CONSIDERATIONS IN DESIGN AND ANALYSIS OF EXPERIMENTS TO MEASURE STOMATAL CONDUCTANCE OF WHEAT

Stomatal conductance of wheat (Triticum spp.) is frequently measured i n studies of adaptation, The objectives of this study were to determin e the effects of diurnal environmental variation on observed differenc es in stomatal conductance among durum wheat (T. turgidum L. var. duru m) genotypes and to assess techniques to improve precision in irrigate d field trials. Stomatal conductance measurements were made with a ste ady-state porometer, usually on clear, calm days because conductance d eclined by as much as 25% as clouds briefly obscured the sun. Variatio n in leaf temperature and photosynthetically active photon flux densit y were not consistently effective covariates for adjustment of observe d conductance values. Adjustment for environmental variability in stom atal conductance with a spatial analysis technique improved precision relative to randomized complete-block analysis in five of six cases an d improved correlation among sampling dates. Increased replication rat her than increased sub-sampling within replications would facilitate g rowing trials with more treatments without loss of precision or additi onal cost.