RELATIONSHIP BETWEEN NET PHOTOSYNTHETIC RATE AND STOMATAL CONDUCTANCEIN LEAVES OF FIELD-GROWN MAIZE SUBJECTED TO SOIL COMPACTION OR SOIL DRYING

Leaf water potential (Psi(l)), stomatal conductance for CO2 (g(l)) and net photosynthetic rate (P-N) Were measured in maize (Zea mays L.) in field conditions after silking, in 1987, 1990 and 1991. Plants grown on either a favourable or a compact soil structure (1987) were compare d to those produced under soil drying (1990) or not drying (1991). All the observed (P-N, g(l)) couples decreased with decreasing predawn Ps i(l), regardless of the origin of the stress (soil compaction or water stress). For a given g(l) little differences occurred in PN in the mo rning and in the afternoon, which could be interpreted as an effect of air humidity on the relationship between P-N and g(l). P-N was correc ted (P-c) for the transpiration efflux (E) in order to have a simplifi ed expression for the relationship between P-c, g(l), and mole fractio n for CO2 in air (c(a)) and in intercellular spaces (c(i)). A unique r elationship between P-N and c(i), and c(i) and g(l) was modelled under saturation irradiance and leaf temperature between 26 and 31 degrees C, for all treatments by using a hyperbola as the P-c versus g(l) resp onse curve. Hence in maize under optimal conditions of temperature and irradiance all the processes involved in net CO2 assimilation and sto matal aperture were strongly related.