Crop Science 42:237-241 (2002)
© 2002 Crop Science Society of America
FORAGE & GRAZING LANDS
Rate of Leaf Appearance in Crimson Clover
Twain J. Butler*,a,
Gerald W. Eversb,
Mark A. Husseyc and
Larry J. Ringerc
a Texas A&M University Agricultural Research and Extension Center, 1229 N. Hwy 281, Stephenville, TX 76401
b Texas A&M University Agricultural Research and Extension Center, P.O. Box 200, Overton, TX 75684-0200
c Dep. of Statistics, Texas A&M University, College Station, TX 77843
* Corresponding author (t-butler{at}tamu.edu)
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ABSTRACT
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Understanding factors that affect growth and development of crimson clover (Trifolium incarnatum L.) are important for the development of management practices to optimize forage utilization. In a 3-yr field experiment at College Station, TX, we evaluated the effects of planting date on rate of leaf appearance of an intermediate- and late-maturing crimson clover. We wanted to determine if growing degree days (GDD) or a photothermal index (PTI) could be used to predict growth. Leaf appearance rates (LAR) did not differ between ‘Tibbee’ and ‘Columbus’ crimson clover. Leaf appearance rate was primarily controlled by temperature or GDD, which accounted for 90 to 99% of the variability within each planting date. Photoperiod did not consistently influence the rate of leaf appearance under normal daylengths of 10 h 12 min to 14 h 6 min used in this study. Predictions of LAR were not improved when photoperiod was combined with temperature in a photothermal index than with predictions that used GDD alone. Leaf appearance rate of crimson clover was generally higher when planted in October, November, and December and lower when planted in September, February, and March.
Abbreviations: DL, daylength • LAR, leaf appearance rate • GDD, growing degree days • LN, leaf number • PLS, pure live seed • PTI, photothermal index
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INTRODUCTION
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CRIMSON CLOVER is one of the most important clovers for overseeding into perennial warm-season grass sods because of its excellent seedling vigor, early forage production, and early maturity. There have been few studies reported on the growth and development of annual clovers. Leaf appearance rates are direct measurements of morphological development that could be used to quantify factors affecting growth. Understanding and predicting crop growth is beneficial to optimize management, production, and utilization.
Leaf appearance rate on the whole-plant level is a valuable tool for studying growth and development of some plants. Past studies with grasses have utilized phyllochron measurements to quantify morphological development (Kiniry et al., 1991; Van Esbroeck et al., 1997). The phyllochron is defined as the interval in growing degree days (GDD) between the appearance of successive leaves on a stem. Typically, phyllochron research has been done on crops with a single main stem, however, it is often difficult to distinguish a main stem in plants that have a rosette growth habit. Therefore it is more accurate to make comparisons on the whole plant level for such plants.
The GDD is calculated by summing the average daily temperature minus a base temperature. Various species have specific ranges or thresholds of temperature for growth. The base temperature used is the minimum temperature at which a plant species can grow (Eastin and Sullivan, 1984). In addition, a maximum temperature threshold model using an adjusted maximum temperature can account for reduced growth associated with high temperatures (Dufault, 1997). If the maximum daily temperature exceeds an upper limit, the difference between the upper limit and maximum temperature is subtracted from the upper limit to account for the growth reduction.
Strong linear relationships between leaf number and GDD support the contention that temperature is the primary factor controlling leaf appearance rate (LAR) in grasses, soybeans [Glycine max (L.) Merrill], peas (Pisum sativum L.), cowpeas [Vigna unguiculata (L.) Walp.] and subterranean clover (T. subterraneum L.) (Baker et al., 1986; Van Esbroeck et al., 1997; Sinclair, 1984; Truong and Duthion, 1993; Ney and Turc, 1993; Crauford et al., 1997; Aitken, 1974). Rate of leaf appearance increases with higher temperatures, but the relationship differs among species.
Photoperiod can have varied effects on LAR. Rate of leaf appearance decreased with increasing photoperiods in glasshouse studies during the spring in certain long-day, cool-season perennial grasses, such as: orchardgrass (Dactylis glomerta L.), meadow fescue (Festuca pratensis Huds.), perennial ryegrass (Lolium perenne L.), timothy (Phleum pratense L.), smooth brome (Bromus inermis Leyss), and tall fescue (Festuca arundinacea Schreb.) (Ryle, 1966; Heide et al., 1985; Kiniry et al., 1991).
Photoperiod did not affect LAR in rye (Secale cereal L.) or annual ryegrass [Lolium multiflorum (L.) Lam.] (Kiniry et al., 1991), which are cool-season annual grasses. Conversely, Cao and Moss (1989) reported that increasing photoperiods in the growth chamber increased LAR of wheat (Triticum aestivum L.), which differed from other cool-season annual grass species. In certain short-day species, such as corn (Zea mays L.), increasing photoperiod in a glasshouse study has been reported to increase LAR (Kiniry et al., 1991) and leaf length in bermudagrass [Cynodon dactylon Pers.] (Marousky et al., 1992). In soybean, a short-day, warm-season annual legume species, LAR increased with increasing photoperiods; however, LAR was not affected by photoperiod in cowpea or faba bean (Vicia faba Maris Bead.) (Kiniry et al., 1991; Crauford et al., 1997).
