Although the vibrant, waifish petals of the poppy may appear inviting to the casual observer, a closer look reveals a pricklier message: Stay away! To discourage plant eaters like insects and birds from biting into their leafy appendages, many plant species protect themselves with defense mechanisms, like tougher leaves, distasteful latex, and armor made of prickles. Developing these defense features is part of a plant’s natural growth throughout its lifetime. Some plants, however, are able to activate additional protection when faced with attacking herbivores. The authors of a recent PLOS ONE paper investigated these defense mechanisms in two species of poppy currently found in Hawaii, where natural herbivores have long been extinct. The authors’ results reveal that island poppies may have more “nettle” in the face of simulated adversity than previously predicted.
The authors chose two species of poppy for testing, Argemone glauca, a species native to Hawaii, and Argemone mexicana, a species originally hailing from the North American continent and a recent inhabitant of the Hawaiian islands. Both species come pre-equipped with permanent features that may function as defense strategies. However, permanent defenses are costly to maintain for a plant: They divert energy away from other functions, like reproduction and growth, and are therefore an energy investment for the plant. To combat the cost of maintaining a full suite of permanent defenses, some plants respond to attacks from plant eaters only when they occur by activating additional defenses, known as inducible defenses. Unlike defense features that develop throughout the course of a plant’s lifetime, also known as constitutive defenses, inducible defenses are not permanent, only prompted by specific need.
In this study, the researchers simulated the need for additional defenses by subjecting the two species to various “attacks” to see how the poppies would respond. Plants were assigned to one of four random treatment groups:
- The control group, which received no treatment
- The damage group, where the authors clipped off portions of the leaves
- The Jasmonic acid group, where researchers sprayed the leaves with a harmful solution that inhibits growth
- And the combination group, where authors defoliated plants first and then sprayed them with Jasmonic acid
The researchers then allowed for two new leaves to grow to ensure that the plants had an adequate amount of time to respond.
Although neither species developed additional leaf toughness or produced more natural latex in response to treatments, both species exhibited increased prickle density on new leaves that grew after treatment. To evaluate prickle density, the authors harvested new leaves and counted all the new prickles on the surfaces of the leaves, excluding prickles found along the leaf edge. They also quantified the leaf area and performed statistical analyses to identify patterns in the various groups.
The authors found that Hawaiian native A. glauca responded more intensely to treatment by developing significantly more prickles than its continental North American counterpart, A. mexicana. The authors report that prickles for A. glauca were 20x more dense and 2.7x higher than A. mexicana.
Plant defenses are selected for over time due to snacking pressures from herbivores. On the Hawaii islands, however, natural herbivores of A. glauca, such as flightless ducks and beetles, are now extinct. The lack of natural predators for island plants has given rise to the idea that island plants have ‘gone soft’ over time. The authors consider A. glauca’s robust response to external attacks evidence that island plants may be better defended than previously thought.
Although it may be impossible to determine whether these island defenses have been selected for by herbivores of the past, no longer present, the inducibility of prickles in A. glauca and A. mexicana demonstrates that these poppies have the mettle to fight back against attackers and snackers.
For more on how herbivores and plants interact, check out this EveryONE blog post on snail mucus.
Citation: Hoan RP, Ormond RA, Barton KE (2014) Prickly Poppies Can Get Pricklier: Ontogenetic Patterns in the Induction of Physical Defense Traits. PLoS ONE 9(5): e96796. doi:10.1371/journal.pone.0096796