Published in Crop Sci. 44:209-213 (2004).
© 2004 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
TURFGRASS SCIENCE
Endogenous Cytokinin Levels and Growth Responses to Extended Photoperiods for Creeping Bentgrass under Heat Stress
Zhaolong Wanga,
Qingzhang Xub and
Bingru Huang*,b
a College of Agricultural and Biological Sci., Shanghai Jiao Tong Univ., Shanghai, 201101, China
b Dep. of Plant Biology and Pathology, Rutgers Univ., New Brunswick, NJ 08901
* Corresponding author (huang{at}aesop.rutgers.edu).
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ABSTRACT
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Summer bentgrass decline is associated with inhibition of plant growth and cytokinin production. This study was designed to investigate whether shoot and root growth and cytokinin production for creeping bentgrass (Agrostis stolonifera L.) exposed to heat stress would be improved by extending photoperiod. ‘Penncross’ plants were initially grown in growth chambers at 20/15°C (day/night) and at a 14-h photoperiod for 60 d, and then exposed to 33/28°C (heat stress) with three different photoperiods: 14 (control), 18, and 22 h (extended photoperiod) daily for 32 d. Root number, fresh weight, viability, turf quality, and shoot extension rate declined during heat stress at the 14-h photoperiod. The decline was delayed and suppressed by extending photoperiod from 14 to 18 or 22 h. Root growth, turf quality, and shoot extension rate increased with extending photoperiod. Cytokinin content in roots and leaves also decreased during heat stress at the 14-h photoperiod. Plants at the 22- and 18-h photoperiods maintained higher cytokinin content in roots at 16 and 32 d of heat stress. The differences in total cytokinin content in leaves among photoperiod treatments were mainly due to the variations in trans-zeatin [6-(4-hydroxy-3-methylbut-2-enylamino) purine], zeatin riboside {6-[(E)-4-hydroxy-3-methylbut-2-enylamino]-9-ß-D-ribofuranosylpurine} and isopentenyl adenosine [iPA, 6-(3,3-dimethylallylamino)-9-ß-D-ribofuranosylpurine]. Cytokinin content in leaves at the 14- and 18-h photoperiods decreased at 16 d of stress, but at the 22-h photoperiod it did not decrease until 32 d. These results demonstrated that extended photoperiod increased root and shoot growth and endogenous cytokinin levels under heat stress, suggesting that extended photoperiod could alleviate heat injury in creeping bentgrass.
Abbreviations: DHZ/DHZR, dihydrozeatin/dihydrozeatin riboside • iP, isopentenyl adenine • iPA, isopentenyl adenosine • PAR, photosynthetically active radiation • TTC, triphenyltetrazolium chloride • Z/ZR, trans-zeatin/zeatin riboside
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INTRODUCTION
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TEMPERATURE AND DAYLENGTH are two main factors controlling leaf initiation and growth, especially following defoliation or mowing (Menhenett and Wareing, 1977). Cattani and Struik (2001) reported that under favorable temperature conditions, increasing the photoperiod from 8 to 16 h daily increased tiller density, number of stolons and green leaves, and shoot dry weight in creeping bentgrass. Similar results were also found with Kentucky bluegrass (Poa pratensis L.) (Hay and Heide, 1983; Aamlid, 1992), Timothy (Phleum pratense L.) (Hay and Pedersen, 1986), and some subtropical grasses (Sinclair et al., 2001).
Creeping bentgrass is widely used on putting greens and fairways. Because of its cool-season characteristics, turf quality and shoot and root growth of creeping bentgrass often declines during summer (Xu and Huang, 2000, 2001). Extended photoperiod has been found to delay leaf senescence in detached western larch (Larix occidentalis Nutt.) needles (Rosenthal and Camm, 1996) and increased cold hardiness in yellow cypress [Xanthocyparis nootkatensis (D. Don) Farjon et al.] seedlings (Puttonen and Arnott, 1994). However, it is unclear whether shoot and root growth of creeping bentgrass could be improved by extending the photoperiod under heat stress conditions.
