INTRODUCTION
Solar radiation intercepted by the canopy is the energy input utilized by the crop for production of dry matter (DM). The amount of DM produced by a crop can be calculated when the total amount of radiation intercepted by the canopy and the effectiveness of the crop to utilize the intercepted radiation are known. An analogue of Beer's law can be used to estimate the fraction of incoming radiation intercepted (f) by the canopy (Monsi & Saeki Reference Monsi and Saeki1953):
where k is the canopy extinction coefficient and LAI is the leaf area index. The extinction coefficient k depends on the leaf angle distribution in the canopy and the angle of radiation (zenith solar angle), and has been reported to be specific to crop type and stage of development (Goudriaan Reference Goudriaan1988).
The effectiveness of a crop to convert the light intercepted to DM is termed radiation use efficiency (RUE) and is defined as the amount of DM produced (g) per unit of radiation (MJ) intercepted by the crop canopy. The relationship between DM and intercepted radiation has been described by Monteith (Reference Monteith1977) as linear. Reported average values of RUE, range from 2·0 to 3·0 and 3·0 to 4·0 g/MJ of absorbed photosynthetically active radiation (PAR) for C3 and C4 plants, respectively (Gallagher & Biscoe Reference Gallagher and Biscoe1978; Kiniry et al. Reference Kiniry, Jones, O'Toole, Blanchet, Cabelguenne and Spanel1989). For cotton (Gossypium hirsutum L.), estimated values of RUE for the cultivars Siokra 1–4 (okra leaf) and Deltapine 90 (normal leaf) ranged from 1·70 to 1·92 g/MJ of intercepted PAR across two experiments in Australia (Sadras & Wilson Reference Sadras and Wilson1997). However, Rosenthal & Gerik (Reference Rosenthal and Gerik1991) reported RUE values for the cotton cultivars Acala SJ-2, Deltapine 50 and Tamcot CD3H in Texas of 1·46, 1·60 and 1·31 g/MJ of intercepted PAR, respectively. In a CO2-enriched environment, RUE of cotton increased from 1·56 g/MJ at 370 mg/kg CO2 to 1·97 g/MJ of intercepted PAR at 550 mg/kg CO2 (Pinter et al. Reference Pinter, Kimball, Mauney, Hendrey, Lewin and Nagy1994). Furthermore, Sadras (Reference Sadras1996) reported values of RUE ranging from 1·18 to 1·71 g/MJ of intercepted PAR for the cultivars CS7S, Siokra S324 and Siokra V-15. Sadras (Reference Sadras1996) also presented average seasonal values of the canopy extinction coefficient k varying between 0·51 and 1·14. For the cultivar Sicala V-2i, across three experiments including nitrogen treatments, RUE was estimated from 0·89 to 3·10 g/MJ of intercepted PAR, with k ranging from 0·51 to 0·99 (Milroy & Bange Reference Milroy and Bange2003). In other studies, the coefficient k was calculated as 0·64 (Stanhill Reference Stanhill and Monteith1976) and c. 0·60 until 120 days after sowing (DAS), with an increase thereafter (Milroy et al. Reference Milroy, Bange and Sadras2001).
The application of plant growth regulators is a common and widely used practice in cotton production for controlling plant growth, increasing yield and improving management efficiency. The plant growth regulators used in cotton production generally have an effect on crop growth, both vegetative and reproductive, and DM partitioning. However, there have been no reports of effects of plant growth regulators on RUE. It is logical to assume that any chemical that affects canopy dynamics by changing light interception or photosynthetic efficiency will alter RUE.
Mepiquat chloride (1,1-dimethylpiperidinium) is the most widely used plant growth regulator in cotton for control of excessive vegetative growth by inhibiting gibberellin biosynthesis. Previously reported research indicated that mepiquat chloride caused height reduction, earlier maturity and a small yield advantage (Oosterhuis et al. Reference Oosterhuis, McConnel and Bonner1991). Reduced leaf expansion and shorter main stem and branch internodes, and therefore more compact plants, have been reported after the application of mepiquat chloride (Walter et al. Reference Walter, Gausman, Ritting, Namken, Escobar, Rodriquez and Brown1980; Reddy et al. Reference Reddy, Baker and Hodges1990). Advantages of mepiquat chloride applications include yield enhancement, improved lint quality, earlier maturity, increased early boll retention, decreased boll rot and improved light penetration (Gausman et al. Reference Gausman, Rittig, Namken, Rodriguez, Escogar, Garza and Rahman1978; Stuart et al. Reference Stuart, Isbell, Wendt and Abernathy1984; Kerby Reference Kerby1985; Hake et al. Reference Hake, Kerby, McCarty, O'Neal and Supak1991). The application of mepiquat chloride changes leaf colouration to a darker green (Gausman et al. Reference Gausman, Rittig, Namken, Rodriguez, Escogar, Garza and Rahman1978) and increases leaf thickness (Reddy et al. Reference Reddy, Baker and Hodges1990; Zhao & Oosterhuis Reference Zhao and Oosterhuis2000). The product Pix Plus contains mepiquat plus 10·5×109 colony units/l of Bacillus cereus.
