False spring, false colors: What does purple turf mean? Part 2

By Dominic Petrella

As discussed in Part 1 of this blog, turfgrass leaves can accumulate anthocyanins and turn purple/red during specific times of the year and under specific types of stresses, usually as seasons are changing in the fall, late winter, and early spring. This specific stress is the combination of low temperature and high-intensity light that can occur during clear to mostly sunny days during these times of the year, leading to what is called low-temperature photoinhibition. Normally, very high-intensity light (˃1,000 µmol m-2 s-1) given over long periods of times are needed to stimulate the production of anthocyanins, but low temperatures reduce this requirement and more moderate-intensity light can stimulate anthocyanin production under cold conditions.

We are currently examining low temperature photoinhibition and anthocyanin accumulation in 12 creeping bentgrass cultivars in a collaborative project led by Michelle DaCosta at the University of Massachusetts funded by the United States Golf Association (Understanding Factors Associated with Successful Re-Establishment of Golf Course Putting Greens Following Winterkill [Project ID # 2019-17-687b]). Winterkill of turfgrass surfaces often forces turfgrass managers to re-seed in the spring when conditions are not great for establishment. Part of what we wanted to examine was if some creeping bentgrass cultivars establish better at low spring-time temperatures and if some of these cultivars are more prone to low-temperature photoinhibition and accumulate more anthocyanins than others.

six trays of potted turfgrass plants
Figure 1. Under 50˚F air temperature and high-intensity light (~800 µmol m-2 s-1) all bentgrass cultivars accumulated anthocyanins, with cultivar ‘T1’ accumulating the least. Reducing the amount of light led to reductions in anthocyanins (visibly none to very little) even at 50˚F, and plants grown under 68˚F air temperature did not accumulate anthocyanins, regardless of the quantity of light.

When 14-day-old creeping bentgrass seedlings were treated under 50˚F air temperature and high-intensity light (~800 µmol m-2 s-1), all cultivars accumulated anthocyanins to some extent, but ‘Memorial’, ‘L-93’, and ‘Barracuda’ were the most purple and ‘T1’ accumulated the least amount of visible anthocyanin pigment (Figure 1). When we reduced the amount of light these plants were exposed to, no anthocyanins accumulated under 50˚F air temperature, and if the plants were grown under 68˚F air temperature anthocyanins didn’t accumulate, regardless of the amount of light (Figure 1).

Additionally, plants exposed to lower-intensity light under 50˚F air temperature exhibited less photoinhibition throughout the experiment (Figure 2). We are still analyzing these data, but initial results indicate that cultivars that were the most purple also exhibited the least amount of photoinhibition, showing the protective nature of anthocyanin pigments. While the anthocyanin pigments provide some protection from low-temperature photoinhibition, these purple plants tend to establish slower, even when the stress is removed. Covering newly seeded areas of creeping bentgrass with shade cloth that reduces light intensity may improve spring-time establishment when conditions are perfect for low-temperature photoinhibition. This may be a potential new management practice, but further research is needed to better define how this can be properly implemented.

nine pots of creeping bentgrass grown under different shade regimens
Figure 2. Measurement of low-temperature photoinhibition in 14- to 21-day-old of creeping bentgrass cv ‘Memorial’. We acquired data before plants were treated with 50˚F air temperature (Day 0; all plants were grown at 800 µmol m-2 s-1 at this point in time under 60˚F), 3 days after treatment with 50˚F air temperature under 800 (no shade), 400 (50% shade), or 100 (90% shade) µmol m-2 s-1, and 3 days after treatment with 50˚F air temperature and the before mentioned light treatments. A) Plants grown at 50˚F at no shade were visually more purple at each day compared to plants grown under 50 and 90% shade. Plants at 50 and 90% also visually established better compared to plants under 0% shade. B) When examining levels of low-temperature photoinhibition, plants grown under shade exhibited higher FvFm data. Higher FvFm levels indicate less photoinhibition, and the maximum level of FvFm is generally 0.800. In B), green pixels indicate lower FvFm compared to blue pixels, and darker blue pixels indicate higher FvFm compared to lighter blue pixels.

Summary and Conclusions

The combination of low air temperature and high-intensity light often seen on clear to mostly sunny days in the fall, later winter, and early spring can lead to low-temperature photoinhibition and anthocyanin accumulation in turfgrass leaves. Newly seeded turf may be more prone to low-temperature photoinhibition, especially turf seeded in early spring due to winterkill. Although the natural form of protection provided by anthocyanins is useful, purple seedlings may continue to establish slowly once temperatures warm. Covering newly spring-seeded turf with shade cloth to reduce light intensity may be a way to help to encourage establishment when the conditions are perfect for low-temperature photoinhibition, but more research is needed to refine this potential management practice.