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Agronomy Journal 93:936-943 (2001)
© 2001 American Society of Agronomy

GRAIN AND OIL CROPS

Plant Growth Regulator Effects on Spring Cereal Root and Shoot Growth

A. Rajala* and P. Peltonen-Sainio

University of Helsinki, Dep. of Plant Production, Section of Crop Husbandry, P.O. 27 Univ. of Helsinki, FIN-00014, Finland

* Corresponding author (ari.rajala{at}mtt.fi)



   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIAL AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Plant growth regulators (PGR) shorten the straw of cereals, but their effects on other traits of plant stand structure have been inconsistent. To arrive at an assessment of whole-plant response, experiments were conducted in the greenhouse to study the effect of PGR applications on root and shoot growth of spring cereals. Ethephon, chlormequat chloride (CCC), and trinexapac–ethyl were applied during early growth stages to barley (Hordeum vulgare L.), oat (Avena sativa L.), and wheat (Triticum aestivum L.) cultivars grown either in sand mixture or in clay illitic topsoil. The effects on root and shoot growth (mg plant-1), root/shoot ratio, tiller number, and weight per main shoot were studied in all cereal species. The response of CO2 exchange rate (CER) (µmol CO2 m-2 s-1) and formation of yield potential to PGR treatment were studied more closely in "Mahti" wheat. The effect of CCC application on root area (cm2), length (cm), specific root length (SRL cm/mg), and width (µm) at soil depths of 0 to 20, 20 to 40, and 40 to 60 cm was studied in Mahti wheat. Plant growth regulator applications reduced main shoot growth in barley and wheat up to 20%. Tiller production was enhanced by ethephon and TE treatment in all species, but not adequately to compensate for PGR-induced reduction in main shoot growth. Carbon dioxide exchange rate was reduced temporarily by ethephon and TE treatments in Mahti wheat. Plant growth regulator applications have modest potential for modifying traits of spring cereal plant stand structure other than straw length.

Abbreviations: CCC, chlormequat chloride • CER, CO2 exchange rate • Exp, experiment • DAT, days after PGR treatment • GS, growth stage • GY, grain yield • HI, harvest index • intmed, intermediate height • LSMEAN, least square mean • PGR, plant growth regulator • SGW, single grain weight • SRL, specific root length • TE, trinexapac–ethyl • ZGS, Zadocks growth scale


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIAL AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
PLANT GROWTH REGULATORS (PGR) are traditionally used in high-input cereal management to shorten the straw and, consequently, increase lodging resistance. This often has a beneficial effect on quantity and quality of harvested grain. To control lodging, PGRs are applied at the beginning of stem elongation (CCC) or at the flag leaf stage (ethephon and trinexapac–ethyl). There are some indications that when applied at earlier growth stages, PGRs may have more versatile effects on plant growth other than being mere antilodging agents.

Application of PGRs, mainly antigibberellins, to cereals has resulted in increased root growth (increased root length, greater root mass) or a higher root/shoot ratio under field conditions (Humbries et al., 1965; De et al., 1982; Bragg et al., 1984; Steen and Wünsche, 1990) and in pots (Naylor et al., 1986; Yang and Naylor, 1988; Enam and Cartwright, 1990). However, the impact on grain yield has been inconsistent. De et al. (1982) postulated that application of chlormequat chloride (CCC) to wheat grown in arid conditions increased root growth, resulting in more efficient water extraction from the deeper soil layers and thereby higher grain yield. However, Bragg et al. (1984) and Steen and Wünsche (1990) did not record significant yield increases as a consequence of enhanced root growth.

Cooke et al. (1983) reported that CCC treatment increased root/shoot ratio in winter wheat seedlings grown hydroponically. This was due to reduced shoot growth and enhanced root growth (Cooke et al., 1983). The longer the shoot of the studied cultivar, the greater the increase in root/shoot ratio following application of CCC. However, under closer monitoring of two wheat cultivars, no significant response of root growth to CCC treatment was observed (Cooke et al., 1983). Contrary effects of PGR on root growth have been reported, indicating that PGR effects on root and shoot interaction are complex. Application of ethephon alone and in a mixture with mepiquat chloride, either as a seed treatment or foliar application, inhibited root growth in barley when measured 2 to 3 wk after application (Woodward and Marshall, 1987, 1988).

