Seeds of change

Most people developing and/or introducing new plant cultivars want to provide a desirable and/or beneficial product to the consumer that at the same time preserves the environment. We all know that mistakes have been made, mainly because of not completely understanding the plants’ behavior and reproductive characteristics. For example, Bermudagrass (Cynodon dactylon) is an introduced grass species that was and to some extent still is considered a weed in fields where row crops are cultivated. In the early 1940s, Dr. Glenn Burton released ‘Coastal’ bermudagrass (a more vigorous forage hybrid than either parent). Some thought that he was releasing one of the most dreaded weeds that farmers had to deal with in cultivated fields. History tells us that ‘Coastal’ bermudagrass, an almost seed sterile (due to self-incompatibility) vegetatively propagated hybrid was not weedy and revolutionized the beef and dairy industries by providing an abundance of high-quality grass for producing nutritious meat and milk, and had a major positive impact on reducing soil erosion and preserving the environment.

Yet, we all know of species that were released without thorough testing and have had major detrimental effects on our ecosystems and/or environments. Kudzu (Pueraria montana) was released for its conservation potential and forage production but ended up harming our environment and not being used for its intended purposes.

One can easily confuse the terms weed and invasive. A weed has been described as “a plant considered undesirable in a particular situation.” The National Invasive Species Information Center describes an invasive species as “non-native (or alien) to the ecosystem under consideration” and “whose introduction causes or is likely to cause economic or environmental harm or harm to human health.”

Mitigating invasiveness

Vinings, et al. (2012) suggested a number of genetic methods for mitigating invasiveness by inducing male and female sterility in woody ornamental plants by induced polyploidy, interspecific hybridization, mutagenesis, and genetic engineering options. They listed a number of tree species that are weedy or potentially invasive and are likely to be listed or banned in many localities in the near future and indicated that “it would be wise to also develop sterile forms within these genera.” As the capacity to produce highly sterile forms through breeding is proven, it is likely that sterile forms of currently banned varieties will be allowed. For example, an amendment to the Oregon noxious weed quarantine (OAR 603-52-1200) provides that “cultivars with reduced fertility may be grown and sold.”

The University of Georgia and former USDA/ARS Warm-Season Turfgrass Breeding Program at the University of Georgia Tifton Campus has produced cultivars with reduced or no seeds since the 1950s. We have used various approaches to develop these sterile/semi-sterile cultivars. The seed and pollen sterile triploid interspecific commercial hybrid bermudagrass turf cultivars have been used around the world. Although the hybrids are rhizomatous to varying degrees, I have not heard a single complaint about invasiveness during my 50-plus-year career with the program.

We produced 11 commercial trispecific ornamental Pennisetum hybrids by manipulating chromosome ploidy and crossing P. glaucum x P. squamulatum x P. purpureum. The first being ‘Princess Caroline’ and the last being ‘Black Stockings’. All were red/purple colored foliage hybrids, seed and pollen sterile, perennial, and vegetatively propagated.

We were able to reduce the weediness and invasive potential of Miscanthus sinensis by developing ‘Scout’, a reduced seed producing cultivar derived from irradiating stolons of the seed producing cultivar, ‘Gracillimus’. We tested ‘Scout’ at Tifton, GA (elevation 114 m) and Blairsville, GA (594 m). Data in Table 1 shows that we reduced potential seed production at Tifton by 98.2% and at Blairsville by 99.4% compared to the ‘Gracillimus’ check. We took the test one step further and determined how many of the seed from ‘Scout’ had potential to germinate and found that none of the seed set at Tifton germinated (100% reduction in germinated seed per plant) while we observed a 99.7% reduction in germinated seeds at Blairsville. We seldomly observed seedlings volunteering around plants of ‘Scout’ in the research plots, whereas numerous seedlings were observed volunteering around ‘Gracillimus’ plants.