There is a need to determine the relationship between crimson clover growth and environmental factors such as temperature and daylength in order to better plan management decisions. In addition, management practices could be scheduled according to an easily measured environmental parameter such as GDD. If leaf number could be predicted, management decisions, such as planting date, harvest dates, and stocking rates, could be modified to maximize production and profit. The effects of temperature under U.S. climatic conditions on the rate of leaf appearance have not been reported for any cool-season annual clover. Our objectives were to determine the effect of planting date on LAR of an early and late maturing crimson clover, and to quantify climatic parameters that could be used to predict growth and development.
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MATERIALS AND METHODS
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A split-plot design experiment with three replications was initiated in the autumn of the 1997-1998, 1998-1999, and 1999-2000 growing seasons at College Station, TX (30° 38'N, 96° 26'W). Main plots were planting dates, while subplots were cultivars. The experimental site was on a Boonville fine sandy loam soil (fine, montmorillonitic, thermic Ruptic-Vertic Albaqualfs). Intermediate-maturing Tibbee and late-maturing Columbus crimson clover cultivars were planted in 3.0-m rows spaced 30.5 cm apart. There is a 4 wk difference in maturity between Columbus and Tibbee crimson clover (Evers et al., 1995). The daylength at the first planting date was 11 h 48 min. It decreased to 10 h 12 min on 21 Dec. and then increased to 14 h 6 min by the end of the study. Clovers were planted at 16.8 kg PLS (pure live seed) ha-1 at approximately monthly intervals from October through March. Clover planted in the autumn was exposed to a decreasing photoperiod followed by an increasing photoperiod in the spring. Clover planted in the spring was only exposed to increasing photoperiods.
Soil test recommendation did not require fertilization or lime. Plots were irrigated after each planting to facilitate germination and emergence, and on an ‘as needed’ basis to prevent drought stress. Grassy weeds were sprayed with 1% solution of sethoxydim {2-[1-(ethoxyimino)butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one}, while broadleaf weeds were hand-weeded in order to eliminate interspecific competition. Plants were thinned within rows to a minimum distance of 15 cm between plants to facilitate monitoring of leaf number throughout the growing season. Thirty plants from each cultivar and planting date were selected at random and marked so that weekly leaf number counts could be recorded. Counts were discontinued when leaf number per plant exceeded 300.
Maximum and minimum daily temperatures were collected from the Easterwood airport in College Station (approximately 2 km from plot area) to calculate GDD, using a base temperature of 0°C. The GDD was calculated by averaging daily minimum and maximum temperature minus a base temperature for growth (0°C) when the maximum temperature did not exceed 30°C. Fukai and Silsbury (1976) reported that at 30°C, leaf death proceeded more rapidly than leaf appearance of subterranean clover (T. subterraneum L.). When maximum temperature exceeded 30°C a threshold model that used an adjusted maximum temperature similar to the one used by Dufault (1997) was implemented to account for the reduced growth associated with high temperatures:
where Tmax = maximum daily temperature, Tmin = minimum daily temperature, Tbase = base temperature, Tu = upper threshold limit, and Tadj-max = adjusted maximum daily temperature. A photothermal index (PTI) was calculated by summing the ratio of daylength per 24-h period multiplied by the total heat units (Masle et al., 1989).
Leaf number (LN) was recorded every 7 d using a modified decimal system similar to the one reported by Raguse et al. (1974). The number of leaves per plant were regressed against GDD and PTI by a linear and quadratic model of multiple regression. The quadratic model resulted in the highest coefficients of determination (R2) and was used to determine if LAR as defined by the slope differed between cultivars and planting dates (SAS, 1991). The slopes between cultivars and planting dates were compared within each growing season by a partial F-test for pairwise comparisons with 0.05 significance level (Neter et al., 1996).
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RESULTS AND DISCUSSION
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Temperatures were similar for all three years (Fig. 1)
of the study. Average daily temperatures in autumn were similar to the 30-yr average, however, temperatures in January and February were 7°C warmer than normal, with the temperatures never falling below 0°C. Accumulated GDD in each growing season were similar to the 30-yr average early in the growing season, but were higher later especially in the 1998-1999 and 1999-2000 growing seasons. The mild winter temperatures with supplemental irrigation resulted in optimum growing conditions during the study.
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Fig. 1. Monthly average daily temperature and accumulated growing degree days (GDD) by year and long-term average at College Station, TX.
