The ontogeny of defense: Age specific leaf characters and herbivory in sun and shade leaves of Lindera benzoin

Background/Question/Methods

Physiological trade-offs between the allocation of resources for growth versus defense against herbivory can be greatly influenced by leaf age and environmental factors, such as differing light environments.  When photosynthesizing leaves are first developing, they must pass through a period as metabolic sinks before assuming the role of metabolic sources.  During this transition there are many changes in anatomical, biochemical, and physiological leaf properties which greatly affect leaf susceptibility to herbivory.  The objective of this study was to characterize aspects of leaf morphology and ontogeny in L. benzoin in varying light environments and to determine how these leaf traits influence herbivory to ultimately understand the continuum of defense techniques from very young to mature leaves.  Chlorophyll concentration per leaf was determined by SPAD-502, trichome density was counted per inch of leaf edge, and overall leaf thickness, thickness of individual layers of leaves, and mean palisade cell area were determined by Scanning Electron Microscopy for leaves of different ages from sun and shade environments. 

Results/Conclusions

Chlorophyll concentration per leaf was positively correlated with leaf age and increased steadily with leaf expansion, but did not vary with light environment.  Trichome densities were negatively correlated with leaf age, and the trichomes were determined to be non-glandular.  No significant differences were found in overall leaf thickness, the thickness of individual layers of leaves, nor mean palisade cell area with leaf age or light environment.  Thus, neither young nor old leaves in sun or shade were found to have particularly advantageous arrangements of cells for photosynthetic efficiency.  Overall, percent herbivory decreased with leaf age as measured by the node of the leaf from the tip of the expanding branch, indicating leaf defenses do alter throughout leaf development, potentially at the sink-source transition.  This relationship is currently being investigated further to determine if throughout leaf development, plants allocate more resources toward secondary defense compounds such as phenolics or rapidly complete leaf expansion and sclerification to increase toughness resulting in reduced herbivory, as well as if this varies with light environment.