13.6 Concluding remarks

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Atmospheric CO2 represents only a tiny fraction of global carbon reserves and yet variation in concentration over past millennia has had a profound impact on life forms and general biological activity of terrestrial ecosystems. Working as an environmental selection pressure, diminishing levels of atmospheric CO2 over the past 100 million years have led to one major adaptive response about 50 million years ago, namely C4 photosynthesis. In this form of cooperative photosynthesis, bundle sheath Rubisco is working near full capacity in contrast to C3 progenitors.

Despite wide variation in atmospheric CO2, and hence carbon fluxes through ecosystems, and notwithstanding a massive cost in terms of plant nitrogen resources, a Rubisco-based system for assimilation of atmospheric CO2 into biomass has remained highly conserved throughout the evolutionary history of vascular plants. Nature has simply not found a better way of achieving CO2 fixation.

Nevertheless, important variation does exist with respect to photosynthetic acclimation and whole-plant responses, a case where growth and developmental responses to CO2 enrichment elicit photosynthetic adjustment, that is, a ‘sink pull’ rather than a ‘source push’ for whole-plant carbon metabolism. Mechanisms underlying such controls over photosynthesis are not yet understood, and yet empirical selection for genetic variants with greater capacity to utilise elevated CO2 is already underway for energy-intensive greenhouse cropping. While global rise in CO2 concentration may eventually negate a need for CO2 enrichment in well-ventilated protected cropping situations, there will continue to be a clear need in many situations such as, for example, where winter production requires tightly sealed enclosures to conserve heat and CO2 depletion becomes a serious limitation to yield. There is scope for breeding and selecting cultivars specifically adapted for optimal performance under CO2 enrichment given large cultivar differences in CO2 responsiveness. In terms of plant science, CO2 effects on devel-opmental processes which ultimately control form, function and overall plant performance need to be clarified.

Selection pressure of a more subtle but equally pervasive nature is also underway in natural ecosystems. Here, comparative abundance of present-day species will be likely to change in accordance with genotype variation in capacity to accommodate global change. As noted above, a range of environmental factors interact with atmospheric CO2 in determining outcomes, but an inexorable rise in CO2 concentration over coming decades (?centuries) will be one common denominator.