With respect to rising temperatures and their effect on photosynthesis, Kirschbaum states that "all plants appear to be capable of a degree of adaptation to growth conditions," noting that "photosynthesis in some species can function adequately up to 50°C."  In fact, he says that "photosynthesis can acclimate considerably [our italics] to actual growth conditions," noting that "optimum temperatures for photosynthesis acclimate by about 0.5°C per 1.0°C change in effective growth temperature (Berry and Bjorkman, 1980; Battaglia et al., 1996)."  This response, wherein plants adjust the workings of their photosynthetic apparatus to perform better at higher temperatures as temperatures rise, would appear to be especially beneficial in a warming world.

With respect to rising CO2 concentrations and their effect on photosynthesis, Kirschbaum notes that CO2 assimilation rates generally rise as the air's CO2 content rises: by 25-75% in C3 plants in response to a doubling of the air's CO2 content, and by something on the order of 25% in C4 grasses, according to the major review of Wand et al. (1999).  This response, wherein plants adjust the workings of their photosynthetic apparatus to perform better at higher atmospheric CO2 concentrations as atmospheric CO2 concentrations rise, would appear to be especially beneficial in a CO2-acreting atmosphere.

With respect to the synergistic effect of simultaneous increases in both atmospheric CO2 concentration and temperature on photosynthesis, Kirschbaum notes that plant growth responses to increasing CO2 are usually much [our italics] more pronounced for plants grown at higher temperatures," presenting a graph that suggests an approximate six-fold amplification of the aerial fertilization effect of atmospheric CO2 enrichment at an air temperature of 35°C compared to one of 5°C.  Consequently, in a world where both air temperature and CO2 concentration are rising, this response would appear to be hugely beneficial.

In the case of transpiration, or evaporative water loss from leaves, Kirschbaum notes that the air's vapor pressure deficit (VPD), which is the driving force for transpiration, tends to rise with increasing air temperature.  Because of the specific nature of the ongoing rise in planetary temperature, however, where warming is most prominently expressed at the coldest time of day and least expressed at the warmest time of day, he concludes that the resulting decrease in the diurnal temperature range "would have the effect of reducing the expected increase in VPD, or even lead to a decrease."  At the same time, Kirschbaum says it "seems likely, on balance, that stomata of most plants are closing to some extent in response to increasing CO2."  As a result, he further concludes that "for the combination of climatic changes observed to date and anticipated into the future, there most likely would be some decrease, rather than increase, in transpiration from most plant canopies."

In the final analysis, therefore, the photosynthesis and transpiration responses of the vast majority of earth's plants (increased biomass production and decreased water usage, respectively) to concomitant global warming and atmospheric CO2 enrichment appear to be incredibly beneficial.  Together, as we have long indicated, they can truly rejuvenate the biosphere, as they have been doing ever so marvelously since the end of the last glacial maximum (stage one, compliments of nature) and the inception of the Industrial Revolution (stage two, compliments of nature and man: nature for warming, man for rising CO2).

Reference
Battaglia, M., Beadle, C. and Loughhead, S.  1996.  Photosynthetic temperature response of Eucalyptus globules and Eucalyptus nitens.  Tree Physiology 16: 81-89.

Berry, J. and Bjorkman, O.  1980.  Photosynthetic response and adaptation to temperature in higher plants.  Annual Review of Plant Physiology 31: 491-543.

Kirschbaum, M.U.F.  2004.  Direct and indirect climate change effects on photosynthesis and transpiration.  Plant Biology 6: 242-253.

Wand, S.J.E., Midgley, G.F., Jones, M.H. and Curtis, P.S.  1999.  Responses of wild C4 and C3 grass (Poaceae) species to elevated atmospheric CO2 concentration: a meta-analytic test of current theories and perceptions.  Global Change Biology 5: 723-741.