Spores are key to the success of the liverwort

Spores are derived from sexual recombination, which provides the mechanism for new adaptations to the nursery and  greenhouse environment.  A single female reproductive structure produces thousands of spores. They are very tiny (less than 3.5 microns), even tinier than many fungal spores that are adapted for aerial dispersal (Fig 1). In dry air conditions, those tiny spores are released and effectively stay aloft in air currents and can move long distances. 

 

 

Fig 1. Dry liverwort spores are less than 3.5 microns in diameter, smaller than many fungal spores.  

 

 

Eventually spores settle or adhere to something tangible in the nursery. They can stay dormant for a long time if dry. (They can be stored dry in a deep freezer for years).  In the presence of moisture, however, they quickly imbibe the water and can begin to grow. Spore diameter doubled in the seconds that it took for me to apply a single drop of water to the spores on a microscope slide and place and view them with a microscope (Fig 2 and 3).   

 

 

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.  

Liverworts can spread and propagate themselves with tiny sexually-produced  spores and asexual propagules.  The flat green leaf-like body of the liverwort spreads horizontally across the media surface by dividing into two branches at apical notches. Each apical notch contains a meristem (here circled in white), basically a growing-point, that can carry on growth laterally if there is media to grow on. Growing-point fragments that are dislodged or torn away from the mother plant have the potential to start new plants. Rhizoids, similar to root hairs, can establish new connections to the media to absorb water and mineral nutrients. 

 

 

 

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.

 

Liverworts also spread through sexual reproduction. Each individual plant is either male or female. Male plants make reproductive structures topped by a flattened, lobed disc that produce sperm. Female plants make umbrella-like reproductive structures that produce eggs. In the presence of water, swimming sperm from the male plant can be splashed up on a nearby female structure. The fertilized ovum develops and remains attached under the lobes of the female structure. Male and female spores are produced on hair-like structures that move to facilitate spore release and dispersal.

 

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.

Liverwort (Marchantia polymorpha) is a tenacious weed that is well known to Central Coast nursery growers.  They form dense colonies on the soil surface that can impede water infiltration into the pot, use available plant nutrients, and can choke the growth of slow growing plants,  vegetative- cuttings and seedlings. They are especially a nuisance in moist environments, shade houses, and greenhouses. And these weeds are tough. No conventional or biorational chemicals have proven to be very effective in managing them well.

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.