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.
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.
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.
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.
|Treatment||Time periodz||FW RGRy (g g-1 d-1)||RLPRx (lf lf-1 d-1)||Leaf production rate (lf d-1)|
|Treatment||Leaf area (cm2/plant)||Leaf length (cm/plant)||Leaf number (no./plant)||Leaf dry wt. (g/plant)||Root dry wt. (g/plant)|
|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.|