Pennisetum alopecuroides cultivars can produce an abundance of seed and can become weedy. We treated seed of this species with Cobalt 60 gamma radiation to develop five cultivars with significantly less seed (Table 2). Seed per plant for the new cultivars was reduced from 99 to 100% compared to the untreated control. Germinated seeds per plant was reduce to less than 0.5% compared to the control. One cultivar (‘Hush Puppy’) was completely seed sterile.

We used gamma radiation to reduce seed set in cold tolerant citrus. ‘Changsha’ tangerine (Citrus reticulate) and ‘Ichang’ lemon are widely grown in backyards from Texas to North Carolina. ‘Changsha’ is a tasty fruit with 30 to 40 seeds per fruit. We reduced the seed per fruit to less than 5 and released ‘Sweet Frost’. ‘Ichang’ lemon is a large juicy lemon with 50 to 60 seeds per fruit. We reduced the seed per fruit to less than 5 and released ‘Grand Frost’.

We have been able to develop numerous cultivars of triploid interspecific bermudagrass hybrids, 11 trispecific Pennisetum hybrids, and one P. alopecuroides cultivar that have been completely seed sterile. ‘Scout’, two citrus cultivars, and four cultivars of P. alopecuroides have shown 99.5% or greater seed sterility.

Evaluating selections

There are a number of factors to consider when evaluating a cultivar for its weed or invasive potential. These include rhizome production and vigor, adaptation, perennial strength in an area, cold tolerance, pollen production, and seed set (both per inflorescence and per plant, the latter being the most important), among other factors. Any reduction in seed set of a cultivar has some advantage, but most would probably agree that complete seed sterility is desired.

What is the potential for producing seed (and pollen) sterile cultivars in the future? It seems like one’s imagination may be the mainly limiting factor. Seed sterility is always a possibility if the cultivar can be vegetatively propagated.

The objectives in crossing different perennial species are first to combine desirable characteristics and second to produce hybrids with unstable chromosome segregation at egg cell and pollen formation. This can be accomplished by crossing species with different number of chromosomes or with unrelated or partially unrelated chromosomes. Knowledge of the chromosome numbers of the species being crossed and the relatedness of these species are helpful in determining the success of this approach. One may not know how to make a chromosome squash or operate a flow cytometer to determine chromosome numbers, but someone does know how, so seek help. Although I mentioned perennial species at the beginning of this paragraph, it is possible to take advantage of characteristic from annual species. Some of the red color in the trispecific Pennisetum hybrids discussed above came from annual P. glaucum. Colchicine is an old chemical used to double chromosome numbers of certain species to make crosses possible with other species and can still be used successfully today.

Other approaches to producing seed sterility are by breaking the chromosomes, producing translocation between chromosomes, producing inversion or re-arranging individual chromosome order, etc. These types of aberrations can be induced with mutagens such as Cobalt 60 gamma rays and X-Rays. When using mutagens, it is best to treat the most dormant bud possible to increase the possibility of recovering a sterile sector. A dormant seed is best. However, if you want to sterilize a superior genotype, treat a dormant bud and protect the roots with a lead shield. Regardless, to be successful it will probably be necessary to look at large populations and search for sectors on each plant.

It should be remembered that it is easier to cause male or pollen sterility than female or egg sterility. A male sterile plant without any outside pollen may not produce seed but produce various amounts of seed when viable pollen is available. Therefore, pollen shedding plants should always be present in tests with potential seed sterile plants.

How much seed sterility is needed to reduce the weed potential of a commercial cultivar? Obviously, complete seed sterility for a perennial cultivar is optimal. However, complete seed sterility may not always be attainable, but at least 99% or greater seed sterility should be our goal.

About the author: Wayne Hanna is professor emeritus at the University of Georgia, Tifton Campus. Prior to joining the University of Georgia, he was a research geneticist, research leader, and location leader at the USDA-ARS, Crop Genetics and Breeding Research Unit in Tifton. whanna@uga.edu