Optimal seasonal schedule for producing biogenic volatile organic compounds for tree defense

The leaves of many trees emit biogenic volatile organic compounds (BVOCs) that protect them from various threats, including herbivory, pathogens, and heat stress. In a previous study, we analyzed the optimal seasonal schedule for producing isoprene, a highly volatile BVOC, in leaves to mitigate heat damage and maximize net carbon gain. In this paper, we investigate the seasonal production schedule of BVOCs stored in leaves, such as monoterpenes and sesquiterpenes, which decay slowly. When the leaves are bitten, these chemicals are emitted and help to prevent further herbivory. The optimal seasonal schedule, analyzed using Pontryagin's maximum principle, includes a period of singular control. Producing BVOCs for defense is advantageous if their decay rate is slow and the photosynthetic rate is fast. The amount of BVOCs produced increases with slower decay rate and faster photosynthetic rate. But it does not increase monotonically with the magnitude of the threat. BVOCs are produced earlier than the peak period of the threat for which the chemicals are intended. Based on the results of the model, we discuss the reported variations in BVOC production among different chemical species and tree species, as well as the seasonal patterns of gene expression in different pathways for BVOC production.