Illuminating Long-lived Cycas Cotyledons Reduces Sink Demands on Megagametophytes during Initial Seedling Growth

Cycas megagametophyte resources are translocated to developing seedlings through the persistent green cotyledons. The source activities of cotyledons and megagametophytes following germination are not adequately understood. The decline in megagametophyte weight during seedling development was quantified until ultimate tissue desiccation for two Cycas species. In addition, the influence of illuminating the green cotyledons during germination and initial radicle growth on megagametophyte, root, and cotyledon dry weight (DW) was determined to evaluate the role of cotyledon photosynthesis on growth and megagametophyte source activity. Cycas micronesica megagametophytes declined 64% in DW during initial growth of the radicle and maturation of the first true leaf. The rate of DW decline slowed after this stage but continued for ≈18 months when the seedlings were supported by six leaves. Cycas edentata megagametophytes exhibited similar patterns with a 72% decline in DW when the first true leaf reached full expansion, and the resources were depleted by ≈14 months when the seedlings had five leaves. Illuminating cotyledons during initial seedling growth reduced the rate of megagametophyte DW decline by up to 20% and increased total seedling DW above that of dark-grown seedlings by up to 21%. The results provide direct evidence for the long-term source activity of the Cycas megagametophyte and provide indirect evidence that photosynthetic contributions of illuminated cotyledons reduce the reliance on gametophytes as a source of resources for initial Cycas seedling growth. Improved design of Cycas nursery germination microenvironments may be enabled by this new knowledge.