GAS-EXCHANGE AND ITS FACTORIAL DEPENDENCY IN FIELD-GROWN BRASSICA-NAPUS L

The purpose of the present held study was to derive factorial dependen cies (including sub models) between gas exchange (conductance and phot osynthesis) and plant water potential and micrometeorological factors based on physiological measurements in rape (Brassica napus L.). In ca nopy models, such relationships are useful for scaling up photosynthes is and transpiration from leaf or organ to crop level. The factorial d ependencies were derived from midday measurements during soil drying c ausing different levels of plant water stress and then tested against other sets of diurnal measurements. In leaves and pods, net photosynth esis (A(n)) as a function of light intensity was well. described by em pirical logarithmic functions on the basis of parameters characterizin g initial slope and maximum photosynthesis. By relating A(n) at light saturation to conductance (g(H2O)) for leaves and pods, our data indic ated that in non-water stressed plants, stomates regulated CO2 diffusi on rate, so that the internal carbon dioxide concentration (C-i) durin g photosynthesis was close to the normal C-3 plant CO2 transition poin t of 250 mu l l(-1). However, water stress caused a further decrease i n C-i which significantly increased the slope of A(n) over g(H2O) and caused an increase in the 'instantaneous WUE'. The factorial dependenc y of g(H2O) of leaves and pods on their water potential (psi), photosy nthetic active radiation (PAR) and leaf- or pod-to-air water vapour co ncentration difference (D), was combined in empirical factorial stomat al models. In pods, a close agreement between measured and predicted g (H2O) values was found. However, a closer agreement for short-term flu ctuations could be obtained if the level of photosynthesis was taken i nto account as a factorial parameter. This might reflect a coordinatio n between levels of CO2 assimilation and stomatal conductance controll ed by C-i. In leaves, substantial discrepancies occurred at a high eva porative demand mainly because a low water potential in fully watered plants simulated stomatal closure. The need to relate stomatal respons es to soil water status when simulating stomatal behaviour is discusse d. (C) 1998 Elsevier Science B.V.