Gas exchange parameters of flag and penultimate leaves of bread wheat (Triticum aestivum L.) genotypes under natural drought stress and non-stressed conditions

Two years experiment was conducted to study the effect of drought stress on gas exchange parameters of bread wheat genotypes. Gas exchange parameters measured by using Li-6400XT Portable Photosynthesis System. The rate of photosynthesis (Pn), stomatal conductance (gs), intercellular CO2 concentrations (Ci), and transpiration rate (Tr) were studied in penultimate and flag leaves from booting to grains milky ripe stage of bread wheat genotypes. Gas exchange parameters in penultimate leaf are maintained high during the booting and heading stages, while gas exchange parameters in flag leaf are high during the heading and anthesis stages. If compare the share of flag leaf and penultimate leaf in photosynthesis, it is about 62% and 38% at heading and anthesis stages, respectively. The share of flag leaf in photosynthesis increases under drought stress conditions. An average reduction of Pn, gs and Tr in penultimate and flag leaves due to water stress were 28% and 19.5%, 31% and 20%, 22% and 14% in 2021-2022, 37% and 32%, 49% and 33%, 38% and 20% in 2022-2023 growing seasons, respectively. Mesophyll conductance (gm) decreases, water use efficiency (WUE) first increases and then decreases under drought stress conditions. Correlations between Pn and gm is stronger (r=0.991** and r=0.982**) than correlations between Pn and gs (r=0.604** and r=0.464**), indicating predominance role of mesophyll conductance in regulating photosynthesis rate.