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Agronomy Journal 92:1125-1134 (2000)
© 2000 American Society of Agronomy

INTEGRATED WEED MANAGEMENT

Effects of Temperature and Photoperiod on the Phenological Development of Barnyardgrass

Clarence J. Swantona, Jian Zhong Huanga, Anil Shresthaa, Matthijs Tollenaara, William Deena and Hamid Rahimianb

a Dep. of Plant Agric., Univ. of Guelph, Guelph, ON, Canada N1G 2W1
b Dep. of Agronomy, College of Agric., Mashhad, Iran

cswanton{at}plant.uoguelph.ca


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
An understanding of the environmental variables influencing the phenological development of weeds is essential for simulation model development. Temperature and photoperiod are important variables governing the phenological development of weeds. Growth cabinet studies were conducted to characterize the phenological development of barnyardgrass [Echinochloa crus-galli (L.) Beauv.] in response to variations in temperature and photoperiod and to determine the duration of the juvenile phase and the effect of temperature and photoperiod on reproductive development. Barnyardgrass was adapted to a temperature range of 6.5 to 52°C. Phenological development of barnyardgrass was described in terms of thermal days (cumulative day degrees above a base temperature for leaf appearance, tiller appearance, and shoot elongation). For modeling purposes, three development phases of barnyardgrass at a constant temperature of 20°C were described: (i) a juvenile phase of 1.5 thermal days; (ii) a photoperiod-sensitive inductive phase of 4.1 thermal days; and (iii) a photoperiod-sensitive postinductive phase of 19.5 thermal days. Photoperiod sensitivity of barnyardgrass did not differ with stage of development when expressed as a rate. Interpretation of constant sensitivity to photoperiod will simplify simulation of weed phenology in mechanistic models.

Abbreviations: ANOVA, analysis of variance • DAE, days after emergence • IWM, integrated weed management • LD, long day • PPFD, photosynthetic photon flux densities • SD, short day


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
INTEGRATED WEED MANAGEMENT (IWM) is a systems approach to vegetation management (Swanton and Murphy, 1996). An IWM system requires the capability to predict weed occurrence and its potential impact on crop yield loss (Kiniry et al., 1991). Predictive simulation models of weed occurrence are fundamental and critical for the successful implementation of IWM systems (Swanton and Murphy, 1996). Two types of models have been developed for the prediction of weed occurrence: mechanistic and empirical models. Mechanistic models are usually better than empirical models in assessing the impact of genetic variation and environmental factors on the development processes of the crop and weed (Ghersa and Holt, 1995). Empirical models cannot account for these variations and are therefore of limited predictive ability (Kropff et al., 1992; Swanton and Murphy, 1996). Mechanistic models integrate our current understanding of phenological development of both crop and weed under competition. Crop scientists have repeatedly demonstrated that knowledge of phenological development is critical in understanding crop growth, yield potential, and for prediction of phenology (Grant, 1989; Miller et al., 1993). Similarly, it is essential to understand weed phenology in order to develop predictive models.

Barnyardgrass is a common and troublesome C4 weed capable of infesting a variety of crops such as rice (Oryza sativa L.), soybean [Glycine max (L.) Merr.], cotton (Gossypium hirsutum L.), corn (Zea mays L.), wheat (Triticum aestivum L.), vegetables, beans, root crops, forage crops, orchards, and pastures in most of the agricultural areas of the world (Kempen, 1984; Maun and Barrett, 1986). For example, crop yield losses due to barnyardgrass competition have been reported as 8 to 82% in corn (Spitters et al., 1989) and 30 to 45% in cotton (Kempen, 1984). Phenology is a major factor determining the outcome of crop–weed competition, and temperature and photoperiod are two of the most important environmental factors affecting phenological development (Ghersa and Holt, 1995; Major and Kiniry, 1991; Patterson, 1993). Detailed and systematic information on the effects of temperature on the phenological development, senescence, and life cycle of this weed is necessary to explain its widespread distribution and competitive ability and for the development of mechanistic models. In addition, information on the influence of photoperiod on the juvenile phase and reproductive development of barnyardgrass has not been reported previously. Therefore, to provide biological information for modeling purposes, the objectives of this research were to characterize the phenological development of barnyardgrass in response to variations in temperature and photoperiod by determining the duration of the juvenile phase and the effects of temperature and photoperiod on reproductive development. A similar approach was taken by Swanton et al. (1999) to characterize the phenological development of green foxtail (Setaria viridis L.).


