Dr Geoff Seavers

Two experiments were carried out, one in 1995 and one in 1997, to investigate the competitive abilities of two spring wheat cultivars with Sinapis arvensis L. The spring wheat cultivars (Baldus and Canon) of contrasting growth habit were grown with and without S. arvensis under two different moisture regimes (10% and 70% of field capacity). In 1995, S. arvensis was found to be less competitive when subjected to moisture stress, resulting in smaller wheat yield losses in dry soil than in moist soil. In both years, seed production of S. arvensis was reduced by competition and moisture stress, and the seeds produced by plants that had been grown in drier soil were small and had negligible dormancy. Hence, in dry conditions, the competitiveness of S. arvensis and its potential to produce persistent seed may be reduced. Some differences between the two wheat cultivars were evident: cv. Baldus was more competitive against S. arvensis than cv. Canon. This could be attributed to differences in canopy structure.

Weed suppression characteristics of different winter cereal species and cultivars were studied in two field experiments in two successive years. Two cultivars each of barley, oats and wheat were studied using the weed Galium aparine L. at a range of densities. Significant differences were found in the suppressive abilities of the crop species: oats being the most suppressive, followed by barley and then wheat. Percentage yield loss was related to G. aparine density using a rectangular hyperbolic model. In 1995–96, significant differences between the two wheat cultivars were found, with cv. Avalon being able to suppress G. aparine growth much more effectively than cv. Spark. However, in 1996–97, high rainfall in May, June and July enabled G. aparine to escape suppression by the cultivars. Cultivar competitive ability was associated with high overall leaf area, resistance to loss of tillers under competitive pressure, greater height, canopy structure and development. The oat cultivars were more competitive at early growth stages than could be accounted for by their canopy structure; the possible contribution of allelopathic exudates to their suppressive ability is discussed.

The quality and quantity of light reflected from Nicotinia tabacum L. internodes was monitored as the angle of incidence was varied. Reflectance of incident light, which was either normal or longitudinal to the internode axis, was investigated. Increasing the angle of incidence caused a greater proportion of incident light to be reflected. Light striking N. tabacum internodes was always modified prior to reflection, and smaller incident angles produced greater modification of the reflected light quality. At larger angles, interactions with the internode were reduced. As a parallel investigation, the extension growth rate of light-grown Sinapis alba L. seedlings was monitored continuously using sensitive linear displacement transducers. When the angle of illumination by fibre-optic probes, presenting far-red light to the growing internode, was changed from 0° to 45°, an increase was observed in stem extension rate. There is a possibility that this rate increase was the result of a fall in the red:far-red ratio (R:FR) present inside the plant internode as the angle of incidence was altered. However, it is more likely that it was due to the larger surface area of stem being illuminated. The consequences of these observations are discussed in relation to the potential influence of such modified reflections on canopy light environments and resultant shade responses which may occur when light of known R:FR impinges on plant internodes at angles other than 0°. The possibility is discussed that plants may perceive the quality of reflected radiation from neighbouring plants to be substantially different dependent upon the angle at which it is reflected.