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Crop Science 43:952-956 (2003)
© 2003 Crop Science Society of America

TURFGRASS SCIENCE

Plant Growth Regulators Can Enhance the Recovery of Kentucky Bluegrass Sod from Heat Injury

Xunzhong Zhang*, E. H. Ervin and R. E. Schmidt

Department of Crop and Soil Environmental Sciences, Virginia Tech, Blacksburg, VA 24061-0404

* Corresponding author (xuzhang{at}vt.edu)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
A primary economic concern of sod producers is loss of sod quality during the transportation, storage, and transplant stages of sale. Although a number of cultural methods have been documented for potentially reducing sod heating, many of the biological mechanisms are yet to be investigated. This study investigated the influences of selected plant growth regulators (PGRs) on sod tolerance to stress during storage by examining the relationship between preharvest photochemical efficiency (PE) and transplant rooting of Kentucky bluegrass (Poa pratensis L., KBG) sod. The PGRs, including propiconazole (propiconazole [1-(2-(2,4-dichloropheny)-4-propyl-1,3-dioxolan-2yl)methyl-1-H-1, 2,4-triazole]; PPC), and seaweed (Ascophyllum nodosum Jol.) extract (SWE) plus humic acid (93% a.i.; HA), were applied alone, or in combination, to the KBG 2 wk before harvest in 1999 and 2000. Photochemical efficiency was measured immediately before harvest. The harvested sod was subjected to high temperature stress (37°C) for 72 or 96 h. The heated sod was replanted in the field and transplant visual turf injury and rooting were determined. Foliar application of SWE at 0.5 kg ha-1 plus HA at 1.50 kg ha-1, PPC at 0.44 kg ha-1 alone, or a combination of SWE + HA with 0.22 kg ha-1 PPC, enhanced PE of preharvest sod in both years. Extension of heat duration from 72 to 96 h caused significantly more injury to the sod in 1999. All PGR treatment combinations reduced visual turf injury. On average in 1999 and 2000, SWE + HA, PPC, and SWE + HA + PPC enhanced transplant rooting by 21.8, 34.7, and 44.2%, respectively. Regression analysis indicated that KBG with higher preharvest PE suffered less turf injury and produced greater rooting after transplantation. The data suggest that foliar application of SWE + HA, PPC alone, or in a combination, may improve shelf life and transplant rooting of KBG sod.

Abbreviations: a.i., active ingredient • HA, humic acid • PE, photochemical efficiency • PPC, propiconazole • SWE, seaweed extract • WAH, week after harvest


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
KENTUCKY BLUEGRASS is a widely used cool-season turfgrass species for sod production in the USA and many other regions of the world. High quality sod is the goal of sod producers worldwide. However, sod frequently experiences severe environmental stress, especially supraoptimal temperatures during the transportation and transplanting period. Sod often loses its color several days after transplanting, expressing a condition called transplant shock (Giese et al., 1997; Heckman et al., 2001; King et al., 1982).

Environmental stresses may injure plants through accumulation of free radicals in cells (Zhang, 1997; Zhang and Schmidt, 1997). Sod is often exposed to an adverse environment characterized by high temperature and lack of irradiance after harvest and during transport. Previous researchers have reported that high temperatures, nitrogen levels, and cutting heights were associated with greater respiration, percent leaf kill, and ethylene production (King et al., 1982). During storage and sod transport, such high temperatures could cause carbohydrate reserves to decrease rapidly because of elevated respiration rates (Watschke et al., 1970). When the heat-stressed sod is transplanted into a field with high solar radiation, including ultraviolet (UV) wavelengths, greater injury may occur possibly through UV-induced photooxidation (Smirnoff, 1995; Schmidt and Zhang, 2001).

Transplant rooting is an important parameter for survival and successful establishment of sod (Giese et al., 1997; Goatley and Schmidt, 1991). Selected PGRs have been used to enhance turfgrass growth and tolerance to environmental stresses, including UV radiation and high temperature (Schmidt and Zhang, 2001). Goatley and Schmidt (1991) reported application of benzyladenine and PPC increased rooting of KBG sod as much as 23%. Heckman et al. (2001) noted that trinexapac-ethyl reduced the internal temperature of KBG sod stacks after 48 h of storage. Some PGRs, such as SWE, HA, and PPC may enhance root growth of grass subjected to stress possibly by increasing the antioxidant defense system (Zhang and Schmidt, 1997; Zhang and Schmidt, 1999, 2000a,b; Allen et al., 2001).

