ENVIRONMENTAL AND ENDOGENOUS CONTROLS ON LEAF-LEVEL AND STAND-LEVEL WATER CONDUCTANCE IN A SCOTS PINE PLANTATION

Measurements of leaf level gas exchange and conductance, tree transpir ation via sapflow monitoring, soil moisture and water extraction, pred awn water potential, and xylem abscisic acid (ABA) concentration were carried out over the course of the 1993 and 1994 summer seasons at the Hartheim Pinus sylvestris plantation on the Upper Rhein Plain, German y. Periodic leaf level conductance determinations with porometry estab lished a maximum value of ca 280 mmol m(-2) s(-1) (13.6 mm s(-1)). Hal f maximal conductance was attained at 40 mu mol m(-2) s(-1) and 90 % o f light saturation occurred at ca 500 mu mol m(-2) s(-1) PPFD. Conduct ance decreased strongly with increases in vapor pressure deficit above 10 hPa, while the temperature optimum was 22 degrees C at light satur ation. Strong restrictions on maximum conductance at both leaf and sta nd levels were apparent below a soil moisture content of 16 volume per cent. Although less strongly, conductance also decreased with initial drying of the upper soil layers and decreases in predawn water potenti al from -0.4 to -0.6 MPa. In this range of water potential change, xyl em ABA increased to between 200 and 500 nmol L-1. Thus, an immediate l eaf-level reaction to the onset of summer weather conditions is observ ed, i.e. leaf conductance and water use decrease. We hypothesize that ABA functions as a key control on water balance, transmitting informat ion about soil water status and endogenously modifying canopy response in order to budget water and avoid extensive cavitation damage in mos t years. Transpiration potential of the stand was reduced by thinning during autumn 1993 in approximate proportion to changes in leaf area i ndex and sapwood area. Simultaneous observations of sapflow and conduc tance have allowed us to view the effects of leaf conductance on whole plant water use, while thinning revealed the effects of stand level p henomena on conductance regulation. ((C) Inra/Elsevier, Paris.).