Bingru Huang, Ph.D., and Patrick Burgess
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Figure 1. Turf quality (rated on a scale of 1-9) under well-watered control and drought stress in 2010 (A) and 2011 (B) for plants treated with Primo Maxx (PM-only + drought), glycine betaine (GB only + drought), and Primo Maxx plus glycine betaine (PM + GB + drought). Vertical bars indicate LSD (least significant difference) values (P ≤0.05) for comparison between treatments at a given day of treatment where significant differences were detected.
Drought-induced turf decline is an increasing concern for turfgrass managers because of the declining availability of fresh water for irrigation and the increased frequency of natural drought events. Two key strategies for maintaining high-quality turf stands with a limited water supply are reducing water loss from the turf canopy or lowering water consumption rates and facilitating water retention within plant cells as soil water becomes limited. Understanding the physiological factors controlling water use and water retention is critically important for enhancing turfgrass survival during drought stress and for maintaining functional aesthetics with limited water resources.
One of the major determinants of the plant water-use rate is the amount of leaf area available for transpiration, which is closely associated with vertical growth rate of shoots. Cultural practices that suppress vertical shoot growth or reduce transpirational leaf area may reduce the plant water-use rate.
Trinexapac-ethyl is one of the most widely used plant growth regulators for suppressing shoot growth and reducing clipping accumulation (5) and has also been found to reduce evapotranspirational water loss (1,2). Exogenous application of trinexapac-ethyl before plant exposure to stress has been shown to improve turf quality under combined drought and heat stress (6) or drought stress alone (1,7,9) in several turfgrass species in controlled- environment growth chambers. The application of trinexapac-ethyl before drought events or preconditioning of turfgrass plants with trinexapac-ethyl may slow rates of water use and water depletion from the soil and promote plant survival for longer duration as soil becomes dry.
The water-retention capacity of plants is regulated by the accumulation of compatible inorganic and organic solutes commonly referred to as osmoregulants or osmolytes, as they contribute to osmotic adjustment for increased or maintained cellular water retention. Glycine betaine, a quaternary ammonium compound, is a major osmoregulant that controls osmotic adjustment for plant adaptation to drought stress (8). Foliar application of glycine betaine has also been found to strengthen the antioxidant defense system of creeping bentgrass (Agrostis stolonifera) and Kentucky bluegrass (Poa pratensis) under drought or salinity stress (10). Exogenous application of glycine betaine has been shown to be effective in improving drought tolerance of various agronomic crop species such as rice (3), but information on the use of glycine betaine for improving turfgrass drought tolerance is limited. Studies in controlled-environment growth chambers have demonstrated positive effects of exogenous glycine betaine on growth and physiological activities of creeping bentgrass and Kentucky bluegrass during prolonged periods of drought and salinity (10). It is anticipated that glycine betaine application at the onset and during soil dry-down periods may alleviate leaf dehydration and sustain active growth and improved turf quality. Currently, many products that claim to be beneficial to plant health include an osmoregulant such as glycine betaine in their formulation.
As previously demonstrated in controlled environments, when either trinexapac-ethyl or glycine betaine is applied singly, each has shown some positive effects on drought tolerance of plants. The question is: Can sequential application of trinexapac-ethyl and glycine betaine be more effective than using either product alone in field conditions? We conducted a field study in 2010 and 2011 to evaluate sequential applications of trinexapacethyl before drought stress and applications of glycine betaine at the onset of drought stress and during drought stress for effectively promoting turfgrass performance and physiological adjustments when water is withheld.
Materials and methods
Figure 2. Drought-stressed turf plots treated with: Top, water only (untreated control), Primo Maxx (PM
only) and glycine betaine (GB only); and Bottom, water only (untreated control) and Primo Maxx plus
glycine betaine (PM + GB). Photos by P. Burgess
The experiment was performed May through August in 2010 and 2011 on 4-year-old field plots (5 feet × 6 feet [1.52 × 1.83 meters]) established with 007 creeping bentgrass. Turf was mowed weekly at a height of 0.4 inch (10 millimeters) with clippings removed and watered three times per week to maintain soil water content at field capacity (30%) before the drought treatment. The soil type was a Nixon sandy loam (fine-loamy, mixed, semiactive, mesic typic hapludults).
