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<br />ROOD, BRAATNE AND HUGHES
<br />Table 3. Recent studies of physiological responses to drought stress imposed by river damming and flow diversion, water table depression, abrupt
<br />water table decline or other experimental treatments.
<br />Response
<br />Duration
<br />References
<br />Altered shoot growth
<br />Weeks to season(s)
<br />Mahoney and Rood 1991, 1992, Braatne et al. 1992, Segelquist et al. 1993,
<br />Shafroth et al. 1994, Barsoum and Hughes 1998, Somogyi et al. 1999,
<br />Hughes et al. 2000, Rood et al. 20006, Amlin and Rood 2002
<br />Reduced shoot elongation
<br />Days to weeks
<br />Mahoney and Rood 1991, 1992, Busch et al. 1992, Willms et al. 1998,
<br />Scott et al. 1999, Hughes et al. 2000, Rood et al. 2000b, Horton et al. 2001 c,
<br />Amlin and Rood 2002
<br />Reduced leaf area
<br />Days to weeks
<br />Smith et al. 1991, Braatne et al. 1992, Mahoney and Rood 1992,
<br />Busch and Smith 1995, Van Splunder et al. 1996, Scott et al. 1999,
<br />Rood et al. 20006
<br />Reduced trunk expansion
<br />Weeks to season(s)
<br />Stromberg and Patten 1990, 1991, 1992, 1996, Willms et al. 1998,
<br />Scott et al. 1999, 2000
<br />Leaf senescence and
<br />Weeks
<br />Mahoney and Rood 1992, Scott et al. 1999, Somogyi et al. 1999,
<br />mortality
<br />Amlin and Rood 2003, Cooper et al. 2003
<br />Branch sacrifice
<br />Weeks to season(s)
<br />Scott et al. 1999, Rood et al. 2000a, Horton et al. 2001 b, Cooper et al. 2003
<br />Crown die -back
<br />Multiple seasons
<br />Stromberg and Patten 1991, 1992, Scott et al. 1999, Shafroth et al. 2000,
<br />Horton et al. 2001 b, 2001 c, Cooper et al. 2003
<br />Altered root growth
<br />Days or longer
<br />Mahoney and Rood 1992, Shafroth et al. 1995, 2000, Van Splunder et al. 1996,
<br />Hughes et al. 2000, Rood et al. 20006
<br />Mortality
<br />Weeks to season(s)
<br />Mahoney and Rood 1991, Segelquist et al. 1993, Shafroth et al. 1998, 2000, 2002,
<br />Scott et al. 1999, 2000, Somogyi et al. 1999, Taylor et al. 1999, Rood et al. 20006,
<br />Vandersande et al. 2001, Amlin and Rood 2002, Guilloy - Froget et al. 2002,
<br />Sprenger et al. 2002, Cooper et al. 2003
<br />Stomatal closure
<br />Minutes
<br />Smith et al. 1991, Braatne et al. 1992, Mahoney and Rood 1992,
<br />(reduced transpiration)
<br />Busch and Smith 1995, Van Splunder et al. 1996, Loewenstein and Pallardy 1998,
<br />Sparks and Black 1999, Zhang et al. 1999, Rood et al. 2000b,
<br />Horton et al. 2001 a, 2001 b, 2001 c, Vandersande et al. 2001,
<br />Amlin and Rood 2002, 2003, Cooper et al. 2003
<br />Reduced photosynthesis
<br />Minutes to hours
<br />Horton et al. 2001 a, 2001 b, 2001c
<br />Reduced midday
<br />Minutes to hours
<br />Smith et al. 1991, Busch and Smith 1995, Sparks and Black 1999,
<br />xylem water potential
<br />Rood et al. 20006, Horton 2001 a, Amlin and Rood 2003, Cooper et al. 2003
<br />Reduced predawn
<br />Hours to day(s)
<br />Rood et al. 20006, Horton et al. 2001 a, 2001 b, 2001 c, Cooper et al. 2003
<br />water potential
<br />Increased xylem cavitation
<br />Days to weeks
<br />Braatne et al. 1992, Tyree et al. 1994, Sparks and Black 1999, Rood et al. 2000a
<br />Altered phytohormones
<br />Hours to days
<br />Loewenstein and Pallardy 1998, Rood et al. 2000b
<br />Reduced 13C in tissues
<br />Weeks to year(s)
<br />Leffler and Evans 1999, Horton et al. 2001 c
<br />that are compared with their interspecific hybrids.
<br />Aspects of the water relations of cottonwoods that have
<br />been extensively investigated by one or more of these ap-
<br />proaches are summarized in Table 3. The physiological re-
<br />sponses of native cottonwoods to groundwater depletion have
<br />been fairly confidently established, and can be summarized as
<br />follows.
<br />As the balance between groundwater availability and tran-
<br />spirational demand becomes unfavorable, xylem water poten-
<br />tial declines (Table 3). Usually, this leads to rapid stomatal
<br />closure, although there are substantial differences across cot-
<br />tonwood species and genotypes (Bassman and Zwier 1991,
<br />Tschaplinski et al. 1994, Sparks and Black 1999, Dunlap and
<br />Stettler 2001), and preconditioning may be required, particu-
<br />larly in R trichocarpa T. & G. (Schulte et al. 1987). Stomatal
<br />closure, as indicated by reduced stomatal conductance, is the
<br />primary mechanism by which most plants reduce transpiration
<br />to conserve water during drought cycles. With stomatal clo-
<br />sure, COZ uptake is also reduced, which can reduce photosyn-
<br />thesis and subsequent growth.
<br />In severe or sustained drought conditions, stomatal closure
<br />is insufficient to maintain a favorable water balance. Subse-
<br />TREE PHYSIOLOGY VOLUME 23, 2003
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