   Materials and methods
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results and discussion
 Conclusions
 REFERENCES
 
Plant Material and Growth Conditions
Experiments were conducted in growth cabinets using seeds of barnyardgrass collected in 1994 from Woodstock, ON, and stored at 4°C. Seeds were soaked for 24 h in water at 20°C before planting. The seeds were planted at a depth of 0.5 cm in 20- to 25-cm diameter 6-L plastic pots containing a general purpose growth medium (75–85% Canadian sphagnum peat moss, perlite, vermiculite, dolomite, calcite, and a wetting agent) and placed in controlled growth cabinets. Pots were watered as required and supplied with a nutrient solution containing N, P, K, Ca, Mg, and chelated micronutrients 6 d after emergence (DAE) followed by a second dose 15 DAE. Irradiance was supplied by a sliding bank of Sylvania Cool White and Vita-Lite Duro-test fluorescent lamps and Westinghouse 40-W incandescent bulbs. Photosynthetic photon flux densities (PPFD) were measured at the top of the canopy using a line (LI-191SA line quantum sensor, LI-COR, Lincoln, NE) or a point quantum sensor (LI-190SA point quantum sensor, LI-COR, Lincoln, NE) and data were stored in a data logger (LI-1000, LI-COR, Lincoln, NE).

Temperature Response
Pots were initially maintained in a growth room at 20°C, 16-h day length, and 400 µmol m-2 s-1 PPFD. Emerged barnyardgrass seedlings at the 1-leaf stage of growth were thinned to two seedlings per pot, of which one seedling was used later for the determination of floral primordia appearance (Kirby and Appleyard, 1984). The experiment was arranged as a completely randomized design using six growth cabinets with 18 pots. Six day/night temperature treatments were established: 12/2, 17/7, 23/13, 29/19, 35/25, and 44/35°C at 16-h day lengths with 400 µmol m-2 s-1 PPFD. Shoot height was measured to the tip of the youngest leaf or to the tip of the inflorescence. Leaf and tiller numbers on the mainstem were recorded at 3-d intervals until seed maturation of the mainstem inflorescence. Times of floral primordia differentiation, heading, flowering initiation, initiation of seed set (green seeds in early milk to soft dough stage), and end of seed maturation (seeds fully ripe, kernels hard and yellow) on the mainstem inflorescence were recorded. All parameters were recorded from the day after emergence (initial appearance of coleoptile) of the seedlings. Each plant was later harvested at seed maturity, and the fresh and dry weights of the shoots were measured. The dry weights of the plants were determined after they were dried to a constant weight in a forced-air oven at 80°C. Two extra pots with approximately 15 seedlings for each treatment also were maintained in the growth cabinets to determine initial floral primordia appearance. The growing points of these plants were dissected periodically under a microscope to determine the growth stage. It was assumed that the presence of 15 seedlings in a pot would not affect the time of floral primordia appearance because phenological development is primarily controlled by temperature and photoperiod (Hodges, 1991a; Patterson, 1995). Also, it has been reported that effect of plant density on phenological development is of minor importance relative to the effects of photoperiod and temperature (Major and Kiniry, 1991; Roché et al., 1997).

Photoperiod Response
Fifteen pots were selected from the growth room and emerged barnyardgrass seedlings at the 1-leaf stage were thinned to two seedlings per pot, of which one seedling was used for the determination of floral primordia appearance. Three pots were placed in each of five separate growth chambers at 20°C with 8-h core irradiance at 400 µmol m-2 s-1 PPFD. Photoperiod was supplemented by two 40-W incandescent bulbs that provided approximately 40 µmol m-2 s-1 PPFD. This level of irradiance was high enough to affect photoperiod response but not sufficient to have an impact on photosynthesis. This design eliminated the potential confounding effect of differing high light-mediated growth rates on phenology (Tollenaar, 1999). Day/night photoperiods of 8/16, 10/14, 12/12, 14/10, and 16/8 h were established. Similar measurements were made and recorded as in the temperature response study.

