As for increased CO2 you can see the paper attached. Nevertheless I advice you to first look at the results separately and only after try to interpret the synergistic results from both stimulus.

In some cases it depends on the plant type.

An example is in the case of C3, C4 and CAM.

An elevated CO2 would cause the rubisco enzyme in C3 plants to be more efficient. Instead of acting as an oxygenase, it will act more as a carboxylase. This increases the rate of photosynthesis of the plants by facilitating the carlvin cycle instead of the alternative photorespiration. On the other hand, C4 and CAM plants have a carbon concentrating mechanism which inhibits rubisco to act as an oxygenase and therefore no major photosynthesis-related change occurs. Thus at high CO2 w/ all other factors remain optimum, C3 plants perform

In the case of high temperature, enzyme specificity is affected and rapid water loss occurs. Some CAM plants, in this case, fully close their stomates. On the case of C4, they are able to partially close their stomates because PEP Carboxylase brings in CO2 faster and so does not need to keep stomata open as much for the same amount of CO2 gain for phot synthesis.Thus, reducing transpiration rate and increasing water use efficiency. This does not occur in C3 plants causing high transpiration rate. Aside from this, enzyme specificity is affected as an increase in temperature reduces the activation state of rubisco. It also decreases the solubility of CO2, relative to O2. Thus, increasing photorespiration and decreasing photosynthesis.

Overall as a rule of thumb w/ all other components as optimum, C4 plants always perform better (higher PS rate) than C3 plants even at high CO2 concentration when there is elevated temperature (above optimum).

Good luck!

http://books.google.com.ph/books?id=0_3XqlcKPpwC&pg=PA320&lpg=PA320&dq=high+temperature+and+photosynthesis+c4+c3&source=bl&ots=yQFQtBoI00&sig=QzaAFHcGz4Zx-vY_raoaqlgf83A&hl=fil&sa=X&ei=50CuU7H6JcLClAWJ_4GgCg&ved=0CFQQ6AEwBw#v=onepage&q=high%20temperature%20and%20photosynthesis%20c4%20c3&f=false

there are many Dr Dwivedi and you can check the many excellent reviews on the subject (starting with the Drake et al., 1997 Ann Rev Plant Biol for CO2 and the Atkin et al., 2003 trends in ecol and evol for temperature).

I will give some examples for CO2. At the leaf level, reduced photorespiration and stomatal conductance may downregulate rubisco levels but still increasing photosynthesis and water use efficiency. Respiration is another matter with downregulation of enzymes, no consistent chnges in respiration rate (Gonzalez-Meler et al., 2004) and less efficient energy conservation at high CO2 (Gonzalez-Meler et al., 2009).

At the plant level there are changes in growth rate, NPP and LAI (Norby et al., 2010) which increases whole plant or canopy photosynthesis and maintains canopy transpiration. There is more root production in some cases (Iversen et al., 2010) but not in others (Matamala et al., 2003). There may be more allocation of C to belowground tissues at high CO2 (Hopkins et al., 2013; Lynch et al., 2013) enhancing microbial decomposition of organic matter (Chen et al., 2014). A few studies have measured the carbon use efficiency (the amount of gross primary production invested in biomass DeLucia et al., 2007) of plant systems and found that CUE is not altered by CO2 (DeLucia et al., 1999; Hamilton et al., 2002; Gonzalez-Meler and Taneva, 2005; Gonzalez-Meler et al., 2014).

This is a summary of aggregated responses but as Bernardo indicated variability is large particularly if looking at annual versus perennial plants or herbaceous versus woody plants.

Good luck with your research and keep me posted of your findings

Miquel

increased in the CO 2 level and temperature leads to increases in the photosynthetic activity but which level it will increase is still unknown.