PARENT SESSION
Oral Session # 77: Plant Ecology VI: Temperature and Water Stress.
Presiding: C Ford
Thursday, August 7. 1:30 PM to 5:00 PM, SITCC Meeting Room 102.

A mechanistic basis for lower freeze tolerance under elevated [CO₂].

Loveys, Beth^{1, 2}, Egerton, John^{*,1, 2}, Pippen, Wayne^{1, 2}, Ball, Marilyn^{1, 2}, ¹ Australian National University, Canberra, ACT, Australia² CRC for Greenhouse Accounting, Canberra, ACT, Australia

ABSTRACT- Recent studies have shown that elevated [CO₂] can adversely affect freeze tolerance in evergreens such as snow gum (Eucalyptus pauciflora). The mechanism, however, is unknown. In snow gum, lower stomatal conductance under elevated [CO₂] results in an increase in leaf temperature of approximately 3 ^oC. Here we test whether such higher leaf temperatures during the photoperiod can influence the freeze tolerance of plants subject to the same minimum night temperatures. A 3.5 ^oC increase in daytime leaf temperature was simulated in the field using a Free Air Temperature Increase (FATI) system. Freeze tolerance in these seedlings grown with or without warming was compared with that of seedlings grown in adjacent open-topped chambers (OTC) flushed with air containing either 350 or 700 ppm [CO₂]. The experiment began at the start of autumn, prior to the onset of freezing temperatures. Freeze tolerance was assayed by change in electrolyte leakage and photosynthetic efficiency (Fv/Fm) of leaf disks as a function of minimum freezing temperature. Seven days after planting, freeze tolerance was lower in leaves of plants grown with either warming or elevated [CO₂]. However, 27 days after planting, there were no differences in freeze tolerance between treatments. Thus, the elevated leaf temperature caused either by lower stomatal conductance in response to elevated [CO₂] or by FATI warming increased the time required to achieve freeze tolerance in snow gum.

Key words: OTC, CO₂, FATI, freeze