Juvenile Response
The length of juvenile phase was determined using a reciprocal transfer study, i.e., long day (LD) to short day (SD) and SD to LD as described by Wilkerson et al. (1989). Sixty pots were selected from the growth room and divided into two groups. The seedlings were thinned to two per pot, of which one seedling was used for the determination of the appearance of the floral primordia. Two cabinets receiving a core irradiance of 8 h at 400 µmol m-2 s-1 PPFD and 20°C were selected. One cabinet was designated for SD (8-h photoperiod) and the other for LD (16-h photoperiod). A 16-h photoperiod was provided to the LD cabinet by supplemental light from three 40-W incandescent bulbs. The first transfer between SD and LD cabinets was made 3 d after the coleoptile penetrated the soil surface. Exchanges were made repeatedly at 3-d intervals up to and including 27 DAE. Six pots were selected randomly of which three were transferred from SD to LD and three from LD to SD 3 to 27 DAE. Three pots were left in each of the SD and LD cabinets as controls. Similar measurements were made and recorded as in the temperature and photoperiod response studies.

Determination of Cardinal Temperatures and Thermal Days
Polynomial regressions were used to estimate the cardinal temperatures (base, optimum, and maximum temperatures) for leaf appearance, tiller appearance, and shoot elongation. The model was

where x = mean daily temperature. Coefficients a, b, c, d, and e provide estimates of the relative effects of mean temperature on growth rate. The base temperature (Tb) and the maximum temperature (Tmax) were determined by the upper and lower points where the polynomial regression curve intersected the x-axis. The peak of the regression curve was deemed as the optimum temperature (Topt). The Tb for leaf appearance, tiller appearance, and shoot elongation was used to describe the phenological development of barnyardgrass in terms of thermal days.

Data Analysis
All experiments were conducted twice using different growth cabinets. Data on effects of temperature on shoot elongation and tiller and leaf number were analyzed using analysis of variance (ANOVA) procedures (SAS Inst., 1990). Data from each treatment were linearly regressed against time. Leaf appearance, tiller appearance, and shoot elongation rates as affected by temperature were estimated from the slope of the regression. Rates were then regressed against the mean treatment temperature (e.g., 35/25°C at 16/8 h day/night photoperiod equals a mean temperature of 31.7°C). Multiple regressions were used to simulate growth rates of barnyardgrass against the mean treatment temperatures.

Data from the photoperiod study were subjected to ANOVA to identify significant effects of photoperiod on days to floral primordia differentiation. Data were converted to rates by calculating the inverse of time taken to reach a particular stage. The intervals consisted of the floral primordia to heading (tip of inflorescence emerged from sheath), heading to first flower, first flower to initiation of seed set (green seeds in early milk to soft dough stage) and end of seed set (seeds fully ripe, kernels hard and yellow). Development rates for each interval were normalized to the maximum rate within each interval.

Data from the juvenile phase study were subjected to ANOVA to identify significant effects of transfer time on thermal days to floral primordia. Data analyses were similar to the methods employed by Ellis et al. (1992) and Wilkerson et al. (1989). A series of paired data sets were generated by splitting data (individual replicates) at the 3, 6, and 9 DAE treatments. Linear regressions were fit to the various data sets to determine the set that provided the least combined sums of squared residuals. For example, linear regressions were first fit to the 0, 3, 6, d, and 9, 12, 15, 18, 21, 24, and 27 d data, then to the 0, 3, 6, and 9 d, and 12, 15, 18, 21, 24, and 27 d data and so on. The point of interception of the two regression lines, at which the sum of squared residuals was a minimum, was taken as the time of initiation of photoperiod sensitivity (i.e., end of juvenile phase). The mean date of initiation of photoperiod sensitivity was calculated along with an associated standard error.


