6.5.4 - Suboptimal environments

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.

Fig6.22.png

Figure 6.22. Low nitrogen (supplied as nitrate) reduces RGR in both fast-growing and slow-growing grass species. Photosynthesis and respiration (mass basis) also decrease, but the percentage of daily fixed carbon that is lost via respiration is higher on low nitrogen due to a greater investment of photoassimilate in roots. Photosynthetic CO₂ gain is expressed as net photosynthesis plus shoot respiration (assuming shoots respire in daytime at the same rate as that measured in darkness). Values for CO₂ exchange per unit plant mass were calculated from whole-plant measurements and proportions of plant biomass allocated to shoots and root, respectively. Based on Poorter et al. (1995) Plant Soil 171, 217-227

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.