Cytokinins may play an important role in photoperiodic improvement of plant growth under environmental stresses (Pons et al., 2001). An extended light period increased cytokinin levels in long-day plants such as Sinapis alba L. (Lejeune et al., 1988) and Solanum tuberosum subsp. andigenum (Juz. & Bukasov) Hawkes (Machackova et al., 1998). Studies with the long-day plant Sinapis alba and the short-day plant Xanthium strumarium L. suggested that cytokinin synthesis is photoperiodically controlled and may serve as root-to-shoot signal in affecting shoot growth and development (Kinet et al., 1993).
Cytokinins are synthesized primarily in root tips and transported via the xylem to the shoots, where they exert major regulatory influences on growth and development (Haberer and Kieber, 2002). Synthesis of cytokinins in roots is among the most sensitive processes to heat stress. Heat stress for 5 h inhibited cytokinin synthesis in roots of Phaseolus vulgaris L. (Udomprasert et al., 1995). Brief heat treatment (47.5°C for 2 min) to roots of maize (Zea mays L.) inhibited chlorophyll accumulation and photosynthetic activity in shoots, and adverse effects were reversed by applying cytokinins to plants (Caers et al., 1985). Root-produced cytokinins mediated cell wall synthesis and photosynthesis of leaves (Itai et al., 1973). Cytokinin production in roots of creeping bentgrass declined with heat stress; application of cytokinin to the root zone alleviated leaf senescence and improved turf quality under heat stress (Liu and Huang, 2002; Liu et al., 2002).
The objective of this study was to investigate whether shoot and root growth and endogenous cytokinin production for creeping bentgrass exposed to heat stress would be improved by extending photoperiod.
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MATERIALS AND METHODS
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Sod pieces of Penncross creeping bentgrass were collected from the Turfgrass Research Center at North Brunswick, NJ, and transplanted into clear polyethylene bags (10 cm in diameter and 41 cm in length) that were placed in opaque polyvinylchloride tubes. The bags were filled with sterilized fine sand. Plants were grown in growth chambers at 20/15°C (day/night), 600 µmol m–2 s–1 photosynthetically active radiation (PAR), and a 14-h photoperiod for 30 d before treatments were imposed. Before and during treatments, turf was mowed daily at a 3.8-mm height with an electric hand clipper, watered daily, and fertilized weekly with 50 mL of full-strength Hoagland's nutrient solution (Hoagland and Arnon, 1950).
Plants were initially grown in growth chambers set at 20/15°C (day/night) at a 14-h photoperiod for 60 d. During the photoperiod treatment, plants were exposed to a day/night temperature of 33/28°C (heat stress) with three different light durations: 14, 18, and 22 h of light during each 24-h cycle for 30 d. The 14-h light duration is the average daylength during summer months in our area, which was considered as the control. The PAR at the canopy level was maintained at 600 µmol q m–2 s–1, which was 
70% of the light saturation point for C3 plants (Salisbury and Ross, 1992).
Each photoperiod treatment was replicated four times in four different growth chambers. At any one time, each light treatment was performed in one growth chamber. The three treatments were randomly assigned to different growth chambers. All measurements were made on two subsamples for each treatment at various times of treatments (varied with parameters, shown in the table and figures) to study time response. The experimental design was a completely randomized design with repeated measures. Effects of photoperiod, duration of treatment (photoperiod and temperature), and the interaction between photoperiod and duration were determined by ANOVA according to the general linear model procedure of Statistical Analysis System (SAS Institute, Cary, NC). Differences among photoperiod treatments at a given day of heat stress and between the initial level (0 d of heat stress) and various durations of heat stress were determined by the LSD test at the 0.05 probability level.
Turf quality was visually rated based on color, density, and uniformity on a scale of one (worst, most plants died) to nine (best, healthy and green plants). Grasses rated at six or above were considered to have acceptable quality. Vertical shoot extension rate was estimated by measuring the difference in mean canopy height in a 2-d period with a ruler. The number of primary roots was counted at 10, 21, and 31 d of treatment for roots visible on the surface of the clear polyethylene bags in two 2 x 3 cm areas marked on the opposite side of each bag at a 5-cm depth from the soil surface. Root density was calculated as root number per unit soil surface area (cm2).
