Idiosyncratic phenomenon of regeneration from cotyledons in the idiot fruit tree, Idiospermum australiense

Idiospermum australiense (Diels) S. T. Blake, is a rare species that produces very large diaspores (up to 225 g) comprising 2-6 cotyledons. Anecdotal evidence suggested that each cotyledon within the diaspore could develop an independent root/shoot axis in natural populations and under glasshouse conditions. To investigate this potential, 13 diaspores were collected. Six diaspores were kept intact and cotyledons in the remaining seven were separated, planted into soil and scored for germination. All shoots were allowed to grow for 12 weeks. To test for differences in growth response from seedlings emerging from intact diaspores and single cotyledons, plant height, shoot mass, stem mass, leaf mass and total leaf area were compared between groups using both absolute values and values standardized to initial reserve mass. To examine the effects of reserve mass in more detail, height to stem mass, leaf mass to stem mass and specific leaf area was also compared between groups. All intact cotyledons and 24 of the 27 single cotyledons produced a functional root/shoot system. Time to germination was not different between groups. Shoot mass, stem mass and leaf mass were significantly greater in intact diaspores than in single cotyledons, although no difference was found in any parameter when standardized to initial reserve mass. Shoots arising from individual cotyledons were significantly taller per gram initial weight, because of a significantly greater relationship between height and stem mass. Shoots arising from single cotyledons also had a significantly greater investment in leaf mass per unit stem mass, even though there was no difference in specific leaf area between groups. Thus, seedlings arising from individual cotyledons grew relatively taller and produced a greater area of leaf tissue relative to stem than those arising from intact diaspores.