Dylan Korczynskyj and Byron B. Lamont

Dept. of Environmental Biology, Curtin University of Technology, GPO Box U1987, Perth, Western Australia 6845.

The vertical annual growth rate of the grasstree, Xanthorrhoea preissii, is rarely greater than 2 cm, yet following fire the same plant can produce an inflorescence (spike) in excess of 3 m long in less than a quarter of the time. This contrast in growth rates raises the question: how is it achieved? Leaf and spike growth, caudex (stem) starch reserves and the effect of restricting light to reproductive plants on spike elongation were assessed for a sample of naturally occurring grasstrees in the jarrah forest and in the banksia woodland near Perth, Western Australia. The emergence of the spike from within the plant's apex triggers a reduction in leaf production of up to 4.6 times that of a vegetative grasstree, which is sustained until seed release 4.5 - 5 months later. Over this period the starch reserve held within the desmium tissue of the stem is depleted. Experimentation in the banksia woodland revealed that, although the developing spike is itself photosynthetic, it is the daily production of photosynthates by the surrounding foliage that contributes most significantly to its growth. And when light is prevented from reaching the leaves the starch stored within the stem is not a sufficient substitute, evidenced by a significant reduction in spike biomass of 41%. An explanation for how such an energy expensive process can be achieved in a relatively short period of time and the trade-offs are discussed: the inflorescence biomass as an alternative sink to foliage for carbon supplied by the desmium.

Keywords: Xanthorrhoea preissii, grasstree, reproduction, spike, starch.