True-to-type maintenance of open-pollinated varieties

Ian Magnus talks about maintaining heirloom seed integrity.

Beetroot ‘Chioggia’

Until recent times, the Earth’s human population has been largely agrarian.

For thousands of years, the world’s farmers have been saving seeds for the next year’s crop, selecting plants suited to their environmental conditions and, in the process, creating what we term ‘landraces’ and a cornucopia of diversity in food plants.

Until shortly after the turn of the 20th century, all plant species were open pollinated. Gregor Mendel’s work with plant hybridisation led to the new science of genetics and stimulated research by many plant scientists dedicated to improving crop production through plant breeding.

In the 1960s, the introduction of hybrid maize, “semi-dwarf wheat”, “short-statured rice” and improvements in sorghum and alfalfa led to the Green Revolution.

By the end of the century, hybrids dominated commercial agriculture.

Today we have open-pollinated breeding (OP), hybrid breeding (F1 and CMS), and genetically modified breeding (GM). However, open-pollinated strains continue to be the breeding material for hybrid varieties.

Just as they do in the wild, as plants have been cultivated by man in different environments and conditions, they have adapted.

Sometimes these adaptations have produced a unique strain, which displays characteristics that are different than its parents. The beet family (Beta vulgaris) includes common table beets, silver beets, sugar beets, and fodder beets. All of them share a common ancestor — the sea beet (Beta vulgaris subsp. maritima).

Originally domesticated more than 2,000 years ago, the leafy types came first. The root vegetable types were developed next, sometime in the 16th century, and in the 18th century the larger fodder types were selected and developed.

The sugar beet is a newcomer, having been developed a scant 200 years ago for its sucrose content. Within each of these four groups, there are many distinct cultivars (e.g. table beets ‘Chioggia’, ‘Bull’s Blood’, ‘Burpee’s Golden’).

Varieties often occur in nature and most varieties are true-to-type. That means the seedlings grown from a variety will also have the same individual characteristic of the parent plant (e.g. Beta vulgaris var. rubra).

A cultivar is a selection that has been cultivated by humans, in fact the word means ‘cultivated variety’. A cultivar may originate as a sport, mutation, or hybridisation and may not be true-to-type.

Developing a cultivar from seed requires that, over a series of generations, the plant is grown out and seed saved only from the plants with the desired characteristics until those characteristics have been stabilised (e.g. Beta vulgaris var. rubra ‘Chioggia’). Depending on the species, this may take years or decades of patient, careful work.

Once a new cultivar has been stabilised, it has a set of characteristics that define it. Succeeding generations that display those distinct characteristics are known as being true-to-type. Maintaining a cultivar through open-pollinated breeding requires a knowledge of:

1) Whether a plant is an annual, biennial or perennial.

2) Whether a plant is dioecious, that is having separate pollen-bearing (male) and fruit/seed-bearing (female) plants, e.g. spinach, asparagus, and hops; or monoecious (having separate male and female flowers but on the same plant), for example sweet corn, pumpkins and cucumbers; or has perfect flowers with both male and female parts in one flower, such as peas, beans, tomatoes, and beets.

3) Means of propagation: is the species a cross-pollinator, a self-pollinator, or a bit of both?

4) Means of pollination: while most species rely on insects, others such as corn, rye, and beets rely on the wind.

5) Isolation distance: how much space is necessary between cultivars in the same species to prevent cross pollination?

6) Population size: a minimum number of plants is necessary to maintain the overall genetics or biodiversity of a given cultivar.

Bringing this back to our beet cultivar, our considerations for growing seed in order to maintain the true-to-type characteristics of a cultivar are: the species is biennial and will set seed in the second year.

It has a perfect flower, cross pollinates between plants with pollen carried by wind. Each species in the Beta family will cross with any other, so only one cultivar can be flowering at a time and, because the pollen is wind borne, the isolation distance is very large: 1.6km in the absence of physical barriers. The number of plants necessary to maintain the cultivar is 20-50.

Periodically and prior to flowering, each plant will be assessed to ensure it meets the true-to-type characteristics of the cultivar. Any plants at variance with the characteristics will be removed, leaving only the true-to-type plants to flower and set seed.

Depending upon the family and species, growing for true-to-type seed of a given cultivar can be a more or less exacting and painstaking process and have very different considerations than growing for produce.

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Ian Magnus

Ian is the former Trials Manager at the Diggers Preservation Garden in Dromana and has written several pieces for Diggers on heirlooms, trials and soils.

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