Photosynthetic Response of Pentagramma triangularis to Summer Desiccation

Abstract Ferns are thought to have been some of the first vascular plants to develop approximately 390 million years ago. Since then, ferns have speciated and developed methods of survival especially in relation to water stress. Research conducted in 2016 by Helen Holmlund showed that eight California fern species fell into four classes of survival strategies, one of those being resurrection. Pentagramma triangularis is one of the resurrection-type ferns and is endemic to the western coast of the United States. As P. triangularis desiccates during the summer it enacts a physiological response by which it curls its fronds and closes its stomata. In this way it enters deep dormancy and preserves its carbon stores for the duration of the drought period. It is unknown how photosynthetic elements respond to this process of desiccation. The primary method of monitoring photosynthetic performance used in this experiment was measuring the biochemical and photochemical limiting factors of photosynthesis. These are approximated through a carbon response curve conducted in situ on intact fronds of the fern. The carbon response curve, known as an A/Ci curve, is based on the assumptions and calculations established by Graham Farquhar and others and refined by Thomas Sharkey and plots carbon assimilation of the plant compared to internal CO2. The question asked in this experiment focuses on what is occurring on a molecular level of photosynthesis to allow for the distinctive survival method observed on a macro level in the curling of the fronds. We found that the biochemical pathway of rubisco carboxylation became the limiting factor of photosynthesis by an increasing margin as the plant desiccates while photochemical limitations remained statistically unchanged. This gives interesting insight into the molecular workings of the P. triangularis, suggesting a unique photosynthetic response that governs that outward response. Further research is focusing on the resurrection process in order to more completely understand the internal processes of this fern