Roots were harvested at 10, 21, and 31 d of treatment to determine fresh weight and activity. Total fresh weight of all roots in each PVC tube was determined after roots were washed free of soil and blotted dry with paper towels. Relative root activity was evaluated by measuring dehydrogenase activity with triphenyltetrazolium chloride (TTC) reduction technique (Knievel, 1973). Roots (0.15 g) were washed free of soil and incubated in 0.05 M Na2HPO4–NaH2PO4 buffer (pH 7.6) containing 0.6% TTC (w/v) at 34°C for 20 h. Samples were washed twice with distilled and deionized water and then extracted in 95% ethanol at 55°C for 2 h. Absorbance was measured at 490 nm.
Extraction and quantification of cytokinins in leaves and roots followed the methods described by Setter et al. (2001) with modifications. Briefly, plant samples were extracted in 80% [v/v] methanol, and then partially purified with reverse phase C18 column packed with 15 mg of 40-µm-diam. C18 silica material (J.T. Baker Chemicals, Phillipsburg, NJ). Samples were loaded in 100 µL of Solvent A (10 mM tri-ethylamine-acetate, pH 3.4). Hydrophilic contaminants were washed out with 200 µL of Solvent B (85% Solvent A, 15% methanol, v/v), and then cytokinin-containing fraction was eluted with 200 µL of Solvent C (65% solvent A, 35% methanol, v/v). Corresponding radioactive chemicals (Amersham Co., Arlington Heights, IL) were added to each sample to monitor the loss of radioactivity during the purification step. Recovery of each cytokinin compound averaged >90% on the basis of analysis of radioactivity in noncytokinin fractions.
An indirect competitive ELISA was used for quantification of three kinds of cytokinins, trans-zeatin/zeatin riboside (Z/ZR), dihydrozeatin/dihydrozeatin riboside (DHZ/DHZR, dihydrozeatin/dihydrozeatin-9-ß-D-riboside), and iPA as previously described by Setter et al. (2001). Monoclonal antibody for Z/ZR, DHZ/DHZR, and iPA were from clone no. t-ZR-J3-I-B3, clone no. DHZR-J23-II-B1, clone no. iPA-J40-IV-C4, respectively (Eberle et al., 1986).
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RESULTS
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Turf Quality and Shoot Growth Rate
Turf quality declined with the duration of heat stress to below the initial (0 d) level at 10, 18, and 24 d of treatment for plants exposed to 14-, 18-, and 22-h photoperiods, respectively (Fig. 1). Turf quality was highest at the 22-h photoperiod, lowest at the 14-h photoperiod, and intermediate at the 18h-photoperiod during prolonged periods of heat stress. Turf quality was maintained above the minimum acceptable level (6.0) until 28 d of heat stress at the 22-h photoperiod, but decreased to below this level at 18 d at the 14-h photoperiod and 24 d at the 18-h photoperiod.
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Fig. 1. Effects of extended photoperiod on turf quality of creeping bentgrass under heat stress. Vertical bars on the top indicate LSDs (P = 0.05) for treatment comparison at a given day of treatment. Vertical bars on the right indicate LSDs (P = 0.05) for turf quality comparisons across time within a treatment.
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Vertical shoot extension rate declined with duration of heat stress to below the initial level at 10 d of heat stress at the 14-h and 18-h photoperiods and 24 d at the 22-h photoperiod (Fig. 2). The decline was less dramatic at the 22-h photoperiod than that at the 14-h photoperiod. Shoot extension rate increased as photoperiod increased from 14- to 18- or 22-h.
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Fig. 2. Effects of extended photoperiod on shoot extension rate of creeping bentgrass under heat stress. Vertical bars on the top indicate LSDs (P = 0.05) for treatment comparison at a given day of treatment. Vertical bars on the right indicate LSDs (P = 0.05) for comparisons across time within a treatment.
