Plants vary in their intrinsic growth rate, and may under the same environment differ three-fold in the relative growth rate. Some species are inherently fast-growing, and some inherently slow-growing. This section investigates the reason for this, and in particular the energy efficiency of the various species.

Fast-growing species tend to have higher rates of photosynthesis, but also use respiratory energy more efficiently for maintenance, growth and ion uptake. Variations in efficiency of energy use reflect differences in the proportion of whole-plant respiration that is allocated to these three processes and/or the specific costs of each process.

Fast-growing species achieve a higher RGR under optimum conditions than do slow-growing species under similar conditions. Carbon loss via respiration is considerable with genetic differences in generation and utilisation of respiratory energy contributing to these differences in RGR. Fast-growing species achieve a higher RGR than slow-growing species because their net rate of CO₂ uptake per unit of shoot and whole-plant mass is greater (Figure 6.18). By definition, net carbon fixed per day must depend to some extent on the proportion of fixed CO₂ that is subsequently lost by respiration, so that differences in respiratory CO₂ loss have an important impact on net carbon gain, and can be linked quantitatively to RGR.

Fig6.18_edit.png

Figure 6.18. Daily carbon economy of plant species that differ with respect to inherent maximum RGR (g g^-1 d^-1). The fast-growing grass and fast growing herb both exhibit higher rates of gross photosynthetic CO₂ uptake per unit mass (i.e. net photosynthesis plus shoot dark respiration) than their slow-growing counterparts. Fast-growing species lose a smaller percentage of daily fixed carbon via respiration. Based on data in Atkin et al. (1996) Funct Ecol 10, 698-707 for slow-growing Australian alpine and fast-growing lowland Poa species, and Poorter et al. (1990) Plant Physiol 94, 621-627 for the slow-growing herb Pimpinella saxifrage versus the fast-growing herb Galinsoga parviflora).

Data shown in Figure 6.18 can be used to calculate RGR for each species from photosynthesis and respiration measurements if the plant’s carbon concentration is known.

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

\[\frac{\text{d}W}{\text{d}t}=A-R\tag{6.25}\]

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

\[\text{RGR} = \frac{1}{W}\frac{\text{d}W}{\text{d}t} = \frac{A}{W} - \frac{R}{W} \tag{6.26}\]

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

\[\text{RGR} = (A-R)/C_{wp} \tag{6.27}\]

where C_wp  is plant carbon concentration in mmol carbon g^–1 dry matter. R can be separated into R_shoot and R_root.

Whole-plant RGR can now be linked to gas exchange data for shoot assimilation (A), shoot respiration (R_shoot) and root respiration (R_root) according to the expression

\[\text{RGR} = (A-(R_{\text{shoot}} + R_{\text{root}})/C_{wp} \tag{6.28}\]

A and R can be determined from direct measurement of whole-plant gas exchange. The example below uses a value for C_wp of 34.8 mmol C g^–1 dry matter.

Taking the fast-growing grass in Figure 6.18, where A is 11.1 mmol C g^-1 plant d^-1, and R for shoots and roots is 1.75 and 1.68 mmol C g^-1 d^-1 , then RGR = 0.22 g g^–1 d^–1 .

This prediction of 0.22 d^–1 for RGR represents an instantaneous value derived from whole-plant gas exchange measurements, whereas 0.255 d^–1 in Figure 6.18 represents an average RGR from growth analysis over several days. Gas exchange values are generally within 10% of RGR values from sequential harvests.

Herbs and grasses can differ in the degree to which respiratory losses account for differences in RGR. Considering grasses (Figure 6.18, left side), 56% of daily fixed CO₂ is lost by respiration in the slow-growing alpine species whereas only 30% of daily fixed CO₂ is respired by the fast-growing lowland grass species. Over half of the carbon loss is attributable to roots in both species and, overall, respiration rate per unit plant mass is slightly higher in the slow-growing grass species.

Herbs in Figure 6.18 (right side) differ from grasses because the fast-growing herb respires faster than the slow-growing herb (on a mass basis) so that differences in percentage loss of carbon between these species cannot be due to differences in respiration rates per se. Significantly, however, the fast-growing herb still loses a smaller percentage of daily fixed carbon due to whole-plant respiration because daily CO₂ assimilation (mass basis) is especially high. A notably higher SLA in this fast-growing herb contributes to faster photo-synthesis on a mass basis (Figure 6.18).

A lower percentage loss of daily fixed carbon due to respiration in fast-growing grasses and fast-growing herbs does imply that carbon metabolism is more effective in these species than in their slow-growing counterparts, and serves as a model for generalisations. Such fast-growing plants may be more efficient in how they generate and/or use respiratory energy.

It is likely that an inherent capacity for fast growth confers a selective advantage for plants in favourable environments such as warm moist lowlands, but would be selectively neutral in restrictive environments such as nutritionally poor sites or alpine regions.