Journal of Experimental Botany, Vol. 52, No. 357, pp. 829-838,
April 15, 2001
© 2001 Oxford University Press
Original Papers |
The response of the high altitude C4 grass Muhlenbergia montana (Nutt.) A.S. Hitchc. to long- and short-term chilling
Department of Botany, University of Toronto, 25 Willcocks St., Toronto, Ontario, Canada M5S3B2
The acclimation of C4 photosynthesis to low temperature was studied in the montane grass Muhlenbergia montana in order to evaluate inherent limitations in the C4 photosynthetic pathway following chilling. Plants were grown in growth cabinets at 26 °C days, but at night temperatures of either 16 °C (the control treatment), 4 °C for at least 28 nights (the cold-acclimated treatment), or 1 night (the cold-stress treatment). Below a measurement temperature of 25 °C, little difference in the thermal response of the net CO2 assimilation rate (A) was observed between the control and cold-acclimated treatment. By contrast, above 30 °C, A in the cold-acclimated treatment was 10% greater than in the control treatment. The temperature responses of Rubisco activity and net CO2 assimilation rate were similar below 22 °C, indicating high metabolic control of Rubisco over the rate of photosynthesis at cool temperatures. Analysis of the response of A to intercellular CO2 level further supported a major limiting role for Rubisco below 20 °C. As temperature declined, the CO2 saturated plateau of A exhibited large reductions, while the initial slope of the CO2 response was little affected. This type of response is consistent with a Rubisco limitation, rather than limitations in PEP carboxylase capacity. Stomatal limitations at low temperature were not apparent because photosynthesis was CO2 saturated below 23 °C at air levels of CO2. In contrast to the response of photosynthesis to temperature and CO2 in plants acclimated for 4 weeks to low night temperature, plants exposed to 4 °C for one night showed substantial reduction in photosynthetic capacity at temperatures above 20 °C. Because these reductions were at both high and low CO2, enzymes associated with the C4 carbon cycle were implicated as the major mechanisms for the chilling inhibition. These results demonstrate that C4 plants from climates with low temperature during the growing season can fully acclimate to cold stress given sufficient time. This acclimation appears to involve reversal of injury to the C4 cycle following initial exposure to low temperature. By contrast, carbon gain at low temperatures generally appears to be constrained by the carboxylation capacity of Rubisco, regardless of acclimation time. The inability to overcome the Rubisco limitation at low temperature may be an inherent limitation restricting C4 photosynthetic performance in cooler climates.
Key words: Chilling, C4 plants, Muhlenbergia, photosynthesis, temperature response, Rubisco.
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