Calcium sulfate deposits associated with needle substomatal cavities of container-grown longleaf pine (Pinus palustris) seedlings

Extracellular calcium sulfate (CaSO4) formations associated with substomata l cavities of longleaf pine (Pinus palustris Mill.) are described. Longleaf pine seedlings were grown with two levels of soil nitrogen (N) (40 or 400 kg N ha(-1) yr(-1)) and water stress (-0.5 or -1.5 MPa xylem pressure poten tial) in open-top field chambers under two levels of atmospheric CO2 (365 o r 720 mu mol mol(-1)). Needles were subjected to scanning electron microsco py after 12 mo exposure to experimental conditions. Crystalline to fibrilla r formations, appressed to surfaces of guard cells facing the interior of t he needle, were observed in all treatments. In some cases, both crystalline and fibrillar formations were observed to occur within the same needle cro ss section. Formations were characterized as calcium sulfate using energy-d ispersive spectrometry. Crystal-like CaSO4 appeared to originate from guard cells in the vicinity of the stomatal aperture. Formations may arise from evaporation of plant water at the interface between stomatal antechambers a nd substomatal cavities, leaving Ca and SO4 behind to precipitate. Many que stions remain regarding their ecological and physiological significance as well as their occurrence and prevalence in both time and space.