Lower Canopy Temperature is Associated with Higher Cytokinin Concentration in the Flag Leaf of Wheat

ABSTRACT Canopy temperature difference (CTD) is used to select wheat (Triticumaestivum L.) varieties tolerant to drought and heat stresses. As CTD reflects the stomatal conductance and transpiration of wheat leaves, and root‐synthesized cytokinin has been shown to be distributed into the aerial parts by the transpiration stream, we therefore investigated the associations of CTD with trans‐zeatin (Z) and trans‐zeatin riboside (ZR) concentration and tolerance of a leaf to high light and high temperature (HLHT) stresses in the flag leaf of different wheat genotypes in field trials. The measurements at 14 d postanthesis (DPA) in 2009 and at 7, 14, 21, and 28 DPA in 2010 showed that Z plus ZR level increased as canopy temperature decreased. This could be at least partially explained by the fact that the flag leaves of low canopy temperature (LT) genotypes received higher delivery rates per unit leaf area of Z plus ZR from the roots and had lower cytokinin oxidase activities than those of high canopy temperature (HT) genotypes. The flag leaves of LT genotypes also exhibited longer photosynthetic durations, higher activities of antioxidant enzymes, and stronger tolerance to HLHT stresses than those of HT genotypes, especially at the late grain filling stage. Hence, CTD shows the potential to be an index for selecting wheat with high cytokinin level of the flag leaf favorable for longer photosynthetic duration and stronger tolerance to HLHT stresses.