data suggest that acclimation in the field may be associated with the onset of cold-induced photo-inhibition. We conclude that cold-acclimation of dark respiration in snow gum leaves is characterized by changes in both the temperature sensitivity and apparent 'capacity' of the respiratory apparatus, and that such changes will have an important impact on the carbon economy of snow gum plants.Key-words: Eucalyptus; diurnal variation; leaves; Q 10 ; respiration; seasonal variation; temperature. INTRODUCTIONLeaf respiration represents a major source of CO 2 release in plants. For example, up to 35% of the CO 2 fixed by photosynthesis each day is released back into the atmosphere by leaf respiration at night, in plants grown under controlled-environment, constant-temperature conditions ( Van der Werf, Poorter & Lambers 1994;Atkin & Lambers 1998). However, the extent of daily leaf respiratory CO 2 release may differ under natural conditions where air temperatures vary diurnally and seasonally, as respiration is very sensitive to short-term changes in temperature (Körner & Larcher 1988). Understanding the effect of variations in temperature on respiratory CO 2 loss is therefore a prerequisite for predicting plant growth in a changing global environment.The degree to which leaf respiration changes with temperature is highly variable, with Q 10 values (i.e. the proportional increase in respiration for each 10°C rise) being as low as 1·4 and as high as 4·0 (Azcón-Bieto 1992). Q 10 values differ between species (e.g. Larigauderie & Körner 1995) and are influenced by the metabolic state of the tissue and the growth environment. For example, Q 10 values are higher in plants with high concentrations of soluble carbohydrates (Wager 1941;Breeze & Elston 1978;Berry & Raison 1981; Azcry & Raison 1983) and are often higher in winter than in summer (Breeze & Elston 1978;Sowell & Spomer 1986;Hagihar & Hozumi 1991;Criddle et al. 1994;Stockfors & Linder 1998). Determining the extent of daily leaf respiratory CO 2 release under natural conditions ABSTRACTWe investigated the relationship between daily and seasonal temperature variation and dark respiratory CO 2 release by leaves of snow gum (Eucalyptus pauciflora Sieb. ex Spreng) that were grown in their natural habitat or under controlled-environment conditions. The open grassland field site in SE Australia was characterized by large seasonal and diurnal changes in air temperature. On each measurement day, leaf respiration rates in darkness were measured in situ at 2-3 h intervals over a 24 h period, with measurements being conducted at the ambient leaf temperature. The rate of respiration at a set measuring temperature (i.e. apparent 'respiratory capacity') was greater in seedlings grown under low average daily temperatures (i.e. acclimation 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 much of the year. However...
Growth under elevated [CO 2 ] promoted spring frost damage in field grown seedlings of snow gum (Eucalyptus pauciflora Sieb. ex Spreng.), one of the most frost tolerant of eucalypts. Freezing began in the leaf midvein, consistent with it being a major site of frost damage under field conditions. The average ice nucleation temperature was higher in leaves grown under elevated [CO 2 ] (-5·7°C versus -4·3°C), consistent with the greater incidence of frost damage in these leaves (34% versus 68% of leaves damaged). These results have major implications for agriculture, forestry and vegetation dynamics, as an increase in frost susceptibility may reduce potential gains in productivity from CO 2 fertilization and may affect predictions of vegetation change based on increasing temperature. [CO 2 ]; freeze-induced damage; global climate change; ice nucleation; open-top chambers. Key-words: Eucalyptus pauciflora; elevated atmospheric INTRODUCTIONAtmospheric CO 2 concentrations have been increasing since the industrial revolution, and are predicted to reach twice present levels late next century (Boden et al. 1994). It is thought that temperature will increase as [CO 2 ] rises, and that warming may be greater in winter than in summer (Plummer, Lin & Torok 1996). This could lengthen the growing season if evergreen species were able to take advantage of warm conditions earlier in spring when below-ground resources are relatively abundant. However, predictions of plant responses to global