Altitudinal Variation in the Photosynthetic Characteristics of Snow Gum, Eucalyptus pauciflora Sieb. Ex Spreng. VII. Relationship Between Gradients of Field Temperature and Photosynthetic Temperature Optima in the Snowy Mountains Area

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Abstract
Elevational gradients of several field temperature parameters were examined and compared with estimated and observed photosynthetic temperature optima for a range of elevational populations of E. pauciflora. The gradient of maximum temperature (9.68°km-1) was found to be very close to the dry adiabatic lapse rate, regardless of whether annual, seasonal or monthly conditions were examined. In contrast, the gradient of minimum temperature (2.06°km-1) was much flatter and was affected by topographic location, particularly at sites exposed to nocturnal temperature inversions. The gradient of the square-wave conversion of the day-time temperature curve (Tsqw), found previously to be useful in mediating the photosynthetic temperature response, was intermediate in slope, 7.20°km-1. The gradient of the phytotron-derived preferred temperature for photosynthesis, Tpref (6.26°km-1), was very close to that for Tsqw. The acclimation coefficient, α, showed no apparent change with elevation. Estimates of field photosynthetic temperature optima, based on Tpref, α and Tequiv (which was the value of Tsqw averaged for 10 days prior to the date of measurement) were in close agreement with measured values. The close agreement between the gradients of Tpref and Tsqw, together with the effectiveness of estimation of field temperature optima by means of Tequiv suggested that a temperature function resembling Tsqw may exert effective selection pressure on the photosynthetic temperature response of E. pauciflora in this environment.