Morphological and Physiological Plasticity of Cycas Cotyledons Contributes to Carbon Recovery

The cotyledons of cycad seedlings are green if they are illuminated during germination, and their gas exchange traits have not been studied to date. The influence of developmental light level on cotyledon morphology and physiology was assessed for three Cycas species. Germination occurred under 0%, 25%, 47%, or 100% of incident light, and the cotyledon traits were quantified after primary root growth but prior to emergence of the first true leaf. The cotyledons were devoid of stomates. The cotyledon area declined and the dry weight increased as the developmental light increased. No photosynthesis occurred in the dark-grown cotyledons, but zero net-positive photosynthesis of light-grown cotyledons reversed respiratory loss of carbon dioxide, evincing re-fixation of internal free CO2 pools. The dark respiration, photosynthetic induction time, and light compensation point increased as the developmental light increased from 25% to 100%. In contrast, the apparent quantum yield increased as the developmental light decreased from 100% to 25%. The greatest plasticity occurred in the dark respiration and specific leaf area. These photosynthetic Cycas cotyledons exhibited plastic structural and functional responses to the developmental light level similar to those of true leaves. These findings provide a theoretical basis for the light management of germinating cycad seedlings, potentially improving the growth and survival of endangered cycad seedlings.