In the context of the recent climate, effective water utilization and nighttime water-saving strategies can be vital for crops in arid and semi-arid regions. A diverse set of 30 genotypes, from the Canadian bread wheat heritage panel, was laid out in a randomized complete block design at Llewellyn Farm, Saskatoon in the 2021 field season under rainfed conditions. The study was conducted to elucidate mechanisms, which could best explain the variation of traits determining water-use efficiency and nighttime physiological processes. For assessing the productivity of genotypes, morpho-phenological, flag leaf characteristics, and yield attributing traits were recorded. Diurnal measurements for transpiration and photosynthesis-related traits were conducted at an interval of 3-hours at different crop developmental stages and nighttime observations were pooled for analyses. The carbon isotopic composition in flag leaves and grain was also determined. Analysis of variance (ANOVA) was conducted using the mixed-effect model and data were analyzed using different packages in R v 4.1.0.

The combined ranking of morpho-phenological and yield traits showed superior performance of modern genotypes AAC Alida, AAC Starbuck, Superb and AAC Magnet. Considering grain yield alone, AAC Prevail (2,343.19 kg·ha-1) yielded the most, whereas genotype Red Fife (1,509.46 kg·ha-1) yielded the least. Traits such as days to heading, days to maturity, thousand-grain weight (gm), and the number of spikelets per spike showed very high (>0.90) broad sense heritability while grain yield exhibited heritability of 0.60. In terms of nighttime physiological traits (stomatal conductance, transpiration rate, Photosystem II efficiency, electron transport rate, spike quantum yield, and canopy temperature); Laura, AC Barrie, Pasqua, and AAC Concord were promising genotypes for preferred traits. Nighttime canopy temperature variation was relatively smaller and ranged between 18.2°C and 18.6°C on average. Genotypes Carberry, Superb, AAC Brandon, and AAC Starbuck were top-ranked for superior flag leaf traits (higher flag leaf dimensions and more leaf mass area). Genotype Canuck (0.257 mg H2O·cm-2·hr-1) exhibited highest residual transpiration, while Stettler (0.058 mg H2O·cm-2·hr-1) showed lowest rate of residual transpiration from flag leaf.  The 13C isotopic composition (δ13C) for flag leaf and grain showed significant variation. For flag leaves, δ13C values ranged between -25.07‰ (AC Majestic) and -26.76‰ (Harvest); and was negatively correlated with the grain yield (R2=-0.32, P<0.05) and thousand-grain weight (R2=-0.36, P<0.05). Grain δ13C was positively correlated with grain protein content (R2=+0.51, P<0.01). Observed distinct differences justified the importance of diurnal measurements of physiological traits on a temporal basis.