Dihaploid Poas – Evolution, Adaptation, and Management
By Jason Pick, BASc., ODH
Perhaps the most appropriate quote which applies in the context of poa annua management programs, is a statement by Charles Darwin ; “It is not always the strongest that survives, nor the fittest, but the species that is most adaptable to change” . The objective of this article is to provide an understanding of the evolution and adaptation of poa annua, as programs used to manage or eradicate demand a vocabulary of genetics, selection forces, and polyploidy.
The invasion of annual bluegrass into bentgrass greens of has kept turf managers modifying their management practices for more than 100 years and continues to be a challenge. It persists in as diverse of climates and management regimes as those superintendents who grow them; an ongoing dilemma which suggests its genetic diversity is a result of continuous adaptation to the management philosophies of our sports turf managers and superintendents.
It is poa’s adaptability to change, that has made it so competitive and opportunistic. Recent DNA mapping confirmed poa’s parents – poa supina and poa infirma, were a uniquely diverse combination. Poa Infirma (early meadow grass), from the Mediterranean regions of southern Europe, and poa supina from the mountainous regions of central Europe.
Poa infirma, has a bunch type growth habit (similar to early stage poa encroachment into bent greens), with no use as a sports turf due to an intolerance to mowing. It is a true annual, typically dying at the 8-10 week stage. In contrast, Poa supina is a true perennial, and is extensively used in athletic fields and can manage regular mowing. Annual bluegrass’s evolution has transitioned from winter annual called “rough-type” to the perennial referred to as “greens-type”. However, these two types are not exclusive – they each exist on our putting greens, representing unique attributes of both parents through the various stages of its evolution.
As a true winter annual, the rough-type dies in the summer and germinates its progeny in the fall. This annual characteristic utilizes most of its energy to produce seed. The purpose is to ensure that where death occurs, the seed bank remains full – and can quickly re-establish in those bare areas. The winter annual is also subsequently favored by surface disruption, opportunistically waiting for surface injury, a less densely vegetated area to invade. Still easier to kill, the rough-type poa annua is extremely sensitive to heat, cold and drought, low shoot density and coarse texture, not ideal for consistent putting surfaces.
For most grasses, propagation occurs through either self or sexual propagation. Where self pollination produces identical offspring, meiosis or sexual propagation produces offspring which share a combination of paired homologous alleles. The ploidy level, refers to the number of chromosomes within cells. When two paired species, (referred to as homologous pairs) combine, complete set of chromosomes combine to create the diploid cells or genome. However, where too diverse of pairs combine, the resulting offspring is sterile. This is evident in the hybrid Bermuda grasses, which cannot produce seed, and must be propagated vegetatively.
However, occasionally in nature where diverse parents like poa supina and poa infirma combine, this non-homologous pairing or “in-equivalent” chromosomal combination, can still produce fertile offspring. In the case of poa annua, a whole genome duplication occurred WITHOUT cell division, and subsequently produced multiple sets of chromosomes, double the chromosomal attributes within each cell, from both parents. Subsequently, that whole genome duplication event, created those previously “unpaired” partners of poa supina and poa infirma a homologous pairing, and restored fertility through meiosis – the recombination of both diploid cells from parental chromosome. This created that fertile hybrid ; Poa annua. While relatively rare in nature, 80% of all plant species have shown polyploidy somewhere in their evolutionary history, and is what makes poa annua so successful.
Poa annua now carries both sets of genes from poa supina and poa infirma, and subsequently has the ability to express either attributes from its parents when faced by a selection force. Each time the poa hybrid reproduces, it results in modified gene expression or genetic diversity to avoid that selection force. With many possible expressions among the many poa biotypes, older poa putting greens can exceed 50+ biotypes on a single surface. Examples of many different poa biotypes are commonly seen as “patchy” greens, the darker, lighter, or denser poa patches on greens. Each biotype on your greens, are responding to the same selection forces, but in a different stage of evolution. The gradual development of biotypes continues to improve their “fitness”, instigated by the selection forces, in efforts to avoid them. We can define selection forces as all the cultural programs we employ to maintain upright, fast and firm surfaces, and which may threaten the plant; adversely impacting its growth or growing environment. Perhaps the most obvious selection force, is close mowing; and the evolution of the poa biotype to re-direct resources from top growth to lateral growth, thereby avoid the repeated wounding associated with daily mowing.
