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Ecoscape Nursery Fall 2013

Seed Propagation of Aloe polyphylla
Notes and Discussion

Alan C Beverly
Fall 2013

Since my return from Lesotho in 1977 I have been fascinated with all aspects of this species, the individual plant, habitat, and ecology. I started with a small can of seed harvested from several populations and began experimenting with seed germination. Little did I realize that this endeavor would give me such horticultural knowledge that influenced my primary profession as a Landscape Contractor and Arborist.

There are many "gates" to open to propagate A.p. from seed. Asexual propagation is not possible because in the genus Aloe leaves do not have a meristem at the base so only stem cuttings can be used. Since there is no stem this is not an option for nurserymen. Hobby growers may explore the phenomena of multiple offshoots and the binary division which adult plants sometimes spontaneously perform. Some of the factors making A.p. a difficult species to grow from seed are ; A.p. only flowers sporadically beginning at the 90 leaf stage and thereafter do not predictably flower in successive years. The species only produces viable seed from crossing two differing parents. A single flowering plant can self pollinate, but I cannot confirm the viability of selfed seed.

The pollination biology is fascinating. The developing inflorescence appears between leaf at mid spiral and projects upward as the tissue differentiates. Individual flowers are held upright and fall pendulous to await their turn in the sequential opening from lower to upper flowers. Anthers appear first and are elongated to project outside the corolla. Once the anthers dehisce and release pollen only then does the style elongate and protrude the stigma beyond the corolla lip and become receptive. This sequence separates pollen release and stigma receptivity in space and time, ensuring cross-pollination.

There may be other biochemical barriers to selfing. Once pollinated the flower then rises to be borne vertically as the capsule develops. There are 3 locules of 17 seed each when full. The capsule has photosynthetic tissue. When ripe the capsule dries and splits from the top to release seed. Each seed has an outer seed coat which is extended into three wings. This presumably aids in dissemination from the parent plant but it can not go far. It is this outer seed coat which while intact, slows gas exchange and imbibition of water.In habitat some decay occurs by water and or microorganisms to prepare the seed for germination over the cold wet winter.

Cold-wet stratification is a seed pretreatment which can release an embryo from dormancy. I cannot confirm this for A.p. seed at this time, but I will try again. My net germination rate is below 10% when seed is scarified and placed on moist media or slightly covered with at least 60F. I have tried many pretreatments; peroxide, alcohol, HCl, hydroxide may only increase the permeability of the inner seed coat but do not increase net germination. Gibberellic acid soak did not increase germination.

Kozlowski and Pallardy (1) have surveyed the literature regarding exogenous chemical treatments to coax seeds toward germination and conclude, "Although exogenous growth promotors often are associated with the breaking of seed dormancy,the weight of evidence now indicates that other endogenous factors are more directly involved in controlling seed dormancy ". Upon hydration the embryo cells enlarge,enzymes are activated and the cells begin a metabolism which supports cell division. The radicle emerges and the apical and root meristem cells begin to divide. The low net germination rate I observe supports one or both conclusions; that there is an embryo dormancy factor which must be overcome or that only a small % of seed is actually viable.

Restated inversely, the combined parent genotypes will enable hybrid vigor or maybe not. In a population of seedlings there is great variability in performance. Some of them are able to develop into a 35-40 leaf plant in 8 months, while the rest are slower but catch up in 18 months. When seedlings in the same growth chamber exhibit differential growth rates you may correctly conclude that some specific genotypes have an advantage over others. One year (1992?) I witnessed ~70% germination using the scarify method of pretreatment only. I have not been able to recreate this result. I am left with the conclusion that I had created by chance a "jackpot" combination of genotypes I cannot identify to this day.

Interested nurserymen, frustrated with these difficulties, used tissue culture(TC) to "end run" the limitations, but subsequent observations found developmental problems which should be highlighted; tissue cultured plants grow slowly and show errors in leaf sequence for the first 7 years. This Spring I observed the first flowering TC plants and again they showed anomalies (malformation of flowers and opening too early) which prevent seed formation. TC is a phenomena to wonder about, how does a glob of stem cells figure out who is the lead meristem cell and who is the root meristem cell and go from there to organizing a plantlet with differentiated cells? This is why TC plants develop slower than embryos in seeds. TC will not become a preferred method of propagation, they have none of the hybrid vigor of seedlings. Clones are not the way to preserve an endangered species, creating a diversity of genotypes is.

A survey of internet websites and discussion forums offering horticultural advice reveals more about the individuals' training than horticultural truth. I have purchased seed from two different sellers and have not received important accompanying information enabling me to judge seed quality. I would advise not purchasing seed from a seller who cannot supply the following; date of harvest, method of storage, pollination method and germination/handling instructions with a prediction of net germination %.

At this date I cannot confirm that cold-wet stratification increases net germination over my normal "scarify-keep wet and 60-70 F temp" method. I will repeat the cold-wet stratification experiment though. A seller in Cape Town sent me seed which attained a 3% net germination and is "frozen" at the 4 leaf stage, indicating defective seed.

The expression of seed vigor is influenced by heredity, seed development, harvest date, and storage conditions. How long can you store A.p. seed and still retain viability? My initial effort to answer this became a thesis which could not answer the question definitively. Lack of seed has limited progress and research since then. As seeds age structural, biochemical and genetic changes occur which diminish its' viability. How does one distinguish this from an unknown seed dormancy?

A dormancy mechanism can be crucial to the survival of many other species in habitats which offer a narrow time for seed germination. From a nurserymen's point of view seed dormancy is an unwelcome extra step which must be overcome. Seed dormancy results from:

  • Immaturity of the embryo, which requires time to be viable
  • Impermeability of seed coats which must be physically or chemically altered to permit gas exchange and imbibition
  • Mechanical resistance to expansion of radicle
  • Metabolic blocks within the embryo
  • Leaching of germination inhibitors
  • Combinations of the above

From my habitat harvested seed I grew adult plants (150+leaf) to maximize the opportunity to create quality hybrid seed. I have 25 adult plants. I have harvested ~40,000 seed from 13 plants I cross pollinated this year and will make this available for $250.00 per tablespoon (~550 seed). I will send this to international destinations for an extra postage fee. E-mail me if interested. I hope this effort will standardize the trade in A.p. seed.

Continuous creation of recombinants ensures the survival of a population of any species, and has been the main evolutionary strategy of all species. When an endangered species' habitat deteriorates (desertification) faster than it can evolve more "fit" individuals, the classic prediction of extinction applies. The Malachite Sunbird pollinator co-evolved with A.p. and is also endangered. Now it is up to mankind to cultivate this beautiful species and ensure its' survival in private and public gardens.

Photos A and B, below show early seedling development. The first seven leaf are equitantly arranged, opposite each other in a bilateral arrangement, like Aloe plicatilis and A. suprafoliata. The next (8th) leaf has a keel on the abaxial (backside) which unequally divides the leaf, thus the twist begins and the plant starts to form a radial symmetry by leaf 12-15. This young plant will form 5 rows of leaf, which spiral Right or Left.

Photo C: exerted anther | Photo D: exerted stigma and style

Photo E: Pendulous flowers with either anthers or stigmas, and vertically held capsules in the lower section of inflorescence.

Photo F: Germinating seed with radicle beginning to differentiate and producing chlorophyll.| Photo G seeds with membranous wings.

    References:
  1. (1) T.T.Kozlowski & S.G. Pallardy 1997
    Growth Control in Woody Plants
    Academic Press pg 55