Seasonal Variation in Iridoid Glycosides in Leaves, Flowers, Fruits and Seeds of Lonicera x bella
Different tissue types within plants can vary substantially in both the concentration and composition of defensive compounds . The Optimal Defense Theory, which posits that allocation to defense of plant tissues occurs in relation to tissue value and probability of attack, predicts preferential allocation of defenses to fruits and seeds relative to other tissues. Based on the few comparisons that have been made, seed chemical defense is generally as high or higher than that of leaves and higher than that of stems and roots. As a result, allocation of defense compounds to seeds likely impacts overall patterns of plant resource allocation. Thus, investigation of the evolution of plant defense investment requires consideration of seeds, which is rarely done. Most often, what is known about patterns of allocation is formed from temporal snapshots. Allocation, however, is not static through time. Defense levels within adult plants have been shown to fluctuate dramatically. Temporal variation may be due to changes in resource availability throughout a season or may represent a response to changing requirements for defense. Allocation changes seasonally; but what has rarely been investigated is how seeds change as fruits ripen or how changes in allocation correspond with fruit and seed development. I am determining how allocation of chemical defense to leaves, flowers, fruits, and seeds changes seasonally, focusing on important phenological time-points. |
Iridoid Glycosides in L x bella seeds and incidence of fungal attack
Iridoid glycosides levels in Lonicera x bella fruits have been correlated with decreased damage to the seeds contained in those fruits. Hemipterans feeding upon developing fruits may introduce fungi into fruits, which can attack the seeds within. The chemistry of fruits may confer protection but what of the seeds themselves? Fruit and seed chemistry are often considered in isolation but both are likely important for seed defense. I am determining whether seed chemistry, in addition to fruit chemistry, correlates with incidence of fungal attack.
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Scales of variation in seed defense
Plants are made up of repeated modular structures; thus every individual contains an array of multiple leaves, stems, and often flowers and seeds. Variation within tissue- types, such as differences in defense between the flowers on an individual, has been largely ignored or disregarded as 1) minor in comparison with variation on larger scales and 2) ecologically unimportant. However, there is growing evidence that this within-plant variation is not only quite large but may be important for interactions with herbivores. For example, limitations imposed by plant vasculature, coupled with environmental heterogeneity, such as in sun exposure, can result in differences, such as in concentrations of defense compounds, between leaves of different branches. Across-scale comparisons are rarely made for any plant trait, though those that have been made have shown that within-plant variation can be equal to or greater than variation among individuals and even among sites. If this pattern is common for traits important to herbivores and pathogens, then variation in plant traits on smaller scales requires increased attention for the study of plant-enemy interactions. I will compare the variation in seed chemical defense and seed size across scales, from populations to individuals, to within individuals, using Lupinus argenteus, a forb native to Colorado.
Temporal variation in selection on seed defense
Seeds function as propagules for dispersal and are more mobile than most adult plants; thus they will likely exist in a greater diversity of environments than adult plants: seeds must be defended as they develop on the maternal plant, during dispersal, on the surface of the substrate to which they are dispersed, and during their residence in the soil. In each of these stages a seed is likely to encounter different suites of enemies and selective pressures, such as specialist seed predators during development, gut microbes during dispersal, and pathogens in the soil. Despite these potentially very different selective environments, seeds may be armed with an unchangeable set of defenses throughout their lifetimes. Thus, seeds present a unique opportunity to study temporal variation in selection between life phases, as the suites of enemies change but a key trait that may influence the outcomes of interactions with these enemies remains the same.
I am investigating patterns of selection pre- and post-dispersal for Lupinus argenteus. This work focuses on the identities of enemy groups (animals and pathogens) important during each phase and the nature of selection on chemical defense traits, both on the trait means and trait variances, including how selection pre-dispersal influences selection post-dispersal. |