Intercropping has been seen as an advantageous strategy in sustainable agriculture. Plants however interact with one another both above and below ground with members of the same species (intraspecific) or members of a different species (interspecific) for nutrients, water and light. It is therefore essential to understand these interactions when intercropped. The objective was to examine the above and below ground interactions between onion and lettuce in monocrop and intercrop systems. We examined the various possible interactions (no competition, above ground, below ground, or full) using a full factorial randomized design under greenhouse conditions. Onion yield was highest in intraspecific above ground competition and lowest in below ground and full interspecific competition with lettuce. Dry weight of onions in above ground competition with lettuce was significantly greater than that of the control group. Fresh weight of lettuce leaves were highest in below ground and full interspecific competition treatments. The hectare model and yield results suggest that there is strong below ground competitive effect between onion and lettuce in intercrop. Asymetric interspecific facilitation was found: facilitation by onion led to increased lettuce yield but a negative effect of lettuce on onion yield was observed. Knowledge of competitive interactions between component crops can have several applications in sustainable agricultural as it helps to match the most efficient species under specific conditions.
Intercropping is an agricultural technique based on the management of plant interactions to maximize crop yield and involves cultivating two or more crops in the same field within the same growing season [
Competitive interactions between two plants, brought about by a shared requirement for soil nutrients, water or light, can have negative effects on either species by decreasing growth rate, survival, reproduction or yield of one or both plants [
In order to study competition in intercrop treatments and fully explore the yield potential of intercrops, we must examine and distinguish between the different types of interactions [
Research into cereal-legume intercrops is extensive and common in tropical and subtropical regions such as Africa and Asia. There is however limited research dedicated to vegetable or non-legume intercrops, intercrop competition and the specific interactions occurring between intercropped species, especially in temperate re- gions [
We used a complete random factorial design in a greenhouse setup where both intraspecific and interspecific competitions were examined using 11 treatments with 10 replicates: one single plant of onion 1) or lettuce 2) acted as the control groups; two plants of onion 3) or of lettuce 4) or one onion and one lettuce 5) planted in the same pot with full (above and below ground) interactions; two plants of onion 6) or of lettuce 7) or one onion and one lettuce 8) planted in a pot where the above ground portions were separated with greenhouse plastic (no above ground interaction); and two plants of onion 9) or of lettuce 10) or one onion and one lettuce 11) planted in two different pots but placed side by side (no below ground interaction). Seeds were sown directly into pots: onion at a depth of 2 cm and lettuce at a depth of 1 cm (according to the seed company). Seeds were spaced ap- proximately 10 cm from each other in either the same pot for below ground and full competition, or in two pots for above ground competition treatments (one per pot) (note that the volume of soil was the same for all plants, whether in a larger pots for two plants or smaller pots for single plants). Plants were watered when the soil was dry to the touch. Fertilizer (20-20-20) was mixed into the water once every two weeks. Temperature varied be- tween 15˚C and 25˚C with photoperiod of 14 h day/10 h night.
Lettuce was harvested after 55 days (on May 1, 2013). Fresh leaves were weighed and the samples were placed into a drying cabinet at 45˚C for two weeks, at which point dry weight was recorded for each sample. The onion plants were left to grow for another 33 days and were harvested on June 3 - 4, 2013. Fresh bulb weight was immediately recorded, samples were placed in the drying cabinet, and after two weeks the dry onion samples were removed from the oven and weighed.
Fresh and dry weight data were transformed using a natural logarithmic transformation to satisfy normality and compared between treatment groups with an analysis of variance followed by a multiple range Dunnett T3 post- hoc test.