Emerald fungicide (boscalid, BASF) was applied at the manufacturer’s recommended rate of 7.8 ounces/acre (548.9 grams/hectare) in late April to preventively control dollar spot disease before trinexapac-ethyl application. Urea (46-0-0) was also applied in late April at 24 ounces nitrogen/1,000 square feet [7.3 grams/square meter] to promote spring greenup. No fungicides or fertility were applied during the experimental period of 2010 or 2011 to avoid confounding effects. After the study was terminated in 2010, the field was again treated to control dollar spot and a spoon-feeding fertility regimen was implemented to deliver 8 ounces nitrogen/1,000 square feet (2.4 grams/ square meter) every two weeks until the growing season ended.
In 2010, irrigation was withheld for 31 days. In 2011, irrigation was withheld for 48 days. The duration of drought differed between years due to weather conditions. The 2010 trial experienced hotter days with more wind during the drought period, causing rapid drought onset compared to 2011.
Each main plot of drought was divided into replicated subplots that were treated with Primo Maxx (trinexapac-ethyl, Syngenta Professional Products), glycine betaine or Primo Maxx + glycine betaine. In 2010 and 2011, Primo Maxx was applied twice, biweekly during the month of May (May 17 and 31, 2010; May 16 and 30, 2011) as per manufacturerrecommended rates for creeping bentgrass turfgrass at 10.9 fluid ounces/acre (0.8 liter/ hectare) Primo Maxx (0.25 fluid ounce/gallon [1.95 milliliters/liter] [v/v]; a.i. trinexapac-ethyl = 11.3%). Glycine betaine (200 millimoles/ liter) was applied weekly four times over the 31-day dry-down period in 2010 (June 1, 8, 15, 22) and seven times over the 48-day dry-down period in 2011 (May 31; June 7, 14, 21, 28; July 5, 12). The carrier volume was 87.12 gallons/ acre (815 liters/hectare) applied with a pressurized (40 pounds/square inch [276 kilopacsals]) backpack sprayer. The concentration of glycine betaine used was chosen based on a preliminary test that showed positive effects on creeping bentgrass growth at the 200 millimoles/liter concentration in a growth chamber study.
Results and discussion
Figure 3. Leaf relative water content under well-watered control and drought stress in 2010 (A) and 2011 (B) for plants treated with Primo Maxx (PM only + drought), glycine betaine (GB only + drought), and Primo Maxx plus glycine betaine (PM + GB + drought). Vertical bars indicate LSD (least significant difference) values (P ≤0.05) for comparison between treatments at a given day of treatment where significant differences were detected.
Creeping bentgrass performance measured as visual turf quality (on a scale of 1-9, where 1 is dead, brown grass and 9 is best-quality turf) based on color, density and uniformity during drought stress was significantly improved by the sequential application of Primo Maxx before drought stress and glycine betaine during drought stress in 2010 and 2011 (Figure 1). Turf treated with Primo Maxx alone also maintained higher water content during early drought stress, but the effectiveness diminished following prolonged periods of drought stress in both years (Figure 2). The effects of Primo Maxx on improved turf performance were mainly due to increases in the amount of green leaves or canopy density (Figure 2).
Glycine betaine applied singly during drought stress resulted in significantly higher visual turf quality (Figure 1) and leaf water content (Figure 3), but did not have significant effects on the canopy density (Figure 2) in both years. The improvement in drought tolerance by the application of glycine betaine was mainly due to enhanced water retention and suppressed leaf dehydration. Turf treated with glycine betaine appeared to be brighter green with more active growth rather than dull green with ceased growth as seen in the untreated control treatment (Figure 2).
The beneficial effects of sequential application of Primo Maxx and glycine betaine were more pronounced than when either product was applied singly. The two chemicals had additive or synergistic effects in maintaining higher-quality turf under prolonged drought stress. The additive effects of sequential Primo Maxx application before drought stress followed by glycine betaine application during drought stress in creeping bentgrass were manifested as enhanced turf quality and leaf hydration in creeping bentgrass exposed to drought in both 2010 and 2011 (Figures 1-3). Combining Primo Maxx for improving turf density and reduction in water consumption before drought stress and glycine betaine for improving leaf water retention and antioxidant activity ultimately improved turf quality during prolonged periods of drought. Incorporating both plant growth regulators and osmoregulants into turfgrass management protocols in a programmatic approach could be beneficial for managing cool-season turfgrass species in environments with prolonged periods of drought stress.
This research was supported by the O.J. Noer Research Foundation, the Rutgers Center for Turfgrass Science and the New Jersey Agricultural Experiment Station at Rutgers University.
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Bingru Huang, Ph.D., is a professor and Patrick Burgess is a laboratory researcher in the department of plant biology and pathology at Rutgers University, New Brunswick, N.J.