The response of the high altitude C-4 grass Muhlenbergia montana (Nutt.) AS Hitchc. to long- and short-term chilling

The acclimation of C-4 photosynthesis to low temperature was studied in the montane grass Muhlenbergia montana in order to evaluate inherent limitatio ns in the C-4 photosynthetic pathway following chilling. Plants were grown in growth cabinets at 26 degreesC days, but at night temperatures of either 16 degreesC (the control treatment), 4 degreesC for at least 28 nights (th e cold-acclimated treatment), or 1 night (the cold-stress treatment). Below a measurement temperature of 25 degreesC, little difference in the thermal response of the net CO2 assimilation rate (A) was observed between the con trol and cold-acclimated treatment. By contrast, above 30 degreesC, A in th e cold-acclimated treatment was 10% greater than in the control treatment. The temperature responses of Rubisco activity and net COP assimilation rate were similar below 22 degreesC, indicating high metabolic control of Rubis co over the rate of photosynthesis at cool temperatures. Analysis of the re sponse of A to intercellular CO2 level further supported a major limiting r ole for Rubisco below 20 degreesC. As temperature declined, the CO2 saturat ed 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 photosynt hesis was CO2 saturated below 23 degreesC at air levels of CO2. In contrast to the response of photosynthesis to temperature and COP in plants acclima ted for 4 weeks to low night temperature, plants exposed to 4 degreesC for one night showed substantial reduction in photosynthetic capacity at temper atures above 20 degreesC, Because these reductions were at both high and lo w CO2, enzymes associated with the C-4 carbon cycle were implicated as the major mechanisms for the chilling inhibition. These results demonstrate tha t C-4 plants from climates with low temperature during the growing season c an fully acclimate to cold stress given sufficient time. This acclimation a ppears to involve reversal of injury to the C-4 cycle following initial exp osure to low temperature. By contrast, carbon gain at low temperatures gene rally appears to be constrained by the carboxylation capacity of Rubisco, r egardless of acclimation time. The inability to overcome the Rubisco limita tion at low temperature may be an inherent limitation restricting C-4 photo synthetic performance in cooler climates.