Another plant growth regulator that has become increasingly important in cotton production in the USA is Chaperone™, which consists of sodium 5-nitroguaiacolate, sodium o-nitrophenolate and sodium p-nitrophenolate. In plants, the phenolic compounds play a central role in metabolism and growth, and are known to increase photosynthetic electron transport, improve and protect membrane integrity, increase enzyme/protein production, increase fruit retention and act as a part of lignin biosynthesis (Robinson Reference Robinson1980). In some cases, lint yields have been reported to increase by 8% after foliar applications of Chaperone (Lackey et al. Reference Lackey, Oosterhuis, Brown, Dugger and Richter2004; Oosterhuis & Brown Reference Oosterhuis, Brown, Li, Zhang, Dobermann, Hinsinger, Lambers, Li, Marschner, Maene, McGrath, Oenema, Peng, Rengel, Shen, Welch, von Wiren, Yan and Zhu2005). In addition, increased nitrate–nitrogen, total soluble proteins and Bt endotoxin levels in response to Chaperone have been documented in transgenic cotton cultivars (Oosterhuis & Brown Reference Oosterhuis, Brown, Li, Zhang, Dobermann, Hinsinger, Lambers, Li, Marschner, Maene, McGrath, Oenema, Peng, Rengel, Shen, Welch, von Wiren, Yan and Zhu2005).
Considering the response of the cotton crop to plant growth regulators, it was hypothesized that cotton RUE will be affected by the application of plant growth regulators due to changes in crop growth and canopy dynamics. The objective of the present study was to determine the effect of mepiquat chloride and nitrophenolates on RUE of cotton.
MATERIALS AND METHODS
The effect of plant growth regulators on the RUE of cotton was studied in 2006 and 2007 at Fayetteville, AR (University of Arkansas Agricultural Research and Extension Center) (36°4′N, 94°9′W; 410 m a.s.l.) (Captina silt loam, Typical Fragiudult). The cotton cultivar DP444BGRR (Delta and Pine Land Company, Scott, MS) was planted on 20 May 2006 and 17 May 2007 at a population density of 10 plants/m2. The fertilization programme was determined according to pre-season soil tests and recommended rates. Weed and insect control were performed according to state recommendations and furrow irrigation was applied according to the Arkansas irrigation scheduler programme (Tables 1 and 2), which is based on soil moisture balance and evapotranspiration (Cahoon et al. Reference Cahoon, Ferguson, Edwards and Tacker1990). The experimental plot size was four rows 10 m long, with 1 m between rows. Treatments consisted of: (i) an untreated control, (ii) Mepiquat with B. cereus (as Pix Plus® at 363 ml/ha; BASF Corporation, Research Triangle Park, NC, USA; hereafter referred to as mepiquat) and (iii) mixed nitrophenplates (as Chaperone™ at 582 ml/ha; Asahi Chemical Manufacturing Co., Ltd, Osaka, Japan; hereafter referred to as nitrophenolate) and were arranged in a randomized complete block design with five replications. Plant growth regulators were applied at the pinhead square stage (PHS) of growth, 10 days later (PHS+10) and at the beginning of flowering (FF) with a CO2 backpack sprayer calibrated to deliver 94 litres/ha.
Table 1. Crop management details for 2006
Spraying equipment was calibrated to deliver at a rate of 93·5 litres/ha.
Table 2. Crop management details for 2007
Spraying equipment was calibrated to deliver at a rate of 93·5 litres/ha.
RUE was determined for the period between the PHS and 3 weeks after first flower (FF+3), by the slope of the increase in DM over the accumulated intercepted radiation, when DM data were plotted against intercepted radiation from all plots of each treatment. DM was determined every 10–15 days by collecting plant samples from 1 m2 ground area and oven dried at 55°C for 48 h. Leaf area of the plant samples was measured using an LI-3100 Area Meter (Li-Cor, Lincoln, NE). Intercepted radiation was calculated by multiplying the incident radiation, measured by a WatchDog 2475 weather station (Spectrum Technologies Inc., Plainfield, IL) located at the edge of the field, with the fraction of intercepted radiation. The fraction of light intercepted by the crop canopy was estimated weekly, starting at PHS, by measuring PAR above and below the canopy in unobstructed sunlight, close to solar noon, using an LI-191S line quantum-source quantum sensor (Li-Cor, Lincoln, NE). For each experimental plot, measurements of fractional light interception were plotted against days after PHS (day zero) and a regression line was fitted (Fig. 1). The quadratic equation developed for each plot was used to estimate fractional light interception for each day of the measuring period. The canopy extinction coefficient was calculated from Eqn (1), solving for k.
