The response of net photosynthesis per unit leaf area to growth and measurement temperature was measured for Eucalyptus species from diverse natural habitats in Australia and the results compared to Nerium oleander, a species known to exhibit a large degree of temperature acclimation. Eucalyptus species did not show the same degree of temperature acclimation observed for N. oleander in this and previous studies. Photosynthetic acclimation to temperature varied with species of Eucalyptus, apparently in relation to their subgeneric groupings. The estimated parameter, the preferred temperature for photosynthesis, correlated broadly with temperature conditions in the natural habitat. Estimates of two biochemical parameters, Rubisco (estimated from the initial slope of the A/pI curve) and RuP2 regeneration capacity (calculated from the A/pI curve at high CO2) were found to parallel changes in assimilation rate. High temperature acclimation in the Eucalyptus species differed from that of N. oleander. Low temperature grown species, except E. miniata, had a higher temperature threshold for reversible inactivation of Rubisco. In E. miniata, Rubisco, but not regeneration capacity, was sensitive to high temperatures in low temperature grown plants. High growth temperatures appeared to affect the thermolability of the photosynthetic components of the two E. pauciflora provenances. Low temperature acclimation was similar for N. oleander and most of the Eucalyptus species and was accompanied by an increase in both biochemical parameters. Regeneration capacity was sensitive to low temperatures in high temperature grown E. miniata.
The photosynthetic responses of three altitudinal populations of snow gum, E. pauciflora Sieb. ex Spreng., were examined on material grown at a range of day/night temperatures from 8/4 to 33/28°C. The pattern of the photosynthetic responses to growth temperature was generally similar for all populations but the material from the lowest-elevation, warmest, site showed the highest temperature optimum and significantly higher rates of net photosynthesis at the highest growth temperature. In a corresponding way, the material from the highest-elevation, coldest, site showed the lowest temperature optimum, and significantly higher rates of net photosynthesis at the lowest growth temperature. This pattern, also reflected in the responses of rI, the intracellular resistance, and rI, the gas-phase resistance, supported the view that E. pauciflora shows continuous variation in physiological responses through its altitudinal range. The peak values of net photosynthesis were high for all populations, but were greatest, 81 ng cm-2 s-1, in the lowest elevation material and decreased to 72 ng cm-2 s-1 in the highest-elevation material. Corresponding values of rI ranged from 2.5 - 3.0 s cm-1, and for rI from 2.4 - 3.3 s cm-1. These levels compare favourably with levels reported for other woody species.
Established, field-grown, seedlings of Eucalyptus pauciflora were transferred from high- and low- elevation field sites to a controlled-environment greenhouse in Canberra (maximum/minimum daily temperature range 26/15°) and the pattern of photosynthetic acclimation observed. Levels of net photosynthesis, Pamb, intracellular resistance rI, and leaf gas-phase resistance to CO2 transfer (r1) were monitored, as were the temperature optima for these parameters. Acclimation proceeded most rapidly in the material grown at the warmer, low-elevation, site (955 m), and in the low-elevation population. Daily maximum/minimum temperatures at this site for the 10 days prior to transfer averaged 23/11°. With this material, levels of, and the temperature optimum for, Pamb reached control levels within 6 days of transfer from the field environment. By comparison, Pamb in the high-elevation population grown at the high-elevation (tree-line) site (1910 m) where the 10-day temperature averaged 15/7°, did not reach control levels until 14 days after transfer, and the temperature optimum for Pamb required 20 days to reach the control level. In general, the patterns of change in rI and r1 paralleled those in Pamb. Both the level of physiological activity in the field, and the temperature differences between the field and greenhouse environment, appeared to affect the rate of acclimation. Immediately after transfer from the field, the temperature optima of the high-and low-elevation populations were close to the daily maximum temperature of the respective field environments. The temperature optimum of the high-elevation material grown at the low-elevation site was intermediate in value. At the conclusion of the acclimation period, the temperature optima of both high-elevation populations had converged to a value similar to that of the high-elevation control (about 22°); similarly, the temperature optimum of the low-elevation populations had reached the level of the low-elevation control (27°) These various temperature optima are interpreted on the basis that each population has a 'preferred' temperature which can be modified by different effective growth temperatures to yield different optima in different thermal environments. In the field, the effective temperature appears to be intermediate between the prevailing maximum and minimum temperatures.
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