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A significant (P < 0.05) year, planting date, and year by planting date interaction was observed for LAR, however, no difference in LAR was observed between cultivars. Because a year by planting date interaction was observed, data were analyzed by year. Due to the absence of cultivar by year or cultivar by planting date interactions, Tibbee and Columbus cultivars were pooled across planting dates.
Rate of leaf appearance was best related to GDD by a quadratic model with R2 values ranging from 0.90 to 0.99 (Table 1). This is in agreement with Knight and Hollowell (1958) who reported that the number of leaves per plant increased as night temperature increased from 10 to 18°C. Fukai and Silsbury (1976) reported that the number of green leaves on an individual plant of subterranean clover increased with increasing temperature in growth chambers from 15 to 20°C, remained constant at 25°C, and decreased at 30°C. The use of a photothermal index, which resulted in R2 values ranging from 0.89 to 0.99 did not differ from using GDD, therefore only GDD will be discussed. Within each year, LAR of crimson clover plants differed at each planting date, suggesting that both daylength and temperature influence LAR.
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Table 1. Relationship of leaf appearance rate of Tibbee and Columbus crimson clover cultivars with growing degree days (GDD) by year for planting date.
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In 1997-1998, crimson clover that was planted from October to December had a slower rate of leaf appearance than crimson clover planted in February and March (Table 1; Fig. 2)
. With the autumn planting dates, the LAR of crimson clover increased as planting date was delayed from October to December, whereas delayed planting in the spring from February to March (under an increasing photoperiod), caused LAR to decrease. Average daily temperatures were lower in 1997-1998 than the other two years, resulting in slower accumulation of GDD (Fig. 1).
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Fig. 2. Influence of growing degree days (GDD) on leaf number per plant by planting date for Tibbee and Columbus cultivars in the 1997-1998 growing season at College Station, TX.
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In 1997-1998, leaf counts were terminated in early March from the fall planting dates because of damage by rabbits (Sylvilagus spp.). Due to the curvilinear relationship between GDD and leaf number, the abbreviated counts reduced the overall LAR. In addition, clover planted in the spring was exposed to temperatures greater than 30°C, so a threshold model similar to the one used by Dufault (1997) was used to account for the senescence and reduced growth. This, as well as the lower temperatures, may account for much of the differences between 1997-1998 and the other two years.
In 1998-1999, crimson clover planted from October to December had a faster LAR than the clover planted in September, February, and March (Fig. 3)
. Leaf appearance rate increased as planting date was delayed from September to December, whereas delayed planting from February to March resulted in a slower LAR. In 1998-1999, the September and spring planting dates had similar LAR's while crimson clover planted in late autumn had faster development rates. This implies that crimson clover planted in the mid- to late-autumn had accelerated development because of more favorable autumn temperatures for growth. High temperatures in both September and late spring inhibited growth of crimson clover.
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Fig. 3. Influence of growing degree days (GDD) on leaf number per plant by planting date for Tibbee and Columbus cultivars in the 1998-1999 growing season at College Station, TX.
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In 1999-2000, clover planted in September had the slowest LAR, which is similar to the previous year (Table 1). However, clover planted in October and December differed from the previous two years. Clover planted in October had a faster LAR than November and December, and December had a slower LAR than November, which is contrary to the previous two years (Fig. 4)
. The reason for the reversal of LAR of crimson clover in October and December is not known. As planting date was delayed from February to March in the spring, the LAR of crimson clover decreased, which is similar to the other two years.
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Fig. 4. Influence of growing degree days (GDD) on leaf number per plant by planting date for Tibbee and Columbus cultivars in the 1999-2000 growing season at College Station, TX.
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Although there was an interaction among years, LAR of crimson clover followed a general trend. The highest LAR occurred when crimson clover was planted in October, November, and December. A September planting date that was used in 2 of the 3 yr resulted in a low LAR similar to the spring planting dates. It appears the high temperatures that occurred in College Station (30° 38'N) during September limited crimson clover seedling growth and development. Delayed planting in autumn resulted in increased LAR with GDD, while delayed planting in the spring produced a decreased LAR with GDD. This is consistent with previous findings where the number of leaves per plant of crimson clover increased as photoperiod decreased from 18 to 14 hr (Knight and Hollowell, 1958).
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CONCLUSION
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Generally, as planting date was delayed in autumn, the rate of development and leaf appearance was accelerated. This corresponds to a shorter growing season, which could equate to reduced production. From a practical viewpoint, it is important to plant early in the growing season as soon as moisture and lower temperature allows, to maximize the length of the growing season and possibly total production in grazed environments. Planting after 1 January resulted in a shorter growing season and fewer leaves per plant. The rate of development or leaf appearance was not different for intermediate- and late-maturing cultivars. Therefore, another way to extend the growing season is to use later maturing cultivars, which will remain vegetative for a longer period.
Received for publication March 14, 2001.
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ABSTRACT
INTRODUCTION
