Nitrogen limitation decreases absolute rates of shoot and root respiration in both fast- and slow-growing species (Figure 6.22) but the decrease in gross photosynthesis is much greater. Thus, the percentage of daily fixed CO2 lost during respiration increases under nitrogen limitation. This mainly results from a greater allocation of photoassimilate to roots. Slower growth of whole plants on low nitrogen is therefore due to both slower photosynthesis due to less Rubisco coupled with more costly nitrogen acquisition.
The proportion of daily fixed CO2 that is respired may also increase under other stressful conditions such as drought, high temperature and ion toxicity. Challenged by such stresses, a greater proportion of respiratory energy is being used to support cellular maintenance in place of growth.
In conclusion, this chapter has shown that respiratory costs are high, for both formation of new tissues and maintenance of old ones. Plants profit from shedding old leaves and roots, where the costs of maintenance outweigh the benefits of their function. Future research into ways to minimise costs while maximising functions may produce more efficient plant forms. Quantitative growth analyses will be essential in developing new plants or improving management practises for higher yields in both optimal and suboptimal environments.