6.5  Respiratory efficiency and plant growth

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Table 6.14

Production of photoassimilate depends upon capture of light energy but subsequent use by plants necessitates expenditure of metabolic energy. Fixed carbon meets this need, so that costs associated with growth and maintenance of vascular plants can be represented as biomass equivalents. Generalised values for such dry matter utilisation during growth and development (Table 6.14) show that respiratory demand is substantial. According to these estimates, a germinating seedling with starting biomass of 1 g would in one day gain a further 0.2 g in structural growth plus 0.05 g in storage, resulting in an RGR of 0.25 g g–1 d–1. However, respiratory losses supporting that strong RGR would have been equivalent to 0.10 g g–1 d–1. Using similar logic, the young vegetative plant in Table 6.14 has achieved an RGR of 0.2 g g–1 d–1 at a respiratory cost equivalent to 0.08 g g–1 d–1, and in a mature plant with storage organs that are importing photoassimilate, RGR has fallen to 0.15 g g–1 d–1 with a respiratory cost equivalent to 0.04 g g–1 d–1.

During such growth and development (Table 6.14) a downward drift in RGR has been accompanied by a similar fall in whole-plant respiration, although component costs have changed. Structural growth decreased whereas storage increased. Overall, respiration accounts for a significant fraction of photoassimilate. Commonly one-third and under stressful conditions as much as two-thirds of a plant’s daily fixed CO2 can be respired during the same period (van der Werf et al. 1994).

Processes supporting a net gain in new biomass (dW, g) per unit time (dt, d) can be represented as:



where A is daily carbon assimilation and R is whole-plant respiratory loss, so that net gain per unit existing plant biomass per unit time (or RGR, g g–1 d–1) becomes



If A and R are expressed as mmol carbon g–1 dry matter per day, then Equation 6.26 becomes



where Cwp is plant carbon concentration in mmol C (g dry matter)–1.

A and R can be determined from direct measurement of whole-plant gas exchange, and the example below uses a value of 34.8 mmol C (g plant)–1. Whole-plant RGR can now be linked to gas exchange data for shoot assimilation (A), shoot respiration (Rshoot) and root respiration (Rroot) according to the expression