6.4.4  Respiratory losses

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Notwithstanding genetic differences in component processes of photosynthesis, net efficiency of light-energy conversion to biomass will impose a ceiling on CGR. Respiratory losses will feature in that overall net efficiency and must be included in any process-based model of crop growth. Taking well-documented cases of canopy light climate and combining those profiles with light response curves for photosynthesis by single leaves, early modellers further assumed that respiratory loss would also be proportional to LAI and predicted an optimum LAI for different crop types.

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Figure 6.28 Community gas exchange by cotton plants in a growth cabinet (duplicate measurements at 20 °C; low (200), medium (350) and high (550) photon irrandiance (µmol m-2 s-1) plus dark respiration as indicated) shows an asyptotic relationship to LAI (ratio of canopy area to ground area) with maximum net assimilation reached around LAI = 3.5. Additional measurements at higher temperatures (30 °C and 40 °C) amplified differences due to photon irradiance and showed some reduction in net photosynthesis at high LAI. Respiration at zero LAI represents CO2 efflux from stems and roots (Based on Ludwig et al. 1965)

 

Experience showed otherwise (Figure 6.27) with CGR increasing asymptotically with LAI for a wide range of crop species rather than showing an optimum. Why is there this discrepancy between theory and practice? In a classic case where model making was no substitute for experimentation but did suggest what experiment had to be done, flawed estimates of respiration proved responsible.

Using a highly novel approach to this issue, Ludwig et al. (1965) started with the intact canopy of an artificial community of cotton plants and varied LAI by removing successive layers of foliage from bottom to top. They demonstrated that respiratory losses from lower (shaded) leaves in artificial cotton communities downregulated in proportion to light attenuation. Lower leaves, both older and more shaded than their better-exposed counterparts towards the top of a canopy, thus impose a smaller respiratory load than would be predicted by LAI alone. Consequently, daytime CO2 assimilation (net photosynthesis) and night-time respiratory loss by an entire community both show an asymptotic relationship with increased LAI (Figure 6.28). King and Evans (1967) subsequently confirmed this same relationship for artificial communities of wheat, lucerne and subterranean clover where community net photosynthesis approached a maximum at LAI values of about 8, 9 and 5 respectively.

By implication, there is no clear optimum LAI for CGR either, although harvest index (shoot HI in Section 6.3.3) can decrease in dense plantings (high LAI) due to restrictions on reproductive development by individual plants. Grain yield per unit area of land can thus show an optimum LAI even though CGR tends to an asymptote.

Respiratory costs associated with plant growth and re-productive development are thus crucial to both biomass accumulation and yield outcomes, representing a surprisingly large fraction of carbon fixed by leaf assimilation and especially under suboptimal growing conditions. Genetic differences in respiratory efficiency thus interact with environmental con-ditions in determining growth and reproductive success in nature as well as the comparative performance of crop plants. Underlying processes responsible for such differences in pro-duction and utilisation of respiratory energy are discussed in Section 6.5.

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