Cone thermogenesis and its limits in the tropical Cycas micronesica (Cycadaceae): Association with cone growth, dehiscence, and post-dehiscence phases

• Premise of the study: Thermogenesis is a prominent pollination-related feature of cycad cones and is generally assumed to play a role in pollination. Although typically studied just before, during, and immediately after the cones’ pollination phase, thermogenesis may be present in other cone developmental phases.• Methods: We assayed thermogenesis in Cycas micronesica, Guam’s endangered cycad, over successive cone developmental phases by measuring temperatures in shaded and unshaded in situ cones for up to 7 wk. We also studied the effect of ambient conditions on cone thermogenesis in laboratory experiments and estimated the cones’ metabolic heating rates.• Key results: Pollen cones exhibit a continuous, but small, metabolically generated thermogenesis for multiple weeks, including a single thermogenic peak temperature greater than peak ambient each day. The magnitudes of those daily peak temperature elevations above ambient reach maxima twice during cone development: a few days before dehiscence and approximately 1 wk post-dehiscence. Excised cones in dark, fixed temperature environments generated multiple thermogenic events (∼24 h period) over ∼10 d. Cones appear to initiate a protective temperature regulatory response at temperatures ≥∼38°C.• Conclusions: Cycas micronesica pollen cones exhibit several thermogenic attributes not reported in other cycads, including continuous thermogenesis for many weeks. These cones grow in a hot tropical environment that likely confines their metabolically generated temperature increases to a small thermogenic window beyond which they encounter heat stress. These findings suggest the presence of thermogenic functions not strictly related to pollination and a potential vulnerability to warming climates.