Friday, August 1, 2014

The Trouble with Seeds (2/3)

For someone interested in breeding plants, that some plants often don't produce seeds can be a major barrier. A breeder would be limited to looking for selectable mutations, rather than using the more general mixing and segregation of genetics to increase crop diversity.

Many of our common vegetables, fruits, and landscape plants are traditionally propagated by clonal divisions. In some cases (apples, pears, etc.) the complex genetic diversity of the crop means that every seedling would produce a distinct plant. The market demands for consistent production then encourage growers to clone the plants. In other cases, the clonal tradition is enforced by the plants themselves due to their inability to produce seeds. (I'll later discuss a third category of plants which have such long life-cycles that breeding projects become difficult to undertake.)

Researchers have figured out tricks to get seed and allow breeding to be done with plants that might otherwise prefer not to.

The Trouble with Seeds (1/3): Garlic, Horseradish, Potato Onion, Walking Onion, and Banana.
The Trouble with Seeds (2/3): Pineapple, Lily of the Valley, Potato, and Sweet Potato.
The Trouble with Seeds (3/3): Babington's Leek, Crosnes, and Bur Oak.

Most of this post is an accumulation of information from other sources (in colored quotes below) about how to get seeds from these crops. I've also included my thoughts and experiences where I felt something needed to be clarified or extended.


    How to get pineapple (Ananas comosus) to set seed.

    [1] "Pineapple is largely vegetatively propagated. Sexual reproduction is rare in nature because pineapple is self sterile; seeds if produced by self fertilization germinate slowly with low vigour and young seedlings are fragile due to inbreeding depression."

    "Self-sterile" generally doesn't mean there won't be any seeds, but rather that there will be very few seeds.   Plants are somewhat flexible in such things. The examination of one I recently purchased revealed three seeds.

    [2] "Pineapples are usually propagated vegetatively. Seeds are only used by commercial growers for breeding purposes. They are viable for 6 months, are unreliable and difficult to germinate. Pineapples from the store are hybrids and if you do manage to get the seed to germinate and grow into a plant and eventually fruit (2-3 years) the fruit will not resemble what you bought. It still could be a lot of fun. Nick the seeds and plant as deep as it is long in moist (not soggy) potting soil and place the pot inside a plastic baggie. Keep it warm (75-80 F). It will take a long time (up to 6 months to germinate)."

    The best method for germinating the seeds from [3] was to place the seeds in a glass jar with some water, sealed with plastic wrap and stored on a heating-mat.



    If you just want to grow another pineapple like the one you got from the store, simply cut off the leafy top and transfer into soil [4]. Give it plenty of light, keep it moist, and if you're lucky, the new plant will grow you a new fruit in a couple of years. If you're unlucky, the new plant will in time turn into a mold festival.
    1. http://www.ogtr.gov.au/internet/ogtr/publishing.nsf/content/pineapple-3/$FILE/biologypineapple08_2.pdf
    2. http://uk.answers.yahoo.com/question/index?qid=20101129055936AAuEhEl
    3. http://www.youtube.com/watch?v=Z2xK9LigVJQ
    4. http://www.youtube.com/watch?v=vpJHgXaPzFA



    How to get Lilly of the Valley (Convallaria majalis) to set seed.

    Lily of the Valley flowers profusely, but rarely produces fruit. This leaves most propagation of the plant to be via vegetative means. The abundant small rooted bulbs are referred to as 'pips' and readily take to being transplanted.

    I've come across two methods which might trick Lily of the valley to set fruit.

    The first is two have two different clones growing close to each other, so that each will pollinate the other. This is based on the idea that the plant will prefer to cross-pollinate to maintain genetic diversity. I've come across this idea elsewhere, but I am not now able to find any references.

    The second method is to convince the plant that it is dying and thus will have no chance to reproduce except by producing fruit via self-polination. I potted up a single pip from a pink-flowered form, intending only to transfer it to a new garden bed. I placed the pot inside under some fluorescent lights, as I was in the process of moving and didn't have a garden for it, and wasn't that consistent with making sure it had water. Once I finished moving into my new place, I brought over the plants and discovered this one was growing a fruit. The conditions were not consistent with insect based cross-pollination, leading me to the conclusion that the fruit probably derived from a self-polination.