Beneficial effects of SWE on grass tolerance to stress have been documented (Zhang, 1997, Zhang and Schmidt, 1999; Fike et al., 2001). Its enhancement of plant growth and stress tolerance has been attributed to its hormonal activity (Sanderson and Jameson, 1986; Yan, 1993). Humic acid is one of the three fractions (humic acid, fulvic acid, and humins) of humic substances that is insoluble below pH 2.0, but soluble above. There is increasing evidence showing that humic acid exhibits auxin-like activity in stimulating root growth (O'Donnell, 1973; Cacco and Dell'Agnola, 1984; Young and Chen, 1997; Clapp et al., 1998). Propiconazole, a synthetic fungicide, has been shown to have plant growth regulating properties (Fletcher et al., 1986; Yan, 1993). These materials have been shown to enhance PE and root and shoot growth of KBG and creeping bentgrass (Agrostis palustris Huds.; Goatley and Schmidt, 1990, 1991; Zhang and Schmidt, 2000a). However, there are few studies concerning the influence of these PGRs on PE and transplant sod survival and rooting. The objectives of this study were to investigate the influence of selected PGRs on transplant sod survival and rooting of KBG sod following simulated heating and to examine the relationship between preharvest PE and transplant rooting of KBG sod subjected to postharvest stress.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Three-year-old Kentucky bluegrass (cv. Georgetown), grown on a Groseclose silt loam soil (clayey, Kaolinitic, mesic Typic Hapludult, pH 6.2, OM 2.2%, P 67 kg ha-1; K 280 kg ha-1) at the Virginia Tech Turfgrass Research Center, Blacksburg, VA, was used for this study. The turfed area was irrigated as needed to prevent visual wilting and was mowed weekly at 5 cm with a rotary mower; clippings were returned. The herbicides 2,4-D (2,4-dichlorophenoxyacetic acid) at 4.6 kg ha-1 plus dicamba (3,6-dichlorophenoxyacetic acid) at 2.3 kg ha-1, and pendimethalin [N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobezenamine] at 2.7 kg ha-1 were applied in late April 1999 and 2000 to control weeds, and the insecticide chlorpyrifos at 4.6 kg ha-1 was applied in late June of both years to control insects. Nitrogen was applied at 49 kg N ha-1 as urea in May 1999 and 2000. An additional 49 kg N ha-1 was supplied with a 10-10-10 fertilizer in October of 1998 and 1999.

The PGR treatments consisted of the following: (i) SWE + HA (0.5 + 1.5 kg ha-1); (ii) PPC (0.44 kg ha-1); (iii) SWE + HA + PPC (0.5 + 1.5 + 0.22 kg ha-1); and (iv) control. Seaweed extract, a dry powder, was supplied by Acadian Seaplants Limited (Dartmouth, Nova Scotia, Canada). Humic acid (a.i. 93%) was provided by Plant Wise Biostimulants, Inc. (Louisville, KY). Propiconazole (a.i. 14.3%) was supplied by Syngenta (Greensboro, NC). The PGRs were applied over the foliage with a CO2-pressurized boom sprayer delivering 784 L ha-1 liquid at 290 KPa 2 wk before harvest in both 1999 and 2000.

Plots (1.8 by 1.8 m) were arranged in a randomized complete block design with four replications. Data were analyzed with analysis of variance and mean separations were performed with a protected LSD test. Linear or polynomial (quadratic or cubic) regression analysis was performed for each data set with Sigmaplot 6.0 (2000).

For the heat treatment, two pieces of sod (0.3 by 1.8 m) were removed from each plot on 23 Aug. 1999 and 13 Aug. 2000, rolled and placed in a storage building set to maintain a uniform 40°C. This building has been engineered to heat sod similar to that stored during summer months in the center of pallet stacks or rolls except that the temperature increases gradually from the outside to the inside of each sod roll. The sod was placed on a bench (1 by 4 m) whose top is made of metal screen (1 by 1 cm). The bench is 0.5 m high. Two plastic pipes (15-cm diam) are used to deliver heat and a box fan is used to enhance uniform heat circulation within the building. A thermocouple (Omega model HH23, Stamford, CT) was used to measure and confirm uniform temperature conditions for each roll at 48 h (37.2 ± 1.1°C) and 72 h (36.7 ± 1.1°C). A metal bar (5-mm diam by 35 cm long), connected to the thermocouple, was inserted into the center of each roll and temperature measured at 48 and 72 h.

