Pigments are naturally water insoluble, so they possess a relatively strong ability to stick and persist on the leaf surface. However, many pigment products can still be separated by the types of binders also included in the product.
Over the past several years, there has been an exponential increase in the amount of pigment and dye products (collectively referred to as colorants) being used on turfgrasses. Still, many turfgrass managers question the potential benefit, and whether or not colorants are worth the extra cost. In fact, there are actually many potential benefits of using these products, and as time goes on new research is becoming readily available illustrating the benefits. Ultimately, turfgrasses can be effectively maintained without spraying pigments or dyes. However, colorants can be an effective tool for a number of situations, and with looming changes in governmental regulation these products may help to fill many gaps.
From a chemistry standpoint, pigments and dyes are relatively opposite, but their potential uses have some overlap. By definition, a pigment must meet two criteria: it must be insoluble in water, and it must absorb/transmit a given wavelength of light and reflect back a different wavelength(s) (Zolinger, 2003). What we see reflected back is “color.” For example, chlorophyll primarily absorbs red and blue light, while mostly reflecting green; therefore, plants appear green because of this selective reflection of green light. Pigments and dyes both have the same reflective/absorptive capabilities, but they vary greatly in their ability to stay visible on turf.
Compared to pigments, dyes are very soluble in water, e.g., food coloring. Therefore, when sprayed, a dye won’t adhere to the leaf surface for very long. Dyes will be easily washed off by irrigation, dew and any type of traffic. On the other hand, pigments will stick to the leaf relatively easily; however, most pigment products still contain additional chemical additives to help extend their appearance even longer. For example, many pigment products are classified as paints. These products contain strong deposition aids (stickers) that not only help the pigment adhere to the leaf, but also help the pigment last for a longer period of time. Comparatively, products that contain less deposition aid (low-binding, LB) function somewhere in between dyes and high-binding products (HB or paint). Low-binding products will adhere very well to the leaf, but will tend to wear off faster than HB products, therefore they should be sprayed on a shorter interval to maintain color.
For the most part, dyes will not function as effectively as pigments, and this is due to their high degree of water solubility. Once sprayed, most dyes will not be effective past a couple of days, and in order for a dye or a pigment to be functionally effective the color increase must be visible to the naked eye. If you can no longer see the added color, then the colorant is essentially gone, or is at too low of a concentration to have any impact. Beyond increasing color, how can colorants be beneficial to a field manager? There’s a large span of potential uses for color- ants on sports turf, from protecting the turf from potentially harmful light to allowing for early and extended playing seasons to masking physical or phytotoxic damage and even helping to decrease water use.
High-binding (HB) and low-binding (LB) pigment products can vary drastically in their final color (A = untreated, B = LB, C = HB (2x rate), D = HB). The plots pictured here were treated with one application of their respective treatment 24 hours prior to this photo being taken, and the evidence of variable color is clear.
Photoprotection – protecting the turf from harmful and excess light
From a light perspective, it’s fairly well known that ultraviolet (UV) wavelengths of light are damaging; it’s the reason we wear sunscreen to protect our skin. Plants are just as susceptible to UV light as we are. However, for plants the visible light, the light that’s used for photosynthesis, also has the potential to be stressful (Hakala-Yatkin et al, 2010). Cool-season turfgrasses are only able to utilize a given amount of light for photosynthesis and energy production. During periods of high light intensity turfgrasses will only be able to use a small percentage of the incoming light, and when light availability is beyond what can be used plants are said to be light saturated (Sharkey et al, 2010). When levels of photosynthetic active radiation (PAR) exceed saturation, there is a high probability that the excess light will cause damaging chemical reactions that will be highly stressful (Hakala-Yatkin et al, 2010). By decreasing excessive light during high stress periods, we can potentially reduce overall turfgrass stress.
Pigments have the innate ability to filter, absorb and reflect damaging or excess light energy. Synthetic colorants mimic the function of naturally occurring compounds that plants already produce to filter and reflect light. Both the waxy cuticle layer and leaf hairs are anatomical structures that function in reflecting UV and PAR light. However, when levels of light stress become excessive, many plants will induce the synthesis of extra quantities of carotenoids, anthocyanins and flavonoids. These molecules are natural plant pigments that function in filtering UV and PAR that isn’t blocked by the cuticle or used for photosynthesis (Demmig-Adams and Adams III, 1992). When it comes to protecting the plant from UV and excess light, the idea of adding extra pigment to the leaf isn’t that crazy of an idea, especially when you realize that the plant may be already trying to do the same thing.
Thermal imaging is a great tool for analyzing turfgrass stress. Here a comparison can be made between a treated (A) and an untreated (C) plot. Computer analysis of the thermal images (B and D) allows for the determination of canopy temperature. This image shows that there is no decrease in canopy temperature in the treated area (B) compared to the untreated (D) area. This data has held true for many different pigment products, with measurements taken at different times of the day.
Canopy and soil temperatures – extending the playing season
During late winter or even early spring, many sports field managers have been testing whether or not pigment applications help increase early-season growth. While Ohio is acclimated for winter, many of our turfgrasses here are light green to straw colored. Because of this, leaf tissue will not absorb as much radiant heat energy, therefore, the plant isn’t able to maintain much heat. By darkening the leaf with a synthetic pigment, the plant may have a greater ability to increase biological activity and growth. However, in order for any possible effect, the turf must be visibly darker than it was before it was treated, so choosing an appropriate product, rate and interval are essential to gain any benefit.
