- Author: Steven A. Tjosvold
The last post demonstrated the remarkable ability and unique features of aphids that allow them to rapidly boost their numbers and colonize their hosts in favorable conditions. What about weeds? What features give them the ability to rapidly colonize a potted crop or planted field? Many plants become weeds because they have the powerful trick of producing many many seeds. Seeds are dispersed by wind, insects, animals, or by their association with our nursery tools and machinery. Often, these seeds are long-lived in the soil. Consider these statistics:
Probably some unfortunate graduate students or field assistants in 1954 were given the task to count weed seed of hundreds of common weed species from about 50 plant families in North Dakota. The table above is just a sampling (Stevens 1957). In most cases a single plant, judged to be of average size and growing where competition was low, was harvested at maturity or when a maximum number of seeds could be obtained. The plants were air dried for two weeks or more, threshed and cleaned to re-move immature seeds, empty florets, etc. All of the sampling methods are described in the Stevens reference given below.
So basically left on their own, weeds have a profound ability to produce seed. Some seed are not viable, some germinate immediately, and some persist, perhaps for years, in the soil as a “seed bank”. This bank represents the holdings of weed seeds in the soil. Place a “deposit” of seed in this bank, and your “interest” is compounded in a big way. An interesting experiment with velvetleaf (Abutilon theophrasti) an important weed in soybean crops demonstrated this (Hartzler, 1996).
Velvetleaf is a prolific seed producer and seeds are long-lived. In 1990, replicated experimental plots were planted with soybean and then with one of three velvetleaf densities: 0, 0.2, and 0.4 plants per square meter. In subsequent years, the experiments were maintained in a corn-soybean rotation. Weed densities were determined at crop harvest for four years. As seen above-- even with competition from the crop plants-- velvetleaf density increased dramatically for years following the very sparse initial planting of the weed. There were even some velvetleaf plants seen in the untreated “0” plots, even though the plots were hand weeded to reduce seed production for 5 years prior to initiating the study.
The number of weed seeds in in the soil can range from near 0 to over 1,000,000 per square yard, and most weed seeds are between 0 and 5 years old. A small number of seed can remain viable for decades or more. With this knowledge, one of the most important principles of weed management is to “never let weeds go to seed”. Never.
Stevens O.A., 1957. Weeds, Vol. 5, No. 1 (Jan.), pp. 46-55
Hartzler R.G. 1996. Velvetleaf (Abutilon theophrasti) Population Dynamics following a Single Year's Seed Rain
- Author: Steven A. Tjosvold
Unlike most weeds that growers deal with, liverworts are very primitive plants. Land plants arose from freshwater green algae around 500 million years ago. Bryophytes, consisting of liverworts, mosses, and hornworts, were some of the earliest groups of land plants. They have features distinct from those of other land plants: they lack a vascular system and lignified (hardened) cell walls, they have motile sperm that can swim in water, and their life cycle is dominated by a haploid stage. Liverworts are mostly composed of cells with nuclei that only contain one set of chromosomes (haploid). More evolved plants, the crops and weeds on land that we are familiar with, are primarily composed of cells that have two sets of chromosomes (diploid). Only their eggs and sperms are haploid.
It turns out that liverworts with their haploid nature, production of spores, ease of culture, and quick regeneration time-- the same characteristics that make them weeds-- are the same characteristics that help scientists study the most advanced forms of experimental molecular plant biology. Here is where liverworts shine. In 2015, Japanese molecular plant biologists created liverwort mutants by bombarding them with radiation. These mutants were studied with genetic markers to elucidate how land plants might use their phytochrome system and different wavelengths of light to regulate plant development. In addition, the liverwort's chromosomes have been analyzed. This has revealed that most of the genes that regulate growth and development in higher land plants are also found in liverworts. It has been suggested recently that many of the fundamental features of land plants appeared in bryophytes first and were then co-opted in vascular plants. So in a distant way, the crop plants you grow today have a little bit of liverwort genetic material in them. The next time you cuss out those tenacious liverworts growing in your nursery crops, think about how intrinsically important they are to the crops you grow!
For the next several Wednesday blogs, I will discuss liverwort biology and management. Don't forget to subscribe to receive these posts in a timely manner.