A sod roll from each treated plot was removed from the building after being heated for 72 h or 96 h, respectively, at approximately 37°C and transplanted 30 cm apart onto a prepared soil. A piece of sod (0.3 by 0.3 m) was cut from the center of the roll and a 0.3- by 0.3-m sheet of expanded metal was inserted under this piece of sod for subsequent rooting determination. The metal rooting sheet has uniformly distributed diamond-shaped openings (3.8 cm long by 1.6 cm wide at middle) for roots to grow past (Goatley and Schmidt, 1991; Schmidt et al., 1986). There was a distance of 30 cm between each piece of sod. Siduron [1-(2-methylcyclohexyl)-3-phenylurea] was applied at 6.1 kg ha-1 over and between the sod pieces for preemergent weed control. Irrigation was applied as needed to prevent desiccation. Three weeks after transplanting, turfgrass injury was rated based on a visual scale of 1 to 9, with 9 indicating the most injury. Six weeks after transplanting, transplant rooting was determined by measuring the energy required to vertically lift the expanded sheet of metal through which the sod roots had grown (Goatley and Schmidt, 1991; King and Beard, 1969). Briefly, hooks were attached to the corners of the steel squares and the amount of force required to lift the roots free from the soil was recorded with a 100-kg hand-held push/pull scale.

Photochemical efficiency (Fv/Fm) was determined on preharvest sod by measuring chlorophyll fluorescence with a dual wavelength fluorometer (OS-50, Opti-Sci. Inc., MA.). The ratio of variable fluorescence to maximum fluorescence at 690 nm Fv690nm/Fm690nm or Fv/Fm) is an indicator of the photochemical efficiency of photosystem II or relative photochemical efficiency (Bolhar-Nordenkampf and Oquist, 1993; Miles, 1990; Zhang and Schmidt, 2000a). Chlorophyll fluorescence was measured on the whole turfgrass canopy consisting of mature and actively growing leaves at approximately 1400 h on 23 Aug. 1999 and 13 Aug. 2000 under clear-sky conditions. The canopy area in each plot was selected randomly and covered for 15 min by a polyvinylchloride ring (10 cm dia. by 5 cm high) filled with polystyrene (10 mm thick) for dark acclimation. An opening the size of the probe (10-mm diam.) was made in the polystyrene of each polyvinylchloride ring and covered by a plastic plate. After the canopy was subjected to dark acclimation, the plastic plate was switched and the probe for the actinic light source (500 µmol m-2 s-1) was inserted immediately into the opening. Then the ring was rotated 90 degrees three times after each reading and another fluorescence measurement was collected. The values of Fv/Fm were calculated based on an average of the three measurements per experimental unit.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Photochemical Efficiency
All PGR treatments enhanced PE significantly in 1999 and 2000 (Table 1). On average, application of SWE + HA, PPC, and a combination of SWE + HA with a low dose of PPC increased PE by 14.1%, 17.6%, and 19.8%, respectively.


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  		Table 1. Photochemical efficiency (PE) of Kentucky bluegrass (Poa pratensis L.) sod before harvest as influenced by plant growth regulators (PGRs).{dagger}

 
Visual Turf Injury
Extending the heat treatment to 96 h caused higher transplant visual turf injury in 1999, but not in 2000 (Table 2). This was most likely due to the milder environmental conditions during the harvest and transplant period in 2000, especially lower irradiance (Table 3), which implies less total UV irradiance resulting in less photooxidative damage on leaf tissue.


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  		Table 2. Visual turf injury and transplant rooting of KBG (Poa pratensis L.) sod after heating as influenced by plant growth regulators.

 

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  		Table 3. Comparison of temperature and rainfall between 1999 and 2000 (Blacksburg, VA).

 
The interaction between heat duration and PGR treatment for visual turf injury was significant in 1999 (Table 2). The high rate of PPC alone (72 h) or both treatments that included PPC (96 h) provided the best protection in 1999. In 2000, there was no significant interaction between heat duration and PGR treatment, possibly because favorable climate conditions allowed more rapid recovery of sod, reducing the effects of heat duration on injury. However, when averaged over heat durations, all PGR treatments reduced transplant visual turf injury.

Transplant Rooting
All PGR treatments increased transplant rooting in 1999 following 72- and 96-h heat treatment (Table 2). Averaged over both years, SWE + HA, PPC, and SWE + HA + PPC enhanced transplant rooting by 21.8, 34.7, and 44.2%, respectively. The rooting of treatments containing PPC was consistently higher. As with the visual turf injury data, no PGR x heat duration interaction for rooting was observed in 2000. However, when averaged over heat durations in 2000, all treatments again increased transplant rooting.

