6.5.1  Carbon economy of fast- versus slow-growing plants

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Figure 6.29 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 (values shown above each respiration bar) (Based on data in Atkin et al. (1996) for slow growing Australian alpine and fast growing lowland Poa species, and Poorter et al. (1990) for the slow-growing herb Pimpinella saxifraga versus the fast-growing herb Galinsoga parviflora)

An inherent capacity for fast growth confers a selective advantage on 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. Accordingly, fast-growing species achieve a higher RGR under optimum conditions than do slow-growing species under similar conditions. In either case, 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 6.29). 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. Data shown in Figure 6.29 can be used to calculate RGR for each species at the time of photo-synthesis and respiration measurements if the plant’s carbon concentration is known. As outlined above, RGR is related to photosynthesis, respiration and carbon content where

Taking the fast-growing grass in Figure 6.29, RGR = 0.22 g g–1 d–1 according to:

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 6.29 represents an average RGR from growth analysis over several days. Gas exchange values are generally within 10% of RGR values from sequential harvests.

Net carbon gain per day and hence NAR is clearly a consequence of daily photosynthesis minus whole-plant respiratory loss, but herbs and grasses differ in the degree to which respiratory losses account for differences in RGR. Considering grasses (Figure 6.29, 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 6.29 (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 dif-ferences 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 6.29).

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.