Why are the stomata of ponderosa pine open at night?
Alonso, Rocio*,1, Nguyen, Tham1, Cascio, Chiara2, Grulke, Nancy1, 1 USDA-Forest Service, PSW Research Station, Riverside, CA2 University of Florence, Firenze, Italy ABSTRACT- Ponderosa pine (Pinus ponderosa Laws) is a widespread conifer in western U.S. This species experiences significant drought stress in late summer in Mediterranean regions of California (var. ponderosa). We describe the phenomena of the lack of stomatal closure during the night in early summer in trees in the Mediterranean region at two sites that experience significant pollutant exposure (ozone, nitrogen oxides), but not at a third site with low pollutant exposure. Nighttime stomatal conductance (gs) ranged from one third to one half that of midmorning values. Young trees (40-yr-old) had greater gs than mature trees (250-yr-old) at the two polluted sites. In late summer, mature trees did not have measurable gs during the night at any site, but gs of young trees was approximately one sixth that of midmorning values.
