Tuesday, January 09, 2007

Chemical Factors Affecting Germination and Seedling Growth

The gesneriphiles@lists.ibiblio.org list yesterday posted an interesting comment that made me think of something that I've been wondering about for quite some time now. I hope you will find this line of thought (although a little long) worth pondering too.

In a posted conversation discussing the seeding of gesneriad and African violet seeds, comments were made about the effects of moving new seedlings as a way of either increasing their growth or stimulating further germination of other seeds that were sown, but hadn't germinated yet. Karen C. stated "Another reason for gently taking seedlings out of the pot when they are small is this leaves room for a possible 2nd set of germination from the seed. I’ve found this happens more often with species than with hybrids and I think it is natures way of making sure that if the first batch of seedling die off that the conditions may allow the 2nd batch to succeed." This remark about species seeds and nature providing a method of ensuring germination if something should cause the first seedlings to fail prompted me to think of a couple more comments that relate to gesneriad young and our interaction with them as home hobbyists.

Dale Martens has commented that a sure way to get a new seedling to grow faster is to "disturb" them. She moves/removes the seedlings from their pot and resets them every couple of weeks. At first people were skeptical, but many have shared the usefulness of this technique as really working well for them. I recall some speculation as to why this would work. Perhaps it was some sort of "stress" reaction by the plant. If it felt disturbed it put out more of something, perhaps hormones or growth regulators, to speed rooting and growth to ensure the plant's survival after disruption. Again, a plant reacting to conditions to ensure continued survival.

I was also talking to a fellow grower about growing streptocarpus. She said that she brushed up against the leaves and moved them occasionally like they might move if they were in the wind outdoors. She was quite satisfied with bothering to take time to do this because it seemed to produce stronger, healthier leaves. Yet another plant reaction to environmental conditions?

These three somewhat different lines of thought regarding the improved growth in rate and vigor of gesneriad young may all be connected. Why would moving plant material, be it seed, seedling or young affect growth?

A quote from the Annals of Botany states "The vertical distribution of seeds in sand determines the proportion of seeds that germinate after precipitation and acts to maintain seed banks over multiple years." I take this to mean that something about the position in the soil makes the seed either germinate or not as conditions change. Like the comment about the species gesneriad seed. When removing some of the seedlings a whole second set of seedlings might be produced. Are there chemical inhibitors produced by the germinating seedlings that stop other seeds (from either the same species or different ones) from growing? Is it an environmental condition such as position in the soil that makes the difference?

From the Florida Cooperative Extension Services document HS944 "Allelopathy refers to the beneficial or harmful effects of one plant on another plant, both crop and weed species, by the release of chemicals from plant parts by leaching, root exudation, volatilization, residue decomposition and other processes in both natural and agricultural systems." They go on to further state that different plant parts, not only the roots can produce chemical agents to affect growth.

Allelopathy is usually studied to determine the negative effects one plant reacting to the presence of another, but for our purposes what if there are beneficial compounds that are present when gesneriads experience movement. (Either by their roots in the soil or by the stems and leaves.) The folks at the Florida Extension Service are mainly talking about using one plant as a potential inhibitor for another, but for our discussion their points about the plant's ability to produce compounds to enhance growth is what is interesting to me. Can we as gesneriad growers learn anything useful from their observations? The publication went on to summarize the point much better that I could: "Furthermore, physiological and environmental stresses, pests and diseases, solar radiation, herbicides, and less than optimal nutrient, moisture, and temperature levels can also affect allelopathic weed suppression. Different plant parts, including flowers, leaves, leaf litter and leaf mulch, stems, bark, roots, soil and soil leachates and their derived compounds, can have allelopathic activity that varies over a growing season. Allelopathic chemicals can also persist in soil, affecting both neighboring plants as well as those planted in succession."

In conclusion, wouldn't it be interesting to see what was going on with the gesnerial seed and young from a chemical point of view? If we could locate what the plant produced naturally could we seriously speed up production of the plants that we enjoy keeping in our homes? Could identifying these particular growth regulators have any other interesting applications in relation to the home hobbyist?

Sources that were used: The comments about soil distribution: Annals of Botany 2005 95(4):649-659; doi:10.1093/aob/mci060

The comments about allelpathic activity are from:
This document is HS944, one of a series of the Horticultural Sciences Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Publication date: July 2003. Please visit the EDIS Web site at http://edis.ifas.ufl.edu.

2. James J. Ferguson, professor, Bala Rathinasabapathi, associate professor, Horticultural Sciences Department, Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, 32611.

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