Morphological indicators and the time of transition to flower in Ptilotus nobilis (Amaranthaceae)

Abstract

Floral initiation of Ptilotus nobilis (Lindl.) F.Muell. was investigated by both scanning electron and light microscopy to develop a template for the transition from vegetative to reproductive growth. Four developmental stages were identified, including two vegetative and two reproductive stages. Bract initiation was accompanied by a significant increase in meristem area and diameter, and was defined as the onset of flowering. At the transition, meristem diameters and areas increased 1.5 and 2.3 times, respectively. Most morphological traits, including leaf number, mean leaf area and total leaf area, increased gradually at the time of transition and were further investigated in a subsequent glasshouse trial. Floral initiation occurred very early and at 25 days after sowing all plants had entered the reproductive phase. At the onset of flowering leaf and node number increased significantly with 3.6 and 4.2, respectively. Branching and leaf area expansion were identified as post-initiation processes, and increased by 2.8 (total foliage area), 3.3 (total leaf area) and 1.8 (mean leaf area). During the vegetative phase a discolouration of the shoot apical meristem was noted and underlying reasons are discussed.