Long-term contributions from pre-formed leaves in cycad seeds

The most vulnerable phase of a plant’s life is the short time period that follows germination. During this phase the newly emerged seedling relies on the maternal parent to provision the food that is required for initial growth. A recent study from the Plant Physiology Laboratory at the University of Guam has determined that cycad seeds offer a unique approach for boosting seedling performance, and the results have been published in the January issue of the journal HortScience.

The pre-formed leaves found inside seeds are designed to contribute to successful establishment of newly germinated seedlings. These leaves are called cotyledons by botanists and are central players in a seedling’s quest to become a sustained member of the forest community. Cycad seedlings use the two cotyledons found inside of cycad seeds to transfer the food resources from the seed’s storage tissues into the growing seedling.

“These cycad cotyledons persist for long periods of time and are green when exposed to light,” said Thomas Marler, author of the study and retired professor from the University of Guam. “Our experimental approach was designed to determine if the cotyledons were also able to contribute new food for seedling growth through photosynthesis.”

The source-sink plant physiology research discipline relies on several experimental approaches to more fully understand the sources of food and the growing areas that act as sinks of that food. These approaches include direct measurement of photosynthesis, quantifying dry matter accumulation, and blocking sunlight from the organs that make new food through photosynthesis.

The measurement of plant growth while photosynthetic tissues receive darkness is an indirect approach for quantifying the amount of food that is contributed by photosynthesis.

“This was one of our experimental approaches which revealed cycad cotyledons indeed contribute to early seedling success through photosynthesis,” Marler said. “The protocol is reliable because photosynthesis depends on light energy to drive new food production.”

Cotyledon strategy has largely been studied through the lens of a tradeoff between a storage role versus a photosynthesis role. In reality, a continuum may exist among plant species and the full spectrum of how cotyledons contribute to early seedling establishment deserves more research, Marler said. The newly published cycad research reveals a plant group that uses cotyledons to contribute to the storage role of seeds but also to the photosynthesis role for extended periods of time.

The full range of mechanisms that enable coexistence of diverse forest species remains poorly studied. The few new seedlings that survive in a competitive forest community provide the pool of new recruits that persist into the sapling stage, ultimately contributing to biodiversity. The unique cotyledon strategy employed by germinating cycad seeds indicates a robust cotyledon strategy for improving seedling persistence.