6.5.7  Suboptimal environments

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Figure 6.35 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 (values above respiration bars) 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)

Nitrogen limitation decreases absolute rates of shoot and root respiration in both fast- and slow-growing species (Figure 6.35) but the percentage of daily fixed CO2 lost during respiration increases. Such increase on low nitrogen results from a greater allocation of photoassimilate to roots which in turn show an intrinsically higher rate of respiration (mass basis). Slower growth of whole plants on low nitrogen is therefore due to slower photosynthesis (mass basis) coupled with more costly nitrogen acquisition.

The proportion of daily fixed carbon that is respired will also increase under other stressful conditions such as excess salt or aluminium. Challenged by such stresses, plants increase their demand for energy to exclude the toxic compound or repair damage, and grow more slowly. For example, wheat roots increase respiration rates and grow more slowly due to high concentrations of aluminium (Collier et al. 1993). According to this model, a greater proportion of respiratory energy is being used to support cellular maintenance in place of growth under stressful conditions.