.76 MPa) than the plants in the control treatment (−0.64 MPa). Drought did not affect midday leaf water potential of seed sources. Trees native to the Lost Maples State Natural Area in Vanderpool, TX (designated LMP5), had the greatest leaf area (1236 cm2) among plants from all sources, while those native to Logan Canyon in Cache County, UT (designated UW2), had among the smallest leaf area (216 cm2). Leaf area ratio (LAR) was highest in plants from LMP5 (24.23 cm2·g−1), which suggests that they have potential for more carbon assimilation than the other plants tested. Plants from LMP5 had the highest leaf area/xylem diameter ratio (135 cm−2·mm−1). This ratio was 5.8 times higher than that of UW2, which had among the lowest leaf area/xylem diameter ratios. The high leaf area/xylem diameter ratio of LMP5 plants relative to UW2 plants indicates that LMP5 plants had a larger surface area of tissues that transpire relative to those that transport water. Treatment did not affect stomatal conductance (g S) or transpiration, but g S and transpiration were positively correlated for both drought-stressed (r 2 = 0.801) and well-irrigated plants (r 2 = 0.759). Plants from New Mexico (designated DS) had the lowest transpiration rate (2.32 mmol·m−2·s−1), lowest g s (52.1 mmol·m−2·s−1), largest xylem diameter (11 mm), and had among the largest shoot dry weight (DW) and plant height. Plants did not differ either among sources or between treatments in the ratio of variable to maximal fluorescence (mean = 0.64), relative water content (averaged 57%), specific leaf weight, stem DW, root DW, and plant DW. Carbon isotope discrimination (Δ) averaged −26.53‰ and did not differ among plant sources or irrigation treatments. This suggests that Δ might not be effective in screening bigtooth maples for drought tolerance. Low transpiration rate, g S, and high shoot dry weight in DS plants and traits, such as a high LAR in plants from LMP5, suggest that plants selected from these provenances might effectively endure deficit irrigation.

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Dawn M. VanLeeuwen, Rolston St. Hilaire and Emad Y. Bsoul

Statistical analysis of data from repeated measures experiments with missing factor combinations encounters multiple complications. Data from asynchronous cyclic drought experiments incorporate unequal numbers of drought cycles for different sources and provide an example of data both with repeated measures and missing factor combinations. Repeated measures data are problematic because typical analyses with PROC GLM do not allow the researcher to compare candidate covariance structures. In contrast, PROC MIXED allows comparison of covariance structures and several options for modeling serial correlation and variance heterogeneity. When there are missing factor combinations, the cross-classified model traditionally used for synchronized trials is inappropriate. For asynchronous data, some least squares means estimates for treatment and source main effects, and treatment by source interaction effects are inestimable. The objectives of this paper were to use an asynchronous drought cycle data set to 1) model an appropriate covariance structure using mixed models, and 2) compare the cross-classified fixed effects model to drought cycle nested within source models. We used a data set of midday water potential measurements taken during a cyclic drought study of 15 half-siblings of bigtooth maples (Acer grandidentatum Nutt.) indigenous to Arizona, New Mexico, Texas, and Utah. Data were analyzed using SAS PROC MIXED software. Information criteria lead to the selection of a model incorporating separate compound symmetric covariance structures for the two irrigation treatment groups. When using nested models in the fixed portion of the model, there are no missing factors because drought cycle is not treated as a crossed experimental factor. Nested models provided meaningful F tests and estimated all the least squares means, but the cross-classified model did not. Furthermore, the nested models adequately compared the treatment effect of sources subjected to asynchronous drought events. We conclude that researchers wishing to analyze data from asynchronous drought trials must consider using mixed models with nested fixed effects.

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Emad Bsoul, Rolston St. Hilaire and Dawn M. VanLeeuwen

Ecological traits such as an extensive range of natural distribution and tolerance to varying soil conditions, suggest that bigtooth maples (Acer grandidentatum Nutt.) could be popular landscape trees. But information on the tolerance of bigtooth maples to environmental stresses, such as drought, is virtually nonexistent. We studied physiological, growth and developmental traits of bigtooth maple plants from 15 trees native to Arizona, New Mexico, Texas, and Utah. Plants were grown in pots in a greenhouse and maintained as well-irrigated controls or exposed to drought and irrigated in cycles based on evapotranspiration. The ratio of variable to maximal fluorescence (Fv/Fm) was not different between drought-stressed and control plants, but the low Fv/Fm in plants designated as LM2 from the Lost Maples State Natural Area in Vanderpool, Tex., suggests these plants were relatively inefficient in capturing energy at PSII. Plants from another tree (LM5) originating from Lost Maples State Natural Area maintained similar predawn water potentials between drought-stressed and control plants after five cycles of drought. Plants from Dripping Springs State Park in Las Cruces, N.M., and those from LM2 had a strong, significant linear relationship between transpiration and stomatal conductance. Drought-stressed plants from Dripping Springs State Park, two plant sources from the Guadalupe Mountains in Salt Flat, Tex., designated as GM3 and GM4, and plants from trees designated as LM1 and LM2, had high relative growth rates and net assimilation rates. Drought-stressed plants from three of the four Guadalupe Mountain sources (GM1, GM3, GM4) had among the longest and thickest stems. Drought reduced shoot and root dry weight (DW). Although bigtooth maples showed several provenance differences in drought adaptation mechanisms, the lack of an irrigation effect on biomass allocation parameters such as root to shoot DW ratio and leaf area ratio implies that altered biomass allocation patterns may not be a common drought adaptation mechanism in bigtooth maples. Plants from selected provenances from the Guadalupe Mountains and Lost Maples State Natural Area in Texas, and to a lesser extent, provenances from Dripping Springs State Park in New Mexico might hold promise for selecting bigtooth maples for arid environments.