    Both these methods require more experimentation, but are consistent with evolutionary theory and long-term survival of the plant's genes.

    This plant is highly toxic, with reports of fatal poisonings due to eating only a few of its berries, so take care with how you handle and where you store your plants.


      How to get Potatoes (Solanum tuberosum) to set seed.

      Typical garden potatoes are propagated by saving tubers from year to year. A side effect of this process is the gardener is also saving potato viruses from year to year, inside the tubers. Over several years, the amount of viruses can build up and result in stunted plants which don't produce well. There are laboratory techniques to clean viruses from potato tissue clones, allowing the professionals to restore the varieties they grow to high production. Another method is to routinely grow potatoes from true seeds. This will eliminate the viruses which can interfere with production and will let you grow a diverse population of new potato types.

      To do this, you have to get seeds from the potatoes and not all potato varieties are forthcoming about flowering and producing seeds for you. One approach to get potatoes to make seeds for you relies on better living through chemistry.

      [1] "The effect of gibberellic acid containing mixtures, silver thiosulphate and extended photoperiod on flowering induction in 16 non-flowering potato genotypes and on flowering enhancement in 14 normally potato flowering genotypes was studied in sub-tropical plains of India during short-day autumn crop season of 2000-2001 and 2001-2002. Extended photoperiod alone was not successful in induction of flowering. Silver thiosulphate in combination with extended photoperiod effectively induced flowering in 16 potato genotypes studied for flower induction. Induced flowers of some genotypes were male fertile. Normal berry setting was observed on induced flowers and seeds obtained from such berries germinated normally. Gibberellic acid containing treatments were not very effective in flower induction as they induced some flowers only in few genotypes. In the normally flowering genotypes silver thiosulphate enhanced maximum flowering and duration of flowering to a great extent."

      A second approach takes a bit longer and dispenses with the chemistry. If you have varieties which don't produce berries, you eat the tubers and end those genetic lines. Before long, you will only have potato lines which do produce seeds easily and their progeny will also be likely to produce seeds easily. It will only take a few generations to get rid of the gene variations which prevent flowering/seed-production in typical potatoes.

      [2] "The easiest one to deal with is potatoes - mostly commercial varieties - that set a huge load of spuds which act as a photosynthate sink absorbing all the energy and nutrients the plant produces. This prevents berry set because there are not enough resources to go around. Azul Toro is a good example. The way to get these varieties to produce seed is to change the load balance. Plant these potatoes a few weeks later than normal and push them just barely into the surface of the soil. The resulting plants won't have room to make stolons and therefore will have extra resources to devote to producing seed. I accidentally stumbled on this method a few years ago and have used it several times since.

      This probably explains why I found seeds in berries produced by potatoes that were in my garden when I bought the place. They were growing right at the surface, slightly protruding in fact. This may also be why the plants I grew from seed have themselves set seed. They, too, are growing right at the surface."

      This third approach appears to work by playing with the physiology of the potato plant by how they are planted. The mechanism of disrupting the typical source-sink migration of sugar in the plant appears to be a generally useful idea that can apply in various ways to many species.
      1. http://link.springer.com/article/10.1007%2Fs10681-005-9050-y
      2. http://alanbishop.proboards.com/thread/8356/restoring-potato-fertility?page=1#ixzz3Vvu4F6Cb



      How to get Sweet Potatoes (Ipomea batatas) to set seed.

      Sweet potato vines will sometimes flower and set seed, depending on the variety and growing conditions. If you want to encourage flower/seed production, you have to break out the chemical weapons.

