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Photosynthetic Adjustments to High Temperatures in C4 Plants
Reference
Dwyer, S.A., Ghannoum, O., Nicotra, A. and von Caemmerer, S. 2007. High temperature acclimation of C4 photosynthesis is linked to changes in photosynthetic biochemistry. Plant, Cell and Environment 30: 53-66.

What was done
The authors grew well watered specimens of two C4 grasses (Panicum coloratum and Cenchrus ciliaris) and one C4 dicot (Flaveria bidentis) from seed for eight to ten weeks in 5-L pots filled with a sterilized general-purpose potting mix within controlled-environment walk-in growth cabinets maintained at either moderate (25/20 C, day/night) or high (35/30 C, day/night) temperatures, after which several types of physiological and biochemical measurements were made on the plants' leaves, including measurements of net photosynthesis or assimilation (A) and leaf nitrogen (N) concentration.

What was learned
Dwyer et al. report that "when compared at growth temperatures, the A/N ratio [was] consistently higher for the high-temperature-grown plants," and that "this [occurred] in spite of significant reductions in leaf N and photosynthetic capacity." Therefore, as they conclude, "when environmental conditions allow for it, C4 plants acclimate in such a way as to forsake the opportunity of fixing more CO2 (at the expense of using more N) for the more conservative option of fixing CO2 at a slightly higher rate while using less N." In either case, as they continue, "A/N improves; however, the latter acclimation strategy places a greater premium on saving N," which "is reminiscent of the well-documented acclimation of C3 photosynthesis in response to growth at elevated CO2 concentration."

In addition, they report there is "an increase in the temperature optima of A," which means that the plants grown at elevated temperatures come to actually prefer higher temperatures (where prefer is implied to mean "assimilate CO2 better"), as a result of their "acclimation" to the higher temperatures.

What it means
The Australian researchers conclude that C4 plants will show "modest changes in photosynthetic rates in response to changes in growth temperature, such as ... the warming anticipated as a result of global climate change," where the photosynthetic rate changes, of course, will be increases in response to the projected increases in global air temperature. Hence, we can expect both C3 and C4 plants to perform much better in a warmer and CO2-enriched world of the future than they do in the world of today, where they already do much better than they did in the days of the cold and CO2-deficient world of the Little Ice Age that preceded the Current Warm Period (see, in this regard, our Editorial of 11 July 2001).

Reviewed 4 April 2007