Increase in canopy temperature on dormant turfgrass has already been documented using various pigment products sprayed on dormant bermudagrass (Cynodon dactylon). A study published in the September 2012 USGA green section performed at Arizona State University showed that spraying various pigment products on dormant bermudagrass (once per week or biweekly) not only increased color, but also increased canopy and soil temperature (Whitlark and Umeda, 2012). Compared to an untreated control, pigment applications increased canopy temperature by 2 to 6 degrees Fahrenheit and increased soil temperatures by 1 to 2 degrees Fahrenheit. Maintaining these slight increases in temperature hypothetically allows for biological activity to increase at a faster rate, therefore, spring green-up and growth can potentially occur much earlier than normal. Still, it’s necessary that the pigment darkens the turf, allowing for increased absorption of radiant energy. In this situation, a dye on its own wouldn’t prove as effective as a pigment because of the dye’s high degree of water solubility. Pigment products that contain a strong deposition aid (Green Lawnger, Warm-Up, etc.) have the ability to not only last longer with only one to two treatments, but they also provide a darker more concentrated color.
Even though the work done by Whitlark and Umeda was performed on bermudagrass, the same idea holds true for cool-season turf. However, more research into the subject is needed to determine if these increases in temperature actually lead to boosts in biological activity. Ultimately the goal of “warming up” the turf is to provide a longer/earlier growing season. However, whether or not the potential increase in temperature leads to enhanced biological activity is still unknown.
Masking – covering up turfgrass damage
Whether it’s from misapplication or inclement weather, damage to your turfgrass is inevitable. Even when applying essential pesticides there is a chance of mild phytotoxicity. As turfgrass managers we know that the turf will recover, but many will still complain because of the decrease in turf quality. Pigments and dyes are excellent choices for masking short-term discoloration/phytotoxicity while not affecting plant physiology.
Embark (mefluidide) from PBI Gordon is a plant growth regulator (PGR) used to suppress seed head production and slow growth. However, Embark application will also lead to slight discoloration and phytotoxicity (Danneberger et al, 1987). While Embark works very well, many avoid its use because of the unwanted side effects. As the manager of an athletic field, you may not use Embark. However, the same idea still holds true: applying a pigment product will relieve unwanted discoloration without entering the plant.
Some still have questioned whether or not a HB pigment product could interrupt the absorption of a foliar-applied pesticide or fertilizer. Since Embark inhibits seed head production, it serves as a great indicator to test whether or not pigment products (both HB and LB) affect pesticide translocation. This straightforward approach was evaluated last year at The Ohio State University. Results showed that whether the pigment was applied before, with or after Embark, there was no decrease in Embark activity. The pigment application also resulted in a large decrease in visible phytotoxicity, where symptoms only showed up if the pigment wore off. Essentially, this data shows that a layer of pigment, HB or LB, doesn’t result in the disruption of foliar absorption. However, it’s still a good idea to set up your own test plots and evaluate different pesticides case by case.
Pigments or dyes also serve as a tool for decreasing fertilizer input. Without sacrificing plant health, fertilizer rates or total applications can be decreased to a certain level to where there is only a loss in color. To make up for this, a pigment or a dye can be applied to increase color while on a low-fertility program. Using this approach could be more expensive, but with fertilizer restrictions hanging over our heads this serves as a good option for maintaining visually appealing turfgrass.
A study performed at The Ohio State University (2013) showed that some pigment products were able to significantly increase photochemical efficiency over the untreated control. The products-Green Lawnger, Pigment Blue and Pigment Green-contained higher concentrations of pigment relative to the other products tested, showing that in theory products that have a higher pigment concentration may do more to protect the plant from light stress.
Other potential uses
Many pigment products claim decreased water use by the plant, and this could be true. As mentioned earlier, HB products contain strong deposition aids that allow the pigment to persist on the leaf for a longer period of time. These same deposition aids will also physically block transpiration from the leaf. Some common deposition aids that are combined with pigments include: silicon emulsions, synthetic latex, resins (plant terpenoids), silicon dioxide, metal oxides and even oils (Hazen, 2000). Many turfgrass mangers claim less watering when certain pigment products have been applied. However, decreased water use is not a property of the pigment, but instead is caused by the deposition aid physically blocking water loss (Gale and Hagan, 1966). In the short term lower ET rates can be helpful, but in the long run it could lead to other physiological dysfunctions, therefore excessive applications could be physiologically damaging.
The rate to which a pigment product protects the plant can be measured as change in photochemical efficiency, a measure of how much of absorbed light is used for photosynthesis. As the day progresses, photochemical efficiency will show a decline as midday approaches and will recover near the end of the day. For the most part, photochemical efficiency exhibits a maximum of around .800, and will naturally decay as the sun rises.
On the same page as water use is the potential for colorants to decrease canopy temperature under high air temperature. Some companies claim that the potential increased light reflection (especially infrared light) from the colorant also leads to decreased canopy temperature. However, when evaluating canopy temperature throughout the day, pigment applications show no significant decrease in temperature compared to an untreated control. Canopy temperature can be difficult to measure because of constant fluctuations due, in part, to slight changes in cloud cover, wind speed and sun angle. However, even when using thermal imaging technology the answer stays the same, there is no significant decrease in temperature. As mentioned earlier, warming temperatures during winter and fall are a different story, and darker colored turf has been shown to consistently produce small increases in canopy temperature.
There is a wide variety of applications that both pigments and dyes can be used for. Even though research into the subject is still young, these products are relatively safe to experiment with on your own fields. Much of the research currently being conducted has actually stemmed from the tips and tricks that have been used by field managers for quite some time. Therefore, it’s essential to set up your own test plots and test your ideas. Colorants have become versatile in their functionality, and the research is only starting to catch up. Many of the tools we use to measure colorant effectiveness aren’t specifically designed for turf, so in the near future colorants may prove to be even more beneficial than we currently think.
Dr. Dominic Petrella’s research at Ohio State University focuses on turfgrass stress physiology and how turfgrasses respond to environmental stress.