Applications of PGRs have modified tiller number, especially when applied as a seed treatment or during early growth stages (Cartwright and Waddington, 1982; Hutley-Bull and Schwabe, 1982; Koranteg and Matthews, 1982; Waddington and Cartwright, 1986; Woodward and Marshall, 1987, 1988; Ma and Smith, 1991). Ethephon, CCC, and mepiquat chloride, alone and in mixtures, stimulated tillering and occasionally increased the number of spike-bearing tillers (Cartwright and Waddington, 1982; Koranteg and Matthews, 1982; Waddington and Cartwright, 1986; Naylor and Saleh, 1987; Woodward and Marshall, 1987, 1988; Craufurd and Cartwright, 1989; Ramos et al., 1989; Ma and Smith, 1991; Moes and Stobbe, 1991; Peltonen and Peltonen-Sainio, 1997). In some experiments, this resulted in higher grain yield (Cartwright and Waddington, 1982; Waddington and Cartwright, 1986; Ramos et al., 1989; Peltonen and Peltonen-Sainio, 1997), whereas in others it did not (Woodward and Marshall, 1988; Foster et al., 1991; Ma and Smith, 1991; Moes and Stobbe, 1991). Increase in tiller formation may be due to changes in responsiveness to day length (Hutley-Bull and Schwabe, 1982; Craufurd and Cartwright, 1989), inhibition of biosynthesis and transport of apical dominance initiating auxins (Morgan and Gausman, 1966; Lyon, 1970; Evans, 1984), or changes in assimilate and nutrient availability (Woodward and Marshall, 1987, 1988). Under Nordic growing conditions, high seeding rates (500–600 viable seeds m-2) concurrent with prolonging light period during early growth stages suppress tiller formation (Hutley-Bull and Schwabe, 1982; Peltonen-Sainio and Järvinen, 1995). To overcome this and to improve root growth in spring cereals, early application of CCC is recommended by Finnish agrochemical corporations.

Although PGR treatments alter crop growth, research on their effects on photosynthesis is limited. According to Höfner and Kühn (1982), CCC alone and in mixture with ancymidol slightly decreased 14CO2-assimilation in wheat, which was compensated for by prolonged duration of photosynthesis and equal or increased assimilate partitioning to the grains. Therefore, grain yields of treated and control plants did not differ (Höfner and Kühn, 1982).

This study, comprising several experiments, is aimed at monitoring the possibilities of antigibberellins, CCC and trinexapac–ethyl, and ethylene-releasing ethephon, applied at early growth stages (ZGS 12-13), to modify root and shoot growth and tiller formation of barley, oat, and wheat cultivars varying in stem height and dry matter partitioning. Furthermore, the potential of PGRs to modify grain number and weight in main shoot and tiller spike, root growth pattern at varying soil depths, and photosynthetic capacity was studied more closely in wheat cultivar Mahti. In part, these experiments were carried out to reveal if early applications of PGRs have beneficial effects on plant stand structure of spring cereals as claimed by the agrochemical companies in Finland and to support or discourage their use for this purpose.


