Effects of herbivore exclusion and jasmonic acid-induced resistance on plant growth, reproduction, and defense in field-grown Datura stramonium
Plant resource allocation to growth, reproduction, and defense varies based on abiotic and biotic factors. Herbivory can decrease the amount of energy a plant has to allocate to life processes (e.g. by reducing photosynthetic tissue). Herbivory may also alter energy allocation (e.g. by shifting resources towards defense). Plants have a variety of defense strategies for resisting herbivore damage and although the jasmonic acid pathway is well known as a mediator of plant defenses against insects, the trade-offs and costs associated with jasmonic acid (JA) induction and herbivory under field settings has not been examined.
In order to quantify trade-offs in plant resource allocation, Datura stramonium from three populations were grown in the field using a 2x2 factorial design of JA pre-treatment and herbivore exclusion. JA was applied only once at the start of the experiment to increase plant defense (which is costly to the plant). Insecticide was applied throughout the experiment to exclude herbivores, with the expectation that insecticide would reduce insect damage without the costs associated with defensive JA induction. The plants were followed in the field from the juvenile stage through senescence to test for 1) growth/defense trade-offs, 2) changes in flower traits and 3) overall reproductive allocation.
Both JA pre-treatment and herbivore exclusion successfully protected plants against damage. Plants receiving JA pre-treatment showed less early season leaf damage but had fewer leaves (growth/defense trade-off). By mid-season, although the protective effects of JA had worn off, JA pre-treated plants remained smaller until late in the season. As expected, plants receiving insecticide had low levels of damage throughout the experiment and had the highest number of leaves. These results show that although priming with jasmonic acid successfully decreased early season herbivory damage, it resulted in long-lasting growth/defense trade-offs. Additionally, because JA-treated plants displayed a growth/defense trade-off in the absence of leaf tissue loss, there is evidence that plant defensive induction is costly.
Herbivory also altered reproductive allocation such that plants exposed to herbivory produced similar numbers of flowers as plants in the herbivore-exclusion treatment, but flowered earlier with flower morphologies that promoted selfing. For all treatments, late-season flowers had morphologies that promoted self-fertilization rather than outcrossing. For a mixed mating species like D. stramonium, this indicates that floral allocation varies temporally in a way that likely maximizes outcrossing early in the season (when pollinators are more abundant) but also that propensity to outcross is reduced when plants experience herbivory.