6.4.3  Potential crop growth rate

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Genetic factors dictate potential yield, which in turn is set for every genotype by the intrinsic efficiency of light-energy conversion and net generation of photoassimilate. In well-nourished crops, yield is ultimately limited by community use of light energy. Such utilisation can be represented at successive levels of organisation (cf. Warren Wilson 1969) as follows. Take an annual irradiance of 3.30 × 103 MJ m–2 year–1 as representative of mid-latitudes (10–30°). Consider a perennial tropical crop that maintains a complete canopy for 90% of each year, so that light energy available to that crop will be 0.9 × 3.30 = 2.97 × 103 MJ m–2 year–1. Taking LAI = 5 with an extinction coefficient k of 0.46 (recall Equation 6.24), intercepted energy will be 0.9 × 2.97 = 2.67 × 103 MJ m–2 year–1. Taking an efficiency of light-energy conversion to dry matter (e) of 4.15 g MJ–1 (recall rice in Table 6.12), dry matter production should be 4.15 × 2.67 × 103 or about 11 000 g m–2 year–1.

Compare that estimate with observed values for both natural and managed ecosystems (Table 6.13) where total dry matter production per year ranges from 8000 g m–2 year–1 in perennial tropical crops down to 1500 g m–2 year–1 in temperate deciduous forests. Soil–plant–atmosphere water relations, nutrient supply, canopy light climate and duration of growing season will all contribute inter alia to variation in Table 6.13, but limitations imposed by light-energy conversion efficiency will be common to all. Photosynthetic energy transduction has an absolute requirement for 8–12 quanta per molecule of CO2 fixed, but this photochemical restriction is compounded to a varying extent by CO2 diffusion limitations. Some scope thus exists for improving dry matter production via leaf physiology, and in greenhouse crops via CO2 enrichment. Greenhouse microclimate is conducive to year-round production, with annual productivity commonly two to three times higher in greenhouse than in field, and even further enhanced under elevated CO2. For example, Warren Wilson et al. (1992) compared ambient with CO2-enriched greenhouse crops, and showed that mean efficiency of light utilisation (net photosynthesis per unit intercepted light) for a number of crop species increased from 8.06 to 10.90 µg CO2 J–1. By contrast, well-managed field crops returned on average only 7.10 µg CO2 J–1. Duration of cropping season would amplify these greenhouse–field differences even further in terms of annual productivity.