Thermogenesis and developmental progression of Macrozamia macleayi pollen cones

Abstract

Macrozamia macleayi Miq. (family Zamiaceae) pollen cones generate high thermogenic temperatures that are crucial to pollination of these dioecious plants. However, cone thermal behaviour has not been characterised with respect to developmental stage, so any links with the progression and development of their pollination processes remain unclear. Here we show that after growing to full girth, cones progress through thermally active stages of slow/no growth, rapid lengthening, dehiscence and early post-dehiscence, each with a distinct thermal response. During slow/no growth cones exhibit a small late afternoon peak thermogenic temperature elevation above peak ambient, and remain elevated overnight. During rapid lengthening the late afternoon/night-time temperature elevations disappear, and mid-day thermogenesis commences. During dehiscence the midday cone temperature elevations become large, approaching 10°C near the day of maximum dehiscence rate, and then decrease daily. Pollen cones generate their large, dehiscence stage thermogenic temperature elevations synchronously with the diel ambient temperature peak, thus maximising the peak cone temperature. This likely enhances the expulsion of their pollen bearing obligate mutualist thrips pollinator, thus boosting pollination rates. Thermogenic events are fuelled by carbohydrates only, and significantly increase the pollen cone water loss – yet the percentage of water in sporophylls remains nearly constant (~63%) throughout all developmental stages. Similar coordinated pollen cone developmental stage and thermogenic responses are also present in Cycas micronesica K.D. Hill (family Cycadaceae), suggesting a conserved physiological response across cycad families.