Index | Search | Home | Table of Contents

Callan, E.J. and C.W. Kennedy. 1996. Intercropping stokes aster: Seedling growth under a soybean canopy. p. 363-367. In: J. Janick (ed.), Progress in new crops. ASHS Press, Arlington, VA.

Intercropping Stokes Aster: Seedling Growth under a Soybean Canopy*

E.J. Callan and C.W. Kennedy

    1. Culture
    2. Environmental Conditions
    3. Data Collection
    4. Experimental Design and Statistical Analysis
  5. Table 1
  6. Table 2
  7. Fig. 1
  8. Fig. 2

The perennial Stokes aster (Stokesia laevis, Asterallae) has the potential to become a new industrial oilseed crop. The vernolic acid (12,13-epoxy-cis-9-octadecenoic acid) contained in its achenes is easily converted to epoxy oil, a material widely used in the chemical industry (Perdue et al. 1986). Stokes aster's favorable agronomic traits, such as an upright growth habit and achenes in bracts that do not shatter, could facilitate its use as a crop (Campbell 1981; Gunn and White 1974). The fact that it is a perennial could be a potential advantage, as the crop would not need to be replanted for up to 5 years (Campbell 1981, 1984). However, Stokes aster will not flower or produce an economic yield during the first year of growth (Campbell 1981).

One management technique to overcome first-year expenses would be to intercrop Stokes aster seedlings with an annual such as soybean. Success with such a system would depend on the ability of Stokes aster plants to tolerate shade. Controlled studies of shading help distinguish the different effects of an intercrop system on the component crops (Newman 1986). By separating out the effect of light on the crops, the performance of the intercrop in the field can be better understood.

Determining appropriate relative planting dates for the intercrop species is also important. Optimum planting dates for the components of an intercrop appear to be highly dependent on the specific intercrop. A roselle (Hibiscus sabdariffa L.) and oilseed intercrop performed better when the oilseed was planted later (Roy et al. 1990). However, a rice (Oryza sativa L.) and legume (various species) intercrop performed best when legumes were planted early, so the rice was not overly competitive with the legumes (Mandal et al. 1990). A cotton (Gossypium hirsutum L.) and mungbean [Vigna radiata (L.) R. Wilczek] intercrop was most successful using late maturing cotton cultivars, as early season bean growth caused less damage to these compared to short-season cotton varieties (Sulistyowati 1989).

Our objective was to evaluate the growth of Stokes aster seedlings of different ages during and after exposure to the shade of an intercrop by (1) measuring Stokes aster's growth under a soybean canopy and (2) evaluating Stokes aster's recovery from shade effects after the soybean canopy was removed.



A greenhouse experiment was conducted at the Louisiana Agricultural Experiment Station in Baton Rouge from Feb. 11 to Aug. 21, 1992 and repeated from Sept. 1, 1992 to Mar. 12, 1993. Germination of achenes of USDA Stokes aster accession BSLE2 was initiated at two different times: 30 days before soybean seed germination was initiated [early planting (EP)] and the same day soybean seed germination was initiated [concurrent planting (CP)]. Seeds were started in germination paper, then transferred to styrofoam seedling trays. Forty-two days after planting seedlings were transferred to 3-liter buckets filled with a 0.2-strength Hoagland's solution (Hoagland and Arnon 1950) changed every 10 days. There were three seedlings per bucket, and aeration was achieved by pumping humidified air through tubing.

Maturity group V 'Delta Pine 105' soybean seeds were inoculated with Bradyrhizobium japonicum and germinated in germination paper, transferred to peat press pots then transplanted to 22-liter tubs filled with gravel and sand and covered with a thin layer of potting soil. Plants were spaced at 50-mm intervals in a single row with a total of 9 plants per tub. Two tubs were aligned to produce a soybean row 0.91 m long. Three such rows were spaced to produce 2 interrows with a width of 0.76 m each. Soybeans were supplied with a nutrient solution without NO3 consisting of 1 mM MgSO4, 1 mM KH2PO4, 0.5 mM NH4SO4, 1 mM CaCl2, 1 ml/liter stock micronutrients, and 1 mg/liter Fe as iron chelate. This solution was later doubled in concentration. The nutrient solution was applied by trickle irrigation every 30 minutes and was recycled from a 19-liter bucket, the contents of which were replaced every 2 to 5 days. The 3-liter buckets containing Stokes aster were placed in the interrow spaces between the soybean rows at the time of plant transfer from seedling trays. The height of all containers was about the same.