   Results and discussion
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
Conclusions
 REFERENCES
 
Vegetative Development and Temperature
Vegetative development of barnyardgrass occurred over a wide temperature range. Number of leaves on the mainstem increased linearly with temperature at a rate of about 0.5 leaves per °C from 12/2 to 35/25°C, but declined from 35/25 to 44/35°C (Fig. 1a) . This trend in increase of leaf number is similar to that reported by Tollenaar and Hunter (1983) in another C4 plant, corn. The authors showed that total leaf number in corn increased by 0.2 leaves per °C from 15 to 30°C. The average number of tillers plant-1 also increased with temperature, reaching a maximum at 29/19°C, and declining at higher temperatures (Fig. 1b). The maximum tiller number plant-1 (43) at 29/19°C was attained at 38 DAE, whereas the maximum tiller number of 2 and 12 at 12/2°C and 44/35°C was attained at 80 and 69 DAE, respectively. These maximum leaf and tiller numbers for barnyardgrass at 35/25°C and 29/19°C, respectively, are greater than those reported by Kacperska-Palacz et al. (1963) and Maun and Barrett (1986). They found that the vegetative phase of barnyardgrass consisted of up to a maximum of eight leaves and 15 tillers, whereas Keeley and Thullen (1989) reported a maximum of 23 tillers. Norris (1996), however, reported 225 basal tillers in barnyardgrass and attributed the differences in tiller numbers to differences in regional biotypes. Shoot height also increased with temperature but declined at the highest temperature (data not shown). Maximum shoot height (117 cm) was attained 50 DAE at 35/25°C, whereas the shoot height at 12/2°C was 5.8 cm 90 DAE. The maximum shoot height of the plants was similar to earlier findings (Keeley and Thullen, 1989; Vengris et al., 1966). Norris (1996), however, found that the average plant height of barnyardgrass was 150 cm when grown in the absence of competition in a common garden environment.



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  		Fig. 1 Effect of various temperature treatments on the number of leaves on the (a) mainstem and (b) number of tillers plant-1 in barnyardgrass

 
The rate of leaf appearance and shoot elongation of barnyardgrass increased with temperature up to a maximum of 35/25°C and declined at higher temperatures (Table 1) . The rate of tiller appearance was maximum at 29/19°C and declined at higher temperatures. At 44/35°C, the rate of leaf and tiller appearance, and the rate of shoot elongation declined to 0.26, 0.17, and 0.90 d-1, respectively. At 12/2°C, seedling growth was delayed and the plants produced one to two tillers with about five pale-yellow leaves (Table 1 and Fig. 1). The plants did not complete their life cycle at this temperature. Similar responses have been reported for other weed species. For example, Muldoon et al. (1982) reported that the relative growth rate and accumulation of shoot dry weight of Japanese barnyard millet (Echinochloa utilis Ohwi et Yabuno) and paisa (E. frumentacea L.) were positively correlated with an increase in temperature from 15/10 to 33/28°C. Rate of leaf appearance in velvetleaf (Abutilon theophrasti L.) also increased with temperature from 12/4 to 26/18°C and declined at higher temperatures (Patterson, 1992). Similarly, Swanton et al. (1999) reported that the rate of leaf and tiller appearance and shoot elongation of green foxtail increased with temperature up to 35/25°C and declined at higher temperatures.


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  		Table 1 The effect of temperature on the rate of leaf and tiller appearance and shoot elongation (±SE) of barnyardgrass.{dagger}

 
Cardinal temperatures varied for leaf and tiller appearance, and shoot elongation (Table 2) . For example, Tb for leaf appearance was lower than that for tiller appearance or shoot elongation. Similarly, the Tb for shoot elongation was lower than that for tiller appearance. The Topt for leaf and tiller appearance was similar and lower than the Topt for shoot elongation. Tiller appearance occurred up to a Tmax of 44.5°C, whereas leaf appearance and shoot elongation continued up to 51.0 and 52.0°C, respectively. This finding helps in explaining the results of Vengris et al. (1966), who reported that the increase in plant height of barnyardgrass was directly related to temperature. They observed that the rate of plant growth was slow when temperatures were low and very rapid in the summer. The Tb and Topt for leaf appearance of barnyardgrass are very similar to those for corn (Tollenaar et al., 1979). Predictive phenological models of barnyardgrass occurrence can be developed according to the accumulated thermal days. Phenological models have been developed with a considerable degree of success for wheat, corn, sorghum [Sorghum bicolor (L.) Moench.], rice, cotton (Hodges, 1991b), and green foxtail (Swanton et al., 1999).