   MATERIAL AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIAL AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Five experiments were conducted in a greenhouse at Viikki Experimental Farm, University of Helsinki, Finland. Species, cultivars, experimental designs, and PGR treatments are given in Table 1. In the greenhouse, photoperiod was set to 17 h and the minimum temperature to 20°C. In every experiment conducted, except Exp 4, 15 seeds were sown. Following emergence, seedlings were thinned to 10 pot-1. Plants were grown either in clay, tentatively classified as fine Typic Cryaquept (Yli-Halla et al., 2000), topsoil from the experimental field, or in a mixture of graded sand (0–0.6 mm and 0.1–0.6 mm, 1:1). All PGR applications were applied with a battery-operated small-scale atomiser (Wagner PiCO-Bel, Germany) at 8 mL pot-1, except for the replicate trial in Exp 3 (4 mL pot-1), and in Exp 4 (1 mL plant-1). In experiments carried out in sand mixtures, liquid fertilizer (6.4–5–26, N–P–K) was applied during watering at 0.4% (v/v). Root and shoot growth were measured 14 DAT in Exp 2, 3, and 4, and 14 d after last PGR application, at the flag leaf stage (ZGS 39, Zadoks et al., 1974) in Exp 1. In Exp 5, Trial 2, shoot biomass was recorded 5 DAT. All root and shoot samples were dried overnight at 100°C. Results in tables and figures are presented as weight (mg plant-1) or number per main shoot. Statistical analyses were carried out with SAS (SAS Inst., North Carolina, USA). The PROC GLM method was used for data analysis in Exp 1, 2, and 3, and in Exp 4 and 5 a repeated measures method of PROC MIXED was employed (SAS Inst., 1996). All factors involved in the Exp 4 and 5 were considered fixed effects.


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  		Table 1. Species, cultivars, cultivar type, PGR treatments with active ingredients, timing of treatment, growth media, and experimental design of different experiments carried out in greenhouse.

 
In Exp 1, the effect of PGR application time (Table 1) on root and shoot growth was studied in three oat cultivars differing in their growth habit. Root and shoot dry weights were determined 14 d after the final PGR application (Table 1). Two trials were carried out in Exp 2 to study the impact of early PGR application, conducted at the three-leaf stage (ZGS 13), and the effect of PGR on root and shoot growth of barley, oat, and wheat. Experiment 3 was conducted to study the effect of early application of PGR (ZGS 13) on root and shoot growth of spring wheat (cv. Mahti). The first trial was conducted in the greenhouse and the replicate trial outdoors nearby the greenhouse. Two weeks after PGR application, half of the pots were removed and root and shoot growth (mg plant-1) was recorded. At maturity, root and shoot growths of plants from the second set of pots were analyzed. Phytomass (mg plant-1), grain yield (mg plant-1), grain number (no. of grains head-1), and single grain weight (mg) of main shoot and tillers were measured separately. Harvest index (HI%, the proportion of grain to phytomass) and the total number and number of spike-bearing tillers (no. plant-1) were also recorded.

Experiment 4 was carried out to monitor root growth in more detail. Plants were grown in a 5.4-L glass-fronted wooden box (30 by 60 by 3 cm). To avoid soil loss and to permit water to enter the soil, a dense net was placed on the base of the box. One pregerminated seed was sown per box, which contained clay illitic topsoil from the experimental field in the first trial and mixture of graded sand (0–0.6 and 0.1–0.6 mm, 1:1) in the replicate trial. Wooden boxes were placed in a plastic container and were ground watered frequently. Two weeks after CCC application, aboveground plant parts were cut and dry weight (mg plant-1) of shoots was recorded. The soil was divided in to three sections according to depth from soil surface, 0 to 20, 20 to 40, and 40 to 60 cm. In the first subtrial, root sections were first separated from the soil using forceps and then washed to remove the soil particles. In the replicate trial, sand samples were placed on a sieve plate (r = 1.2 mm) and roots were separated from the sand under running water. After removing the soil and sand, the root samples were stored in 15% ethanol at 4°C. Root samples were stained in malachite green for 2 d, and then placed uniformly on a tray where a subsample was taken by cylinder (Ø 60 mm). The subsample was placed uniformly on a glass tray and was scanned (Corel PHOTO-PAINT). Image files were analyzed using ROOTEDGE 2.0C (Iowa State Univ. Res. Foundation, Inc.) to determine the root area (cm2), average root width (µm), and total root length (cm). Following scanning, root samples were dried overnight at 100°C before determining dry weight (mg plant-1). Specific root length ratio (SRL cm mg-1) and root/shoot ratio were calculated.