A model was created to estimate the yield of lettuce and onion in intercrop and monoculture in a simulated area of 1 hectare (100 m × 100 m). The hectare model uses average yield from both the onion and lettuce, con- trols, intraspecific full competition and interspecific full competition treatments. In the onion monoculture model, rows were spaced 40 cm apart, and onion plants were spaced 10 cm apart within rows, in accordance with onion- growing fact sheet [
The Relative Interaction Index (RII, [
where Mw is the fresh mass of plant matter (lettuce leaves or onion bulbs) in competition with neighbours, and Mo is the fresh mass of isolated plants (control). RII has values ranging from −1 to 1, is symmetrical around zero and is negative for competition and positive for facilitation [
Onion bulbs exposed to intraspecific above ground competition had the highest fresh weight. In interspecific competition, fresh bulb weight was the lowest in below ground and full competition with lettuce (
When individual onions were grown in pots, i.e. with no competition, and extrapolated for yield in a field of 1 hectare, the results predicted the highest yield with a mean of 247 kg/ha (
Lettuce yield was greatest in below ground and full interspecific competition with onion compared to intraspe- cific or no competition (
The effect of lettuce on onion was competitive in the below ground and full competition treatments, but was
neutral in the above ground competition treatment. Onion monoculture was facilitative in above ground interacttion as well as below ground interaction, but when paired in full competition, the effects were slightly competi- tive (
When lettuce was paired with onion above ground, there was a neutral effect, while a facilitative effect was observed when lettuce was paired with onion in below ground and full interaction treatments. The effects of let- tuce in intraspecific interaction above ground and below ground were competitive, while there was a neutral ef- fect for lettuce paired with lettuce in full interaction (
Competitive interactions among plants of the same or different species can have either a positive or a negative effect on growth, reproduction, and survival [
This unbalanced benefit of both crops in intercropping has been previously observed, showing the complexity of the system and its interactions [
The results suggested that the interactions were not necessarily similar for above and below ground. In interspecific competition, onions faired best in above ground interaction with lettuce. We expected that lettuce plants would shadow
Treatment | Fresh Bulb Weight (g) | RII | Interaction |
---|---|---|---|
Onion Control | 0.99 | ||
Onion-Onion Above | 1.66 | 0.25 | Facilitative |
Onion-Lettuce Above | 1.09 | 0.05 | Neutral |
Onion-Onion Below | 1.45 | 0.19 | Facilitative |
Onion-Lettuce Below | 0.59 | −0.25 | Competitive |
Onion-Onion Both | 0.74 | −0.14 | Slightly Competitive |
Onion-Lettuce Both | 0.45 | −0.38 | Competitive |
Treatment | Fresh Leaf Weight (g) | RII | Interaction |
---|---|---|---|
Lettuce Control | 51.47 | ||
Lettuce-Lettuce Above | 40.68 | −0.12 | Facilitative |
Onion-Lettuce Above | 47.44 | −0.04 | Neutral |
Lettuce-Lettuce Below | 39.43 | −0.13 | Facilitative |
Lettuce-Onion Below | 70.13 | 0.15 | Competitive |
Lettuce-Lettuce Both | 51.75 | 0.00 | Slightly Competitive |
Lettuce-Onion Both | 70.71 | 0.16 | Competitive |
onions, but it appears that below ground competition with lettuce limited onion growth more than above ground competition. The combined effect of full competition is therefore dominated by the below ground portion. A similar reduction of onion bulb yield due to intercropping with vegetables was noted by [
Our comparison between individual onion and onion in full or below ground competition also suggest that onions required greater spacing than what was stated by the seed company due to mainly below ground effect. Our results were similar to Peach et al. [
It was expected that broader and more horizontal leaves of lettuce would lead to increased shading, as well as respiration and maintenance costs, thus likely reducing yield in intraspecific above ground competition [
Intercropping has been shown to have yield advantage presumably resulting from both above and below ground facilitation between intercropped species [
In the face of interactions or changes in environmental cues, plants can respond by emitting different biogenic volatile organic compounds or by modifying their internal hormonal balance [
Results from this study have promising applications in crop yield optimization and sustainable agriculture. While intercropping has been practiced in some countries for centuries, several questions remain unanswered regarding optimization of plant combinations, optimal density, spatial arrangements, or selective breeding ne- cessary to produce maximum yields in full scale agricultural systems [
We thank V. De Luca for his involvement in the planning and execution of this study and the technical assis- tance of C. Carpenter-Cleland, A. Shidu, B. Amout. K. Dundas and A. Gheorghe. Funding to LV provided by Brock University.