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  		Table 2 Cardinal temperatures for leaf and tiller appearance and shoot elongation

 
Reproductive Development and Temperature
Barnyardgrass completed its reproductive development and produced mature seeds at temperatures ranging from 23/13 to 35/25°C (Table 3) . Within this temperature regime, heading occurred at 33.1 to 37.1 thermal days after seedling emergence and anthesis occurred 0.8 to 2.1 thermal days after heading. Initiation of seed set required 6 to 6.2 thermal days after anthesis, and seed maturity (seeds firm and brown in color) required an additional 8.6 to 9.6 thermal days. These results agree with the findings of Rahn et al. (1968) who reported that barnyardgrass began heading about 40 DAE, and producing mature seeds about 20 d after heading. No reproductive development occurred at 12/2°C (Table 3). At 44/35°C, although differentiation of floral primordium and heading occurred 39.5 thermal days after emergence, the plants failed to flower and to set seeds. It has been reported that processes such as respiration, photosynthesis, transpiration, inactivation of enzymes, and formation of sexual organs and spores are affected by extreme temperatures (Daubenmire, 1974; Fitter and Hay, 1987). Therefore, the transition from vegetative to reproductive phase may have been influenced by stresses of low or high temperatures at 12/2 and 44/35°C.


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  		Table 3 Number of thermal days (±SE) required for phenological development of barnyardgrass at selected temperatures and a constant photoperiod (16/8 h d-1)

 
Photoperiod Effects
Phenological development of barnyardgrass was affected by photoperiod (Table 4) . Similar to the findings of Maun and Barrett (1986) and Vengris et al. (1966), our results also indicated that barnyardgrass was a quantitative, SD plant. Phenological development of plants exposed to 8-, 10-, 12-, and 14-h photoperiods was similar. Therefore, the data for these photoperiods were combined. The phenological development of barnyardgrass was expressed in thermal days. For example, using a photoperiod of 8 h, the number of thermal days from seedling emergence to initiation of floral primordium, initiation of floral primordium to initiation of heading, initiation of heading to initiation of flowering, initiation of flowering to end of flowering, end of flowering to initiation of seed set, and seed set to end of seed set was 4.5, 8.3, 0.5, 4.2, 1.4, and 5.8 thermal days, respectively. At this photoperiod a total of 25.2 thermal days was required for barnyardgrass to complete its life cycle. Swanton et al. (1999) reported that green foxtail required a total of 19.6 thermal days at this photoperiod to complete its life cycle. Plants exposed to the 16-h photoperiod took longer to reach the same phenological stages. The number of thermal days required from seedling emergence to initiation of floral primordium was 20.1 when grown under a photoperiod of 16 h compared with 4.5 under an 8-h photoperiod. Floral primordium differentiation occurred at 2.8- to 3.1-leaf stage under conditions of 8- to 14-h photoperiod, whereas at 7.8- to 8.5-leaf stage under a 16-h photoperiod (Fig. 2c) . An average of 5.3 thermal days was required from initiation of seed set to end of seed set under the 8 to 14 h photoperiods compared with 10 thermal days when grown under a 16-h photoperiod. Sensitivity of barnyardgrass to photoperiod was apparent from the seedling stage to seed set (Table 4). This finding is in contrast to green foxtail, where sensitivity to photoperiod was apparent only up to initiation of heading and further phenological development was independent of photoperiod. The number of thermal days required to reach initiation of flowering, seed set, and end of seed set was similar across all photoperiods (Swanton et al., 1999). Our results for barnyardgrass, however, agree with the findings of Vengris et al. (1966). They reported that plants under a LD (16 h) condition headed and produced mature seeds much later than similar plants under SD (11 h) conditions. In an 11-h photoperiod, barnyardgrass headed about 35 DAE and produced mature seeds at about 52 DAE, whereas under a 16-h photoperiod, heading occurred at about 53 DAE and produced mature seeds at 90 DAE. Similarly, cultivated Japanese barnyard millet and paisa experienced delayed heading, flowering, and seed (or fruit) set when exposed to 14 to 16 h photoperiod (Muldoon, 1985).


