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Seed biology

Persoonia fruits are knowns as drupes. Within the outer fleshy layers is a hard, woody endocarp that protects the seed. The endocarp must break down in the soil before germination can occur. This process can take 12-24 months to occur (depending on environmental conditions) and also makes Persoonia difficult to germinate in the lab.

Persoonia seeds have two dormancy types:

Mechanical dormancy (genus wide) is created by a woody endocarp that protects the seed and restricts germination. We know that at least 50% of the endocarp must be removed for germination to occur. When dispersed from the plant, the endocarp is strong enough to withstand up to 40-50kg of force before cracking!

Physiological dormancy (species specific) occurs as the seeds themselves may require specific temperatures to relax dormancy and germinate. In some cases, the temperature requirement can be overcome by using gibberellic acid (GA3). Some species may experience dormancy cycling between seasons as observed in Persoonia pauciflora.

Cross section of a Persoonia hirsuta drupe. 
A diagram showing the structures inside the drupe. Highlighted in bold is the endocarp which protects the seed and must break down or be removed for germination to occur. 

Just as Persoonia seeds take a long time to germinate in the wild, they also very time-consuming and slow to germinate in a laboratory. This is because we need to remove the fleshy outer layers and the endocarp. Currently, we remove the endocarp one fruit at a time.

Our current methodology at the Australian PlantBank has been simplified and can be replicated outside of a laboratory (with some modifications), and fruits can be batch processed or done individually. 

Persoonia seed processing
1.  Fruits should be separated from any significant detritus, placed in sealed plastic bags and left at room temperature to ferment for 2-3 weeks.
2.  Soak fruits in sterile water for 15 minutes to soften the outer exocarp and fleshy mesocarp layers.
3.  Remove fruits from the water and physically macerate fruits to separate the pyrene (endocarp + seed) from the fleshy layers. If working with small numbers of fruits:
               a. Squeeze the pyrene from the outer layers using forceps; or,
               b. Peel away the fleshy layers from the pyrene using forceps.
4.  Wash and clean pyrenes in sterile water with a weak bleach solution (~1-2%).
5.  Rinse and pat dry pyrenes and place in a drying room (15°C and 15% relative humidity) for at least one week.

The pyrenes (endocarp + seed) can then be vacuum-sealed and stored as an ex situ conservation collection or used for propagation work. There are also several issues that can occur when germinating Persoonia seeds in the lab, including prolific microbial contamination, very low seed fill and damage when cracking the endocarp. Surface-sterilising seeds can help reduce microbial contamination.

We find there is little if any increase in germination success when seeds are pre-treated with gibberellic acid (GA3) or smoke. When germinating Persoonia seeds, we often achieve greatest success mimicking spring temperatures.