Fig. 1. An example of calculating daily fractional light interception. For each experimental plot, measurements of fractional light interception were plotted against days after PHS (day zero) and a regression line was fitted, as shown above. The quadratic equation developed for each plot was used to estimate fractional light interception for each day of the measuring period.
Statistical analysis was performed with the JMP 6 software (SAS Institute Inc., Cary, NC). Means were separated with Student's t test (P⩽0·05). Statistical differences were evaluated between each plant growth regulator treatment and the untreated control and not between plant growth regulators. Regression analysis was used to test differences in productivity of DM and RUE between each plant growth regulator treatment and the untreated control. Differences across years were tested using stepwise linear regression analysis.
RESULTS
The crop reached PHS at 44 DAS in 2006 and 49 DAS in 2007, with the beginning of flowering at 65 and 71 DAS for 2006 and 2007, respectively. Foliar application of nitrophenolate did not significantly affect plant height or leaf area index (LAI) in either year of the study (Table 3). In contrast, mepiquat applications significantly decreased plant height in 2006 (P=0·020) and 2007 (P=0·003). LAI was close to being significantly decreased by mepiquat application in 2006 (P=0·059) and was significantly decreased in 2007 (P=0·034). While no differences were observed for the nitrophenolate treatment, mepiquat application also affected the crop canopy structure, with higher values of canopy extinction coefficient recorded in 2006 (P=0·005) and 2007 (P=0·003) (Fig. 2).
Fig. 2. Canopy extinction coefficient (k) values for 2006 and 2007, estimated at the FF stage of growth. ±1 s.e.d. bars are shown for treatment comparison (1, untreated control to chaperone; 2, untreated control to mepiquat chloride).
Table 3. Plant height and LAI measured at FF+3
* Comparison of each plant growth regulator treatment with the untreated control. ns=not significant.
Accumulation of DM, for the duration of the study, did not differ between the untreated control and the two plant growth regulator treatments in either year (Table 4). However, the partitioning of DM was altered by the mepiquat treatment in 2007, with the fraction of total DM partitioned to stems being significantly decreased (P=0·007) and to fruit significantly increased (P=0·022) compared with the untreated control (Fig. 3).
Fig. 3. DM partitioning between leaves, stems and fruit measured at FF+3 for 2006 (a) and 2007 (b). ±1 s.e.d. bars are shown for treatment comparison (1, untreated control to chaperone; 2, untreated control to mepiquat chloride).
Table 4. DM production and intercepted PAR for the period between the pinhead square of growth and FF+3
* Comparison of each plant growth regulator treatment with the untreated control. ns=not significant.
The amount of intercepted radiation by the crop canopy, for the duration of the study, was not significantly affected in 2006 by either nitrophenolate (P=0·169) or mepiquat (P=0·073) treatment compared with the untreated control (Table 4). Similarly, in 2007, nitrophenolate applications did not alter (P=0·282) the amount of intercepted radiation by the cotton crop (Table 4), but mepiquat significantly (P<0·01) lowered the amount of radiation intercepted (Table 4).
Differences in total intercepted radiation can be explained by differences in the fractional light interception of the crop for the PHS, FF and FF+3 (Fig. 4). Higher fractional light interception (P=0·010) was observed in 2006 for the nitrophenolate treatment compared with the untreated control at FF+3, while the mepiquat treatment significantly decreased fractional light interception (P=0·008) at the FF stage. However, the increase in fractional light interception of the nitrophenolate treatment did not lead to an increase in the intercepted radiation, possibly due to the small degree of the increase (6·6%) and the small amount of time available for accumulation of radiation. In 2007, a significantly lower (P=0·004) fraction of incident radiation was intercepted by the mepiquat treatment at FF+3.
Fig. 4. Fractional light interception measured at PHS, FF and FF+3 in 2006 (a) and 2007 (b). ±1 s.e.d. bars are shown for treatment comparison (1, untreated control to chaperone; 2, untreated control to Pix Plus).
