“Don’t plant peas or beans with onions in your garden”: Fact or Myth? Catrina Frey, Department of Biological Sciences Introduction Review of LiteratureHypotheses and Expected Results Literature Cited Acknowledgements I would like to thank Dr. Karl Kleiner for all of his help and support. H 1 : When planted separately, compared to when grown in the presence of onion, the legumes will have a higher dry biomass. All plants will be planted separate in their own container. Then all three cultivars will be planted within the same container. One by alternating species randomly and the other by setting up a row of each species, onions being the middle row. ER 1 : The peas and beans will have a lower dry biomass when planted with onions. H 2 : The presence of isothiocyanate in the soil will inhibit Rhizobium growth on legume roots. The legumes will be planted in a pot together and treated with a ½ :1 solution of isothiocyanate and H 2 O. ER 2 : The isothiocyanate will suppress the Rhizobium growth on the legumes (Figure 1). H 3 : Spacing between the onion and legume species will have a direct correlation with their growth. A group of onions will be planted at one end of a container and each legume species in a row to the other end, by using a full bench of soil. ER 3 : The mean dry mass of the legumes will increase as their distance is increased from the onions (Figure 2). H 4 : Nitrogen additives to the soil will increase growth of the legumes. A nitrogen supply will be added to the soil of legumes planted in containers without onions. ER 4 : The increase in nitrogen will augment legume growth (Figure 3). Project Summary H 5 : Adding fertilizer to the soil, without onions, will increase the growth of legumes. A Miracle Grow fertilizer will be added to the soil of both legumes in separate containers. ER 5 : The legumes biomass will serve as a control mass for the next hypothesis. H 6 : Fertilizer in the soil with legumes and onions will cause the legumes to grow near the control’s dry biomass. A Miracle Grow vegetable fertilizer, with other nutrients in addition to nitrogen, will be added to the soil in pots containing both legume and onion species. ER 6 : The biomass of the legumes will be lower than the control dry mass (Figure 3). Harvesting Procedures: After harvesting each species will be weighed for dry mass, which includes the roots and the fruit portion. Rhizobium count will be determined by counting one large root and then multiplying it to receive and average number of Rhizobium nodules. Figure 2 Figure 1 The gardening literature is replete with admonishments not to plant peas (Lathyrus grandiflorus) or beans (Phaseolus coccineus) with onions (Allium cepa). However, experimental evidence for this planting strategy is lacking. In fact, there is no exact mechanism explaining why you shouldn’t plant these species together. There are many possible reasons as to why this lore may have been established, such as allelopathy or negative feedback between legumes and onions. Nitrogen and secondary compounds may play a role in the inhibition of Rhizobium, legume and onion growth. The objectives of this study are to determine if Rhizobium is affected in the presence of onion and to see if legume growth is in turn affected by the presence of onion and its secondary compound, isothiocyanate. Another aspect of this study will examine the addition of nitrogen to the soil and its effect on both the legumes and onion growth. The end result to this study will give rise to a specific growth pattern so that the legumes and onions can be planted together. Figure 3. Onions are companions to a lot of plants, but they are not compatible with peas and beans (Garden Guides, 2003). Allelopathy is the release of chemicals, usually biochemical reactions that take place between plants (Robson, et. al., 2000). The effects from allelopathic conditions include reduced growth and the inhibition of normal symbiotic relationships such as the nitrogen fixation by legumes (Robson, et. al., 2000). Changes in the soil, due to microbes, that leads to decreased growth in a species in relationship to other species is deemed negative feedback (Mills and Bever, 1998). Legumes secrete various compounds including flavonoids when in the presence of NodD factors or “specific lipo-chito-oligosaccharides” (Limpens, et al., 2003), thus causing nodulation of Rhizobium onto the roots of legumes. Flavonoids come from the plant and interact with the Rhizobium NodD proteins that act as the environmental activators to begin nodulation, thus responsible for the symbiotic relationship between the two (Broughton, et al. 2003). The increase of nitrates and other nitrogen fertilizers can limit the nodulation of Rhizobium (Broughton, et al. 2003). Isothiocyanate is a secondary chemical compound that is found in onions (Cornell University, 2001). Broughton, William J., Feng Zhang, Xavier Perret and Christian Staehelin Signals exchanged between legumes and rhizobium: agricultural uses & perspectives. Plant and Soil. 252: Cornell University Methyl isothiocyanate voluntary product cancellation 5/92. [online]. /fumigant/methyl-isothiocyanate/mitc-5-92-rer.html. Garden Guides Vegetable Companion Chart. [online]. Accessed: Nov Limpens, Erik, Carolien Franken, Patrick Smit, Joost Willemse, Ton Bisseling and Rene Geurts. October, LysM domain receptor kinases regulating rhizobial nod factor-induced infection. Science. 302(5645): Mills, Katherine E. and James D. Bever Maintenance of diversity within plant communities: soil pathogens as agents of negative feedback. Ecology. 79(5): Robson, M.C., Robinson, D., Litterick, A.M., Watson, C., and Leitch, M Investigations into allelopathic interactions of white lupin (Lupinus albus). Accessed: 13, November An old wives tale states that legumes should not be grown in the presence of onions. Although an abundance of information exists on companion planting, a scientific explanation is lacking as to why these species should not be planted together. To determine the effect of legumes and onions on each other, beans and peas will be grown alone and together with onions. Isothiocyanate is a secondary compound that may play a role in blocking the nodulation of Rhizobia. To test if Rhizobia are being affected, the legumes will be treated with isothiocyanate. The cultivars Everlasting Pea, Scarlet Runner Bean and the Common Onion will be weighed and the number of Rhizobium on the legume root systems will be counted. Understanding the mechanism will lead to knowledge that will determine spatial planting patterns so they can be grown in the same gardens. General Procedures The study will be conducted inside in a large greenhouse. The containers will be 3m X 3m X 10cm. Each will be filled with a non-fertilized soil. One bench will be filled with soil to accommodate a specific trial. There will be heat lamps placed above the containers to ensure germination, and then they will be removed and the greenhouse will be kept at 70 degrees Fahrenheit.