Relationship between PE before Harvest and Transplant Visual Turf Injury or Rooting
There were negative quadratic or cubic regression relationships between pre-harvest PE and transplant visual turf injury (R2 = 0.9081** and 0.7738**) in both 1999 and 2000 (Fig. 1). The data suggest that sod with higher preharvest PE generally was injured less after being subjected to a period of high temperature stress and transplanted.



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  		Fig. 1. Relationship between photochemical efficiency (PE) before harvest and transplant visual turf injury of KBG sod in 1999 and 2000 (R2 value marked with ** is significant at P = 0.01).

 
Positive quadratic regression relationships between preharvest PE and transplant field rooting of the KBG sod 6 wk after heat treatment were obtained (R2 = 0.5196** and 0.7801**) in both years. The data indicate that KBG sod with higher PE before harvest generally produced better rooting after being subjected to heat stress (Fig. 2).



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  		Fig. 2. Relationship between photochemical efficiency (PE) before harvest and transplant rooting of KBG sod in 1999 and 2000 (R2 value marked with ** is significant at P = 0.01).

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
The results of this study indicate that foliar application of SWE + HA, PPC, or a combination of SWE + HA with a low dose of PPC, significantly improved PE of field grown KBG sod. This is in agreement with the results of Zhang and Schmidt (2000a)(b) who reported that foliar application of SWE or PPC improved PE of KBG and creeping bentgrass in both field and glasshouse conditions. Speculation regarding the cause of rooting and PE increases involve the hormone-like activity of these compounds or their effects on plant hormone balance. Propiconazole, like other triazoles, interferes with the isoprenoid pathway through inhibition of C-14 demethylation reactions which block sterol and gibberellin biosynthesis (Fletcher and Nath, 1984). Additionally, a complex of cytokinins and indole–3-acetic acid (IAA) have been identified and quantified in extracts of A. nodosum (Tay et al., 1985; Sanderson and Jameson, 1986; Sanderson et al., 1987). Bioassays also have indicated that A. nodosum extracts exhibit cytokinin-like activity (Sanderson and Jameson, 1986; Allen et al., 2001). Polyamines and IAA have also been quantified in humic acid preparations and its hormone-like activity in stimulating root growth has been shown in bioassays (O'Donnell, 1973; Cacco and Dell'Agnola, 1984; Young and Chen, 1997).

A shift in the balance of plant hormones, including gibberellins, abscisic acid, and cytokinins, may account for their plant growth regulating activity (Fletcher and Hofstra, 1988). Thus, exogenous applications of PPC, SWE, and HA may be causing endogenous shifts in the balance of hormones, increasing cytokinins, IAA, and ABA levels while decreasing gibberellins. Previous work has demonstrated that a consequence of this supposed shift is an increase in antioxidants such as superoxide dismutase, {alpha}-tocopherol, and ascorbic acid resulting in greater tolerance to various stresses (Mackay et al., 1987; Zhang and Schmidt, 1999, 2000a, b).

PGR-induced PE increases before sod harvest may offer protection of photosystem II during sod heat treatment and during transplantation (Table 1; Zhang and Schmidt, 2000a). Higher PE reflects better electron transport and more efficient ATP and NADPH synthesis, and eventually CO2 reduction (Miles, 1990). Greater PE found in the sod treated with PGRs suggests that the sod may utilize radiant energy more efficiently during summer transplant periods characterized by high temperature and irradiance.

The technique used in this study simulates a type of stress (high temperature and dark environment) that sod experiences during transport periods in the summer months. Previous research showed that high temperature is one of the major factors causing sod quality decline (King et al., 1982). Heckman et al. (2001) indicated that suppression of turfgrass respiration rate might decrease sod injury under high temperature stress. When heat stressed sod is exposed to a full sunlight environment, photoinhibition and photooxidation most likely occur (Demmig-Adams and Adams, 1992; Smirnoff, 1995). The results of this study indicate that application of the PGRs reduced transplant visual turf injury. This is consistent with the findings by Schmidt and Zhang (2001) who showed that a decline of PE caused by UV irradiation of KBG could be alleviated partially by application of selected PGRs. This suggests that the PGR applications improved PE, resulting in better resistance to UV radiation stress after transplantation. Further work characterizing the relationship between the field UV environment, PE, PGR-treatment, and subsequent leaf injury is warranted as are investigations regarding the supposed hormonal mode of action of these materials.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 




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