Switchgrass Photosynthetic Response to Simulated Insect Defoliation Differs During Drought

Insect herbivory is one of few important factors affecting the physiology of plants and yield performance. In response to gross tissue removal, photosynthesis by remaining leaf tissue has been observed reported to remain unaffected, decrease or increase. Some plants recover photosynthetic rates by enhancing their stomata l conductance, performance, transpiration and leaf senescence, whereas others do not have the ability to overcome tissue losses caused by insect herbivory. Moreover, it has been observed  according published data that the differences in plant response occur because of the injury patterns. Switchgrass (Panicum virgatum L.) is a potential biofuel and is consumed by a number of chewing insects. We conducted research to evaluate the photosynthetic response of upland switchgrass cultivar (Shawnee) and lowland switchgrass cultivar (Kanlow) to herbivory, defoliation treatments which simulated different patterns of insect feeding were applied and by determining the photosynthetic rates of remaining leaf tissues were measured during a year of normal precipitation (2011) and a drought year (2012). In both cultivars, overall photosynthetic rates were far lower during the drought year.  Defoliation of upland cultivar leaves during normal precipitation did not affect photosynthetic rates of remaining tissue until day 12, when double-edge cut leaves had a significantly reduced photosynthetic rate. In the lowland cultivar, treated defoliated leaves had lower photosynthesis rates than controls after 12 days, but photosynthetic rates did not differ among defoliation patterns.  During drought, defoliation did not reduce photosynthetic rates of either cultivar.  Based on these results, pest management programs for switchgrass must account for cultivar, water stress, and type of insect herbivore when managing insect pests.