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  		Table 4 Effect of photoperiod on the thermal days required for phenological development of barnyardgrass at 20°C

 


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  		Fig. 2 Constant photoperiod effect of 8-, 10-, 12-, 14- and 16-h at 20°C on the (a) leaf number, (b) shoot height, (c) leaf stages to floral primordium initiation, (d) shoot dry weight, (e) mainstem inflorescence dry weight, and (f) seed number of barnyardgrass

 
A single photoperiod sensitivity parameter can be used to characterize barnyardgrass development. When data from Table 4 were expressed in terms of development rates (i.e., inverse of duration of development) (Table 5) and compared across phenological stages, we found no differences (P < 0.05) among photoperiod sensitivities. This finding agrees with the observation in green foxtail (Swanton et al., 1999) but differs from that of Slafer and Rawson (1996), who suggested that photoperiod sensitivity will vary with growth stage. Their comparison, however, was based on the impact of photoperiod on duration of growth stages. In Table 4, a similar conclusion could have been drawn if we also had based our comparisons on duration. If phenological development is calculated as a rate function (i.e., inverse of duration), then a single normalized rate of 1.0 can be used to calculate development of barnyardgrass under SD conditions (Table 5). The rate of development changed under LD conditions. For example, the normalized rate of development from seedling to first floral primordium was 1.0 under SD conditions compared with 0.22 when grown under a 14- to 16-h photoperiod. The normalized rate of phenological development from flowering to seed set was 1.0 compared with 0.47 when grown under a 8- to 14-h and 14- to 16-h photoperiod, respectively. A rate change of -0.53 indicated that the effect of photoperiod was apparent from flowering to seed set. These results differed from green foxtail in which reproductive development after heading was found to be independent of photoperiod (Swanton et al., 1999).


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  		Table 5 Normalized rate of phenological development from seedling to first floral primordium, heading, flowering, and seed set of barnyardgrass as influenced by photoperiod.{dagger}

 
Vegetative development was also dependent upon photoperiod. Shoot height and dry weight increased with increasing photoperiod (Fig. 2b and d). Shoot height and dry weight were greatest for plants grown under a 16-h photoperiod. Under the 8 to 14 h photoperiod, barnyardgrass produced a relatively constant number of leaves averaging 5 to 7 leaves mainstem-1 (Fig. 2a). At the 16-h photoperiod the average leaf number increased to 13 mainstem-1. The rate of leaf appearance was maximum (0.26 leaves d-1) at the 16-h photoperiod, whereas the rate of leaf appearance was similar under the 10 to 14 h photoperiod (Table 6) . Although differences in rate of leaf appearance occurred among photoperiods, they may be attributed to differences in rate of dry matter accumulation rather than to a direct effect of photoperiod (Tollenaar, 1999). Similar patterns of vegetative growth and dry matter accumulation have been reported in Japanese barnyard millet and paisa grown in a 14- to 16-h photoperiod (Muldoon, 1985). Similar findings have also been reported in ryegrass (Lolium perenne L.), orchardgrass (Dactylis glomerata L.), and brown top (Panicum fasciculatum Swartz) (Kiniry et al., 1991).


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  		Table 6 Effect of photoperiod on the rate of leaf appearance (±SE) at 20°C

 
The mainstem dry weight and number of seeds on the mainstem inflorescence increased with an increase in photoperiod (Fig. 2e and f) . For example, average dry weight and seed number of the mainstem inflorescence was 0.25 g and 145, and 1.6 g and 922, respectively, when grown under the 8- and 16-h photoperiod. Previous studies have reported that seed production depended upon photoperiod and was correlated positively with inflorescence size (Keeley and Thullen, 1989; Maun and Barrett, 1986; Norris, 1992, 1996).

