In this last post on liverwort biology and management, we look at some key management strategies and why they can be effective.
Reduce overhead splashing water and flooding pots
Remember from previous posts that there are over 100 gemmae clones in each gemma cup ready to be splashed out and moved. If the gemmae find a suitable environment each one can establish a new individual. (The white arrows lead from the gemma cups and illustrate potential movement). Also remember, that the male reproductive structure produces sperm when exposed to water. Each sperm has a flagella and swim. Can you imagine what happens when irrigation water rates exceed the infiltration rate of the potting mix, and the pot is flooded? Sperm are produced and can move freely to the "waiting" female structure above. (The orange arrows lead from the male reproductive structure to the female structure and illustrate potential movement that can result in fertilization).
Irrigate on the dry side when you can
Remember that liverworts are very vulnerable to drying. They don't have root systems like most plants only root- hair- like structures that are sensitive to drying. If you have a crop that can tolerate drying like lupine, why not irrigate on the dry side to dry out the liverwort?
Left: Lupine irrigated on the dry side resulting in the drying and dying of liverwort.
Use contact herbicides if compatible
On the liverworts upper surface are thousands of open pores that are used to enhance air exchange. Here they are seen as whitish bumps, almost like thousands of orderly spaced pin pricks. (And the gemma cups are clearly visible).
A micrograph of a single pore is seen below.
Left: Here's a micrograph of a liverwort with a pore on the upper side. Liverworts cannot close these pores in dry conditions. They also lack a thick surface layer of wax on the upper cell layer.
The liverwort is vulnerable to contact herbicides such as various herbicidal soaps and botanical oils. The soaps and oils can penetrate the open pores and thin upper cell layer, resulting in cell water loss and death.
Spores are key to the success of the liverwort
Fig 1. Dry liverwort spores are less than 3.5 microns in diameter, smaller than many fungal spores.
Fig 2. Liverwort spores quickly imbibe water and more than double in diameter (same scale as above)
Fig 3 (left) is a more magnified image of the previous image showing some internal structure of the spore.
The sporelings grow rapidly with mineral nutrients and a cool moist environment. In laboratory conditions new reproductive structures could begin to form in just 3 to 4 weeks, and the whole sexual life cycle, from spore to spore, takes about 3 months.
An experiment in 1925 and other more recent confirm that the sexual reproductive phase of liverworts is induced with long days (12 hours or longer photoperiod). Other similar experiments have shown that far-red wavelengths of light also promote the sexual stage and therefore spore formation. Far-red light is mostly invisible to our eye and is found in greater proportions to that of visible light in the shadows, under benches or under plant leaf canopies. This knowledge leads to interesting management options. Could greenhouse covers or supplementary films or sprays be manufactured that reduce or eliminate far red light for the purpose of eliminating liverwort spore production?
In any weed management strategy, look for strengths and potential weaknesses of a weed's biology and life cycle. Since liverwort spores are so important to the success of liverworts, we need to reduce or eliminate reproductive structures. The information on photoperiod tells us that reproductive structures could begin to be formed in the longer days of spring and summer. If you needed to target a critical time to rogue immature liverworts or apply herbicides, it would be in the period before reproductive structures could mature and produce spores.
Liverworts do not have the typical propagules associated with other common weeds. They don't have seeds or asexual propagules such as rhizomes and stolons that can spread and establish as new plants. However, growers wonder how liverworts can become so widely established in the nursery or greenhouse.
Probably the most important asexual spread in nurseries and greenhouses is a result of gemmae. On the upper surface of the liverwort there are 3 to 4 mm cup-like structures called gemma cups, and at the bottom of each gemma cup over a hundred tiny, disk-shaped gemmae can be produced. When water droplets from rain, sprinkler, or an irrigation wand hits the gemma cups, the gemmae are propelled out and can develop into separate plants. Each gemmae produces a clone of its mother plant.
When spores are released they can be spread by air currents. Each female structure produces spores by the thousands. They are very small (less than 3.5 microns in diameter when dry), on the smaller side of most aerially dispersed fungal spores and pollen (which range from 3 to 100 microns). See for yourself in this video below. I recorded in a calm, darkened office. (Be patient for about 20 seconds). When I opened the office door and the fresh air came in, whoosh, away flew the spores. In the office, not a big deal; when it happens in your nursery or greenhouse, it is a big deal. These spores might move significant distances even with the slightest of air movement!
Next week: what happens after the spores land in your nursery or greenhouse, and management options.
Spore Release video. Use full screen mode.
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.