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

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.