Duration of Juvenile Phase
Plant seedlings are insensitive to photoperiod during the juvenile phase; however, this phase is very short in herbaceous plants (Thomas and Vince-Prue, 1984). Barnyardgrass showed sensitivity to photoperiod soon after emergence. Early sensitivity to photoperiod enables weed seedlings to adjust their duration of development according to length of the photoperiod. Data from our experiment in which barnyardgrass was exchanged between noninductive (16 h) to inductive (8 h) photoperiods indicated that the juvenile phase was very short (Fig. 3) . Similar observations have also been reported in green foxtail (Swanton et al., 1999). The duration from emergence to the end of juvenile phase (i.e., initial photoperiod sensitivity) using days to first flowering was estimated to occur 3.1 DAE (corresponding DAE on x-axis at the intersection of SD and c in Fig. 3). This DAE value (3.1) was calculated to be 1.5 (± 0.22) thermal days. Estimates of the duration of juvenile phase were based on the time from seedling emergence to initiation of photoperiod sensitivity. The juvenile phase should account for the time from germination to seedling emergence as well. Based on an average temperature of 20°C, we estimated the time from germination to seedling emergence to range from 1.4 to 2.4 thermal days (data not shown) for an average of 1.9 thermal days. Therefore, the juvenile phase ranged from 2.9 to 3.9 thermal days, averaging 3.4 thermal days. Swanton et al. (1999) reported that the juvenile phase of green foxtail averaged 2.6 thermal days.



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  		Fig. 3 Time to first appearance of (a) floral primordia, (b) heading, (c) flowering, (d) end of flowering, (e) first appearance of seed set, and (f) end of seed set for barnyardgrass transferred from long to short day at different days after emergence (DAE). Horizontal line (SD) indicates time required to flower by plants remaining in short day condition throughout the experiment. The intersect of two lines (SD and c) indicates the estimated duration of the juvenile phase

 
Barnyardgrass required a minimum of 1.9 to 2.9 thermal days to initiate differentiation of the floral primordia after being transferred from LD to SD. This minimum number of thermal days was consistent across varying durations of transfer treatments. There was a linear relationship between day of transfer from LD to SD and day of first appearance of floral primordia, heading, flowering, or seed maturation (data not shown). A delay of each day in transfer from LD to SD resulted in the delay of appearance of anthesis by approximately 0.44 thermal days (Fig. 3).

Vegetative development of barnyardgrass was affected by the duration of reciprocal transfers between SD and LD. The longer the barnyardgrass remained under LD before being transferred to SD, the greater the final leaf number on the mainstem (Fig. 4) . A linear relationship was found between the time of transfer of plants from LD to SD and leaf number. Leaf number increased by approximately 0.26 leaves mainstem-1 d-1 to a maximum of 13 with each day's delay in transfer. Seedlings transferred from SD to LD before 3 DAE (i.e., during juvenile phase) had similar total leaf numbers as the seedlings grown under continuous LD conditions or under SD conditions. Hence, there was no effect of photoperiod on the total leaf number when seedlings were transferred during the juvenile phase. Seedlings that were transferred to LD after 6 DAE in SD had an average final leaf number of 5.3 to 5.8 leaves mainstem-1. Shoot height and dry weight increased as transfer to SD conditions was delayed and declined when exposure to LD conditions was decreased (Fig. 5) . For barnyardgrass, the increase of leaf number, shoot height, and weight mainly depended on the time the plants remained in LD before being transferred to SD. These results confirmed that the optimal photoperiod accelerated the transition from vegetative to reproductive development. The duration of the vegetative development was decreased and the life cycle of barnyardgrass was shortened. This suggests that an increase in the duration of the vegetative phase will result in larger and more competitive barnyardgrass plants. The mainstem inflorescence dry weight and number of seeds increased with time of transfer from LD to SD (Fig. 6) . Norris (1992) found a similar positive correlation between barnyardgrass inflorescence size and seed number. Our study demonstrated that photoperiod was one of the main factors affecting inflorescence size and seed production.



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  		Fig. 4 Leaf number on mainstem of barnyardgrass as influenced by the time of transfer from (a) long to short day and from (b) short to long day

 


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  		Fig. 5 Effects of time of transfer from (closed circle) long to short day and from (open circle) short to long day on shoot height and shoot dry weight of barnyardgrass

 


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  		Fig. 6 Effect of time of transfer from long to short day on dry weight of the (a) mainstem inflorescence and (b) number of seeds on the mainstem inflorescence in barnyardgrass

 
Photoperiod Sensitive Phase
The photoperiod sensitive phase was divided into an inductive phase (during which photoperiod affects initiation of reproductive development) and a postinductive phase (during which photoperiod affects the duration of reproductive development) (Patterson, 1995; Swanton et al., 1999; Wang et al., 1997; Wilkerson et al., 1989). We observed that seedlings kept continuously under SD conditions required 13.4 to 14.4 thermal days for flower initiation (Fig. 3). Seedlings transferred from SD to LD 3 DAE responded similar to plants grown continuously under SD conditions (Fig. 7) . The response of flower initiation to transfer from SD to LD was comparable to the response of number of leaves on the mainstem (Fig. 4).



