Water use patterns and stomatal responses to environment in longleaf pine on contrasting sites
Addington, Robert Norris
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Plants growing in different environments may require variable stomatal behavior to regulate their water loss. This study investigated how variation in water supply at two contrasting sites influenced water use and stomatal responses to environment in mature longleaf pine (Pinus palustris Mill.). At low vapor pressure deficits (D), maximum rates of leaf stomatal conductance (gS), photosynthesis (Anet), sap-flux scaled transpiration (EL), and mean canopy stomatal conductance (GSi) were higher for trees on a sandy (xeric) site relative to trees on a sandy-loam (mesic) site in southwest Georgia, USA. Trees on the xeric site, however, showed a more sensitive stomatal closure response to increasing D, and a greater in crease in instantaneous water use efficiency from morning to afternoon relative to mesic site trees. Use of a multiplicative model that incorporated global solar radiation (Rg), D, and leaf water potential (?L) explained an average 71% of the variation in GSi on the mesic site and 76% on the xeric site, and indicated that soil water limitation played a more important role in regulating GSi on the xeric site. Enhanced stomatal sensitivity to D in xeric site trees is consistent with higher rates of stomatal conductance at low D, and may be necessary in avoiding critically low ?L associated with the soil moisture limitation. Other factors that may contribute to the sitedifferences in maximum leaf gas exchange include leaf nitrogen and/or tree height.