Preliminary results on Rubus chamaemorus germination – update 1

January 10, 2020
The first post on R. chamaemorus (December 2019): HERE

First, a simple, short presentation of the Rubus fruit so that we understand the photos. The fruit is an aggregate of drupes. Each drupe presents a fleshy part (intended as a reward for dispersers) and a ‘pyrene’ in which the seed is protected by the hard fruit endocarp similar to the peach stone. Inside the pyrene, there is a seed covered by a thin seedcoat.
The embryo in Rubus species is fully formed, showing a small radicle and two large cotyledons.

Rubus chamaemorus pyrene

The endocarp of the pyrene is very hard, reason for the difficulties in germinating the seeds.
We shall see how to remove it in a simple way and determine if the seeds germinate at warm or need a cold period, to clarify the contradictory existent sources. I realize now that this may be due to the fact that some people tried to germinate the pyrenes while others tried the actual seeds!
When sowing the pyrenes, 270 days of cold/moist stratification are said to be required for germination, meaning 2 years (Baskin & Baskin).

Two lots of pyrenes were used.
Lot#1 was soaked in water since Dec. 18, with the water changed once in a while.
Lot#2 was soaked and also treated with hot water twice at interval of one week (see the ‘hot water treatment’ post).

I sectioned the first few pyrenes last week just to see how it goes, but the germination experiment was started only two days ago; that means about 3 weeks of soaking in water.

The endocarps were removed with a scalpel under a magnifying lamp. The easy way to remove them is by making a small section on the curved side of the pyrene, towards the rounded end. This way you avoid damaging the embryo radicle, which is at the opposite end.

 Rubus chamaemorus; pyrenes showing where to section to remove the endocarp

Despite the recommendation of the article presented in the first post, I find no need for nicking the seedcoat, which is very thin and sometimes even slides out easily (especially in lot #2).

Findings: the pyrenes treated with hot water were definitely easier to cut than the ones only soaked in water. For lot# 2 some of the seeds were already soft, a sign that the soaking time was too long. It is clear that the hot water treatment is very effective, leading to fine fissures in the endocarp. The soaking time should be reduced a bit, maybe at two weeks.

After removing the endocarps, the seeds were placed right away in containers (with lids) prepared with a layer of moist vermiculite and a coffee filter on top; this will allow for easy observation. I borrowed the idea from a friend who used it for stratification of seeds in the fridge.

The seeds (some still with half of the endocarp) and the embryos are sitting on top of the coffee filter, which can be easily replaced in case some of them develop mold (quite possible)

Considering the aspect of lot#2 of pyrenes and seeds, I wonder if the hot water treatment (done a bit differently) may not be enough to allow the seeds to germinate (?). I still have a small lot of pyrenes and will start one more treatment right away.
Removing the endocarps is not a practical solution for large scale sowing, plus other Rubus species (all with hard endocarps) have very small pyrenes.

So, until the next update – let’s have the kettle going!

Special thanks: to Trond Hoy for providing the bakeapple seeds and to Keith Baldie for sharing with me his method of stratification.