Root respiration of fast- and slow-growing plants, as dependent on genotype and nitrogen supply: A major clue to the functioning of slow-growing plants

In fast-growing herbaceous plants, grown with free access to nitrate, root respiration consumes at least 8% of the total amount of photosynthates produced per day. In herbs that are slower-growing, however, due to either inherent or environmental constraints, this percentage increases to as much as 50%. Slow-growing mutants of Solanum lycopersicum, with low rates of stem and leaf expansion due to low levels of gibberellin, also use relatively more carbon in root respiration. This suggests that slow growth is the cause, rather than the consequence of a large consumption of assimilates in root respiration. The large fraction of carbon required for root respiration in slow-growing herbs and gibberellin-deficient tomato mutants is only partly accounted for by their proportionately larger maintenance respiration. We have investigated if variation in respiratory efficiency, associated with the relative contribution of the alternative respiratory path, offers an additional explanation for the relatively high rates of root respiration of slow-growing herbaceous plants. We also assessed respiratory costs for growth, maintenance and ion transport (including net nitrate uptake) for fast- and slow-growing herbaceous plants and slow-growing woody plants. We conclude that the rather large fraction of carbon required for root respiration in slow-growing herbs and gibberellin-deficient tomato mutants when grown with free access to nitrate is partly due to higher specific respiratory costs associated with ion transport. These higher specific costs are partly accounted for by a high nitrate efflux expressed per net mole of nitrate taken up. When grown with free access to all nutrients, the carbon costs associated with nitrate efflux that need to be compensated for by active influx of nitrate in slow-growing herbs are estimated at 4.5% of the daily assimilated carbon. When the slow-growing woody species Quercus faginea and Q, suber are grown with free access to nitrogen, they use as much as 25% of their daily assimilated carbon in root respiration. This large fraction is accounted for by the proportionately large maintenance respiration. There is no evidence for higher costs of ion uptake than those estimated for fast-growing herbaceous plants. We estimated that fast- and slow-growing herbs, when grown with free access to nutrients, use I to 3% and 3 to 8%, respectively, of their daily assimilated carbon in root respiration via the alternative respiratory pathway. Contrary to the investigated herbaceous plants, the roots of the woody Q. suber show very little capacity and activity of the alternative respiratory pathway. This is not a typical trait of woody species, however. When herbaceous plants are grown at a limiting nitrate supply, the percentage of carbon that is used in root respiration increases, irrespective of the potential relative growth rate of the species. It is likely that the percentage of carbon used in alternative respiration under these conditions increases in a similar manner. The consequences of the observed patterns of respiration associated with the alternative path and with ion uptake for the functioning of slow-growing herbaceous and woody plants are discussed.