Very broad question... Both sides of the story to tell

Climate change affects plants in many different ways. Increasing CO(2) concentration can increase photosynthetic rates.

Increasing temperature also affects photosynthesis, With increasing temperature, vapour pressure deficits of the air may increase, with a concomitant increase in the transpiration rate from plant canopies.

However, if stomata close in response to increasing CO(2) concentration, or if there is a reduction in the diurnal temperature range, then transpiration rates may even decrease.

Soil organic matter decomposition rates are likely to be stimulated by higher temperatures, so that nutrients can be more readily mineralised and made available to plants. This is likely to increase photosynthetic carbon gain in nutrient-limited systems.

All the factors listed above interact strongly so that, for different combinations of increases in temperature and CO(2) concentration, and for systems in different climatic regions and primarily affected by water or nutrient limitations, photosynthesis must be expected to respond differently to the same climatic changes..

Please refer:http://www.ncbi.nlm.nih.gov/pubmed/15143433

Furthermore we can say that important enzymes have their strict temperature optimal range in which they work, (for instance RuBisCO, a key enzyme of the photosynthesis). So an increase in temperature may strongly affect crucial metabolic pathways, slowing or halting important physiological processes as assimilation.

Please click:

http://www.as.wvu.edu/biology/bio463/Melillo%20et%20al%201993TEM%20NPP%20Estimations.pdf

https://books.google.it/books?hl=it&lr=&id=533UOWBU3_AC&oi=fnd&pg=PP1&dq=global+change+photosynthesis&ots=njKa2Gbwy-&sig=Obmt_-YGrtI9Jjk9710V4xyCfF8#v=onepage&q=global%20change%20photosynthesis&f=false

Finally, the topic is very complex. All the impacts related to the climate change may affect distribution and biodiversity of whole plant communities. But this is another story.

Regards

Valerio

In short the climate change is due to increase in the concentration of CO2 (50-60%). The increase concentration of CO2 increases the rate of photosynthesis where water is in sufficient amount. but in drought areas the increase in temperature decrease the level/amount of water in soil.

Increases in temperature predicted by climate change may lead to stomata changes, with high temperature causing stomata closure. Therefore, stomatal conductance decreases affecting photosynthesis, respiration and growth. Although differences among species can be found. Some species open their stomata with high temperature, but eventually, if temperature keeps increasing, stomata close to avoid water loss.

Kenneth.. sir, though we predict with some analytic saying that photosynthesis will increase with increased CO2 , no one knows the reality except plants themselves. So, any answers will not be near to reality..

Dear Kenneth,

I agree that there are many speculations. But I believe that speculations are a good starting point for science. 

Hello,

This discussion is difficult to follow!  There is a huge literature on the topic. Look at David W Lawlor's publications on Research Gate on this topic which will also give literature in the reference lists up to the about 2000.   (e.g. Look for Mitchell RAC, Mitchell V, Lawlor DW, regarding wheat and CO2 and water supply)  Focusing on current C3 plants  increasing CO2 from about 320 to 700 micromole CO2 per mol increases photosynthetic rate, because it increase substomatal [COs] and saturates Rubisco, Therefore wheat and sugar beet dry matter and yield increase substantially, if nutrients, water, and light are adequate. Increasing temperature by 3 or 4 oC above the ambient decreases wheat and sugar beet dry matter and yield  substantially, by about the same amount as CO2 increases it. This is a general effect in many places.  C4 plants do not respond as much (or some times not at all) to CO2 increase because they have a CO2 concentrating mechanism (PEP carboxylase) which provides a large [CO2] to Rubisco under current concentrations. Temperature effects in C4 plants have, I believe, not been so well analysed but increasing temperature above their optimum will, by definition, decrease production. Temperature is generally  complex - for example the special effects on reproductive development.

I hope this adds to the  discussion and clarifies things a bit.

as some extra information, regarding the quality of the product the influence of the greenhouse gases depends on the time of exposure (pre-harvest or postharvest) and also the type of the gases. Some gases such as ozone, SO2 and nitric oxide have negative influence on photosynthesis and also make the crop more susceptible to water stress, while in the postharvest stage they used for keeping quality of the products

It is a known fact that, C3 plants are more efficient than C4, but just shedding the energy via losses in oxygenation process has reduced its potential to be questioned. Mesophyll cells are abundant and bundle sheath cells are low in numbers. This itself will contribute better photosynthetic carbon assimilation in comparison to efficiency supplied by PEPc in c4 to direct RuBISCo in C3 plants.  Over the time, increased concentration of CO2 will favour C3 and saturation of PEPC in c4 remains same irrespective.

A pertinent study for this question

http://advances.sciencemag.org/content/4/5/eaaq1012