Exogenous melatonin alleviates drought stress in wheat by enhancing photosynthesis and carbon metabolism to promote floret development and grain yield

• Exogenous melatonin alleviates drought stress in wheat by enhancing photosynthetic performance and carbon metabolism. • Exogenous melatonin promotes grain number increase by reducing floret degeneration and abortion, and increasing the number of fertile florets. • Determine the differences in spike floret development between irrigation and MT application that irrigation affected number of fertile spikelets, while MT spraying influenced grain number per spikelet. Melatonin (MT) is a novel exogenous plant growth regulator. Spraying exogenous MT can enhance the growth and development of wheat, and alleviate drought stress, but research on the physiological mechanism by which spike floret development into grain under drought stress is limited. Therefore, we conducted a study from 2019 to 2023 in which a foliar spray of 100 μmol·L -1 MT was applied before the peak of floret degeneration under drought stress conditions to investigate the physiological mechanisms by which exogenous MT regulates spike floret development. The results showed that, compared with the drought stress treatments, MT spraying increased the total number of florets, number of fertile florets, and grain setting rate at different spikelet positions in the two types of wheat varieties, with an increase in the number of grains per spike of 19.72% for Zhoumai 22 and 18.65% for Yumai 49-198. MT application enhanced photosynthetic productivity by regulating the leaf photosynthetic performance of wheat to maintain normal metabolic carbon functions. Structural equation modeling revealed that both irrigation and MT application under drought stress could promote spike floret development. Irrigation treatment affected the number of fertile spikelets mainly by regulating soluble sugars, thus promoting an increase in the number of fertile florets. MT spraying influenced mainly the grain number per spikelet by regulating ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity and soluble sugars, facilitating the growth and transition of florets into grains. This study investigated the physiological and metabolic mechanisms by which exogenous MT regulated floret development into grains under drought stress, providing novel insights into the potential of exogenous plant growth regulators to mitigate the adverse effects of drought and increase wheat yield.