In Exp 5, two trials were conducted in the greenhouse to study the effect of early application of PGR on CO2 exchange rate (CER) of spring wheat (cv. Mahti). Carbon dioxide exchange rate (µmol CO2 m-2 s-1) was measured from the youngest fully emerged leaf on 4 plants pot-1. Measurements were conducted 2, 3, and 6 DAT in the first trial, and 4 h and 1, 2, 3, and 5 DAT in the second trial using a portable photosynthesis meter (LI-6400, Licor, Nebraska, USA). Light response curve was conducted and the internal light source was set at 200-µmol photons m-2 s-1 of photosynthetically active radiation in both trials.


   RESULTS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIAL AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
In Exp 1, oat cultivars differing in stem height responded similarly to PGR treatments. The CCC treatment increased root/shoot ratio by 21% at late application (ZGS 39, Zadoks et al., 1974) (data not shown). Earlier applications of PGRs did not affect root and shoot growth when recorded at heading. Tall and conventional height cultivars (Jalostettu maatiainen and Salo) differed in root and shoot weight from the dwarf cultivar (Pal), but not in root/shoot ratio (data not shown).

In Exp 2, most of the measured characteristics differed among trials, species, and treatments. The PGR treatments significantly affected shoot and tiller growth and root/shoot ratio. Early application of CCC and ethephon reduced shoot growth in barley and wheat, and TE markedly reduced shoot growth in all three species compared with the control (Table 2). Reduced shoot weight was partly compensated for by increased weight of tillers in TE-treated oat and ethephon-treated barley. Application of TE to oat and CCC to wheat reduced root growth. Tiller number was increased in all three cereal species by early ethephon and TE treatments. In barley and wheat, root/shoot ratio was increased by TE (Table 2). When analysis of variance was run within species, cultivars differed in their response to PGR treatments for weight of tillers per main shoot (barley, P = 0.002; oat, P = 0.029; and wheat, P = 0.002) and in tiller number (barley, P = 0.002). Tiller weight was increased in Saana by ethephon treatment, and in Veli by TE treatment. Tiller weight was reduced in Kymppi and Tjalve by CCC treatment (Table 3). Tiller number was increased in Kymppi, Veli, and Mahti by TE treatment. Ethephon treatment increased tiller number in Saana, Pal, Mahti, and Tjalve (Table 3).


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  		Table 2. The effect of early application of PGR on mean shoot, tiller and root growth, root/shoot ratio, and tiller number of barley, oat, and wheat measured 14 DAT.

 

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  		Table 3. The effect of early application of PGRs on tiller weight and number per main shoot in barley, oat, and wheat cultivars in Exp 2.

 
In Exp 3, TE treatment reduced shoot and tiller weight, and total phytomass production in wheat cultivar Mahti when measured 14 DAT (Table 4). A significant trial x treatment interaction (P < 0.05) was observed for shoot weight, phytomass, root/shoot ratio, main shoot GY, tiller GY, total GY per plant, main shoot grain number, total grain number per plant, and HI. Significant responses were recorded in the first trial but not in the second (Table 5). Shoot weight, GY of main shoot and tiller, and harvest index (HI) were clearly reduced and root/shoot ratio increased following TE treatment. Reduction in grain yield was associated with fewer grains and lower single grain weight in the main shoot spike and lower single grain weight in tiller spikes. Ethephon and CCC did not have such marked growth-inhibiting effects. Ethephon tended to increase tiller GY, total GY per plant, total grain number per plant, and HI compared to control plants when measured at maturity in the first trial of Exp 3 (Table 5).


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  		Table 4. The effect of early application of PGR on shoot weight, tiller weight, tiller number, phytomass, root weight, root/shoot ratio, and tiller number measured 14 DAT in wheat cultivar Mahti in Exp 3.

 

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  		Table 5. Means for traits measured at maturity for wheat cultivar Mahti in Exp 3.

 
In Exp 4, CCC treatment and the control did not differ for traits measured in wheat cultivar Mahti. However, there was a tendency for a slight increase in root length and root area after CCC treatment at soil depths of 0 to 20 and 20 to 40 cm. At the deep soil layer, from 40 to 60 cm, the effect of CCC tended to be contrary to that in the upper layers (data not shown).