climate and atmospheric change are complicated by weather variability (Katz & Brown 1992) and the extent to which elevated [CO 2 ] might affect plant responses to temperature (Long 1991), particularly low temperatures. Some studies have predicted that frost damage may increase for trees that break dormancy too early in spring (Cannell & Smith 1986;Repo, Hänninen & Kellomäki 1996) Here we report the results of a serendipitous experiment. The experiment naturally occurred during a larger fieldbased study aimed at understanding the effects of elevated [CO 2 ] on the interactions between grass and trees during spring. The results were unexpected and showed that one of the most frost-hardy of broadleaved, evergreen species suffered greater frost damage when grown under elevated than ambient [CO 2 ]. MATERIALS AND METHODS Plant material and growth conditionsSeeds of Eucalyptus pauciflora Sieb. ex Spreng. were collected from three trees growing along the floor of the Orroral Valley at an elevation of 850 m in New South Wales, Australia. The seeds were cold stratified under moist conditions at 3°C for 4 weeks before germinating on sand flats in a mist house. Seedlings of similar size were transferred to individual containers (5 cm diameter, 25 cm deep) and grown out of doors for 6 months before the start of the experiment.In a pasture near Bungendore in southeastern Australia (35°15' S, 149°27' E; elevation 700 m), 10 open-top chambers were installed as five replicate pairs, flushed with air containing either ambient or elevated CO 2 conce...
Juvenile Eucalyptus polyanthemos Schau. which had been established in an open pasture, were surrounded by individual shelters made of different materials: chicken wire, white translucent plastic, and three types of shade cloth transmitting 30, 50 or 70% of incident sunlight. Air temperatures within white plastic shelters were up to 6°C above ambient air temperatures, whereas air temperatures in other shelters differed little from ambient. Irradiance was the main factor which varied between shelters. Leaves were photoinhibited over winter as shown by depression in pre-dawn Fv/Fm. The extent of that decrease in Fv/Fm was directly proportional to irradiance. Pre-dawn Fv/Fm recovered in all treatments during spring. Growth, as measured by stem elongation. occurred to a limited extent during winter, but primarily during spring. During both seasons, stem elongation was greatest in plants sheltered by 50% shadecloth. However, treatment-induced variation in stem elongation during spring was correlated with pre-dawn values for Fv/Fm measured during the previous winter. Reducing sunlight by 50% thus appeared to effect the best compromise between conflicting demands for protection from cold-induced photoinhibition during winter and for absorption of sufficient light for growth during spring.
Respiration from vegetation is a substantial part of the global carbon cycle and the responses of plant respiration to daily and seasonal fluctuations in temperature and light must be incorporated in models of terrestrial respiration to accurately predict these CO2 fluxes. We investigated how leaf respiration (R) responded to changes in leaf temperature (T(leaf)) and irradiance in field-grown saplings of an evergreen tree (Eucalyptus pauciflora Sieb. ex Spreng). Seasonal shifts in the thermal sensitivity of leaf R in the dark (R(dark)) and in the light (R(light)) were assessed by allowing T(leaf) to vary over the day in field-grown leaves over a year. The Q10 of R (i.e., the relative increase in R for a 10 °C increase in T(leaf)) was similar for R(light) and R(dark) and had a value of ∼ 2.5; there was little seasonal change in the Q10 of either R(light) or R(dark), indicating that we may be able to use similar functions to model short-term temperature responses of R in the dark and in the light. Overall, rates of R(light) were lower than those of R(dark), and the ratio of R(light)/R(dark) tended to increase with rising T(leaf), such that light suppression of R was reduced at high T(leaf) values, in contrast to earlier work with this species. Our results suggest we cannot assume that R(light)/R(dark) decreases with increasing T(leaf) on daily timescales, and highlights the need for a better mechanistic understanding of what regulates light suppression of R in leaves.
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