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Root Growth
Root density (Fig. 3) and root fresh weight (Fig. 4) decreased significantly with the duration of heat stress for all three photoperiod treatments, compared with the initial level. The decline was less severe at the 22-h photoperiod than at the 14- and 18-h photoperiods. Both root density and fresh weight were higher at the 22-h than at the 14-h photoperiod throughout the treatment period. The 18-h photoperiod maintained higher root density than the 14-h at 21 and 32 d of treatment.
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Fig. 3. Effects of extended photoperiod on root density of creeping bentgrass under heat stress. Vertical bars on the top indicate LSDs (P = 0.05) for treatment comparison at a given day of treatment. Vertical bars on the right indicate LSDs (P = 0.05) for comparisons across time within a treatment.
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Fig. 4. Effects of extended photoperiod on root fresh weight of creeping bentgrass under heat stress. Vertical bars on the top indicate LSDs (P = 0.05) for treatment comparison at a given day of treatment. Vertical bars on the right indicate LSDs (P = 0.05) for comparisons across time within a treatment.
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Root activity decreased with the duration of heat stress to below the initial level at 10 d at 14-h and 18-h photoperiods, and at 21 d at the 22-h photoperiod. The decrease was more pronounced at the 14- than at the 18- and 22-h photoperiods (Fig. 5). Plants exposed to the 22-h photoperiod maintained highest root activity, followed by those at the 18-, then the 14-h photoperiod.
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Fig. 5. Effects of extended photoperiod on root dehydrogenase activity of creeping bentgrass under heat stress. Vertical bars on the top indicate LSDs (P = 0.05) for treatment comparison at a given day of treatment. Vertical bars on the right indicate LSDs (P = 0.05) for comparisons across time within a treatment.
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Cytokinin Production
Total cytokinin content in leaves (Fig. 6A) and roots (Fig. 6B) decreased during heat stress compared with the initial level. More rapid reduction occurred at the 14- and 18-h photoperiods than at the 22-h photoperiod (Fig. 6). Leaves at the 22-h photoperiod had significantly higher total cytokinin content than those at the 14-h photoperiod at the 16, 24, and 32 d, and the 18-h photoperiod at 16 and 24 d. Extended photoperiod from 14 h to 18 or 22 h increased total cytokinin content in roots at both 16 and 32 d of heat stress, but there was no difference in total cytokinin content in roots between 18- and 22-h treatment at 16 or 32 d.
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Fig. 6. Effects of extended photoperiod on total cytokinin content in leaves (A) and roots (B) of creeping bentgrass under heat stress. Vertical bars on the top in (A) indicate LSDs (P = 0.05) for treatment comparison at a given day of treatment. Vertical bars on the right in (A) indicate LSDs (P = 0.05) for comparisons across time within a treatment. Letters above the columns in uppercase in (B) indicate the differences among photoperiods at a given day of treatment (P < 0.05). Letters above the column in lowercase in (B) indicate the differences between 16 and 32 d within the same photoperiod treatment (P < 0.05).
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The content of Z/ZR in leaves increased with prolonged photoperiod from 14 to 18 or 22 h at both 16 and 32 d of treatment (Table 1). Leaves at 22 h had higher iPA content than those at 14 h at 16 and 32 d, but had higher DHZ/DHZR only at 32 d. At 16 d of heat stress, root Z/ZR and DHZ/DHZR did not differ among three photoperiod treatments, but iPA content at the 22- and 18-h photoperiods was significantly higher than that at the 14-h photoperiod. At 32 d, compared with those at the 14-h photoperiod, roots had higher content of Z/ZR, iPA, and DHZ/DHZR at the 22-h photoperiod and high content of Z/ZR at the 18-h photoperiod (Table 1).
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Table 1. Effects of extended photoperiod on individual cytokinin content in leaves and roots of creeping bentgrass.