In the first trial of Exp 5, the highest TE and ethephon concentrations depressed CER at 2 DAT, whereas CCC treatment had no effect (Fig. 1). At 3 DAT, TE treatments and the highest ethephon concentration still depressed CER. At 6 DAT, plants treated with 1 and 0.5% CCC showed elevated CER compared with the control. In the second trial, TE caused marked inhibition of CER already 4 h after treatment, whereas CCC and ethephon treatments did not induce any such depression. At 5 DAT, only plants treated with the highest concentration of ethephon exhibited depression in CER (Fig. 2).



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  		Fig. 1. Effects of CCC, ethephon, and TE concentration on CO2 exchange rate (CER) 2, 3, and 6 DAT in wheat in Exp 5. LSMEANS (o) with 95% confidence limits (|) are shown.

 


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  		Fig. 2. Effects of CCC, ethephon, and TE concentration on CO2 exchange rate (CER) 0.15, 1, 2, and 5 DAT in wheat in Exp 5. Each PGR tested separately. LSMEANS (o) with 95% confidence limits (|) are shown.

 

   DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIAL AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Plant growth regulators applied during the early growth stages generally reduced growth of main shoot and roots for varying time periods, which was partly compensated for by slightly enhanced tiller production. Depression of photosynthesis following TE treatment in particular was likely to contribute to reduced growth.

Root and Shoot Growth
Ethephon and CCC treatments did not have a marked effect on root and shoot growth of oat. Application of CCC at the flag leaf stage increased root growth and root/shoot ratio. However, considerable differences in root biomass between treatments were not statistically significant due to large variations within treatment, probably resulting from the more variation prone method of washing and extracting soil particles efficiently from the root material.

Root and shoot growth of the oat cultivars differing in stem length clearly differed in Exp 1, but root/shoot ratio was similar in the studied oat cultivars. This is in accordance with the observations of Mac Key (1988), whereas Siddique et al. (1990) noted that the root/shoot ratio was lower at anthesis in modern, more recently released wheat cultivars.

In Exp 2, TE treatment increased root/shoot ratio compared with the control by 27% in barley and 23% in wheat when measured 14 DAT (Table 2). A similar increase (20%) was recorded in Exp 3 in wheat following TE application (Table 4). This was not due to increased root growth, but reduced aboveground phytomass, as also reported by Cooke et al. (1983) and Enam and Cartwright (1990). Early application of TE reduced aboveground phytomass by 13, 11, and 17% in barley, oat and wheat, respectively, when measured 14 DAT. Ethephon and CCC treatments also caused reductions in barley and wheat, but to a lesser degree than TE. Barley and wheat were sensitive to both antigibberellins and ethephon (Table 2). In CCC and ethephon treated plants, growth of root and shoot was reduced in parallel, resulting in unaltered root/shoot ratios, as was also the case for TE treated oat (Table 2).

When root growth of wheat was monitored more closely by fractioning soil layers (in Exp 4), no marked effects on root growth were noted to follow CCC application. However, there was a tendency for root area and length at the 0 to 20 and 20 to 40 cm depth to increase, and for root growth to decrease at the 40 to 60 cm depth after CCC application. This may have effects on both the water and nutrient uptake capacity of the root system (Enam and Cartwright, 1990; Leon and Schwarz, 1992). Thus, our results indicated that through PGR application it is possible to alter root growth and root/shoot ratio of cereals to some extent, but differences between species and cultivar responses to treatments need to be taken into account when designing PGR strategies.

Ethephon and TE applications tended to reduce photosynthesis for 2 to 3 DAT. Ethephon and TE treatments at recommended or doubled treatment concentrations seemed to be more detrimental than CCC treatments (Fig. 1 and 2). The PGRs depressed photosynthesis only temporarily, as at 5 to 6 DAT, no differences between treatment effects were noted.

