The spike plays important roles in the drought tolerance as compared to the flag leaf through the phenylpropanoid pathway in wheat.

The spike photosynthesis plays a curial role in wheat photosynthesis under drought stress. However, the mechanism of drought tolerance in the spike is still unclear. Our study compared the gas exchange parameters, antioxidant system, and phenylpropanoid pathway between the wheat flag leaf and spike in response to drought stress. Compared with the flag leaf, the spike organs exhibited lower reductions in the net photosynthetic rate (Pn), relative water content (RWC), and chlorophyll content (Chl) under drought stress. The activities of phenylalanine ammonia-lyase (PAL), cinnamate 4-hydroxylase (C4H), and 4-coumarate-coenzyme A ligase (4CL) enzymes, and the total contents of phenolics and flavonoids (TPC and TFC, respectively) were enhanced much more percentages in the spike organs than that in the flag leaf under drought stress. Drought also induced the expression of structural genes (TaPAL, TaC4H, Ta4CL, TaCHS, TaCHI, TaFNS, TaF3H, TaFLS, TaDFR, and TaANS) involved in the phenylpropanoid pathway of the spike organs during the middle and late grain filling periods. The spike organs also showed much smaller accumulations of O₂^.-, hydrogen peroxide (H₂O₂), and malondialdehyde (MDA) in treated wheat. Higher activities of antioxidant enzymes (superoxide dismutase, SOD; peroxidase, POD; and catalase, CAT) and more proline content were observed in the spike organs as compared to the flag leaf under drought stress. All these results indicated that the enhanced tolerance to drought stress in spike organs was related to the elevated phenylpropanoid pathway. It could make the spike maintain a better water status and further lead to the relatively higher photosynthesis and lower membrane damage.