Dark respiration is a major component of plant productivity. Approximately 50% of total assimilates acquired by photosynthesis are respired (Amthor, 1997). An unexpected feature of respiration to GEC is an apparent decrease in its rate when expressed per unit of dry matter. Such a decrease is often observed in plants grown in elevated [CO2] (Poorter et at., 1992). Part, or all, of this decreased respiration is probably due to the decreased proportion of metabolic components per unit dry matter as carbohydrates accumulate (Drake et at., 1997). However, this cannot explain why elevated [CO2] also decreases the respiration rate of young wheat seedlings growing in darkness (Natr et al., 1996).
While respiration in wheat responds markedly to temperature in the short term (Mitchell et al., 1991), the sensitivity is much less in the long term (Gifford, 1995). This suggests that division of respiration into growth and maintenance components, with the latter being highly temperature dependent, tends to exaggerate the temperature dependency. Assumption of a constant respiration : photosynthesis ratio regardless of temperature or [CO2] could be more accurate (Gifford, 1995).
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