Exogenous Spermine Promotes Grain Filling in Spring Wheat by Enhancing the Accumulation and Remobilization of Stem Reserves Under Drought Stress Conditions

Stem reserve accumulation and remobilization are crucial traits for grain filling, particularly under drought stress (DS) conditions. Polyamines significantly affect wheat grain filling, but little is known about the involved physiological processes. Therefore, the study aimed to investigate whether spermine (Spm) regulates grain filling by enhancing stem reserve accumulation and remobilization. Three wheat cultivars differing in drought resistance and dry matter remobilization (RE) were used. Spm was applied to plants from anthesis under well-watered (WW) and DS conditions, and its effects on several physiological processes were evaluated. The results indicated that DS significantly increased remobilization efficiency by 2.19-24.98% and the contribution of stem reserves to grain yield by 12.84-68.88% (P < 0.01), despite a significant decrease in RE (P < 0.01). Exogenous Spm significantly promoted RE by 4.33-14.91% under DS and 6.64-10.63% under WW, along with substantial enhancements in the maximum grain-filling rate and active grain-filling period (P < 0.01). Spm substantially increased stem water-soluble carbohydrates (WSC) by approximately 3.79-26.46% under DS and 1.51-12.77% under WW at most of the tested times (P < 0.01), which was accompanied by significant enhancements in chlorophyll content (P < 0.01), indicating that Spm could sustain assimilate supply for grain filling over a longer period. Moreover, Spm application under DS conditions delayed the WSC peak in the upper internodes, i.e., postponing the depletion of stem reserves. The responses to Spm were more pronounced in sensitive cultivars and cultivars with low RE. Based on these results, Spm could partially alleviate the adverse effects of drought on grain filling by enhancing stem reserve accumulation and remobilization.