Improvement Of Rose Seed Germination With Red Light
Earlier version published in The Rose Hybridizers Association Newsletter, Volumn 28, No. 2, Summer 1997, pages 3-4.
In 1980 Paul Jerabek wrote the following to me: "Exposure to red light seemed beneficial in one experiment but subsequent treatments cast doubt on those results."
In the early 1990s I was germinating my rose seeds in a sun room that had windows on the north, west, and east side. I noticed that I had the earliest germination in containers that were placed on the north side near to the west side. In this position the seeds, which were sitting on a wet surface and enclosed in clear containers, received direct sunlight only in the evening (the red light of the setting sun).
In 1995 Y. Yambe, K. Takeno, and T. Saito published a paper titled: "Light and Phytochrome Involvement in Rosa multiflora Seed Germination": in J. Amer. Soc. Hort. Sci. 120,953-955(1995).
The abstract is: " Seed germination percentage of multiflora rose (Rosa multiflora Thunb.) was much higher under continuous white light than in complete darkness. Red light was the most effective in inducing germination, and far-red light was ineffective. Exposure to red light for 1 min increased germination; this effect was saturated at an exposure of 2 min. The red-light effect was reversed by subsequent exposure to far-red light. The results indicate that rose seeds are positively photoblastic, and that the photoreceptor involved is most likely phytochrome."
The actual article includes a very important experimental detail that was not included in the abstract. This experimental detail is that the experiments were done on seeds that were first treated with enzymes to soften (partially dissolve) the seed coat. I would expect that this treatment would facilitate the light penetration into the seed.
Their "Results and Discussion" section starts with the following: "Germination percentage was >60% under light, whereas germination in darkness was negligible". Later in the discussion section they state that "Almost no germination was observed in darkness in the early experiment (Fig.2), whereas about 10% or more seeds germinated in darkness in the later experiments .... This fact suggests that R. multiflora seeds are strictly photoblastic immediately after harvest, but this may change as the achenes age ...".
In their Figure 2 they showed that when seeds were treated with Driselase for 36 hours and then placed under continuous white light that the germination percentages rapidly rose to about 60% in about 6 days and only a few more by day 15.
Although the main study was on rosa multiflora seeds, the authors did make the following statement: "Preliminary experiments with seeds of R hybrida 'Inspiration' resulted in a similar conclusion (data not presented)".
Another paper reports germination data on rosa multibracteatae seeds that support the photoblastic model. In their Figure 2 untreated seeds germinated in darkness had about 30 % germination after 24 weeks while untreated seeds stratified under alternating 14 daylight / 10 hours darkness gave about 37 %. Please note that these data are for seeds that did not have an enzyme treatment. The difference in germination did not become apparent until after the 16 th week of stratification. This would be consistent with a model that the seed coat had to be softened/thinned before the light effect could take place.
Experiments on the germination of embryos removed from immature seeds (embryo rescue) are consistent with the photoblastic model as the final stage of embryo development is done either in continuous light or in alternate light/dark periods, see for example the paper by Mohapatra and Rout.
An explanation for the red light enhancement of seed germination is: nature has provided the seeds with a mechanism by which they can detect whether they are close enough to the soil surface to survive if they would germinate (see for example, page 192, Plant Propagation, Sixth Edition, by H. T. Hartmann, D.E Kester, F.T. Davis, and R. L. Geneve, Prentice Hall publisher, 1997). Of the various components of sun light, it is the red part of the spectrum that is able to penetrate a short distance into the soil.
I initially used fluorescent bulbs and covered my seed trays with red transparent paper (following the procedure outlined by Yambe et.al.). Probably the easiest type to find and use is the red colored form of transparent cling food wrap such as the Handi-Wrap Brand Plastic Film or clear red wrapping paper found in party stores.
I now use red LEDs as the light source. Ebay has been a good source for LEDs.