Prominent models of herbivore regulation focus on predators, low plant biomass, or poor resource quality as factors that limit herbivore populations. We examined predictions of these tritrophic models for herbivores on the understory shrub, Piper cenocladum, which is defended by mutualistic ants (Pheidole bicornis) and three amide secondary metabolites. To examine sources of variation in P. cenocladum amide content and to compare the effects of amides vs. ants on herbivores, we used three linked experiments in the field and in shadehouses. We manipulated light, nutrient availability, and presence of symbionts for experimental plant fragments and shrubs and then quantified leaf amide concentration. We also examined relationships between amide content and damage by the three most common groups of folivores on P. cenocladum: generalist orthopterans, specialist coleopterans, and specialist lepidopterans. For all experiments, enhanced resources and absence of symbionts caused higher levels of amides. These increased chemical defenses had strong effects on generalist herbivores in this system, while the ant predators were more effective at depressing herbivory by specialists. The negative effects of amides on specialist lepidopterans were small, suggesting that these herbivores are adapted to chemical defenses in their host plant. It is possible that our results are part of a more general trend where top-down effects are stronger against specialist herbivores, while chemical defenses are more effective against generalists. We concluded that different models of herbivore regulation were supported by components of the P. cenocladum arthropod community, depending on resource availability and on the portion of the web examined.

Summary1 Salinization is a growing environmental stress in wetland ecosystems world-wide. Several models have been proposed that predict clonal plant responses to stress, including that environmental stress stimulates sexual reproduction. 2 We conducted a common-garden experiment to investigate the effects of salinity on 10 natural populations of Iris hexagona , a clonal perennial endemic to freshwater and brackish wetlands of the North American Gulf Coast. 3 Salinity reduced vegetative growth but either increased or had neutral effects on sexual reproduction, consistent with the clonal stress hypothesis. Salinity of 4 µ g g − 1 more than doubled the number of seeds produced compared with freshwater controls, but flower number and seed mass were unaffected. 4 Salinity reduced total below-ground mass by nearly 50% compared with controls, with no significant change in rhizome numbers. 5 Plants from 10 randomly selected I. hexagona populations differed dramatically in growth and reproduction, independent of salinity. Total biomass that accumulated over the 20-month experiment ranged across all treatments from 52 to 892 g, and flower numbers varied from 2.3 to 11.3 per replicate. 6 Populations did not respond differently to salinity, except with respect to above-: below-ground ratios, thus providing no conclusive evidence for local adaptation to salinity stress. 7 Our results concur with published models of plant reproductive strategies in variable environments, in that environmental stress stimulated sexual reproduction at the expense of growth. However, these models do not predict the observed sharp decline in seed production at near lethal salinity levels.

The accumulation of ultraviolet (UV)-absorbing compounds (flavonoids and related phenylpropanoids) and the resultant decrease in epidermal UV transmittance (TUV ) are primary protective mechanisms employed by plants against potentially damaging solar UV radiation and are critical components of the overall acclimation response of plants to changing solar UV environments. Whether plants can adjust this UV sunscreen protection in response to rapid changes in UV, as occurs on a diurnal basis, is largely unexplored. Here, we use a combination of approaches to demonstrate that plants can modulate their UV-screening properties within minutes to hours, and these changes are driven, in part, by UV radiation. For the cultivated species Abelmoschus esculentus, large (30-50%) and reversible changes in TUV occurred on a diurnal basis, and these adjustments were associated with changes in the concentrations of whole-leaf UV-absorbing compounds and several quercetin glycosides. Similar results were found for two other species (Vicia faba and Solanum lycopersicum), but no such changes were detected in Zea mays. These findings reveal a much more dynamic UV-protection mechanism than previously recognized, raise important questions concerning the costs and benefits of UV-protection strategies in plants and have practical implications for employing UV to enhance crop vigor and quality in controlled environments.

Macroclimate has traditionally been considered the predominant driver of litter decomposition. However, in drylands, cumulative monthly or annual precipitation typically fails to predict decomposition. In these systems, the windows of opportunity for decomposer activity may rather depend on the precipitation frequency and local factors affecting litter desiccation, such as soil-litter mixing. We used a full-factorial microcosm experiment to disentangle the relative importance of cumulative precipitation, pulse frequency, and soil-litter mixing on litter decomposition. Decomposition, measured as litter carbon loss, saturated with increasing cumulative precipitation when pulses were large and infrequent, suggesting that litter moisture no longer increased and/or microbial activity was no longer limited by water availability above a certain pulse size. More frequent precipitation pulses led to increased decomposition at high levels of cumulative precipitation. Soil-litter mixing consistently increased decomposition, with greatest relative increase (+194%) under the driest conditions. Collectively, our results highlight the need to consider precipitation at finer temporal scale and incorporate soil-litter mixing as key driver of decomposition in drylands.