The soybean canopy closed approximately 50 days after planting, and to simulate conditions within a field row of soybeans, 55% neutral density shade cloth was placed across the ends of the rows to prevent extra diffuse light from reaching the Stokes aster plants. Fifty-two days after canopy closure, soybean plants were removed to simulate the duration of a soybean field canopy (Hicks 1978) and recovery of the shade treated plants was measured. Stokes aster plants were harvested 60 days after the soybean canopy was removed.

Environmental Conditions

To prevent premature soybean flowering in the greenhouse, natural daylength was extended during short days with 2-1000W metal halide lamps. To determine photosynthetic photon flux density (PPFD) above and below the soybean canopy, light measurements were taken with a one meter-long quantum sensor at solar noon. Day and night temperatures varied widely. A maximum of 40°C day and a minimum of 18°C night were recorded.

Data Collection

Leaf number per plant and fresh weight (FW) of plants per pot were determined every 10 days. True leaves longer than 10 mm were counted. Fresh weight was measured rather than dry weight (DW) as an experiment large enough to allow destructive measurements was not within reason. The three plants per pot were blotted dry with paper towels and weighed with the plastic cover in which they were growing. Cover weights were then subtracted and the average weight taken to the nearest 0.01 g. Relative leaf production rate (RLPR) was calculated as the number of leaves produced per day per leaf (Garner 1992), leaf production rate, as leaves per day, and FW relative growth rate (RGR) as the increase in fresh weight per day per unit of fresh weight. Harvest measurements of individual plants included leaf area (measured on a Li Cor LI-3000 portable area meter to the nearest 10 mm2, total leaf length (measured to the nearest 10 mm), leaf number, shoot dry weight, and root dry weight (weighed after drying at 70°C for 1 week, to the nearest 0.01 g).

Experimental Design and Statistical Analysis

The experiment was a complete block with a split split plot treatment arrangement and four replications. The repeated experiment was the main plot, light condition (shade or sun) was the sub plot and date of planting (EP or CP) was the sub-subplot. Both repetitions of the experiment were combined as there was no significant interaction between experiments. Leaf production rate, RGR, or RLPR were found using a piecewise multiple regression (Neter et al. 1989), the two parts being during canopy cover and after the canopy was removed. Days after canopy closure and treatment and their interaction were independent variables and means of the three plants making up each replication were dependent variables. Significant differences between growth rates were calculated using standard errors (SE). At each sampling time, SE was also computed for graphing purposes. The General Linear Model procedure was used to analyze harvest measurements and means were separated using the LSD procedure (p <0.05).


Stokes aster seedling growth was greatly reduced by dense shade. Average light intensity under the soybean canopy and available sunlight approximately 30 days after canopy closure was 20 and 440 µmol m-2 s-1 PPFD, respectively. It was apparent Stokes aster plants were only achieving metabolic stasis, at best, in this low light condition (Fig. 1 and 2).

After the canopy was removed, intercropped plants responded with almost a six-fold increase in FW RGR, an eight-fold increase in leaf production rate, and a seven-fold increase in RLPR. During this same time, plants continuously grown in available sunlight underwent a 50% decrease in FW RGR, a 40% decrease in RLPR, and only a 1.25-fold increase in leaf production rate (Table 1). After canopy removal, intercropped plants had non significant but higher average FW RGR and RLPR than sun plants, but they did not reach the size of sun plants during the time of the experiment. Moreover, the effect of shade on FW may be more long term than its effect on leaf number as FW measurements revealed less recovery than leaf number measurements (Fig. 1 and Table 1). Even after recovery from the canopy, absolute growth parameters for intercropped plants averaged 42% of EP sun plants and 32% of CP sun plants (Table 2).

Introducing intense shade to very young Stokes aster seedlings had only a slightly greater effect than introducing shade to slightly older seedlings. Average FW RGR, leaf production rate, and RLPR of CP and EP shade plants were similar, but average FW and leaf numbers were initially larger for EP plants which gave them a slight numerical advantage during most of the growing season. (Fig. 1 and 2 and Table 1). Altering the planting time in relay intercrops has previously resulted in light interception patterns that benefitted one of the components (Steiner and Snelling 1994; Cenpukdee and Fukai 1992). In this case, however, the growth of Stokes aster seedlings is so slow that a difference of 30 days in planting did very little towards increasing the potential to compete with a fast growing annual species for available sunlight.

In field conditions, it seems Stokes aster would recover at least partially from the shade of a soybean canopy. Plants have been shown to adapt to a change in light intensity over a period of weeks because new organs being initiated and differentiated are adjusted to the ambient light level during their formation (Larcher 1983). We have previously found Stokes aster grown in a light intensity of 120 µmol m-2 s-1 PPFD to have, when exposed to higher light intensities, photosynthetic rates similar to rates of plants grown at higher light intensities (1010 µmol m-2 s-1 PPFD) (Callan and Kennedy 1995). This indicated plants grown in low light had excellent capability to rapidly recover when exposed to full sunlight. In this study, however, recovery was not as complete as expected. Perhaps the extremely low level of light available to Stokes aster under the soybean canopy in this study slowed recovery subsequent to overstory removal. Additionally, ambient light intensity in the greenhouse was less than would be expected under field conditions, perhaps also slowing recovery.