Tiller Growth
Growth was temporarily retarded by applications of PGR in Exp 1 and 2 (Tables 2 and 4). Retarded main shoot growth was associated with slightly increased tiller growth. Cultivars differed in their response to treatments. However, tillering and tiller growth did not fully compensate for reduced main shoot growth, as aboveground phytomass was reduced, when compared with control plants (Tables 2 and 4). Improved tiller growth may have been due to the availability of more photoassimilates for tiller growth following retarded growth and reduced sink activity of the main shoot (Woodward and Marshall, 1987, 1988). Tillering and tiller growth may also have been enhanced by PGR-induced changes in hormonal signals, for example, ethylene stimulated breakdown of apical dominance in oat according to Harrison and Kaufman (1982). This may be due to ethylene-induced inhibition of auxin biosynthesis and movement (Morgan and Gausman, 1966; Lyon, 1970; Evans, 1984).

Enhanced tillering may result in higher yield potential due to more spike-bearing tillers per main shoot. Treatments with PGRs have increased tillering and number of spikes per unit area in cereals (Humbries et al., 1965; Cartwright and Waddington, 1982; Waddington and Cartwright, 1986; Ramos et al., 1989; Ma and Smith, 1991; Peltonen and Peltonen-Sainio, 1997), though this did not invariably have a positive effect on grain yield. In our trials, ethephon and TE treatments were followed by slight increases (6–30%) in tiller number compared with the control when measured 14 DAT (Tables 2 and 4). When measured at maturity, CCC and TE had a tendency to increase tiller number, but not the number of spike-bearing tillers in the first trial (Table 5). The TE treatment increased root mass (32%) and root/shoot ratio (50%) in wheat when measured at maturity. This could be associated with enhanced tiller production. The number of tillers was increased, but not their total biomass on a per-plant basis. According to Cannell (1982), tillering probably affects nodal root production.

Number and Weight of Grains
The PGR treatments did not have any marked effect on grain yields produced by tillers and main shoot in the trial conducted outdoors for Exp 3, whereas 42 and 34% decreases in main shoot and tiller-derived grain yield, respectively, followed TE treatment in the first trial (Table 5). Reduced grain yield was associated with fewer grains per main shoot spike (27%) and lower single grain weight of main shoot and tiller spike (23 and 28%, respectively, Table 5). Similar results were obtained by Foster et al. (1991), Moes and Stobbe (1991), Taylor et al. (1991), and Foster and Taylor (1993).

Results from the two trials in Exp 3 varied markedly, especially regarding tiller growth and productivity (Table 5). The first trial was carried out in the greenhouse, whereas the second one was conducted outdoors. In the greenhouse, tillering was vigorous and slightly enhanced by CCC and TE treatments, whereas when plants were grown outdoors, no marked effects on tillering were recorded. Increased tiller number in the first trial did not increase in parallel with tiller biomass, number of spike-bearing tillers, and tiller grain yield (Table 5). This was also the case in the studies of Waddington and Cartwright (1986), Woodward and Marshall (1987), Foster et al. (1991), and Peltonen and Peltonen-Sainio (1997).

In conclusion, early PGR treatments tended to reduce short-term growth of main shoot and root. Tiller production was slightly enhanced, but not sufficiently to compensate for PGR-induced reduction in main shoot growth. In the long term, root and shoot growth rate recovered and differences between treatments disappeared. According to the results of our experiments, PGR applications generally have modest potential for manipulating spring cereal plant stand structure in ways other than shortening the straw length. Therefore, according to our results, early PGR treatments may not be recommended under Finnish growing conditions.


   ACKNOWLEDGMENTS
 
We thank the Finnish Academy of Sciences for funding this study. The Finnish Union of Agronomists is acknowledged for provision of a grant to carry out this work. Markku Tykkyläinen, Susanna Muurinen, Carl-Fride Laxell, Jarkko Hellström, Anne Karhu, and Kalle Knuuttila kindly assisted in this study, and are gratefully acknowledged. Christian Eriksson is warmly thanked for supervision in statistical analyses.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIAL AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Present address of the authors: MTT Agrifood Research Finland, Plant Production Research, FIN-31600 Jokioinen, Finland. This study was funded by the Finnish Academy of Sciences.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIAL AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 




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