In consideration of effective management strategies, the perennial biotype remains active all year long and is favored by less disruption. This “greens-type” variety produces little to no seed, producing several daughter tillers before flowering (and dying). This behaviour is subsequently much slower to flower, and more tolerant of stress than annual types. These perennial bio-types are so well adapted to the cultural intensity of today’s management practices, if they seed they do so only in spring, while maintaining a high shoot density adapted throughout the growing season.
It has also been realized, that the range of perenniality in poa annua plants also show a range among managed surfaces. Where poa in the rough may show perennial attributes, the number of daughter tillers range from 1-3, where a progressive increase in fairways 4-8, and up to >9 daughter tillers can be found in greens. Clearly, as mowing height drops and cultural programs increase, the tillering capacity of poa annua increases, relative to its management intensity. The plant may inhibit gibberellin production, much like the effects of plant growth regulation, ultimately reducing the repeated wounding that superintendents inflict on a daily basis. In the most basic of maintenance programs such as mowing, is enhancing poa annua’s ability to compete, and encouraging its adaptation.
For years now, common practice was to minimize phosphorus applications as this was thought to encourage poas establishment. While we can certainly see the benefits of establishment by phosphorus, in some cases, this may cause a thinning and subsequent establishment of rough-type poas. Poa has adapted to this low phosphorus selection force, by becoming more efficient at extracting that nutrient. While phosphorus exists predominantely in the top 4” of soil, poas rootzone is most effective at this depth. In terms of phosphorus availability, and in contrast to bentgrass physiology, poas rootzone is ideally adapted at shallow depths. Subsequently, those resources are not wasted on deep rooting, but are allocated to tillering or seed production.
2010 US Open Pebble.Beach Courtesy USGA
The gradual evolution of diploid poas can be visible in just 10yrs. The 60+ year poas on older courses can have extremely high shoot densities, and occasionally super high densities 10X that of the tightest bent grasses. The aforementioned selection forces and close mowing over six decades result in the adaptation of the lowest tightest turfs in the world – that adaptation associated with close mowing.
When we review the relative competitiveness of both bent and poa, and efforts to maintain bents over poa, we must realize poa has its competitive advantage in spring and fall during cooler weather. Where bentgrasses have higher fitness in the summer, the seed bank of poa remains ready to establish whenever bare areas in the stand occur and should therefore we should be most diligent in maintaining dense surfaces during poas periods of advantage. To further assess the best management strategies to compete with diploid poas, diploid plugs were cut into bent surfaces and proved that Pennlinks and Pencross were in fact less resistant to spread than A-4 or G series due to their aggressive growth habits. However, having visited many courses over the years, there are many superintendents who still maintain poa free greens in these older bent varieties. Those that have managed traffic, winter kill and recuperation and ensured dense surfaces, have proven the greater advantage against encroaching poas. As a result, without germination of those annual biotypes, the diploid poas have no chance to evolve, having little or no opportunity to escape the seed bank. Applying minimum nitrogen rates around 2.1lbs per year on older Pencross bents also provide an advantage over the early stage poas. When considering interseeding of A’s and G’s and convert old to new, we must also realize that equivalent opportunity for the poas can germinate. Although reviews are mixed, the recurring statements “high rates and diligence” are most common.
For those wanting to favor the diploid adaptation and promote poa greens, once the surface is primarily poa beyond their tenth year, minimizing bare areas will reduce the likelihood of the seed bank germination, minimize annual type germination and keep the existing perennial biotypes adapting. PGRs have also been enhancing poas adaptability to compete on low height of cut surfaces, by forcing the poa plant to level out its “hill and valley” growth pattern. By denying the growth surge, the poa plant is forced to conserve energy over seed pulse period. Saving it for use through the stressful summer periods, this improves its competitive ability to the otherwise summer bentgrass advantage.
While sometimes unavoidable, the seed bank can still release early stage diploids and set you back, but the goal should be to encourage its perennial attributes by aerating in early to late summer during the seed head pulse. Try using 3-6” centers with ½” tines to maximize recuperative times and heal quicker for a dual benefit – getting conditions back up. Where older diploid poas exist, the preferred method for renovation of one or more greens on a predominantly old poa course, is to aerate and sprig out the diploid plugs back onto the new surface. This technique will provide the best opportunity to maintain consistency while undergoing multi-year greens renovations.
Jason Pick is an Instructor in the Turf Management Program at Olds College and WCTA Director