Early growth and development of wild radish (Raphanus raphanistrum L.) in relation to wheat

Raphanus raphanistrum was grown in monoculture and as a range of cohorts of emergence in mixture with wheat. Growth and development were recorded at frequent intervals up to anthesis of the wheat. R. raphanistrum remained shorter than wheat, only over-topping the crop prior to anthesis for 2 of 7 sowing dates. When expressed in terms of photothermal time, growth in monocultures was similar for all sowing dates except those for wheat in mid-summer. Even a short delay in R. raphanistrum emergence in mixture decreased its growth considerably; R. raphanistrum emerging more than 4 weeks after the crop had no detectable effect on the wheat. However, even R. raphanistrum plants emerging 10 weeks after the crop produced some seeds. In mixture, R. raphanistrum had a higher specific leaf area (SLA), reduced leaf partitioning, and taller stems than in monoculture. A physiological growth model based on parameters from monocultures was unable to predict growth in mixtures; inclusion of changes in SLA and height in mixture improved predictions in some cases. It is concluded that competition models based on monoculture parameters, although previously successful for species with similar height and phasic development, are unlikely to work for species that differ in these attributes. Further work is required on the physiology of plants within mixtures.