6.5.5  Energy use by roots

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Figure 6.34 Root respiration is largely devoted to ion uptake and maintenance in slow-growing species (left-side) compared with a predominant allocation to growth in fast-growing species (right-side). (Generalised values comparable to Figure 6.29) (Based on Poorter et al. 1991)

Most respiratory energy is allocated to nutrient acquisition in both fast- and slow-growing species (Figure 6.34) and this proportion increases even further under suboptimal conditions as maintenance costs rise. However, fast-growing species are distinguished by allocating less respiratory energy to nutrient acquisition, and more to growth. Presumably, a lower allocation to ion uptake in fast-growing species arises from lower specific costs. Loss of absorbed nutrients could also be lower in fast-growing species, while cotransport of protons and anions into roots might conserve energy. Maintenance costs also appear to be slightly lower in fast-growing plants (Figure 6.34) but any difference between these two plant categories in allocation to maintenance processes is small and is unlikely to matter overall. Nevertheless, dif-ferences in maintenance respiration will become more important when a plant is exposed to unfavourable conditions which invariably increase allocation of respiratory energy to fine-root turnover and maintenance of those structures.