Stomatal conductance bears no correlation with transpiration rate in wheat during their diurnal variation under high...

A good understanding of the response of photosynthesis rate (P _N) and transpiration rate (Tr) to stomatal alteration during the diurnal variations is important to cumulative photosynthetic production and water loss of crops. Six wheat genotypes were studied for 2 years with pot cultivation in rain-shelter. Among different genotypes, stomatal conductance (g _s) was significantly correlated with both P _N and Tr. But for each genotype, though g _s was significantly correlated with P _N regardless of relative air humidity (RH) status and it was also significantly correlated with Tr under lower RH (LRH, 15.4%) and moderate RH (MRH, 28.3%), it was not correlated with Tr under higher RH (HRH, 36.7%) during the diurnal changes. The conditional correlation between g _s and Tr of wheat evoked new thinking on the relationships among g _s, P _N and Tr. Path analysis was further carried out to clarify the correlations of g _s with the four atmospheric factors, that of Tr with g _s and the four factors and the direct and indirect effects of the factors, during their diurnal dynamic variation. The effects of these factors on g _s or Tr were related to RH. All the four factors had a much higher correlation with g _s under HRH than that under LRH and MRH. Air temperature (T) had a rather higher direct effect than RH and photosynthetically active radiation (PAR). Also, the other factors had a much higher indirect effect on g _s through vapor pressure deficit (VPD) and T. Transpiration rate was highly correlated with g _s under LRH and MRH, with g _s having a higher direct effect on it. In comparison, Tr was not correlated with g _s under HRH but highly correlated with the atmospheric factors, with T, RH, and PAR having a higher indirect effect through VPD.