ESA2014 preview: External chemical defenses in plants

I'll be presenting this at 1:30 PM on Wednesday in the Plant-Insect Interactions II session in the Compagno room. Fellow GGE student and collaborator Billy Krimmel will follow soon after with an interesting talk on tarweeds. 

I've been studying chenopods and their salt bladder system - which is important both physiologically and defensively for the plant - for awhile and with some gentle nudging from my committee, I've been trying to place the chenopod system into a broader context. Namely, what ecologically and evolutionarily differs between a plant which sequesters its chemical defenses (alkaloids, tannins, etc.) in its tissues and one which secretes them onto plant surfaces?

Glandular trichomes (secretory and non-secretory) cover the surfaces of Trichostema laxum.

Coming from New England, where plants with copious exudates are less common, the summer in California is a bonanza of sticky, oily, slimy (!) and otherwise exudate-covered plants. Is this pattern driven by rainfall? Many of these species have congeners elsewhere without copious exudates (e.g. Trichostema, Lessingia, etc.), which begs the question: are exudates effective defenses only in arid environments? Are the defenses liable to environmental removal?

I therefore set up a series of experiments examining these questions. In one, I simulated rain on individuals in a population of Atriplex rosea - a chenopod with defensive exudates - while holding other individuals as controls and rainfall controls (which received water at the base, not on the leaves) and assessed herbivory at the end of the season. Perhaps unsurprisingly, I found a significant increase in herbivory in the group which received rainfall, suggesting that instead of helping these arid, water-starved plants, the rainfall and subsequent removal of exudates (which are entirely water-soluble in A. rosea) actually increased its susceptibility to herbivores.

Chenopods with external defenses (Atriplex prostrata and rosea) and without (Chenopodiastrum murale) at my field site.

Come to my talk to hear more!