Acclimation of snow gum (Eucalyptus pauciflora) leaf respiration to seasonal and diurnal variations in temperature: the importance of changes in the capacity and temperature sensitivity of respiration

We investigated the relationship between daily and seasonal temperature var iation and dark respiratory CO2 release by leaves of snow gum (Eucalyptus p auciflora Sieb. ex Spreng) that were grown in their natural habitat or unde r controlled-environment conditions. The open grassland field site in SE Au stralia was characterized by large seasonal and diurnal changes in air temp erature. On each measurement day, leaf respiration rates in darkness were m easured in situ at 2-3 h intervals over a 24 h period, with measurements be ing conducted at the ambient leaf temperature. The rate of respiration at a set measuring temperature (i.e. apparent 'respiratory capacity') was great er in seedlings grown under low average daily temperatures (i.e. acclimatio n occurred), both in the field and under controlled-environment conditions. The sensitivity of leaf respiration to diurnal changes in temperature (i.e . the Q(10) of leaf respiration) exhibited little seasonal variation over m uch of the year. However, Q(10) values were significantly greater on cold w inter days (i.e. when daily average and minimum air temperatures were below 6 degrees and -1 degrees C, respectively). These differences in Q(10) valu es were not due to bias arizing from the contrasting daily temperature ampl itudes in winter and summer, as the Q(10) of leaf respiration was constant over a wide temperature range in short-term experiments. Due to the higher Q(10) values in winter, there was less difference between winter and summer leaf respiration rates measured at 5 degrees C than at 25 degrees C. The n et result of these changes was that there was relatively little difference in total daily leaf respiratory CO2 release per unit leaf dry mass in winte r and summer. Under controlled-environment conditions, acclimation of respi ration to growth temperature occurred in as little as 1-3 d. Acclimation wa s associated with a change in the concentration of soluble sugars under con trolled conditions, but not in the field. Our data suggest that acclimation in the field may be associated with the onset of cold-induced photo-inhibi tion. We conclude that cold-acclimation of dark respiration in snow gum lea ves is characterized by changes in both the temperature sensitivity and app arent 'capacity' of the respiratory apparatus, and that such changes will h ave an important impact on the carbon economy of snow gum plants.