Regression analysis revealed no significant differences between the untreated control and the nitrophenolate treatment in RUE for either 2006 (P=0·224) or 2007 (P=0·730) (Table 5). However, for mepiquat application, the analysis was run separately for each year and RUE was significantly increased (P<0·05) in 2006 only when compared with the control treatment (Table 5). Statistical analysis performed across the 2 years of the study provided mean values with RUE at 2·26 g/MJ of intercepted PAR for the untreated control, 2.50 g/MJ PAR for the nitrophenolate treatment, and 2·97 g/MJ PAR for the mepiquat treatment, with the effect of mepiquat applications being statistically significant (P=0·021). Reporting mean values of RUE across the 2 years of the study is possible due to lack of a significant treatment×year interaction (Fig. 5). In addition, the mean square error for both years was very similar therefore allowing the analysis to be combined across years, which provides a better indication of the overall effect of a treatment when there is no treatment×year interaction.
Fig. 5. Slopes of the increase in DM over the accumulated intercepted radiation (RUE) of the untreated control, nitrophenolate and mepiquat treatments for 2006 and 2007.
Table 5. Effect of plant growth regulator treatments on RUE of cotton
* Comparison of each plant growth regulator treatment with the untreated control. ns=not significant.
DISCUSSION
In the present study, the effect of the plant growth regulators nitrophenolate and mepiquat on the growth and RUE of cotton was evaluated. None of the parameters recorded appeared to be significantly affected by multiple foliar applications of nitrophenolate, except for an increase in fractional light interception at FF+3 in 2006 only. In contrast, the application of mepiquat (a.i.: mepiquat chloride) altered the crop canopy structure and the efficiency of the crop in converting intercepted radiation to DM. LAI of the crop was decreased by mepiquat chloride, as previously described by Reddy et al. (Reference Reddy, Baker and Hodges1990). A quantitative estimate of the canopy structure is the canopy extinction coefficient, with higher values estimated in this study following mepiquat chloride applications.
DM partitioning of the cotton crop changed in favour of fruiting organs; this was similar to previous reports of increased partitioning to fruits after applications of mepiquat chloride (Walter et al. Reference Walter, Gausman, Ritting, Namken, Escobar, Rodriquez and Brown1980; Zhao & Oosterhuis Reference Zhao and Oosterhuis2000).
RUE values presented in the present paper are in most cases higher than values reported in previous research for cotton, where RUE is measured for the whole season (Rosenthal & Gerik Reference Rosenthal and Gerik1991; Pinter et al. Reference Pinter, Kimball, Mauney, Hendrey, Lewin and Nagy1994; Sadras Reference Sadras1996; Sadras & Wilson Reference Sadras and Wilson1997). However, Milroy & Bange (Reference Milroy and Bange2003) reported values as high as 3·10 g/MJ of intercepted PAR for RUE measured for only part of the season, similar to the values estimated in the present study.
Treatment with mepiquat chloride increased RUE of the cotton crop, the cause of which could be associated with previously reported increased single-leaf photosynthesis (Zhao & Oosterhuis Reference Zhao and Oosterhuis2000) and whole canopy CO2 exchange rates (Hodges et al. Reference Hodges, Reddy and Reddy1991) following application. Furthermore, the application of mepiquat chloride altered the canopy of the crop, with the treated cotton plants being more compact, usually associated with delayed canopy closure. Previous research demonstrated a decrease in the proportion of light intercepted by the upper leaves, with an increase in light penetration to the middle portion of the canopy after the application of mepiquat chloride (Gwathmey et al. Reference Gwathmey, Wassel, Michaud, Richter and Armour1995). It is therefore hypothesized that a larger portion of the canopy, both on the top and at the sides of the mepiquat chloride-treated plants would be exposed to more direct radiation. The higher RUE values reported in the present study may be attributed to light interception differences, as well as to improved carbon assimilation rates due to the application of mepiquat chloride.
Another point of interest is the higher values of canopy extinction coefficient in conjunction with the increased RUE of mepiquat chloride-treated plants in the present study. The higher canopy extinction coefficient may not be the reason for the higher RUE: Milroy & Bange (Reference Milroy and Bange2003) found that k had little impact on the RUE of cotton. In contrast, Sadras (Reference Sadras1996) previously reported that a higher canopy extinction coefficient can lead to lower RUE in cotton. However, this relationship was only demonstrated for the so-called ‘favourable’ conditions of low plant density (5 plants/m2) and high nitrogen (180 kg N/ha), with no relationship found for less favourable conditions (plant density of 12·5 plants/m2 and no fertilizer).
The results show that the effect of mepiquat chloride measured at the leaf level translates to canopy level differences. In addition, it supports the notion that mepiquat chloride affects yield via effects on growth as well as partitioning of DM. To the authors’ knowledge, this is the first report of a commercially used plant growth regulator affecting the RUE of any crop.