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DISCUSSION
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Turf quality, shoot extension rate, root density, fresh weight, and viability decreased during heat stress (33/28°C) compared with their initial levels at 20/15°C. Root dieback under high temperature could result in limited cytokinin synthesis, hence affect shoot growth and senescence (Adedipe et al., 1971). Cytokinin content in both roots and leaves decreased with the duration of heat stress, in agreement with a previous study in creeping bentgrass (Liu et al., 2002). Heat-induced reduction in cytokinin content has also been reported in other plant species (Udomprasert et al., 1995; Zhou and Leul, 1999). Liu and Huang (2002) and Liu et al. (2002) found that cytokinin application to the root zone of creeping bentgrass alleviated the decline of turf quality caused by heat stress. Roots are the primary sites for cytokinin biosynthesis and root-produced cytokinins may serve as root-to-shoot signal in plant responses to environment stresses (Kinet et al., 1993; Haberer and Kieber, 2002). Reduced cytokinin levels in roots contributes to leaf senescence associated with the inhibition of photosynthetic activities and chlorophyll accumulation in leaves when roots of creeping bentgrass are exposed to heat stress (Liu and Huang, 2002; Liu et al., 2002).
Extended photoperiod alleviated the decline in turf quality and shoot growth rate and maintained a more viable and extensive root system than the natural daylength under heat stress conditions. Extended photoperiod, particularly from a 14- to 22-h photoperiod, also increased the level of cytokinins both in roots and leaves exposed to heat stress. Higher cytokinin production in roots and its supply to leaves with extended photoperiod could be associated with the improved shoot and root growth under heat stress. However, it is not clear whether higher cytokinin levels under extended photoperiods were the result or the cause of changes in shoots and root growth. Kinet et al. (1993) compared a long-day plant, Sinapis alba, with a short-day plant, Xanthium strumarium, and suggested the existence of a shoot-to-root signal which is under photoperiodic control and affects cytokinin synthesis in and/or release from the roots. Prolonged photoperiod promotes cytokinin synthesis in roots and transport to shoots, inducing lateral bud formation and floral initiation (Lejeune et al., 1988).
Changes in photoperiod have been reported to control the synthesis of ZR-5'-monophosphate in leaves rather than cytokinin degradation (Machackova et al., 1996). Menhenett and Wareing (1977) reported that increasing daylength tended to reduce levels of cytokinin bases and/or nucleosides, but increased levels of nucleotide cytokinins for Dactylis glomerata L. Hebbar et al. (1994) reported that zeatin and dihydrozeatin were the predominant cytokinins under long-day conditions in carrots (Daucus carota L.). In Sinapis alba, Lejeune et al. (1994) found that isopentenyl adenine-type (iP-type; chemical definition???) cytokinins were enriched in phloem and xylem saps after long-day treatment. The iP-type cytokinins were markedly accumulated in shoot apical meristem tissues after flower induction by long-day photoperiods (Jacqmard et al., 2002). In our study, iPA was the predominant cytokinin in both leaves and roots for all three photoperiod treatments. Root iPA was also more responsive to changes in photoperiod than Z/ZR and DHZ/DHZR. The results suggest that the differences in total cytokinin content in leaves among photoperiod treatments were mainly due to the variations in iPA and Z/ZR (Table 1).
In summary, our results demonstrated that extended photoperiod alleviated and delayed the decline in shoot and root growth. The effects were associated with the reduction in endogenous cytokinin levels in both leaves and roots of creeping bentgrass exposed to heat stress. It is possible that the effects of extended photoperiods in this study were confounded by increased daily accumulative PAR. In any case, our results suggest that extending the photoperiod or increasing light exposure could help maintain quality turf under high temperature conditions by promoting cytokinin synthesis. This could be achieved with supplemental lighting such as growing lights which provide light intensity high enough to meet at least the light compensation point to sustain photosynthesis.
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ACKNOWLEDGMENTS
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The authors express thanks to the Center of Turfgrass Science, Rutgers University, for the financial support of the research, and to Dr. Jane Larkindale, John Pote, and Michelle Dacosta for reviewing the manuscript.
Received for publication May 25, 2002.
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ABSTRACT
INTRODUCTION
