The Canopty Relationship of Pure and Mixed Populations of Barley (Hordeum vulgare L.), White Mustard (Sinapis alba L.) and Wild Oats (Avena fatua L.)

The capacity of plants to intercept light is largely a function of the leaf area and its spatial orientation. Watson (1947) introduced the concept of Leaf Area Index (LAI) as a measure of the leaf area overlying an area of land. Warren-Wilson (1961) showed that spatial arrangement of leaves determines efficiency of light interception of the canopy. William & Soong Nginkwi (1967) evaluated the relative effects of leaf angle and vertical distribution of leaves on light interception, and found that variation in leaf angles had the greater effect. The pattern of foliage display is characteristic for most genotypes, but this may be modified within limits by environmental conditions. Under crowded conditions (higher densities), Saeki (1961) found that there was a tendency for the whole foliage to be aggregated at the top, while in a stand with lower density, the distribution of the foliage was uniform over a wide vertical range. De Wit (1965) recognized four types of plant canopy on the basis of their angular distribution: (a) planophile canopies with horizontal leaves more frequent; (b) erectophile canopies with most leaves vertical; (c) plagiophile canopies with leaves mostly at some oblique inclination; and (d) extremophile canopies with leaves of oblique inclination less frequent. Verhagen, Wilson & Britten (1963) defined four idealized types of foliage on the basis of their light extinction properties: (a) the exponential foliage in which the extinction coefficient (K) remains constant throughout the canopy; (b) the standard exponential foliage in which the extinction coefficient (K) varies with the development of the foliage in such a way that the bottom leaves are always at the compensation point; (c) the best exponential foliage in which the K value changes with each value of LAI to give maximum production; and (d) the ideal foliage where light is distributed evenly on every photosynthetic leaf. In a mixed population, competition for light may be affected by the structural relationships of the species in the mixed canopy. A series of experiments was designed to determine how far the canopy characteristics of species are modified in mixed stands.