PARENT SESSION
Oral Session 10: Physiology I: Temperature, Light, and Growth.
Presiding: C Knight
Monday, August 2, 8:00 AM to 11:30 AM, Meeting Room C 123.

Temperature sensitivity of leaf respiration: Four time scales.

BRUHN, D^{*,1, 2}, EDWARDS, E^{1, 2}, SCHORTEMEYER, M^{1, 2}, EGERTON, J^{1, 2}, HOCART, C², EVANS, J^{1, 2}, BALL, M^{1, 2}, ¹ Cooperative Research Centre for Greenhouse Accounting, Canberra, ACT, Australia² Research School of Biological Scences, Canberra, ACT, Australia

ABSTRACT- As plant respiration rates (R) are sensitive to temperature, global warming is expected to increase R and thereby exert a positive feedback on atmospheric CO₂ concentrations. Thus, understanding how R responds to variation in temperature over diurnal, seasonal and yearly time scales is fundamental to prediction of carbon budgets in future, warmer climates. We therefore studied temperature sensitivity of leaf R in a sub-alpine evergreen tree (Eucalyptus pauciflora) and a sub-Antarctic perennial herb (Pringlea antiscorbutica) at 4 time scales under field conditions. A potential Q₁₀ was measured by instantaneous responses to manipulated temperatures at intervals during a 24 hour period. This temperature sensitivity was compared with concurrent measurements of apparent Q₁₀, i.e., change in R with natural change in temperature during 24 hours. We have calculated degree of direct temperature control (DDTC) of R as (potential Q₁₀ - 1) / (apparent Q₁₀ - 1). DDTC varied diurnally and was as low as 20% at night-time. Consequently, models assuming constant temperature sensitivity can over or under estimate R by as much as 25% over diurnal periods. An intra-annual Q₁₀ can be obtained from R measured after sunset at ambient temperatures throughout a year. Intra-annual Q₁₀ was higher during autumn/winter than spring/summer, indicating seasonal acclimation. Simulation of climate warming can be done in the field with a FATI (Free Air Temperature Increase) system, resulting in different temperature regimes. R measured at the same time across FATI treatments that raise minimum temperatures by 1, 2 and 3^oC can thus simulate how inter-annual Q₁₀ might vary with progressively increasing minimum temperatures in the future. Because of acclimation, R will be less under future, warmer climates than predicted from current Q₁₀ values measured over short time scales. Temperature sensitivity of R differed between the 4 time scales, with inter-annual Q₁₀ ≤ potential Q₁₀ ≤ intra-annual Q₁₀ ≤ apparent Q₁₀.

Key words: Temperature, Model, Respiration, Leaf