Physiological response of Tamarix ramosissima (salt cedar) to Diorhabda elongata (leaf beetle) herbivory in a controlled environment

Background/Question/Methods The effects of Diorhabda elongata (leaf beetle) herbivory on Tamarix ramosissima physiology are largely undocumented. Wounding by the beetle during consumption of leaf tissue and the laying of eggs (oviposition) could increase root and leaf respiration as carbohydrates are mobilized to regrow leaves. Photosynthesis per unit leaf area may increase if there are more resources per unit leaf area (i.e. photosynthetic compensation). Alternatively, photosynthesis may decrease in response to wounding. If herbivory results in significant damage to leaf tissue, then a plant’s ability to regulate water loss is likely to decrease. This would result in greater water stress and desiccation may be the mechanism that promotes early leaf senescence, a response to herbivory observed under field conditions. We conducted a greenhouse experiment to understand how Tamarix ramosissima plants grown in a controlled environment responded to adult leaf beetle herbivory and subsequent herbivory by larval life stages. We hypothesized that plants exposed to herbivory would have greater root and leaf respiration rates, greater photosynthesis per unit leaf area, greater leaf tissue concentrations of nitrogen, increased level of water stress and this may lead to decreases in belowground root production.

Results/Conclusions Preliminary data indicate that predawn leaf water potentials were significantly more negative in plants with herbivory than plants in control treatments (no beetles). This was due to the increased stomatal conductance observed during daytime and nighttime hours. Nighttime conductance was nearly as high as daytime conductance indicating that water loss was severely increased. Our results indicate that beetle and larvae damage minimally affected photosynthesis and respiration, but significantly increased water loss and water stress. The proximate cause for early leaf fall is likely desiccation. This produced plants in the beetle treatment that were unable to retranslocate nitrogen. Additionally, these multiple stresses in the beetle-affected plants resulted in less belowground root growth at the end of the experiment (P=0.08). These results suggest that the competitive advantage of Tamarix ramosissima may be reduced in future growing seasons if there is continued beetle herbivory.