Environmental factors influencing fruit production and seed biology of the critically endangered Persoonia pauciflora (Proteaceae)

Abstract

The factors that influence seed production and seed dormancy in rare plant species are crucial to their conservation yet are often poorly understood. In this study, we examined the breeding system and seed biology of the critically endangered Australian endemic species Persoonia pauciflora through a series of experiments. Pollinator visitation surveys and manipulative hand-pollination treatments were conducted to investigate the breeding system and subsequent seed production. We used an experimental seed burial to examine the breakdown of the woody endocarp and changes to germination over time. Seed germination response under simulated local seasonal conditions was also examined. Persoonia pauciflora was found to be predominantly pollinated by native bees, and cross-pollinated flowers produced significantly more mature fruit (18 ± 3%) than self-pollination treatments (2-3%). The average strength of P. pauciflora pyrenes buried in soil significantly decreased over two years, from 413.3 ± 14.6 N to 130.8 ± 11.8 N. Post-burial seed germination was cyclical, with highest final germination percentage occurring following late-summer exhumations (75 ± 2% and 74 ± 14%, respectively). When seeds were incubated under different local seasonal conditions, germination at local summer temperatures was more likely when compared with autumn/spring and winter temperature-incubated seeds. Our study highlights the importance of ensuring appropriate biotic pollen vectors are present in the local landscape for maximising viable fruit production for this species. In addition, our data indicate that recruitment will most likely occur after the endocarp has suitably weakened, allowing physiological dormancy of the embryo to be relaxed and germination to commence following summer temperatures.

Reproductive biology of the threatened and highly fragmented shrub

“…maxima, P. mollis subsp. nectens, P. pauciflora and P. virgata (Wallace et al 2002;Field et al 2005;Rymer et al 2005;Emery and Offord 2019). Fruit set following open pollination at Glenorie (44.8%) is comparable with that in many previous studies of other Persoonia species that occur in the Sydney region (Cadzow and Carthew 2000;Wallace et al 2002;Field et al 2005;Rymer et al 2005), and was higher than in the manual cross-pollination treatment.…”

“…Maynard (1995) outlined several anatomical features of Leioproctus bees, such as a flattened clypeus that facilitates effective pollen and nectar gathering from Persoonia flowers. Leioproctus has been reported as a common flower visitor for several rare and common, obligateseeding Persoonia species (Wallace et al 2002;Field et al 2005;Rymer et al 2005;Emery and Offord 2019). Notably, this study is the first to report Xylocopa bees and Zizina butterflies visiting Persoonia flowers.…”

“…Although this method has been used in pollination studies of other Persoonia species (Wallace et al 2002), it is possible that the proportion of set fruits in this study are not carried through to fruit maturity. Persoonia fruits require up to 12 months to reach maturity and disperse from the maternal plant, and other studies have suggested that several species have a post-zygotic genetic mechanism that recognises selfed fruits and terminates most of these during the maturing process (Krauss 1994;Cadzow and Carthew 2000;Emery and Offord 2019). For example, a proportion of early set fruits of P. pauciflora were terminated over 6 months, resulting in an overall decline in fruit production (Emery and Offord 2019).…”

“…Our results showed that P. hirsuta is a preferential outcrosser with more fruits being initiated in the crossed (19.4-23.4%) and open (22.8-44.8%) pollination treatments than following self-or autogamous pollination (<10%). Leioproctus (Cladocerapis) bees are reported as the most common and efficient pollinators of Persoonia (Bernhardt and Weston 1996;Wallace et al 2002;Emery and Offord 2019), and were the most frequent genera observed visiting P. hirsuta flowers (Table 1). Maynard (1995) outlined several anatomical features of Leioproctus bees, such as a flattened clypeus that facilitates effective pollen and nectar gathering from Persoonia flowers.…”

“…Persoonia is an Austalian genus of some 99 species that occur in 64 of the country's 87 bioregions (Emery and Offord 2018), and is one of the few Proteaceae groups that have small, singular and inconspicuous flowers that lack a pollen presenter. Reproductive biology is an important consideration for the ongoing management of Persoonia species because plants have a very low rate of fruit set that may also vary significantly within a population (Collins and Rebelo 1987;Chia et al 2015;Emery and Offord 2019), and several species have soil seedbanks that rapidly lose viability in the short term (Auld et al 2000;McKenna 2007;Nield et al 2015). Persoonia species are most frequently pollinated by a diversity of native bees as well as the European honeybee (Apis mellifera; Bernhardt and Weston 1996).…”