12.3.3 Light use efficiency

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Sunlight intercepted is not utilised with similar efficiency by different crops (Section 6.4). There are clear differences in light use efficiency between crop species, particularly between those with C3 and those with C4 photosynthetic pathways. The photosynthetic advantage of C4 species at a leaf level (Section 2.1) is evident here at a canopy level where efficiency is higher by 30–100%. Expressed in terms of dry mass formed (g) per unit of photosynthetically active energy absorbed (MJ), the efficiency of sorghum and maize (C4) in Figure 12.20 was 1.32g MJ–1, while that of rice (C3) was only 0.93g MJ–1.

Canopy structure, and particularly the spatial distribution of leaf angles, has an important bearing on canopy light climate and energy conversion. Large leaf angles, with leaves close to vertical, ensure good PAR penetration when solar angle is high, and a high proportion of leaves receive similar photon irradiance. An even distribution of PAR at leaf surfaces is advantageous for canopy photosynthesis and improves light use efficiency over canopies where upper horizontal leaves intercept most solar radiation and lower leaves experience greatly attenuated levels. Small and erect leaves, particularly in top canopy layers, are thus a key feature of an ideal plant type, or ‘ideotype’ for high-density cropping.

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Figure 12.22 A hypothetical wheat ideotype with features presumed conducive to high grain yield as a crop community

(Based on Donald 1968)

One early wheat ideotype (Donald 1968) embodied several plant characteristics considered to be associated with a high productivity (Figure 12.22). These features included small erect leaves, a semi-dwarf stature, and a large ear on a single stem (i.e. few tillers). Setter et al. (1995, 1996) extended such concepts to rice, proposing a new plant type aimed at better interception of sunlight during grain filling by lower-ing panicle height relative to upper foliage. Topmost leaves actually protrude beyond the panicles. Breeding lines carrying lower panicles returned 15–40% greater yields compared with isogenic lines where high panicles shaded upper foliage.

Canopy radiation climate is especially complex in mixed crops and pastures where species with contrasting forms grow together. In grass–legume pastures, grass is generally taller than the legume component and is better placed to intercept incident radiation. Legumes then exist in permanent shade. Height is therefore an important determinant of light inter-ception within a mixed sward, and thus species composition. In such mixed swards, management options such as nitrogen fertiliser application, grazing time or cutting frequency all affect the relative height and hence radiation interception by component species. High-nitrogen fertiliser tends to favour grass, while clover may become dominant under nitrogen-limiting conditions.

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Figure 12.23 Photon irradiance declines with depth (cumulative LAI) in any plant communify. That rate of decline is accentuated by a preponderance of horizontal leaves. At moderate LAI, a clover-rich sward (no added nitrogen) will show stronger attenuation than a grass-rich sward (nitrogen added)

(Based on Stern and Donald 1962)

Light profiles within a pasture are therefore affected by LAI profiles of component species (Figure 12.23), and a clover-rich sward with more horizontal leaves (no added nitrogen) shows stronger attenuation of sunlight than a grass-rich sward with a preponderance of vertical leaves (nitrogen added). In common with monocultures, pasture productivity is enhanced by a species balance that ensures even distribution of sunlight within a mixed community.

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