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

1. 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.

2. 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.

3. 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%.

4. 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.

5. 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.