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  		Fig. 7 Time from (•) emergence to first appearance of heading, ({circ}) flowering, ({blacksquare}) end of flowering, ({square}) initiation of seed set, and ({blacktriangleup}) end of seed set for barnyardgrass transferred from short to long day at different times after seedling emergence. Fifteen days after emergence (DAE), the minimum number of thermal days was estimated as 5.6

 
Life Cycle of Barnyardgrass in Thermal Days at 20°C
For modeling purposes, the duration of the different phases of plant development has been characterized in terms of thermal days at a constant temperature of 20°C (Swanton et al., 1999). In our study, development rates were most rapid under the 8-, 10-, 12-, and 14-h photoperiods, and since they were equivalent, the phase durations expressed above in terms of thermal days was used to describe the life cycle of barnyardgrass at 20°C. Comparisons of the duration of the phenological stages were based on the information obtained from 10-, 12-, and 14-h treatments of the photoperiod and juvenile phase studies (Table 7) . The purpose of this comparison was to confirm the phase durations from two different experimental approaches. Phase duration estimated from these two studies did not differ. Based on thermal days at 20°C the phenological stages of barnyardgrass were determined as: (i) 1.9 thermal days from seed to emergence; (ii) 4.4 thermal days from seedling emergence to initiation of floral primordia; (iii) 8.8 thermal days from initiation of floral primordia to initiation of heading; (iv) 0.8 thermal days from initiation of heading to initiation of flowering; (v) 4.6 thermal days from initiation of flowering to end of flowering; (vi) 1.3 thermal days from end of flowering to initiation of seed set; and (vii) 5.3 thermal days from initiation of seed set to end of seed set. Also, based on the sensitivity of barnyardgrass to photoperiod, our study determined three phases: juvenile phase, photoperiod sensitive inductive phase, and a photoperiod sensitive postinductive phase (Fig. 8) .


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  		Table 7 Duration of phenological phases of barnyardgrass as determined from the photoperiod and reciprocal transfer (juvenile phase) studies at 20°C

 


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  		Fig. 8 Life cycle of barnyardgrass defined in thermal days at a constant temperature of 20°C

 

   Conclusions
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
REFERENCES
 
Knowledge of phenological development is fundamental in the development of mechanistic weed competition models. In this study we determined how phenological development of barnyardgrass was influenced by temperature and photoperiod. Barnyardgrass was adapted to a wide temperature range and its life cycle was described in terms of thermal days at 20°C (Fig. 8). Barnyardgrass became sensitive to photoperiod after the juvenile phase and this sensitivity continued up to and including seed set. Although barnyardgrass completed its life cycle both in short or long photoperiod, sensitivity to photoperiod was similar across the phases of development when phenological development was expressed in terms of rate. Interpretation of constant sensitivity to photoperiod will simplify simulation of phenological development in mechanistic models.

In future research, the relationships determined in this study need to be incorporated into a photothermal model for field application (Deen et al., 1998a, 1998b). The ability of such a model to explain or predict phenological development of barnyardgrass under field conditions will determine whether photoperiod–temperature interactions or other stresses need to be considered as factors affecting barnyardgrass development. If a photothermal time model is sufficient in describing phenological development of barnyardgrass under field conditions, then we will have a basis for further development of a mechanistic weed competition model.Kackperska-Palacz Putala Vengris 1963; SAS Institute 1990


   ACKNOWLEDGMENTS
 
Funding for this project was provided by the Natural Sciences and Engineering Research Council of Canada, Strategic Grant No. STR0167578, and by Agriculture Canada–Natural Science and Engineering Council of Canada, Partnership Grant No. CRD-102564. We thank Dr. Irena Rajcan for her valuable suggestions and comments on this manuscript. We also thank D. Scholz for his assistance with our growth cabinet studies.

Received for publication August 2, 1999.
   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 





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