Campbell (1981) suggested a size requirement for successful floral induction of Stokes aster. Though it appears Stokes aster can recover from shade, quantification of this size requirement is needed to determine if Stokes aster's growth after overstory removal would provide for successful floral induction.


From these experiments, we have found Stokes aster to be adversely affected by dense shade in the range of 20 µmol m-2 s-1 PPFD. However, when the overstory was removed, Stokes aster plants recovered from shade; FW RGR and RLPR of intercropped plants after overstory removal were slightly greater but statistically equivalent to rates of plants grown in sun.

Introducing intense shade to younger seedlings had a slightly more adverse effect than introducing shade to slightly older seedlings. Planting Stokes aster 30 days earlier than soybeans, if feasible, would marginally improve its growth potential after overstory removal. Under field conditions, however, optimum date of planting is dependant on many environmental conditions, of which light is only one.

Although we did not find Stokes aster to thrive under a very dense soybean canopy, we did find it could survive such a canopy for a period of at least 52 days. In addition, Stokes aster plants recovered well from dense shade. Overall, Stokes aster shows promise as the understory crop of an intercrop depending on the amount and duration of canopy closure of the overstory crop, the duration of growing season after overstory removal, and, marginally, the seeding time of each component.


*Approved for publication by the director of the Louisiana Agr. Expt. Sta. as manuscript no. 94-09-8412.
Table 1. Fresh weight (FW) relative growth rate (RGR), relative leaf production rate (RLPR), and leaf production rate of plants grown in available sunlight (Sun) compared to plants grown under a soybean canopy (Shade). Average of two experiments.

Treatment Time periodz FW RGRy (g g-1 d-1) RLPRx (lf lf-1 d-1) Leaf production rate (lf d-1)
Early plantingw
Shade During canopy 0.005bv 0.002bc -0.029bc
After canopy 0.013ab 0.019a 0.329ab
Sun During canopy 0.021ab 0.015ab 0.286abc
After canopy 0.013ab 0.010ab 0.357a
Concurrent plantingw
Shade During canopy 0.002b -0.005c -0.056c
After canopy 0.018ab 0.023a 0.237abc
Sun During canopy 0.033a 0.022a 0.276abc
After canopy 0.013ab 0.012ab 0.348a
zParameters were calculated from replication means during canopy closure and after the soybean canopy was removed.
yFresh weight relative growth rate; grams per gram per day.
xRelative leaf production rate; leaves per leaf per day.
wEarly planting = Stokes aster seed germination began 30 days earlier than soybean seed; concurrent planting = Stokes aster seed and soybean seed germination began on same day.
vMean separation in column by LSD test, 5% level. All values were determined by piecewise regression on data taken at 10 day intervals.

Table 2. Comparison of Stokes aster previously grown in the shade of a soybean canopy to Stokes aster continually grown in available sunlight. Results are from harvest data after the soybean canopy had been removed and plants recovered in available sunlight for 60 days. Average of two experiments.

Treatment Leaf area (cm2/plant) Leaf length (cm/plant) Leaf number (no./plant) Leaf dry wt. (g/plant) Root dry wt. (g/plant)
Previously Shaded
Early plantingz 333by 296b 29bc 1.60b 0.83c
Concurrent plantingz 216b 198b 20c 0.96b 0.44c
Early planting 808a 654a 51a 4.29a 2.73a
Concurrent planting 704a 573a 44ab 3.87a 2.06b
zStokes aster seed germination began either 30 days earlier than or concurrently with soybean seed germination.
yMean separation by LSD test, 5% level.

Fig. 1. Fresh weight of Stokes aster plants grown under a soybean canopy (shade) compared to plants grown in available sunlight (sun). Stokes aster seed were initially germinated either 30 days earlier than or concurrently with soybean seed. Measurements were continued after the soybean canopy was removed (after canopy). Vertical bars indicate SE. Average of two experiments.

Fig. 2. Leaf number of Stokes aster plants grown under a soybean canopy (shade) compared to plants grown in available sunlight (sun). Stokes aster seed were initially germinated either 30 days earlier than or concurrently with soybean seed. Measurements were continued after the soybean canopy was removed (after canopy). Vertical bars indicate SE. Average of two experiments.

Last update August 21, 1997 aw