Gas exchange and its factorial dependency in field-grown Brassica napus L.

The purpose of the present held study was to derive factorial dependencies (including sub models) between gas exchange (conductance and photosynthesis) and plant water potential and micrometeorological factors based on physiological measurements in rape (Brassica napus L.). In canopy models, such relationships are useful for scaling up photosynthesis and transpiration from leaf or organ to crop level. The factorial dependencies were derived from midday measurements during soil drying causing different levels of plant water stress and then tested against other sets of diurnal measurements. In leaves and pods, net photosynthesis (A(n)) as a function of light intensity was well. described by empirical logarithmic functions on the basis of parameters characterizing initial slope and maximum photosynthesis. By relating A(n) at light saturation to conductance (g(H2O)) for leaves and pods, our data indicated that in non-water stressed plants, stomates regulated CO2 diffusion rate, so that the internal carbon dioxide concentration (C-i) during photosynthesis was close to the normal C-3 plant CO2 transition point of 250 mu l l(-1). However, water stress caused a further decrease in C-i which significantly increased the slope of A(n) over g(H2O) and caused an increase in the 'instantaneous WUE'. The factorial dependency of g(H2O) of leaves and pods on their water potential (psi), photosynthetic active radiation (PAR) and leaf- or pod-to-air water vapour concentration difference (D), was combined in empirical factorial stomatal models. In pods, a close agreement between measured and predicted g(H2O) values was found. However, a closer agreement for short-term fluctuations could be obtained if the level of photosynthesis was taken into account as a factorial parameter. This might reflect a coordination between levels of CO2 assimilation and stomatal conductance controlled by C-i. In leaves, substantial discrepancies occurred at a high evaporative demand mainly because a low water potential in fully watered plants simulated stomatal closure. The need to relate stomatal responses to soil water status when simulating stomatal behaviour is discussed. (C) 1998 Elsevier Science B.V.