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Temperature-response curves for photosynthesis and respiration are useful in predicting the ability of plants to perform under different environmental conditions. Whole crop CO 2  exchange rates of three magnolia ( Magnolia grandiflora  L.) cultivars (`MGTIG', `Little Gem', and `Claudia Wannamaker') were measured over a 25 °C temperature range. Plants were exposed to cool temperatures (13 °C day, 3 °C night) temperatures before the measurements. Net photosynthesis (P net ) of all three cultivars increased from 3 to 15 °C and decreased again at higher temperatures. `MGTIG' had the highest and `Little Gem' the lowest P net , irrespective of temperature. The Q 10  (relative increase in the rate of a process with a 10 °C increase in temperature) for P net  of all three cultivars decreased over the entire temperature range. `MGTIG' had the lowest Q 10  at low temperatures (1.4 at 8 °C), while `Little Gem' had the lowest Q 10  for P net  at temperatures &gt;17 °C and a negative Q 10  &gt; 23 °C. This indicates a rapid decline in P net  of `Little Gem' at high temperatures. The decrease in P net  of all three cultivars at temperatures &gt;15 °C was caused mainly by an exponential increase in dark respiration (R dark ) with increasing temperature. `Little Gem' had a lower R dark  (per unit fresh mass) than `MGTIG' or `Claudia Wannamaker', but all three cultivars had a similar Q 10  (2.46). Gross photosynthesis (P gross ) was less sensitive to temperature than P net  and R dark . The optimal temperature for P gross  of `MGTIG' was lower (19 °C) than those of `Little Gem' (21 °C) and `Claudia Wannamaker' (22 °C). The Q 10  for P gross  decreased with increasing temperature, and was lower for `MGTIG' than for `Little Gem' and `Claudia Wannamaker'. All three cultivars had the same optimal temperature (11 °C) for net assimilation rate (NAR), and NAR was not very sensitive to temperature changes from 3 to 17 °C. This indicates that the plants were well-adapted to their environmental conditions. The results suggest that respiration rate may limit magnolia growth when temperatures get high in winter time.

Temperature Response of Whole-plant CO2 Exchange Rates of Three Magnolia Cultivars