Foliar Sulfur Fertilization at Booting Followed by Potassium Fertilization at Flowering Enhances Grain Quality and Processing Performance in Durum Wheat

Improving durum wheat yield and grain quality is a priority in Mediterranean production systems, where sulfur (S) and potassium (K) deficiencies can limit crop performance. This study evaluates the effects of foliar sulfur (SL) applied at booting (BBCH 45) and foliar potassium (KL) applied at flowering (BBCH 65) on agronomic performance, grain composition, technological quality, free asparagine (ASN) accumulation, grain N relative to fertilizer N input, and economic return in two durum wheat cultivars grown under rainfed Mediterranean conditions. Both SL and KL significantly increased grain yield by 0.47 t ha−1 (+11.5%) and 0.36 t ha−1 (+8.5%), respectively, primarily through increased grain number per unit area, while spike density and thousand-kernel weight were unaffected. SL increased grain protein concentration, glutenin accumulation, the grain N-to-applied N ratio, and dough strength, while reducing free ASN by more than 70%, a response consistent with changes in grain nitrogen partitioning. KL complemented these responses by increasing starch yield, modifying starch composition, and further reducing free ASN. SL significantly increased the net nitrogen-adjusted margin, whereas KL showed a positive but non-significant economic response. Overall, stage-specific foliar S and K applications improved grain yield, grain composition, and technological quality, providing a promising strategy for enhancing durum wheat productivity and quality under Mediterranean rainfed conditions.