Atypical chlorophyll responses to stress in a high light coastal environment

Background/Question/Methods

To provide a better understanding of adaptations to the harsh coastal environment that will elucidate responses to climate change three shrub species were evaluated for chlorophyll responses to two common environmental stressors.  Each species possesses unique characteristics that allow survival in a variable environment with low nutrients.  Myrica cerifera is an evergreen symbiotic nitrogen fixer dominant in mesic swales, Iva frutescens is a halophytic deciduous shrub located at the marsh upland interface, and Baccharis halimifolia is a deciduous generalist small tree or shrub found in swale environments.  Field observations have revealed that microsite differences in stress do not lead to corresponding variations in chlorophyll concentration, a common response to stress.  Laboratory studies evaluated responses to drought and salinity stress in all three species, particularly chlorophyll responses and potential mechanisms to dissipate excess light energy.  Photosynthesis, chlorophyll fluorescence, hyperspectral reflectance, leaf angle and chlorophyll content were measured following drought or salinity treatment. Similar measurements were made in the field at sites determined to differ in degree of stress.

Results/Conclusions

There were no differences in total chlorophyll content following stress treatment or in field conditions for each species.  However, there were species differences in the mechanistic approach to handling incident light as stomata close and in the absence of photosynthesis.  Myrica cerifera uses enhanced thermal dissipation during times of stress as revealed by chlorophyll fluorescence and reflectance measurements.  Iva frutescens adjusts incident radiation by altering leaf position regardless of turgor pressure, while B. halimifolia uses a combination of leaf position while still maintaining some degree of stomatal opening for gas exchange.  Our results demonstrate that chlorophyll breakdown during periods of stress is not advantageous for these woody species for growth in a highly variable environment.  Rather a combination of stomatal response coupled with various protective mechanisms allow for fast photosynthetic recovery that is essential for success in such a dynamic environment.