Elevated UV ‐B alters anti‐herbivore plant defence allocation, while silicon supplementation maintains resistance

The impacts of elevated surface ultraviolet‐B (UV‐B) on ecological processes, such as anti‐herbivore defence in plants, remain poorly understood. Many cereal crops depend on accumulating silicon (Si) in their tissues to resist insect herbivory. UV‐B is known to affect oxidative signalling, which is an early signal triggered by herbivore attack and hence could potentially impact upon subsequent defence allocation. We tested how UV‐B light, Si supplementation and herbivory interact to impact plant defence and insect performance. We conducted a fully factorial glasshouse experiment using hydroponically grown wheat ( Triticum aestivum ), manipulating three UV‐B levels (low, moderate and high) and Si supply, before subjecting plants to herbivory by Helicoverpa armigera . Elevated UV‐B increased oxidative signalling (+52%) and antioxidant capacity but slightly reduced foliar Si accumulation (−8.6%). Silicon supplementation reduced lipid peroxidation and altered defence allocation, lowering UV‐B‐ and herbivory‐induced phenolic production while increasing polyphenol oxidase activity and Si concentration. Phenolics increased under elevated UV‐B (up to +13%), and herbivory increased phenolics only in plants without Si supplementation (up to +30%). Under herbivory, Si reduced phenolic levels (17% lower than −Si plants) and suppressed phenolic induction by 25%. Larvae feeding on Si‐supplemented plants showed a 35% reduction in growth rate, which persisted despite UV‐B‐driven reductions in foliar Si. High UV‐B depleted larval energy reserves, with carbohydrate levels declining by 52%, alongside reduced lipid and triglyceride stores and a 40% increase in protein. Silicon altered larval energy allocation, increasing carbohydrate reserves (2.5‐fold) and glycogen (up to 2.9‐fold), while larvae feeding on −Si plants retained higher lipid and triglyceride stores. Collectively, these results demonstrate that Si helps maintain plant resistance under simultaneous UV‐B exposure and herbivory by altering defence investment and increasing metabolic costs to herbivores. These findings provide broader implications for understanding how crops maintain resistance under future changes in UV‐B light conditions. Read the free Plain Language Summary for this article on the Journal blog.