6.4.2  Light use efficiency

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Again by analogy with growth analysis of single plants where LAR denotes ‘leafiness’ of individuals, LAI represents community leafiness and helps define light profiles within crop communities (cf. Section 12.3). Monsi and Saeki (1953) are credited with formalising an expression analogous to Beer’s law and based on LAI for attenuation of light with depth in crop canopies, namely

where I0 is irradiance above a canopy and I is irradiance beneath a canopy of LAI = L. The extinction coefficient k ranges between about 0.2 and 1.8 according to size, pose and light absorption by individual leaves (larger values for big thick horizontal leaves and smaller values for small thin pendulant leaves).

Notwithstanding wide variation in canopy architecture, Equation 6.23 provides a robust model for canopy light climate and accordingly CGR can now be expressed in functional terms where


Figure 6.26 Crop growth rate (g dry matter m-2 d-1) is linearly related to irradiance absorbed (MJ m-2 d-1) for a wide range of crop communities. Efficiency of light utilisation (epsilon, g MJ-1) is represented by the slope of that relationship and is equivalent to 3 g MJ-1 (or 5%) in this example (Based on Evans 1993)



Table 6.12

The terms in brackets (Equation 6.24) summarise light absorption whereas ε represents the efficiency with which absorbed light is utilised for dry matter production. ε is inferred from the slope of a relationship showing CGR as a function of absorbed light such as that in Figure 6.26. In that particular case, irradiance was used with about 5% efficiency in generating 3 g dry matter per MJ absorbed (i.e. ε = 3 g MJ–1).

LAI (L in Equation 6.24) and the extinction coefficient (k in Equation 6.24) will both vary according to leaf attributes, planting density and subsequent canopy development. Similarly, ε will vary according to mode of photosynthesis, nutrient supply and state of development. Typical values (Table 6.12) range from 4.15 in rice (C3) or 3.40 in maize (C4) down to 1.63 in clover and 1.29 (g MJ–1) in soybean. High efficiency in rice and maize relate to inherently fast photosynthesis in well-nourished crops whereas an apparently low efficiency in clover and soybean reflect the carbon cost of biological nitrogen fixation and generally slower photosynthesis (area basis) in those species.


Figure 6.27 Crop growth rate (g dry matter m-2 d-1) is a function of LAI (ratio of canopy area to ground area) where slope and asymptote vary according to light-conversion efficiency and canopy architecture (Based on Evans 1993)


Crop growth data compiled from a number of sources (Figure 6.27) reveal LAI as a key driving variable, and especially prior to canopy closure where better illumination of individual plants is compounded by vigorous early growth and development. Radiation climate, canopy architecture and light use efficiency would all contribute to these species differences, but in broad terms low values for CGR in cassava and oil palm reflect annual averages and would increase somewhat if leaf litter had been included in above-ground biomass. Even so, perennial plants such as oil palm commonly photosynthesise more slowly than annual crop plants (leaf area basis) and thus achieve rather lower CGR. By contrast, C4 photosynthesis in maize and sorghum obviously confers an advantage on these two species where an inherently high capacity for CO2 assimilation is coupled with higher rates of leaf emergence and expansion plus more effective export of photoassimilate from source leaves. Net efficiency of light-energy conversion to biomass in this particular high-performance maize crop was around 8%, and somewhat higher than data cited in Table 6.12 with ε = 3.40 g MJ–1, representing a light-energy conversion efficiency of 5.7%.