      [1] "Most sweet potato cultivers grown in Zimbabwe are poor in agronomic and quality traits and require improvement through breeding. However, most cultivars rarely flower yet the flowers are crucial in genetic improvements. The aim of this study was to determine the effects of different levels of 2,4-dichlorophenoxyacetic acid (2,4-D) on sweet potato flower induction. A 3*4 factorial experiment in a randomized complete block design with three replications was used. The first factor was landrace with three different landraces and the second factor was 2,4-D with four different concentrations (0, 100, 300, and 500 ppm). The 2,4-D was applied 50 days after planting. Sweet potato landraces that were sprayed with 2,4-D showed morphological and physiological disorders that included temporal drooping, petiole pinasty, stem splitting, shoot dieback and root swelling. Extensive morphological and physiological disorders were observed on landraces that were sprayed with the high levels of 2,4-D (300 and 500 ppm). However, within 30 days, all the landraces that were sprayed with 2,4-D managed to initiate buds and set flowers while the plants that were not sprayed did not flower at all. The Friedman's tests showed no significant differences in bud and flower number among the treatment combinations used. Therefore the lowest concentration of 2,4-D (100 ppm) used in this study is probably close to the optimum concentration for flower induction in sweet potato. Although this concentration is not the actual optimum, at the moment this concentration can be used to induce flowering in sweet potato and thus allow sweet potato breeding initiatives to be launched."

      2,4-D is a very commonly used herbicide, readily available in garden centers or from online vendors. Fortunately, there is a method that doesn't rely on toxic chemicals that you might rather not have around you.

      [2] "It was thought that the slight stimulation of flower production in the nonflowering noted in 1951-52 could be due to the limited storage of carbohydrates in the roots of the easy-floweringstock. This might result in a build-up of materials within the aerial portion of the plant and thus induce bud formation. Following this reasoning, grafts were made in 1952-53 on some closely related species commonly found in many home flower gardens. They were selected because they have no storage roots. These species are: Morning Glory (Ipomea purpurea) variety Heavenly Blue, Cardinal-climber-(Quamoclit sloteri), Moonflower (Calonyction aculeatum), and Cypress-vine (Quamoclit pennata). The method of grafting was the cleft graft. After insertion of the scion, the union was tightly wrapped with a strip of rubber made from a cut rubber band. No other treatment was given. A total of 125 such grafts were tried of which 95% were successful. Grafts were successful on all except Cypress-vine which has a stem smaller than that of the sweet potato; however, even these grafts took and grew for a short time before dying.
      On many of the plants flower buds began to appear about one month after grafting or soon after growth of the scion was resumed. These buds continued to appear, develop, and produce normal flowers. In some cases the plants were dwarfed (plant 103) but in others the growth of the scion was normal. The response of the sweet potato scions was the same regardless onto which ornamental they were grafted."

      This method can be performed easily. The best rootstock (I. tricolor) is readily available, grows quickly, and flowers prolifically. The physiological mechanism suggested is interesting, but later research suggests it is incomplete.

      [3] "Grafting has been used to induce flowering and to induce early flowering. For example, sweet potato (Ipomoea batatas) is routinely grafted to other nontuberous root forming Ipomoea species such as I. ruba, I. carnes, and I. tiliaceae to induce flowering. In this case, the presence of leaves on the scion and rootstock have had a profound influence on the flower-inducing response. Flowering was induced only when the rootstocks had expanded leaves, thus suggesting that flower-inducing substances are synthesized in the rootstock leaves and translocated through the graft union to induce flowering in the scion (Kher et al. 1953; Lam and Cordner 1955). Grafting sweet potatoes onto Ipomoea carnea ssp. fistulosa increased flower numbers, percentage of capsule set and number of seeds in all four tested cultivars…"

      Grafting will take some practice, but it appears to be a good way to start a sweet potato breeding project. I would select for lines that didn't require such heroic efforts to get further seed.
      1. http://www.academicjournals.org/article/article1387208292_Mutasa%20et%20al.pdf
      2. https://ucanr.edu/repositoryfiles/ca707p13-71853.pdf
      3. http://alanbishop.proboards.com/thread/7720/sweet-potato-breeding-project?page=6#ixzz3Q345UFcC


      I discuss this topic with respect to a different set of crops in my first post with the shared name. When I wrote the first post, I was thinking of the classic Star Trek episode, "The Trouble with Tribbles". Tribbles are fictional creatures that replicate rapidly by cloning themselves, having been born pregnant. The plants being discussed are generally clonally reproduced, so the name "The Trouble with Seeds" came to mind.

      Most of the lovely plant diagrams in this post were derived from public-domain images hosted at botanicalillustrations.org. Some other diagrams are public-domain images from the same era that I found via google. I chose the original images which depicted the plants under discussion in the way I appreciated and then subtracted out the yellowed background of the page using my favorite image editor (GIMP)